Note: Descriptions are shown in the official language in which they were submitted.
CA 0222~679 l997-l2-23
W O 97/03215 P~l/~61.
Iron-based shape memory and vibration damping
alloys containing nitrogen.
This invention co. ,ce- ns r-iL u96n-containing shape memory and vibration damping
5 metals, particularly shape memory steels.
In the following text, reference is often made to only shape memory metals or
shape ",e.nory steels, even though this means metals and especi~lly steels whichhave both shape mer"o"/ and damping properties. How great a ~ropo, Lio- ~ can be10 counted as memory properties, or correspondingly as damping properties,
depends on the composition used.
Shape memory metals mean metallic materials, in which a so-called one or two-
way shape .ne,--oiy effect appears. The shape ~e~oly effect is based on the
15 ~ on of a l..a- lensilic ~n~ro...lc.lion. When, in a one-way memory effect, an
austenitic (austel-ilê is a phase that is stable at high temperatures) sample iscoolecl, it forms martensite. If the formation of the martensite does not favour any
direction, for example due to external stress, the shape of the piece does not
change. When the material is deformed (generally less than 10 %), the twin
20 structure of the martensite phase of the material is rea, .anyed so that the twins
that are in an advantageo~ ~s orientation with the stress grow at the expense of the
others and new Illdl lensile can arise as a result of the stress. When the piece is
el,edled above the te,nperdl-Jre of auslenile fo.~alion, the material may return to
its form preceding the deformation.
In some materials, martensite does not arise during cooling, but forms during
cl~ru., . I~Liul 1. Rec~ ~se twinning occurs in three dimensions, the shape of the piece
may even cl ,a"ge during deror---dlion in a very complicated manner, and
nevertheless still return to its original shape when heated. A one-way shape
30 memory effect can be exploited, for example in aLLacl.r.,enl, tensioning and
prestressed structures.
When a rod-like sample dero,.ned by straining is heated to the austenite range,
the s~.-"~le will recover its length before cJeror.naLion, if the shape memory effect
CA 0222~679 1997-12-23
W O 97/03215 PCT/~ 5/~ZI~
is complete. Recovery may also be partial. If, for example the recoverable strain
is half of the strain arising from sl, elcl ~i"y it is said that the recovery rate is 50 %.
The stress ~ Ised by recovery is called recovery stress.
5 In a two-way shape memory effect the maleri21 Ureme,,~bers two shapesr, which
are achieved by heating and cooling. The te""~eralure dirrerel,ce between the
states may even be 1~C. Among the most i"~o, IdnllllelllUIy effect applicaliGI ,s are
so~alled ~ct~ l~tors in active viL,r~lior, damping, in robotics, valves heat relays and
cGr"posile structures.
The most illlpolla"t memory metals used at ,ul-ese,,t are Ni-Ti and Cu-based.
These memory metals are quite eAI~e~ ~sive, which is the reason that the
develop",enl of iron-based memory metals, i.e. memory steels, has been begun.
It is possi~le to divide ",e",oly steels into the following ~ ~sses accordi. ,y to the
15 type of lattice structure in the ",a,lel.sile that is obtained: BCT (Body-centred
tel,dgo,)al), BCC (Body ce"l,~d cubic) and HCP (Hex~gsnal close-packed). In Fe-
Ni-Co-Ti steel, BCT ma,lel)sile is formed from the FCC (Face-centeral cubic)
auslel,ila phase. BCT rllallellsite is generally formed in such an alloy in which
there is a high stacking fault energy. A large change in specific volume is
20 ~ssoci~led with the lrallsron~)alio,-. In this kind of ",~,lensile the derol",alion
mechanism is often slip in ~dition to twinning. The fact that the deformation
based on slipping is non-recoverable weakens the shape ~emo~y properties of
this kind of alloy. If however the material is alloyed in such a way that it has so-
called invar properties slip dero,l"alion is prevented and the memory properties25 may be good.
In Fe-Mn-Si-based "~e"~oly steels HCP r"~,lensile arises in der~"",alion. HCP
martensite generally arises in such alloys in which there is a small stacking-fault
energy and a small change in specific volume. The memory propei lies are based
30 on the fact that derorl"~lion takes place by twinning nor does slip practically
appear.
Examples of such memory steels, in which HCP ~a~lei~sile arises in deror",alion
are given in US Patents 4 780,1~4 4,933,027 and 4 929 289.
CA 0222~679 1997-12-23
W O 97/03215 PCTn~6100408
The first one rerened to is based on an iron-based alloy composed of the
following cG,IsliLuents:
Mn 20 - 40 % (weight %) Si 3.5 - 8.0 % and at least one of the following eler"enls;
Cr-10 % Ni -10 % Co -10 % Mo -2 % C -1 % Al -1 % Cu -1 % which is
balanced with iron and random impurities.
The sec~"d of the pd~enls f~rel, ~ d to is also an iron-based " ,er"o, y steel in which
there is Cr 5 - 20 % Si 2 - 8 % and at least one of the following elements: Mn 0-1 -
~0 14.8 % Ni 0.1 - 20 % Co 0.1 - 30 % Cu 0.1 - 3 % N 0.001 - 0.3 % and in which Ni + 0.5Mn ~ 0.4Co + 0.06Cu + 0.002N ~ 0.67 (Cr + 1.2Si) - 3.
The last patent rerer, ed to depicls an iron-based ~en~o~ y steel in which there is
Cr 0.1 - 5.0 % Si 2.0 - 8.0 % Mn 1.0 - 14.8 % and at least one of the following
ele",enls. Ni 0.1 - 20 % Co 0.1 - 30 % Cu 0.1 - 3.0 % N 0.001 - 0.400 %, and in
which Ni + 0.5Mn ~ 0.4Co + 0.06Cu + 0.002N 2 0.67 (Cr + 1.2Si) which is
balanced with iron and random impurities.
The first of the memory steels referred to achieved a recovery rate of 75 - 90 %.
The addition of at least one element from the group Cr Ni Co or Mo is intended
to improve co" OSiOI, r~sislance. However corrosion resistance is not very good in
these steels on account of the high manyanese coule"L In ~d~ition these alloys
oxidize at high te~ eraL-Ires. Oxidation may occur already when the sample is
being heated to the austenite range to recover the original shape after
deformation. The addition of cl-ro"~ium to the alloy in which there is 20 - 40 %",an~,anese and 3.5 - 8.0 % silicon may lead to the rorl"dlion of a brittle o-phase
which reduces the shape memory properties fo""ability and ductility of steel.
Also the steels according to US Patents 4 933 027 and 4 929 289 do not have
good ductilityvalues and rul",abili4 cha,~-~e,islics. In addition their strengths and
corrosion properties are quite poor. In many cases corrosion resistance is also
insurricienl.
Practical applications require such memory steels that have good shape memory
CA 0222~679 1997-12-23
W O 97103215 PCT/~ 1~8
~,upel lies high s~ ts. Iyll ~ and ductility and good corrosion resistance. They should
also not oxi~ e at high temperatures.
On the other hand the damping of vibration in ",acl,i.,es, equ;t,",e"l and
5 structures has become increasingly il,.po.lanl with industrialization. Vibration
causes both structural fatigue and reduces the performance of equipme~ ll. Further
vibration and noise may be del,i",ei.lal to peo~le's health. An effective way ofreducing the level of vibr~lio- - is to use da-- "~;ng m~lerials in the manufacture of a
",acl-i.,e causing vibration. This is often not possible, bec~use sl~iPhle d~lllping
10 construction materials are not available. The iron-based damping constructionmaterials that are most in use are grey cast irons. Their mecl.a"ical properties,
above all ductility are quite modest which limits their use.
Certain ferrite steels have a high damping capacily. The dam~i"y is based on
15 " ,ayl lelo el~-~ticity. Their use is limited by the fact that their da" ,piny properties are
substantially weakened by derol"~alion or welding. In addition their strength isonly at the level of mild structural steel (Fe37) and they are cold-brittle.
The phase boundary between the c-...a~lel~sile phase appearing in certain iron
20 and manganese alloys and the austenite phase is sensitive to the mechanical
loading of the material. This movement has been shown to damp vibration (C.-S.
Choi et al. Proc. of the Int. Conf. on Mal le~ Isilic Transror",~lions ICOMAT92, ed.
C.M. Wayman and J. Perkins, 1993 pp. 509 - 514). The structure of the e-
llldl lensile phase is hexagonal close-packed and that of auslel ,ile is face-centred
25 cubic. In binary iron-based Fe~n alloys, the highest .lalllpi. ~y ~p~city is achieved
with a composition Fe -17 (mass) % Mn. This col.,~osilion has been selected as
the reference material for this invention.
It is the intention of this invention to create depending on the use either memory
30 steels or damping steels or prerer~Lly both simullal)eously which have the
aforementioned good cl ,aracleri~lics. In other words they have excellent shape
" ,e" ,o~ prùpel lies high slrenylh and ductility and good corrosion r~sisla"ce, as
well as high te,-"~eralure oxidalion resislance. The intention is to also achieve a
high dalllping cA~AciLy. In addition, the steels should retain a high damping
CA 0222~679 l997-l2-23
W O 97/03215 PCT~g6/00408
capacity even when the material is cold-worked. A nitrogen alloy has a central
siyl ,iricance in the achievement of the above prope, lies.
The dror~r"entioned excellent pro~e,lies are achieved using steels with the
5 c hd~d~;teristic features described in the accompanying Claims.
The invention is described in the following text by desc,ibi,)g compositions
accor-Ji"y to the invention, without limiting them ~rec;sely to those des~ iL ed in
any way wl,alsoever. Rerereilce is also made to the accompanying patent
10 draw;. ~y:~ in which:
Figure 1 (a) shows the stress-strain yl ~,uhs of two example steels (curve 1 = steel
number 4 and curve 2 = steel number 2) to be described later,
15 Figure 1 (b) shows the stress-temperature graphs of the same example steels in
Figure 1(a) measured during the heating cycle carried out after the treatment
desc;,il,ecl. During the heating cycle, the length of the samples was kept consla"l.
The treal",e,)ls shown in the figures were carried out five times and the curvesshow the fifth treatment,
Figure 2 shows the length of a 6 % deformed sample of one alloy according to theinvention (steel number 5) as a function of temperature,
Figure 3 shows the damping ~l~cil~ (loga, ill "nic decrement) of one steel (steel
25 number 25) as a function of the vibration amplitude cor"pared to the rererence
steel (steel number 27) and
Figure 4 shows the stress of the same steels as a function of strain.
30 In orderfor HCP ~d~lensile Illelllo~ steel to have good shape memory properties,
the following conditions must be fulfilled:
1. Before deformation. the amount of martensite must be as small as possible.
2. The surface energy of the stacking fault of the austenite must be as small as
CA 0222~679 l997-l2-23
W O 97/03215 PCT/~
possible. In ~ddilicsl 1 r-rl)dl lensiLe must form in the deror",alion and the quantity
of a-",a,le"sile must be as small as possi~le.
3. The sLI er,gU I of the austenite must be as high as possible. In a strong matrix, the
derorllldLion of auslellile through slipping ~ecGIIles difficult.
5 4. The temperature of fol"lalion of malle,lsile Ms must be above the Neel
temperature TN~ at which anlirerl o"~ayl letic orderil l9 takes place.
There is abundant data in theory on the assumed and proven effects of various
el~llellls on the properties of .,-e...û.~ steels. One exd..lple that can be given is
the description in US Patent 4 933,027 referred to above as the state of the artwhich quite e,cte, Isively desca iL,es the significance of dirrer~- IL ele m enl~ in memory
steels.
The aforementioned and other ractor~ have naturally been studied in the
15 develo,c " le, It work on the ~l lemory steel according to the invention. On the basis of
this clesc;, iplio, I and of practical expel il l le, lls, the contents given in the Claims were
arrived at for essentially the following reasons.
1. Manganese. Ma. .yanese stabilizes austenite strongly and increases the
20 s . - Ihility of, .it, oge. " which also stabilizes ausler,ile. When the Mn-content is less
than 5 % a-ma,le,-sile begins to form (in ~d~ition to e-rllarLellsile) to such an
extent that memory and damping ,~,ru,c,ellies begin to s~ nlially worsen. In
chromium silicon and ,,il,ogell-cûlllailling alloys the reduction of the ~l~allganese
conlenl may cause the ro,ll)dlion of o-ferrite during the cooling following melting
25 which leads to the forlll~liol- of porosity ber~se the solubility of nilro~aen in ~-
ferrite is very small. If, on the other hand the ,nd"ya"ese conlenL Pxceeds 50 ~/0
the Neel tel"peraLure rises too much nor can even the addition of silicon and
nitrogen reduce it sufficiently from the point of view of the shape memory effect.
30 2. Silicon. Silicon redoces the stacking fault energy of austenite, increasesstrength and red~ ~ces the Neel temperature. If the content is less than 2 % thedesired ~,ru~Je,Lies are ye.lerally no longer obtained. Nonetheless, thanks to
nitrogen alloying the Illel,lor~/ effect is also presel,L in such alloys in which there
is no silicon at all. At silicon co,-LenLs in excess of 8 % the ductility of steels
CA 0222~679 l997-l2-23
W O 97/03215 PCTn~6/00408
diminishes and the hot and cold-workability is red~ ~cer~
3. Nibogen. NiLIugen has been selected as part of the alloy heG~ se it reinforces
au~l6"ile (and ."a, lensile) more than any other element and stabilizes austenite
5 as well as improving corrosion resistance. Nitrogen improves both shape n,e",o, y
and da" ,,.,i, Iy p, ope~ lies in the alloys accordi"g to the invention. Nill ogen prevents
the fo""alion of the brittle c~-phase which recl~ces ductility. An ~ ro,criale Neel
te""~erdlure can be set by -~le~i-ly a s~-it~hle ratio of llilloyel) and ",a"y~nese.
The alloying of llillc,gen and ",a"gd"ese has o~,uosile effects on the Neel
10 temperature. When the r,il,~ye., colllellt is less than 0.01 % the effects described
above are insignificant. If the CGI ~te~ IL is above 0.8 % the steel becomes brittle.
4. Chromium. The acldilio~- of cl,ror"ium red~ces the stacking fault energy and
improves cGr, OSiOI, r esisla"ce and high temperature oxidation resislal ,ce.
15 Chromium also i"~ ases the solubility of nilrogel). If the chromium content is less
than 0.1 % the above effects are insignificantly small. If on the other hand thechromium conlenl is above 20 % o-ferrite may form during the solidification stage
of the smelting of the steel. In the same way during the solidification or during the
heat treatment stage of steel brittle cJ-phase may form.
5. Nickel. Nickel stabilizes austenite sl~ongly and improves the corrosion
resistance of steel and its high temperature oxidation resisldl,ce. At co"le,1ls of
less than 0.1 % the effects are insiy"irical,l. At conle,.l~ of more than 20 % the
temperature at which "~a~lensile still forms with the aid of derc"",alion becomes
25 very low when the amount of ~"a,lensite rolll,i~,y decreases and finally no
, l lal lensile forms at all.
6. Cobalt. Cobalt improves the memory and hot-working properties of steel. At
conlel lls of less than 0.1 % the effects are insiyl liricantly small while if the conlenL
30 grows to more than 20 ~/0 no further improvel"enls are gained.
7. Copper. Copper stabilizes austenite and improves corrosion resisl~"ce. The
adval,tageous effects of copper appear if the conlel,l is more than 0.1 %. If the
copper conlenl exceeds 3 % the rollllalioll of c-lllallensile in deformation is
CA 0222~679 1997-12-23
W O 97/03215 PcT/~
prevented, becAuse copper ir,~ eases the slackiny fault energy of austenite.
8. Vanadium and niobium. Vanadium and niobium i,~ 3ase yield sl~n~lll. They
also increase the solubility of nil.os~e.. in a molten state, which is i...po,lar,l from
5 the point of view of manufacture. If the cG,-le"ls are less than 0.01 %, the effects
are insignificant, while if they exceed 1 %, shape ~--~r..ory pr~pe,lies and thefGrmaL,ility of the steel weaken. Vanadium and niobium form finely dispersed
I lib ides, which r ,;. I~u.ue steel, which in turn may ina ease the recoverable strain of
the shape ..-e,..Gry effect.
9. Molybdenum. Molybdenum recl~ ~ces the slacking fault energy and improves hightemperature oxiddlion resistance. If the conlenl is smaller than 0.1 %, the effects
are i"siy, .iricanlly small, and if the conlenl is ~aler than 3 %, the memory and
hot-workability properties of the steel worsen.
10. Carbon. Carbon has been selected as an alloying component, because it
reinforces and stabilizes austenite and improves the shape " ,ei "o"/ effect.
Conle"ls of less than 0.001 % have no effect on the properties, and if the content
e~ eeds 1 %, ductility begins to diminish sl ~hsPntially.
11. Rare earth metals (e.g. Sc, Y, La, Ce). Rare earth metals prevent the
prec;,.,il~lion of ele",~l)ls at the grain boundaries, which improves corrosion
resisl~"ce. If the contents are less than 0.0005 %, the effects are insignificantly
small. If the contents are more than 0.02 %, the mechanical properties and
25 workability of the steel weaken decisively.
12. The ratio of the total amount of the elements stabilizing the austenite to the
total amount of the elemenls stabilizing the ferrite.
30 In the steels that are the object of this invention, it is important that the material is
completely ausl~, lilic, or at least that the amount of psssihle a-mal LensiLe is small,
before derc,r",aiion. Due to this, the following equation must be conrul",ed to, in
addition to the above limiLaLions;
CA 0222~679 l997-l2-23
W O 97/03215 PCTn~96/00408
Ni + 0.5Mn + Co + 0.3Cu + 20N + 25C 2 0.3 x (Cr + 2Si + 5V + 1.5Nb + 1.5Mo)
The ability of the ele",e,)ls in the steel to stabiiize auslenile can be cl~ ted by the
nickel equivalent Ni~qu~ which is the left-hand side of the above e~ tion. The
5 right-hand side depi~;t-~ the ability of the elements to stabilize ferrite, this being
termed the cl ,rur..ium equivalent and marked Cr.quh,.
13. Impurities. The phosphorus and sulphur col lle- llS must be less than 0.02 %.
When all of the ~,ope, lies des~ ibed above are taken into account, then the result
according to the invention is a ~emo~y steel cGIl~osiliol~, which, in ~d~ition to
iron, Col .Lains the following elel"e"ls in the cor,le"ls given (weight-%):
Mn5.0-50.0%,SiO-8.0%andNO.01 -0.80%.
In order to improve certain properties, one or more of the following elements may
be added to the composition:
CrO.1 -20.0 %
Ni 0.1 -20.0 %
CoO.1 -20.0%
Cu 0.1 - 3.0 %
VO.1 - 1.0 %
NbO.1 -1.0%
MoO.1-3.0~/O
C 0.001 - 1 .0 %
Rare earth metals (e.g. Sc, Y, La, Ce) 0.0005 - 0.02 %.
The following equation too should be valid:
Ni + Co + 0.5Mn + 0.3Cu +20N + 25C 2 0.3 x (Cr + 2Si + 5V + 1.5Nb + 1.5Mo),
balanced with the aid of iron and ran-lo m impurities.
The nitrogen alloying was observed, according to the invention, to suL,sla"lially
CA 0222~679 1997-12-23
W O 97/03215 PCT/r~5/~
improve not only the shape r"ên ,o. y ~ ~, lies of Fe-Mn-based l"e" ,o"/ steels but
also theim"e~l Idl ~ I prope, lies including da",ping prope, lies. Other ad~,anlages
of "~e" ,o, ~ and damping steels accor Ji. ~ to the invention are ease of manufacture
working and joining by weldills~. BecA~se a weld also has shape Illelllor~
5 properties the areas of the joints do not form points of ~lisconli",~ity in for
example prestressed structures. In Addition ~ ~il- ogel ~ improves co., OSiOI,
resisldnce and high temperature oxid~tion resisld"ce. Other alloy cor,.~.o"e"ts
used in memory steel (such as Mn and Cr) increase the scl ~hility of the llilloyel"
so that the nil,uyen alloyin9 is brought surricienll~ high by using the normal
10 smelting methods used in the steel industry. By carrying out smelting in a high-
pressure nitrogen atmosphere or by using powder metallurgical manl~Actl~ring
Ill~lhGdS, it is possible to inc;.~ase the nitrogen conlenl of steel still further but the
higher cost of the man~ ring ."elhoc~s may then limit the al-pli~li4,,5 of the
steel.
The following ~Jer"o"~lr~les with the aid of examples the effect of nitrogen alloying
on the properties of memory and damping steels. All of the steel examples were
manufactured by conventional induction melting in an argon-nitrogen atmosphere
the partial pressure of the nitrogen being varied in order to obtain a certain
20 t liLI oge n conlênl in the alloy. After smelting the steels were hot-rolled into 5 mm-
thick bars at a te."~ erdl.lre of 1273 - 1373 K and then cold-drawn into 3 mm wires.
When the damping properties were invesfi~te~ the steel alloys were drawn into
1 mm wires which were annealed at a temperature of 1273 K for half an hour and
then quenched in water.
Table 1 Compositions of example steels
A~oy Mn Si Cr Ni V N Co Cu Nb C
No.
1 17.40 5.10 13.00 3.45 - 0.22
2 16.40 5.48 8.09 3.67
3 17.50 5.28 8.56 3.85 - 0.20
4 18.40 5.10 9.70 3.73 0 20 0.20
CA 0222~679 1997-12-23
W O 97/03215 PCT~96/00408
11
13.90 4.68 13.10 4.80 0.20
6 14.90 7.60 0.20 18.00 0.04
7 18.20 5.47 0.40 0.042 17.00
8 16.90 2.87 8.00 12.00 0.12 2.80
9 17.50 5.28 8.56 3.85 0.13
28.80 5.24 0.20 0.11 0.20
11 26.90 5.48 4.00 2.00 0.14
12 24.80 5.44 5.18 0.15 0.51
13 24.00 5.42 8.47 3.80 0.18
1 0 14 34.30 5.87 10.10 0.39
30.10 5.91 8.20 0.20 0.52 0.28
16 40.50 5.94 12.00 0.6 0.20
17 45.00 2.21 18.30 0.59
18 16.00 5.20 9.10 4.30 0.14
1~ 19 18.30 4.50 2.30 2.50 0.50 0.01 0.50
6.00 6.10 12.60 12.60 0.22
21 14.00 11.60 0.23 10.00
22 20.00 6.00 7.00 0.22 1.00
23 20.40 5.10 9.50 3.43 0.20
24 15.30 2.00 0.22
3.70 0.20 0.004
26 15.3 0.11 0.004
27 17.5 0 005
2~ Alloy 27 = reference alloy
Note: Alloy 18 also 1.2 Mo and 0.001 Ce; Alloys 26 - 28 also 0.001 P.
The properties of the above alloys were investigated in the manner described in
the following text:
1. Mechanical properties
The yield and ulli",ale ~l,e"~Lhs and fracture strains are given in Table 2. TheinvesLigdlion of yield ~ nyll, and fracture strains was carried out for several other
CA 0222~679 l997-l2-23
W O 97/03215 PCT/r-.S."C1~~
12
steels. The displacement rate in the testing machine fixture was 1 mm/min. The
measured length of the samples was 100 mm and the thicknesses 0.8 mm.
Table 2 Yield ~lr~ U) (Roa2)~ ultimate tensile ~ l h (Rm) and fracture strain (R30)
5 of example steels 1 - 4
Alloy Yield sl,enyLI, Ultimate tensile Fracture strain
(MPa) sl.enytl, (Mpa) (%)
970 1481 30
2 690 980 34
3 860 1110 32
4 850 1200 11
6 840 24
7 840 31
8 890 28
9 880
850 32
11 870 25
12 860 26
13 900 20
14 900 19
880 24
2~i 16 950 10
17 960 9
18 860 18
19 900 12
780 60
21 1020 25
22 1000 20
23 990 1280
24 460 1080
420 1000
CA 0222~679 l997-l2-23
W O 97/03215 PCTn~6/00408
13
26 380
27 300
~ The nitrogen alloying was shown to increase the yield sllellylll and ullirllale
5 sll ~, Iyl h of the steel but it was not observed to reduce the fracture strain. Figure
4 shows stress-strain plots for materials 25 and 27 (rere, e~ ~ce alloy). Material 25
clearly work-llar~le"s more than ..,dlerial 27 and the y~dlesl value for strain
measured for the r~ilruyen alloyed Illalel ial 25 was more than 50 % gredler than for
the rerere"ce Illalerial 27. The test cler-o--al-dles that rlilluyell alloying cleariy
10 improves precisely the mechanical prol~ellies of dallll~i,ly steel. The damping
r~p:~cjly was retained at quite a high level even in worked steel up to a reduction
of a few percent.
2. Shape ."e..,oly properties
Shape ")e",o,y prupe,lies were investigated on a materials testing r"acl,ir,e using
sampies of 3 mm Ihick ar nealed wire with a dime, ~sional length of 30 mm. The
samples were stretched by 5 mm and then heated above Af temperature (at this
te""~er~lure all rl~a~le~lsile has c;l,anged into austenite). The recovery of the strain
in relation to the original strain (preceding heating) was used as a criterion of the
shape memory properties. Depending on how great this value was three quality
cl-sses were set for this ratio (shape recovery rate).
Class 1: ratio greater than 70 %
Class 2: ratio 30 - 70 %
Class 3: ratio less than 30 %
The cl ~-sses of the steels are given in Table 3.
The effect of nitrogen alloying on the recovery stress was also investigated andthe results are given as graphs in Figure 1 for steels 2 and 4.
The nitrogen-alloyed sample 4 also co"lai,~ed Cr and V-r lill ides. Figure 1 (a)
CA 0222~679 1997-12-23
W O 97/03215 PCTi~-.S/.ClD~
14
shows the i~ ase of stress during al- dil li~ 19. ~-llldl Lel ,siLe arises in derorm~Lion.
The stress level (aLr~n~u,Ll ,) of the nitrogen-alloyed steel 4 is clearly g,~aler than
that of steel 2, which does not conlai. ~ "ilroye". The values for Fe-Mn-Si-based
non-nitrogen steels given in the literature are clearly lower than those of steel 4.
Once the stress was removed, the te" ,per~lure of the samples was raised to about
800 K and after that back to room tel,.l~er~l-Jre, keeping the strain (= length of
sample) CGI laldl ll during the entire cycle. Stress was observed to i- ~c. ~ase at the
beginning of the I .ealing stage, bec~ ~-ce the Illdl lensile beca,ne auslel ~ile and then
10 diminished, as a result of the thermal e~l~a~-sio-- of the sample. The r-~il"um
values for the recovery stress of steel 4 were about 300 MPa, while the value for
the nitrogen-free steel 2 was only about 200 MPa. The recovery stress values
given in the literature for nil,uyel ,-free Fe-Mn-Si-based memGIy steels are 150 -
200 MPa.
Ni~ uge- ,-alloying thus clearly i"creases recovery stress. Recovery stress is a very
i..,po.lanL variable in shape memory steel applications (e.g. tighteners, fasteners
and preslressed structures), often even more i-.,po,Lanl than recoverable strain.
The recoverable strains of nil- oyen-alloyed memory steels are 1.5 - 4 %. Thermo-
20 mechanical cycling, i.e. the so-called lrdini,.g of mel.,o,y steel, was observed to
increase the recoverable strain and to yenerally move the fGr...alion of auslenile
to a lower temperature in nil.ogen-alloyed steels too. In Figure 1 (b), thermo-
mechanical cycling was repealed five times and the curves of the Figure were
measured from the fifth cycle. The ratio between recoverable strain and the
25 original defo-malion also increased as a result of training. Its value was generally
0.6 - 1. Complete recovery was observed with a strain of as much as 3 %, for
example in steel number 22.
When the temperature in Figure 1 was brought back to room te~"peraLure, a
30 pe""d,)enl stress of about 700 MPa remained in nitrogen-alloyed sample 4. In the
.IiLIoge,I-free sa~ le, the stress was less than 400 MPa. In many applications (e.g.
~lLacl.,.,el.Ls, tensioners and the presL.essing of cGncrele), the magnitude of the
residl ~~1 stress is an excellent advantage.
CA 0222~679 l997-l2-23
W O 97/03215 PCTn~g6/00408
Nil,oye"-alloyed steels accorc~ s3 to the invention have good shape memory
properties and mech~"ical ~rupe, lies, even at cryc,ge"ic ter"~,eralures.
Recoverable strains of a few per cent were measured in tensile tests carried out at
liquid rlill o~~eo tel "per~l-Jres.
Nitrogen-alloyed shape memory steels were also observed to have a two-way
shape memory phenomenon. The recoverable strain in one and two-way shape
",emo,y effect is given in Figure 2. The example steel is steel 5 of Table 1. When
a sample ~l~ror",ed by a 6 % stretch is hedled, the sample si,o,lens by 3.5 %,
which is the recoverable strain of a one-way shape ",el"oly effect. When the
sample is cooled and healed after this to between -196~ C - 750~ C, a loop is
obtained, which ~lepic1s a two-way ",el"or~/ ~.I,el,o",el,on. In this steel its
",ay"ilude is about 0.4 % after the third cycle.
3. Vibration damping properties
The damping capacities of rnalelials 25 and 27 (rererence steel) are given in
Figure 3 as ful)uLiuns of the amplitude of the vibration. At a small amplitude with a
value of 0.00005 the damping capacity (loy~rillll-lic decrement) is about 0.02. As
the vibration amplitude increases the damping ca,uacily of both materials
increases but the damping capacity of alloy 25 increases more rapidly and at an
amplitude value of 0.0002 it is more than 50 % ~-~aler than that of alloy 27. The
effect of nitrogen-alloying in improving damping capacily is obvious. A high
d~ Jil lg capaciLy was shown to be retained over a broad range of temperatures.
The damping values of steel 26 were between those of steels 25 and 27.
The properties of shape ",el"o,~ and damping steels according to the invention
are excellent according to all the criteria given. The values also clearly exceeded
the values given in the literature.
The dalllping ~p~ y (1092,iU",.:c decrel"ent) of steels accordi"g to the invention
is typically 0.01 - 0.08 at small vibration amplitudes (relative defor" ,dlion 10~ - 10-
5). At greater amplitudes (c. 10~) the damping ca,uacil~/ is as much as 0.1. Steel
number 23 is an example of a steel which combines excellent mechanical
CA 0222~679 1997-12-23
W O 97/03215
16
~ope~lies corr~sion ,esislance and IllelllGIY properties (a dero""dLion of 2.5 % is
recovered completely) as well as a high da~ iny ca~,acily.
4. Corrosion resistance
The corrosion resisla"ce of the steels was ev~ te~ metallographically after the
Sdll ~l~s had been in the at",ospl)er~ for one year. The steels were divided into
three c~ ~sses on the basis of the following criteria.
10 Class 1: No corrosion products at all observed
Class 2: Cor, USiol, products were observed to some extent on the sur~ace of the
s~" ",1~
Class 3: The surface was entirely coated with co" UsiGn pro~ ctc
15 Table 3 also shows the results of this test.
5. High temperature oxidation resistance
The samples were heated to 600~ C in an air atmosphere and ll ,er~3arler the same
20 kind of ev~ tion as in Section 3 was carried out. The steels were divided into
three cl~sses on the basis of the following criteria.
Class 1: No co" uSiGI, products at all observed
Class 2: Corrosion products were observed to some extent on the surface of the
2~; sample
Class 3: The surface was entirely coated with corrosion products.
The results are given in Table 3.
CA 0222~679 1997-12-23
W O 97tO3215 PCT/r~5/~
17
Table 3 Com~dl iso" of shape me,ool y steels according to the invention
Alloy Shape Co"osio-~ High tel"~,er~ re oxidation
",el"o~y resistance resislance
,~,ropel ly
6 2
1 2 2
11 1 1 2
12
13
14 1 2
16 1 2 2
17 2 2
1 2 2
21 1 2 2
22
23
24 2
26 2
27 - 3 3
Nitrogen-alloyed "~el"oly steels according to the invention are the first shape
memory materials whose propellies and prices permit the exlel,sive industrial
application of shape memory materials. Nitrogen-alloyed memory steels are
CA 0222~679 l997-l2-23
W O 97/03215 PCT/~ 5/~CI~
18
excellently applicable as ~.,aler;als for alla~ enls (e.g. machine components,
stones), te,lsioner:, (e.g. pipe co""ec~ions) and various pre:,l,essed structures
(e.g. co, Icrete reinrorceme,-l steels).
5 The use of ~l~emo~y steels in the above applicalions is based on the fact thatdeformation is carried out on mer"ol~ steel products before they are inst~e~l
Separ~te derol .,~ic" l is not always required, hec~ ~se the normal working of steel,
such as the drawing of wire or the ~ressing, ~or~ y or cold-rolling of a plate or
similar can act as the necess~ry dero,mdlion. This pellllils cc,lsiderdLle savings
10 in costs. During the deror,llalion, mal lel ~sile forms, the twin structure of which is
o, ienled due to the stress held, or the twin structure of rl ,a, lensile that has already
formed is re-oriented. After inst~ fion, the memory steel product is heated to the
austenite range (typically 100 - 350~C), in which case part (or all) of the Illdl lel ,sile
Chdl ,~3es to austenite. The product then tries to retum to its pre~lerc n"dlion shape,
15 which c~uses the desired stress in the structure in which the product has been
installed.
It should be further noted, that nitrogen-alloyed shape memory steels
manufactured accordi"g to the invention have yet one more excellent property, i.e.
20 the ability to exploit the for",alion of nitrides to reinforce the colll~,osilio". For
example, one ,c, ocedure is that after cold-working the steel is aged by healil .9, e.g.
to about 300 - 600~C. The aging ~ses nitrides to form and in this way the
sl, enyl h of the steel is further improved.
2~ The use of shape memory steel, for example in the prestressing of concrele (or
rather in presll essing that is carried out afterwards) provides construction design
with quite new opportunities, be~use shape memory steel can be installed in the
desired shape inside the mass. When the mass has l,arde"ed, the state of
prestressing can be set suitably by heating the steel at a selected point, for
30 example using induction heating or by an electric current led to the steel or other
suitable ",a"ner. The use of ~I~e~l~ory steel as a presllessing method also makes
it possible to increase the stress by means of heating carried out later, if the",a,lensile phase has been left in the steel during the first heating.
CA 0222~679 l997-l2-23
W O 97/03215 PcT/~ c1D~
19
When considering purely damping properties in this case too the pr~ e, lies and
price of r,il,uge,, alloyed steels accor:li"y to the invention permit their wideindustrial arplic~tion. They can be used either as cast cGI"ponenls or as products
that have been worked in various ways. They are s~it~hlc for use in large
5 constructions too bec~ ~se they can be joined by welding.
-
Steels accordi"y to the invention are also suitable for such applications in whichthe .,.alerial must absorb impact energy and shock waves (e.g. v~hic1~5 and
military applicalio"s). An ~ ;liu~ ~al advantage of ~"dl~rial accordi. .y to the10 invention co" "~a, ed to many other metallic damping malerials (e.g. Mn-Cu) is their
high mod~ s of elasticity and high strength.
Practical tests using ~-.emc,.y steels acco~di.-g to the invention were carried out
on some ar pli~tions that were reyarl:leJ as suitable.
As stated above shape memory steels are excellently suitable for many
attacl""enl and tensioning applications. Pre-deformations can be carried out at
room temperature at which it is also possible to store the deformed components
(co""~are Ni-Ti-cryofit connectors). The moduli of elasticity of the steels are high
20 (the moduli of el~sticity of Cu and Ni-Ti-based me",ory metals for example are
suLslzi"lially smaller which means that the greater part of the recoverable strain
of these metals may be in elastic strain). Col- "oar~d to many other memory steels
the advanlages of steels accordi. ,9 to the invention are great recovery forces and
recoverable strain high mechanical sl~ enytl ~ and ductility good cGr. usion
25 ,esislance and high le",per; lure oxidation resistance excellent steel working and
."acl ,i"i"g properties. In addition the steels can be joined by welding. The weld
too has been shown to have a shape l"~",or~r p~ u,~el l~/. This can also be exploited
in applications. A practical demo,-sl, dlion was made by bending a butt weld andstraightening it by heating. Further steels according to the invention can be
30 economically manufactured by conventional methods used in the steel industry.
Shape ",e",o,y metals according to the invention have also an excellent ability to
damp shock waves and impact energy be~ se the defor",alion of auslenile steel
to martensite consumes a great deal of energy. In many applications the
CA 0222~679 1997-12-23
W O 97/03215 PCT~6/00408
derc"llldlions may be very great, he~u-se the dero""alion mechanism is (up to a
certain degree of deror-"dlion) the ror"~dlion of l~,d~lensile, instead of plastic
deformation. Due to this, the limit of defor",2lion of the Illalelidl is very high.
Applications of this include vehicle frame structures and certain military
5 applications.
The presl, essit,g of concrete carried out with the aid of shape memory steel was
tested/de",G"sl,dle.l by man~ ctl~ring two reinrol~ed steel beams (16 x 16 x 60
mm3). Inside both beams there were four 1 mm-thick longitudinal shape memory
10 steel rei"ru' ~" ,enLs acco~ dil ~ to the invention, which were sp~ced at 10 mm from
one another. Ties were placed round the reinforcements at intervals of about 7
mm. Separate pre-deror",alion was not carried out on the memory steel wires,
instead normal cold-drawing of the wire to a thicklless of 1 mm served as pre-
~Jeru".,dlion. The wires placed inside one of the beams were heated to a
15 temperature of 250~C, at which most of the l,~a~lel,sile that had arisen in thedeformation changed to austenite and the wires simultaneously shortened. The
wires placed inside the other beam were not heated. Both steel pillars were placed
inside a form and the form was filled with concrete. The concrete was composed
of normal Portland cement and sand, which was sieved through a 1.5 mm sieve.
20 After casli"y~ the con~ele mass was vibrated to reduce voids. When the concrete
beams had hardened for 6 weeks, they were heated to 250~C. A cG",plessive
stress then arose in the beam, the reinrorcel"ent wires of which had not been
previously heated, as a consequence of the steel wires trying to shorten as the
martensite changed to austenite. When both beams were bent, the prestressed
25 beam broke under a greater load. This demonstrates that prestressing, carried out
with the aid of shape memory steel, works.
Also the attachment of pipe connections/machine col"~onents to an axle can be
carried out using shape memory steel accordi"g to the invention. Machine
30 COIllpGl ,enls, e.g. flywheels and parts of electric motors, are allacl ,ed to an axle by
exploiting ll ,e, Illd~ t,Udl ~SiUI 1. The toleral ~ce required is achieved by either heating
the ",acl ,i"e component or by cooling the axle with e.g. Iiquid nitrogen. The shape
memory eKect can also be exploited in atta~:l""enl. The changes in dimension
achieved with memory steels are much yrealer than those caused by thermal
CA 0222~679 1997-12-23
W O 97/03215 PCTn~96/00408
21
e~ansion. Attachment can take place by means of e.g a sleeve made from shape
memory steel. The sleeve can be pre~erorl"ed by drawing the sleeve in the
direction of the axle. Vvhen the sleeve is placed between the axle and the machine
component and heali"g is carried out to the au~ ile range the sleeve tries to
5 return to its pre-drawing form. The thickness of its wall i"creases and
simulla"eously it tightens the machine co~ onent onto the axle. If the sleeve ismade from two-way shape memory steel the removal of the ~acl ,i"e COIllpOl ,ant
takes place by cooling to a le"~peralure that has been selected with the aid of the
alloying of the ~ "amo~ ~/ steel and lhe~ " ,o" ,ecl ~ ical ll aaln ,e, ll so much lower than
10 the operating temperature of the machine that uni~ ILenliGI ,al detacl ""anl cannot
take place.
A pipe connection made from memol~ steel is a sleeve the inner dian,ater of
which is smaller than the outer diameter of the pipe. The inner dia",aler of the15 sleeve is enlarged to be greater than the diameter of the pipe by deforming the
sleeve e.g. by means of a ~"a"dlal. The enlaryed sleeve is placed over a butt joint
between two pipes. The sleeve tightens the pipes together when it is heated to the
austenite range.
20 The attachment of a memory steel sleeve around an axle and pipe was
de",on~l,aled by manufacturing a sleeve from the l"amoly steel to be palenled
with a length of 10 mm an intemal did",ater of 8 mm and a wall thickness of 2 mm.
Attacl""e"l took place by heating the sleeve to 300~C. The ti~.3hlening sl,asseswere ascertained with the aid of changes in dimension.
A sepa~ale all~;l " "enl cor, ,,uonenl made from rl ~a~ "ory steel is not always required
bec~l Ise in many instances the product itself can be made from memory steel.
It is possible to exploit the invention in the allach~enl of a rivet screw or other
30 allacl ,r"ent member in which a change of dimension takes place in the direction
of the axle. In many attachment applications (e.g. plates machine components)
attachment can be carried out by means of such an attachment member which
has been derc"ned by drawing in the direction of the axle before allacl Iment. After
installation tiyl,lel,i"g takes place by heating the alLachment member to the
CA 02225679 l997-l2-23
W O 97/03215 PcT/k~5
22
austenite range. I le~ ,9 can also take place in such a way that the central part of
the ,ner.ll,er is heated to a higher te,ll,~er~lure than the outer surface. In this case,
rnore au~leniLe arises in the inner parts than in the surface parts. A tensile stress
then arises inside the member and a cor", ressive stress in the surface. Fractures
5 an. d stress co" oSiG,) do not easily arise in a surface subject to col "pressive stress.
=The exploitation of the stress gradients c~l ~sPd by the partial I ,eali,)g of memory
steel is a new innovation, which can be utili~Pd in many applications.
.-~
. ~ ~ .
~, w Cons~truction al-plic~lions dt:",a".li.)g high resisla"ce to fatigue can aiso utilize the
10 ~--~vention. Because nitrogen-alloyed shape memory steels are strong, and they
have so-called super-elastic properties, they easily wilhsland fatigue loa~ yS at
even high loading amplit~ es When steel is sllessed, the derorllldlioll mechanism
is (up to a certain limit) twinning and not slipping. This ...ecl,anis", is recoverable
and ",~terial fatigue is then very small. Be~ ~se steels accordi,)g to the invention
15 are, in addiliGIl, cheap and easily worked and easily welded together, they are
highly sl ~it~hle as construction materials for large steel structures and machines in
which they are subject to great fatigue loadings.
