CA1038653A - Heat-treating method - Google Patents

Abstract

ABSTRACT OF INVENTION
Methods for modifying the temperatures at which metallic o compositions capable of undergoing reversible transformation from the austenitic state to the martensitic state will undergo such transformation and a method for inhibiting the loss of such reversibility are disclosed. The method for inhibiting loss of reversibility between the martensitic and austenitic state com-prises holding a metallic composition which is in the austenitic state at a temperature above the temperature at which transforma-tion to the martensitic state starts for a time sufficient to reduce the loss of reversibility. Articles made form the com-positions may be used as couplings for hydraulic lines.

Description

103~653 BAC~GRO~D OF T-~E Ii'rV~TION

Metallic co~positions which are, generally speaking, alloys which have the properties of being capable of undergoing reversible transformation from the austenitic to the marte~sitic state are knownO
Su~h alloys are, for e~ample, those disclosed in UOS. Patents NosO 3 012 882 (issued 12th Decem~er, 1961); 3 174 851 (issued 23rd ~larch~ l965); 3 351 463 (7th ~ovember, 1967); 3 567 523 (issued 2nd March, 1971); 3 753 700 (issued 21st August, 197~); and 3 759 552 tissued 18th September, l973)~ British Patent NosO
1315652 (published 2nd May, 1973) and 1346046 and 1346047 (both published 6th February, 1974) in the name o~ Fulmer Reaearch Insti-tuteO
Such alloys are also disclosed in l~ASA Publication SP5110, "55-Nitinol-ths alloy with a memory, etcO" (UOSo Government Printing - Office, Washington, D.Cot 1972)9 No Nak~nishi et al, Scri~ta Metallur~ica 5, 433-440 (Pergamon Press 1971), These alloys have in common the feature of undergoing a shear transformation on cooling from a high temperature (austenitic) state to a low temperature (or martensitic)stateO I~ an article made of such an alloy is deformed when in its martensitic state it will remain so

- 2 103~653 deformedO If it is heated to return it to a temperature at which it is austenitic, it will tend to return to its undeform-ed state~ The transition ~rom one state to the other, in each direction, takes place o~er a temperature rangeO The tempera-ture at which martensite starts to form on cooling is designated M8 while the temperature at which this process is complete is designated Mf, each oi these temperatures being those achieved at high, eOgO~ 100a per minute, rates of change o~ temperature of the sample, iOeO, the "basic" Ms, etc~ Similarly, the tem-peratures o~ beginning and end of the transformation to austenite are designated A8 and Afo In some cases, Mf is a lower tempera-ture than As, M8 is a lower temperature than ~, and Ms i9 equal to or higher than A8, ~or a given allo~O The transformation from one form to the other may be followed by measuring one of a number of physical properties of the material in addition to the reversal of deformation described above, for e~ample, its electrical resisti~ity, which shows an anomaly as the trans-~ormations take place. I~ graphs of resistlvity-v-temperature or strain-v-temperature are plotted, a line joining the points M8, Mf, A9, A~ and back to M8 forms a loop termed the hysteresis loopO For many materials M8 and A8 are at approximately the same temperatureO
In certain commercial applications employing heat recoverable alloys, it i8 desirable that A8 be at a higher temperature than M8, ior the ~ollowing reason. Many articles ¢onstructed in the alloys are provided to users in a deformed condition and thus in the martensitic state. For example, couplin~ for hydraulic components, as disclosed in British Patents 1327441 and 1327442 both published on 22nd August, 19739

- 3 -are sold in a deformed (iOeO~ an expanded) stateO The customer places the e~panded coupling over the components (for example, the ends of hydraulic pipe lines) to be joined and raises the temperature of the coupling. As its temperature reacheæ the austenitic transformation range, the coupling return~, or attempts to return, to its original configuration, and shrinks onto the componente to be joined. Because it is necessary that the coupling remain in its austenitic state during use (for e~ample, to avoid stress relaxation during the martensitic tran3formation and because its mechanical properties are superior~, the M8 of the material is chosen to be below that which it may possibly reach in serviceO For this reason, it has to be ~ept in, for example, li~uid nitrogen until it is usedO If, however, the A8 which, as used herein, means that temperature which marks the beginning of a continuous sigmoidal transition, as plotted on a etrain vsO temperature graph, of all the martensite capable of transforming to austenite to the austenitic state, could be raised if only temporarily, for example, for one heating cycle, without a corresponding rise in the M~, then the expanded coup-ling could be maintained at a higher and more convenient tempera-tureO
In copending application NoO 1~6167 of which the present invention is a dlvisional, there is provided an alloy having for at least one heating cycle an A8 higher than its M8, or a raised A8 it the alloy already has an A8 higher than its M~, methods of raieing the A8 of an alloy, at least temporarily, relative to the M8 oi the alloyO Stated another way that inven-tion providee a means for retaining at least a use~ul portion of marteneite attemperatures at which austenite would normally existO
Thus, the physical properties associated with martensite are - ~038653 retained at higher temperaturesO ~he method is termed "precondi-tioning" and the resulting alloy is stated to be "preconditioned"0 The parent application provides a method for raising for at least one heating cycle the As f a heat-redoverable alloy rela-tive to its Ms which comprises slowly, e.gO, less than 1 degree centigrade/minO, heating the alloy from a temperature below its M~ to a temperature in the region where the A~ is desiredO The alloy can then be cooled to any temperature below that to whioh it has been slowly heated or lt can be held at that temperature.
When the alloy is to be used, it is simply heated again at any convenient fa~t rate, eOg., 5 degrees centigrade/minO or greater:
preierably 100 degress C/min. or greater, and the As will be found to be approximately at the temperature to which it was heated 810wl~o The present invention provides a process of inhibiting loss oi reversibility between the martensitic and austenitic states in metallic articles which reversal would otherwise occur at ambient temperatures by holding the article at a temperature above the M8 temperature while in the austenitio state for a time suf-iicient to reduce the loss oi reversibility~
When metallic articles are sub~ected to the process ior in-hibiting loss of reversibility according to the present invention their pseudo elasticity, iOeO, their ability to transform from the austenitic to martensitic state with attendant deformation when sub~ected to stress and to revert to the austenitic state nd recover their original shape9 is improvedO
article is made of an alloy, and preferably the Preferably, the metallic/alloy is an intermetallic compoundO
Among suitable alloys there may be mentioned copper-zinc and copper-aluminum alloys which preierably contain relatively small ~038653 proportions of alu~inu~, silicon, tin, or manganese, or mixtures thereof, which alloys may, it is believed, contain up to about 20 % or more weight percent (based on the weight of copper and zinc) of the third component or the total of the additional componentsO It will be appreciated that the proportion of metals other than copper and zinc affects the transition tem-perature and other properties of the alloysO Suitable alloys ior u~e in the pre~s~t inve~tion include69O7 ~o au, 2603 % ~n,

4 % Al; 6202 % Cu, 37O3 ~o Zn, 0O5 ~o Al; and 8005 % Cu, 1005 J~ A l, 9 % ~In. As e~amples in this specification, there will be dis-cussed in some detail alloys having about 65 qO copper to 35 %
zinc, with optional additions of up to 2 % silioon or up to 2 or 3 ~/o aluminum, these being weight percentages. It will be appre-ciated, however, that such alloys, having Ms temperatures very close to ambient are discussed only as examples and that the processes of the invention are applicable to alloys having, for e2ample, MB temperatures lower than ambient, and to alloys, for e~ample, tho~e based on gold or silver, other than copper-based.
The ~ollowing ~amples illustrate the inventionS
EXAMP~E 1 Several specimens of an~Lloy of composition, by weight, of 6405 ~ copper, 34O5 % zinc, 1.0 ~ silicon were quenched after 5 minutes at 860C into water at 20a, and then aged at 50C for times up to 1 weekO After cooling to below Mf the spe¢imens were reheated at a rate of 10 to 20 degrees C/minuteO ~ittle trans-formation of martensite to ~-phase (as measured by changes in re-sistivity) occurred during heating o~ the specimen aged for 5 minute~. Some transformation took place in the specimen aged for 45 minutes; the specimens aged for 90 minute~ or over trans-formed completely. Other specimens of the same alloy were given the same heat treatment and after aging were deformed in tension at -50C and reheated. The amount o~ heat-recovery was approxi-mately proportional to the amount of martensite which had tran~-iormed in the resistivity test~ on undeiormed specimens. Hence, using the process o~ the invention by aging at lea~t 45 minutes allowed permanent heat-recoverable properties to be imparted to this ~lloy.
~ iter aging 5 minutes at 20C before cooling to -50C, the heat recoverable strain was 2.30 %. Aiter 45 minutes, at +50C
beiore cooling to -50C, the heat recoverable strain was 6.20 ~.
This slowly increased aiter longer aging time~ to 6.50 % aiter 3 hours and 7.0 % aiter 1 week.
EXAMPL~ 2 Several samples oi an alloy oi compoeition by welght oi 66.50 %
copper, 31.75 % zlnc and 1.75 % silicon were also quenched a~ter

5 minutes at 860C into water st 20C. They were then aged at 50C
ior tlmes up to 1 week. Aiter 5 minutes at 50a, the heat recover-able strain was 0.1 %. Aiter 45 minutes at 50C, this remained at 0.1 ~ and aiter 90 minutes had only increased to 0.55 ~. Three hours lncreased the heat recovery strain to 0.70 %, 1 day to 100 %~ and 2 days to 3.9 %. Thus, the increased slllcon content can be ~een to re~uire much longer aging time to produce improved recovery.
D~ ~
The alloy described in B~ample 1 was also used in this E~ample. Its baeic A8was about 15 to 25C, and normally about 75 ~ any heat recovery has taken place by 75C. A sample was heat treated and quenched in the manner described in Example 1 and aged ior about 5 minutes at ambient temperature. It was then cooled to below Mi to the martensitic state, then heated at between '1~03~653 0075 and 100 degree C per minute at 75C~ and then cooled to -50C (iOeo~ below its Mf o~ about -20C)o The sample was then deformed to impart 8 % strain at -50~C~ Approximately half the de~ormation strain was recovered on heating to above 40 Recov6ry was 4 %, about 008 ~0 taking place below, and 302 % above, 75 C~
~x~r~p~Es 4 THRU 7 Samples of the same alloy used in Bxample 1 were heat treated and quenohed to 20C, and aged for 2 days at 50aO They were then cooled to -50C and de~ormed. Samples were then heated to 75C
at the same 810w rate as in ~ample 3, and ~or different periods and heated at 50 to 200C/minute (iOeO~ rapidly) to cause recoveryO

~ample Recovery on Storage A 9 C Recovery on Total NoO Strain Slow geat Ti~e at s Fast geat Recovery to 75 C 20 C to 75 C on Fast Heat ~0 %
4 7040 0~95 5 minO 85 0 5030 6080 1020 90 minO 86 0 4040

6 7065 1 o60 16 hrsO 85 0 403

7 7030 1 o60 168 hr80 86 0 3060 From E~amples 3 thru 7 it can be seen that the alloys may be de~ormed either be~ore or a~ter slow heatingO

~ 8 ~

~03~6~3 SupplementarY Disclosure The term earlier referred to as preconditioning"
is more specifically termed nthermal preconditioning".
In a copending Canadian Application No. 244,815, filed on 2nd February, 1976 there is described another method by which the As temperature of metallic compositions can be elevated. That method comprises holding the composition in a deformed configuration at a temperature above its normal AS-Af range for a length of time sufficient to cause a portion of the deformation to be retained when the constraint is removed. The amount of deformation retained is a function of the temperature at which the composition is held and the duration of the holding step.
The composition can be deformed while in the austenitic state. Typically, however, this requires a great deal of force. Accordingly, it is preferred to deform the composition while it is in a more workable condition that occurs near, within or below the MS-Mf range and then to raise its temperature while restrained to the desired holding temperature.
By analogy to "thermal preconditioning~, this method is referred to as mechanical preconditioningU.
An article preconditioned in this way when heated at a fast rate will recover a portion of the retained strain.

- 103~i53 The loss of reversibility referred to above manifests itself in several ways. In some cases, a sample of a metallic composition that has been cooled to below Mf fails to revert completely or in part to austenite when allowed to warm through its normal AS-Af range. Accordingly, an deformation that has been imparted to the sample while in the martensitic state may fail, at least in part, to recover when the sample is heated under conditions where recovery would be expected to occur.
In other cases, where even though the composition may undergo a reversible transformation to austenite after conversion to martensite followed by fast heating, the composition may not respond to either thermal or mechanical preconditioning when attempts are made to elevate its AS because the reversibility is lost in the preconditioning process.
In addition, the invention also provides a method of improving the response of certain alloys to mechanical or thermal preconditioning (that is increasing the amount of elevated heat recovery) by carefully controlling the aging to be within certain time and temperature limits, even though the total recovery may thereby be reduced.
The optimum aging conditions can be found by routine experiment by those skilled in the art. Suffice it to say that in these compositions, as shown in the examples 10386~3 herein. too short an aging time or too low a temperature can give insufficient useful reversibility as mentioned hereinabove, and too long an aging time or too high a temperature can give insufficient useful elevated reversibility, even though the overall reversibility is improved in the latter instance.
The method of this invention is generally applicable to a wide variety of metallic compositions that undergo reversible austenite-martensite trans-formations. It is particularly suited to metallic compositions that are alloys, and, more particularly, to alloys that form electron compounds. Preferred electron compounds are those corresponding to the Hume-Rothery designation for structurally analogous body-centered cubic phases (e.g. beta-brass) or electron compounds that have ratios of about 3 valence electrons to 2 atoms. See A.S.M. Metals Handbook, Vol.
1, 8th Ed. (1961) at p.4. Preferred alloys include those of from about 60 - 85 wt. % copper with varying amounts of zinc and/or aluminum in combination with silicon, manganese or mixtures thereof, for example, alloys having 0 to about 40 wt. % zinc, 0 to about 5 wt. % silicon, 0 to about 14 wt. % aluminum and 0 to about 15 wt. % manganese that form body centered ubic ~pe structures. Ternary and quaternary alloys of copper can be used.

1~3~653 In the case of ~-phase alloys, the aging temp-erature must be one at which there is no significant transformation of ~-phase to a phase that undergoes no reversible austenite-martensite transformation.
Containing varying amounts zinc, aluminum, silicon, manganese and combinations thereof, those having an Ms below room temperature, aging at from about 50C
to 125C for a time ranging from about 5 minutes to 3 or 4 hours is usually adequate. Aging at higher or lower temperatures or for longer or shorter times can usually be beneficial, however. For other compositions, the time and temperature can vary but optimum results are readily determined by comparing the amount of reverqal between martensite and austenite that occurs in representative samples, for example, by measuring the amount of strain recovered as a result of fast heating a sample.
It will be appreciated that aging need not be carried out at a single temperature, the temperature may be changed one or more times or may be varied continuously during the aging period.
The following examples illustra~e the invention.

A ~eries of experiments was conducted that ~ompares the responqe of various compositions in the Cu-Zn-Si and Cu-Zn-Al systems to the aging process of 1~D38{~53 this invention and the effect on thermal preconditioning.
Alloy samples were cast from melts having different ratios of copper, zinc, and either silicon or aluminum.
The castings were hot-rolled into strips and cut into specimens about 37 mm x 3 mm x O.75 mm. All specimens were heated until they entered the high-temperature, ail-beta phase, then quenched into water. Half the samples were aged at 100C for 10 minutes, the other half were not aged. All the samples were deformed b~-bending at -79C to cause an outer fibre strain of 6%.
After deformation, the ~amples were released and measured to determine how much strain was retained. Specimens from the aged and unaged groups were then heated according to one of the three following methods:
~1~ heated rapidly by immersion in liquid at 40C, cooled to room temperature and measured to determine how much strain was recovered, then heated rapidly by immersion in liquid at 200C and again returned to room temperature to determine how much additional recovery of strain occurred, (2) slowly heated at a rate of 0.25 deg C/min from -79C to +40C, cooled to room temperature, measured to determine how much strain was recovered, then heated rapidly by immersion in liquid at 200C, cooled to room temperature and measured to determine how much additional recovery occurred, or (3) treated as (2), except that the slow heating rate ~03~3653 was 1 deg C/per 24 minutes, instead of 0.25 deg C/min.
A "figure of merit~ for the responsiveness of each composition tested to control of the recovery temperature range is obtained by expressi~g as a percent the recovery occurring above 40C for slowly-heated specimens, less the recovery above 40C for rapidly-heated specimens, divided by 5% ~hich is the ideal recovery after the elastic springback which accompanies release of the bending stress~ i.e., reOovery above recOovery above 40 C in slowly- 40 C in rapidly-Figure of Merit = 100 x heated specimens - heated specimens Compositions found especially suitable for use in the invention will now be described in greater detail with reference to the accompanying drawings, in which:
FIGURES Ia and Ib qhow the effect of aging on alloys comprising copper, zinc and silicon that are thermally precon-ditioned.
FIGURES 2a, 2b, 2c show the effect of aging on alloys comprising copper, aluminum and zinc that are thermally precon-ditioned.
In Figures la and lb, the figure-of-merit is plotted verqus composition in a topographical format.
The longer axes of the zones of conqtant figure-of-~L038fiS3 merit are generally parallel to iso-transformation temperature contours. Compositions with lower trans-formation temperatures are in the upper left while those with higher transformation temperatures are in the lower right of the figure. A distinct optimum appears in Figure 1 in the range 1 8 to 2.7 Si, 66.2 to 67.5 Cu, balance Zn (29.B - 32.0%~. Comparison of Figure la with lb shows that aging 10 minutes at 100C enlarges the optimum from the same general central region. The arbitrary selection of 40C as the end of slow heating apparently disqualifies alloys whose usual transfor-mation range lies above or partially above +40C, those in the lower right portion of the figure, but it will be appreciated that a low figure of merit on the graph does not indicate unsuitability of these alloys for use in the invention, merely that a temperature of other than +40C should be chosen as the preconditioning temp-erature. Similarly, for those alloys in the upper left portion of the figure, a low figure of merit on the graph does not necessarily mean that they are not responsive to the process of the invention. In these cases, a low figure merely means that the rate of slow heating selected was not one that prevented recovery prior to reaching 40C. The choice of 40C causes the iso-figure-of-merit zone to close toward the high trans-formation temperature side (lower right). Alloys in ~03~6S3 the lower right region are in fact responsive to the slow heating process, as the CuZnAl data below indicate.
A topographical presentation of the figure-of-merit results for the CuZnA1 system appears in Figure 2. Again, the constant figure-of-merit zones lie parallel to the iso-transformation ¢ontours. The effect of aging is to spread the optimum in the upper left-to-lower right direction of the graph.
Five alloy compositions having a normal As at or above 40C were used to test the mobility of the recovery range at higher temperatures. Again, the same general test procedure was used, but slow heating was continued to +100C rather than stoppingiat +40C.
Results for aged samples appear in Figure 2c, the new optimum lies parallel to that in Figure 2b, but shifted as would be expected toward compositions with higher transformation temperatures. Although the recovery range is mobile in CuZnAl, thè mobility seems more limited than in CuZnSi.
As the unaged CuZnAl samples lost their memory properties as a consequence ofslow heating to 100C, but the aged samples did not, it is apparent that the aging treatment is successful in preserving recoverability of the transformation in the higher temperature range.
It will be appreciated that the aging periods and conditions selected for Figures lb and 2b result ~0386~i3 in certain compositions having optimum properties and that other aging periods and conditions result in different compositions having the same or broadly similar optimum properties. The aged alloys within the areas bounded by lines 40, 60 and 80 in Figure lb and line 20 in Figure 2b are especially suited for the process of the invention.
EXAMPLE g Sixteen samples of 80.8 wt. % Cu, 10.5 wt. %
Al, 8.7 wt. % Mn were betatized at 800C or 900C for 3 minturs or 6 minutes then quenched into room temper-ature water. Half the samples were aged for 10 minutes at 100C, the others were not aged. All samples were deformed by bending at -79C to give an outer fibre strain of 6%, then the stress was relaxed. Half the samples were heated to 100C at 0.25 deg C/min, cooled to room temperature, then heated rapidly to 200C.
The other half were heated rapidly to 100 C. The rate of rapid heating was recovered during rapid heating to 200C versus the controlled variables indicated that thermal preconditioning significantly increased the proportion of recovery taking place above 100C. For this particular alloy, a statistical analysis indicated that aging had no effect.
Averaged effect~:
Percent Strain recovered above 100C

_ 17 -~o3s6s3 Fast heated 0.39 percent Preconditioned 1.89 percent ~he experiment was repeated on an alloy containing 80,49 wt. % Cu, 10.5 wt. % Al, 9.01 wt. % Mn. Analysis of the strain which was recovered during rapid heating to 200C versus the controlled variables showed signif-icance for aging versus no aging and for non-preconditioned versus preconditioned.
Averaged effects:
Percent Strain Recovered above 100C
Un-aged 1.00 Fast Heated 0.15 Aged 0.36 Preconditioned 1.21.

Samples of an alloy containing 79.2 wt. % Cu, 10.0 wt. % Al and 10.8 wt. % Mn were betatized at 550C
for 5 minutes and quenched into water at 20C. The alloy had an M9 of -20C as a result of this treatment.
Samples were either aged for 5 minutes or 1 hour at 50C, then cooled to -30C, or cooled to -30C immediately after the water quenching without aging. All the samples were deformed 4% in tension at -30C and the stress released.
Half of the samples were immediately heated at a very rapid rate by immersion in liquid9 at 20 C, 40C, 100C and 200C. The incremental-amount of strain recovered as a result of each immersion was recorded.

~Lo3~653 The remaining samples were initially slow heated at 6 deg C/min to 40C, after which they were recorded to -30C and rapidly heated, as in the first set of samples. The results are shown in the table below.

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V~ ~I N r~) d ' U~ ~) ~03B653 Considering first those samples rapidly heated immediately after deformation, recovery was complete by 40C in the samples aged 5 min. and 1 hr., but most recovery took place above 40C in the unaged sample. In the samples initially heated at 6 deg C/min to 40C, no recovery took place by 40C in this first heating cycle in the unaged samples and those samples aged 5 min. at 50C. However, after recooling and rapid heating again, most recovery took place above 40C. The sample aged 1 hr. at 50C showed almost complete recovery in the initial heating cycle of 6 deg C/min to 40C.
These observations demonstrate that aging can lower the As since in unaged samples significant recovery took place above 40C without preconditioning tCompare Results 1, 3 and 5). However, the amount of heat recoverable strain obtained when a sample is thermally preconditioned is improved by aging. (Compare Results 2 and 4). Aging also affects the rate of slow heating necessary for thermal preconditioning. For a sample aged but 5 min. at 50C, 6 deg C/min. was a "slow"
heating rate as there was little recovery before 40C.
(See Result 4~. However, in the case of a sample aged for 1 hour at 50C, a heating rate of 6 deg C/min.
qualified as a fast heating rate as mo~t of the heat recoverable strain was recovered during the attempt 1~38653 to precondition. The combined effect of these results is to demonstrate that for a given alloy, there may be an optimum aging treatment, one, however, that is readily determined by those skilled in the art, prior to thermal preconditioning.

An alloy containing 64 wt. % copper, 35 wt.
% zinc and 1 wt. % silicon was studied. This alloy has an Ms temperature of -40C.
Specimens were betatized for 5 minutes at 860C quenched into water at 20C, and then aged for different times in the metastable beta phase, which in this series of experiments was performed at 50C.
After insertion in the tensile loading device (approx-imately 5 minutes to set up at ambient temperature~
the speciments were cooled to -65C and deformed 8%
in tension. After deformation, a constraint was applied to the tensile rig so that no contraction could take place, but the specimens were free to undergo a spon-taneous expansion if one occurred. The constrained specimen was placed in water at +40C, which provides a very fast heating rate, and was held at that temp-erature for different times before re-cooling to below the Mf. Specimens came free of the constraint during cooling with a slight expansion compared with the original set after deformation. The constraint W8S

~03B653 removed from the apparatus so that specimens, now in th~r "preconditioned" state, could heat recover freely when reheated at a nfast" rate in a furnace set at 600C.
The As temperatures and heat-recoverable strains were measured as a function of the two main variables, aging time at 50C before deformation and the time held under constraint at 40C.
Results of "mechanical preconditioning" are shown in Table II. For each aging time at 50C some specimens have also been fast heated directly after deformation at -65C, in order to compare the effect of "mechanical preconditioning" on the As temperature.
Table II shows clearly the trend that the 2nd A9 temperature, the one induced by mechanical precon-ditioning, was raised as the holding time at 40C
was increased and in many cases exceeded the temperature of 40C. On the other hand, the total heat-recoverable strain (i.e. 1st As to Af) was reduced with increased holding time at 40C, and this loss in recovery occurred mainly in that portion of heat-recoverable strain between the 2nd A9 and Af. Increasing the aging time at 50C, in the metastable ~-phase, greatly improved the overall heat-recoverable strains but had only a slight effect in reducing the 2nd A9 temperature.

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Claims (12)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A metallic composition having a substantially reduced tendency to lose reversibility between its mar-tensitic and austenitic state as compared with the tendency to do so at a given temperature of a material of the same composition as initially quenched into the austenitic state.

2. A method of inhibiting loss of reversibility between the martensitic and austenitic states in a metallic composition which would otherwise occur at ambient temperature comprising aging said composition at a temperature above the Ms temperature while in the austenitic state for a time sufficient to reduce said loss at said ambient temperature.

3. The method of claim 2 wherein, prior to said aging step, said composition is heated to a temperature at which the phase capable of existing in the martensite and austenite states is stable, and is then quenched.

4. The method of claim 3 wherein said quenching temperature is a temperature at which the composition is wholly in the austenitic state.

5. An article, the composition of which has been aged by a method as claimed in claim 2.

6. An article as claimed in claim 5, which is heat-recoverable, the composition of which is substantially in the martensitic state.

7. An alloy having a reduced tendency to lose reversibility between its martensitic and austenitic states as compared with the tendency to do so at a given temperature of a material of the same composition as initially quenched into the austenitic state, which comprises copper and zinc.

8. An alloy as claimed in claim 7, which also comprises at least one element selected from the group consisting of aluminum, manganese, silicon and tin.

9. A composition as claimed in claim 1, which comprises copper and aluminum.

10. A composition as claimed in claim 9, which comprises at least one element selected from the group consisting of manganese, silicon and tin.

CLAIMS SUPPORTED BY THE SUPPLEMENTARY DISCLOSURE:

11. An alloy as claimed in claim 7, which contains by weight from 60 to 85% copper, up to 40% zinc, 0 to 5%
silicon, 0 to 14% aluminum and 0 to 15% manganese.

12. A method as claimed in claim-2, wherein the aging is carried out from a temperature of 50°C to 125°C
and a time from 5 minutes to 4 hours.