IE43345B1 - Alloy treatment - Google Patents

Description

This invention relates to heat-recoverable alloys and methods for treating them.

Metallic compositions, for example, alloys, which have the properties of being capable of undergoing reversible transformation from the austenitic to the martensitic state are known, and some of these may be formed into articles that are heat-recoverable. Such alloys, commonly known as heatrecoverable alloys or memory alloys, are, for example, disclosed in U.S. Patent Nos. 3,012,882; 3,174,851; 3,351,463; 3,567,523; 3,753,700; and 3,759,552, Belgian Patent No. 703,649 and in British Patent Nos. 1,315,652; 1,315,653; 1,346,046 and 1,346,047 in the name of Fulmer Research Institute, which four patents are herein referred to as the Fulmer Patents. The disclosures of all the patents mentioned above are incorporated herein by reference.

Such alloys are also disclosed in NASA;.. Publication SP110, 55-Nitinol-The alloy with a memory, etc. (U.S.

Government Printing Office, Washington, D.C., 1972), N. Nakanishi et al, Seripta Metallurgies 5, 433-440 (Pergamon Press 1971), the disclosures of which are likewise incorporated herein hy reference.

These and other 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 state. The terms martensite and martensitic being used herein to denote the low temperature state and the terms austenite and austenitic being used herein to denote the high temperature state from which the alloy is capable of undergoing a transfor- 2 43345 mation on cooling which is known as a martensitic transformation. Under certain conditions of composition and temperature, both transformable and non-transformable phases may co-exist; only those phases capable of undergoing the transformation are martensitic or austenitic.

If an article made of such an alloy is deformed when in its martensitic state it will remain so deformed. If it is heated to return it to a temperature at which it is austenitic, it will tend to return to its undeformed state, producing a so-called shape memory effect. The transition from one state to the other, in each direction, takes place over a temperature range. The temperature at which martensite starts to form on cooling is designated Ms while the temperature at which this process is complete is designated M^, each of these temperatures being those achieved at high, e.g. 100 deg C/min, rates of change of temperature of the sample. Similarly, the temperatures of beginning and end of the transformation to austenite are designated Afi and Af. Generally is a lower temperature than A , M_ is a lower temperature than A_, and can be equal to, lower than or higher than A , depending on the alloy composition and also on the alloy's thermomechanical history. 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 example, its electrical resistivity, which shows an anomaly as the transformations take place. If graphs of resistivity-v-temperature or strain-v-temperature are plotted, a line joining the points Ms, M^, As, Af and - 3 4 3 3 15 back to K forms a loop termed the hysteresis loop. For many materials Mg and are at approximately the same temperature.

One particularly useful alloy possessing heat recoverability or shape memory is the intermetallic compound TiNi, . U.S. Patent No. 3,174,851. The temperature at which deformed objects of the alloys return to their original shape depends on the alloy composition as disclosed in British Patent No. 1,202,404 and U.S. Patent No. 3,753,700, i.e. by selecting the appropriate composition for the alloy, the recovery of original shape can be made to occur below, at, or above room temperature.

In certain commercial applications employing heat recoverable alloys, it is desirable that be at a higher s temperature than Μ , for the following reason. Many articles o IS constructed of the alloys are provided to users in a deformed condition and thus in the martensitic state. For example, couplings for hydraulic components, as disclosed in British Patent Nos. 1,327,441 and 1,327,442, the disclosures of which are incorporated by reference, are sold in a deformed (i.e. an expanded) state. The customer places the expanded 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 reaches the austenitic transformation range, the coupling returns, or attempts to return, to its original configuration, and shrinks onto the components to be joined.

Because it is necessary that the coupling remain in its austenitic state during use (for example, to avoid stress relaxation during the martensitic transformation and because the mechanical properties of the austenite are superior), the Ms of the material is chosen to be below any which it may possibly reach in service, so that during service the material will remain at all times in th austenitic state. For this reason, after deformation it has to be kept in, for example, liquid nitrogen until it is used. If, - 4 4334S however, the A0 (which, ae used herein, means that temperature which marks the beginning of a continuous sigmoidal transition, as plotted on a strain vs. 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 coupling could be kept at a higher s and more convenient temperature.

In British Patent Specification No. 1,490,181 entitled Heat Treating Method*, the disclosure of whioh is incorporated by reference, there is described a method by which the kg of certain metallic compositions oan be raised for one heating oyole. This method comprises first lowering the temperature of the composition from one at which It exists in the austenitic state to below its Mj, temperature. Then the composition is heated to a temperature at which normally it would exist wholly in the austenitic state, i.e. above the A^ temperature. However, the transformation from martensite to austenite does not occur if the heating rate selected is a slow* one. The definition of a slow heating rate is fully set forth in the above British Patent Specification.. Suffice it to say that it can vary depending upon the nature of the metallic composition but is easily determined by one skilled in the art having the benefit of the disclosure of that British Patent Specification.

If the composition is cooled after slow heating is complete and subsequently reheated at a rapid rate it does not begin to undergo a martensite to austenite transformation until approximately the temperature at which slow heating was terminated, is reached. More importantly, if an article was made from the composition and deformed while in the martensite state either prior to, or after, slow heating is terminated, it will not begin to undergo recovery to the fora ia which it existed in th© austenitic state until it reaches approximately the temperature at which slow heating was terminated. This process Is referred to as thermal preconditioning”.

In. British Patent Specification No. 1,490,182, the disclosure of which is also incorporated herein by reference thers ie explained how the tendency of some metallic compositions to lose martensite-austenite reversibility, e.g. particularly as occurs with some compositions with an M_ of 0°C or higher, can be inhibited. This method comprises aging the composition by holding it at an elevated temperature, typically 5O-15O°C, in which it exists in the austenitic state prior to transforming It to the martensitic state. The aging temperature and the holding time required to inhibit lose of this reversibility vary according to the nature of the composition but may be readily determined by those skilled in the art having the benefit of the disclosure in the British Patent Specification.

As a result of these two discoveries, it has been found possible to prepare useful heat recoverable articles from metallic compositions which as a result of the treatment have a significantly reduced tendency to lose martensite-austenite reversibility and which also have an elevated Ag temperature. However, notwithstanding the many advantages of these discoveries in - δ 43345 order to elevate the Ag temperature for metallic compositions it Is necessary that equipment capable of providing a controlled alow heating rate be employed. Furthermore, it is necessary that some preliminary investigation be done with compositions other than those specifically described in order to determine the optimum slow heating rate. Finally, the slow heating rate necessary to avoid the onset of recovery may necessitate an undesirably long preconditioning period to achieve the desired Ag. Therefore, it would be advantageous to have a method by which an elevated A can be imparted to metallic compositions capable of undergoing a reversible transformation between an austenitio state and a martensitic state that does not suffer these limitations.

Accordingly, it is an object of this invention to provide a further method for imparting an elevated A for at 8 least one heating cycle to metallic compositions that undergo a reversible transformation between an austenitic state and a martensitic state.

The present invention provides a method for treating an article made from a memory metal ccnmosition is capable of undergoing a reversible transformation between austenitic and martensitic states, the method comprising holding the article in a deformed configuration under constraint at a temperature above Me for a time sufficient to cause at least a portion of the deformation to be retained when the constraint is removed. The present invention also provides an article treated by said method.

The method of the present Invention may be termed mechanical preconditioning. The holding period for a given alloy may ba determined by routine experiment. In general, the minimum useful period to obtain the effects desired will depend on the holding temperature, but may be, for example seconds at 200°C, 10 minutes at 100°C, and one hour at room temperature. The amount of deformation retained is a function inter alia of the temperature at which the composition is held and the duration of the holding step.

An article made of the compositions under consideration can initially be deformed while in the austenitic state.

Normally, however, this requires a great deal of force. Accordingly, it is preferred initially to deform the composition while it is in the more workable condition that occurs near or within the range and then to raise its temperature while it is still under constraint to the desired holding temperature above Mg.

After the treatment the article may be cooled to below Ms, preferably to a temperature below the Mg-M^ range, in order to render it heat-recoverable. This is preferably done before !0 the constraint is removed.

A mechanically preconditioned heat-recoverable article so produced will when heated to a fast rate recover at least partially to its initial configuration.

It is known that the application of a load or stress to an >5 article of a metallic composition in the austenitic state, for example by applying tension, compression or tension or by bending the sample, can result in an introduction of strain in the sample by means of a stress induced transformation of a portion of the austenite to martensite. This strain, which disappears when the load is removed, is referred to as pseudoelastic strain because its effects differ from those of normal elastic behaviour in that strain does not vary linearly with stress. See H. Pops, Met. Trans. 1 (1) 251-58 (1970). The strain disappears because the transformation. to martensite induced by the applied stress is reversed back to austenite in an elastic, but non-Hookeian, manner. - 8 43345 Generally, there exists a maximum temperature up to which stress Induced martensite formation will ooour. This temperature, which varies with the metallic composition, is usually and herein referred to as M^.

The reversibility between stress induced pseudoelastic martensite and the austenitic state is a phenomenon that is superficially similar to the shape memory effect observed when a sample of a metallic composition that has been deformed while in a low temperature stable, martensitic state undergoes a return to Its original configuration when heated to a temperature range over which the martensite reverts to austenite. The major differences between this phenomenon and that associated with pseudoelastically produced martensite are that, in the latter, the formation of martensite is localized at the area of stress and the transition from martensite to austenite, as well as the reverse, is an isothermal one.

For the latter reason, reversible pseudoelastic strain, while of theoretical interest, does not lend Itself to the practical applications that are possible when utilization . is made of the thermally recoverable strain achieved by deforming a sample of a metallic composition below its and holding it there until it is desired to recover the strain. However, as pointed out above, this latter process will frequently require that the Bample be kept at a relatively low temperature i.e., below A , to prevent recovery until desired unless the 8 temperature at whioh the onset of the transition to austenite - 9 normally occurs (A ) can be advanced sufficiently to allow the sample to be kept and handled without recovery at ambient temperature. The only method other than by the present invention by which this has been possible is the hereinabove discussed method of thermal preconditioning.

By using the mechanical preconditioning method of the present invention the reversion (or recovery) of a deformed metallic article to is original configuration in the austenitic state is prevented from occurring until it reaches a temperature above the normal A , i.e. the normal reversion temperature associated with the given metallic composition. This is achieved by deforming the article from an original configuration and holding it in that deformed condition at a temperature above Mq, but, preferablyfbelow M^, for a period of time sufficient that it retains at least a portion of the originally imparted strain when the holding stress is released. Subsequent fast heating of the article, i.e. at a rate that precludes further elevation of A_ by thermal preconditioning, normally and preferably l00°C/min or greater, will result in the recovery of at least a portion of that retained strained. Thus the metallic composition from which the article is made has an A-A- range that is elevated in comparison to the A-A- range normally associated with it and its hysterises loop is thereby expanded.

Generally speaking, the method of this invention is applicable to the wide variety of metallic compositions that undergo reversible austenite-martensite transformations. It is particularly suited to metallic compositions that are alloys. - 10 43345 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. β-brass) or electron compounds that have ratios of about 3 valence electrons to 2 atoms. See A.3.M. Metals Handbook. Vol. 1, 9th Ed. (1961) at p.4.

Among suitable alloys may be included β phase alloys, for example, those typified by the copper-zinc and copperaluminum alloys that form β alloys of the body-centered cubic type associated with β-brass. Among these are those alloys of copper zino or copper aluminum in which zinc and aluminum may at least partially replace each other and which themselves can he partially replaced hy other alloying elements for example, silicon, tin, manganese or mixtures thereof. Some alloys within this description are discussed in detail in the aforementioned British Patent Specification disclosing the thermal preconditioning process. Preferred alloys include those containing (apart from incidental impurities) from 60-85 wt. % copper witn varying amounts of zinc and/or aluminum, advantageously in combination with silicon, manganese or mixtures thereof, for example, alloys that form body centred cubic type structures having up to 40 wt, $ zinc, and/or up to about 14 wt. % aluminum together with 0 to about 5 wt. $ silicon, and/or 0 to about 15 wt. ·/> manganese . Ternary, quaternary, and more complex alloys of copper can he used. In the Examples, a number of specific alloys that fall within these limits will he discussed in greater detail. However, it should he understood that the method - 11 4 3 3 15 of this invention may he applied beyond the limits of the preferred embodiments. For example, it is within the scope of this invention to apply the method of the present invention to alloys based on metals other than copper.

Alloys of this type are obtained in a β-phase by methods well known to the art. Usually the β-phase is obtained by rapidly quenching the alloy from an elevated temperature at which it exists in substantial part as a stable β-phase to a temperature at which it will exist as a metastable β-phase. If the quenching rate is too slow, extensive amounts of a second phase nay form which does not undergo the reversible austenite-martensite iransformation. However, an alloy that is at least substantially .n the β-phase, e.g., over 70# beta, may still possess to a lubstantial extent the same useful properties as the pure β-phase itructure.

If the alloy is quenched to below its Mg temperature, he ability to be subsequently rendered heat recoverable can be dversely affected. Accordingly, it is desirable to quench the lloy to a temperature above M at a rate such that no significant -phase formation will result. For alloys with an Mg below about ' 3C, a quenchant temperature of about 20°0 is satisfactory. This m be achieved, for example, by quenching the alloy In water at )°C.

The chosen alloy employed is fabricated into an article iving the shape desired after heat recovery. The deformation ’ the article into the configuration from which heat recovery - 12 is desired, i.e., a configuration which will ultimately be that of the heat unstable (i.e., heat recoverable) state, is preferably accomplished at temperatures below the temperature. Tor example, the deformation can be accomplished while the article is in the austenitic state whereby the initial strain introduced into the article will be of the pseudoelastic type, i.e. its unduly rapid release would result in the deformation undergoing the previously described isothermal recovery. Nevertheless, by holding the article in the deformed condition for a suitable length of time, at least a portion of the originally pseudoelastio strain will be converted into strain that is retained after the stress is removed. That portion of the originally pseudoelastic strain that is not retained can be referred to as springbaok.

To recover the retained strain, the sample having been returned to the martensitic state is rapidly heated, as hereinbefore described, through the temperature range in which the transformation to austenite occurs. Any portion of the retained strain that does not recover, a not uncommon occurrence in the case of martensite-austenite transformation, is referred to as non-recoverable strain. The rate of heating necessary to recover the strain must be sufficiently fast to avoid the effect of thermal preconditioning as previously described because if an article is heated unduly slowly recovery will not occur. Since a suitable rate will vary according to the nature of the alloy, it is not possible to specify absolute rates of heating which would qualify as slow or fast for all -13 3 4 5 alloys. However, the significance of these terras will he clear from the previous discussion herein and from a consideration Of the British Patent Specification relating to thermal preconditioning, the disclosure of which has been incorporated herein by reference. With that information, a rate of heating qualifying as fast is readily ascertained.

If held long enough in the deformed state essentially all the original strain will be retained when the stress is removed The length of time necessary to have significant retained strain at a given temperature varies according to the composition and the thermomechanical history of the alloy. Generally speaking, for a given alloy the length of the necessary holding time decreases as the holding temperature increases. Nevertheless, there may be a penalty incurred if the holding temperature is too high as a significant portion of the retained strain may be rendered non-reeoverable. However, mechanical preconditioning has been carried out at temperatures as high as about 200°C. From this discussion, it will be apparent that the optimum combination of holding temperature and period of constraint, i.e., the period during which the article is under stress, will depend on the alloy hut that this combination can he readily ascertained. In an optimum case up to about 10$ heat recoverable strain is achievable with articles treated hy the method of this invention.

. In the case of thermal preconditioning, the temperature of the elevated A. referred to as A , is often approximately at the temperature where the slow heating is terminated. This is - 14 4 3 3 4 5 not the case with the mechanical preconditioning method of this invention. It can be below, at or above the holding temperature. In general, it increases as the length of the holding time is increased. Routine experiment with a given alloy will make it possible to determine the amount of preconditioning necessary to achieve the desired elevation in A . Storage at ambient temperature after mechanical preconditioning may result in a loss of some heat recoverability but does not affect the elevated A temnerature. s As indicated above, in a presently preferred embodiment of this invention, the article is deformed from its original configuration while in the austenitic state, i.e., under conditions where the initial strain induced in the article may be regarded as essentially pseudoelastic. However, metallic compositions suitable for use in.this invention are usually more easily deformed as their temperature is lowered from the holding temperature, e.g. until near, within or below the Μβ-Μ^ range. Accordingly, it is within the scope of this invention initially to lower the temperature of the article, for example, to below the Μβ-Μ^ range, to facilitate its deformation, deform it and then to heat it while using constraining means to keep it deformed while the desired holding temperature above the normal Ag-Af range is reached, and maintained for the required time.

In contradistinction to the situation with the thermal preconditioning process, the rate of heating to reach the elevated holding temperature need not be a slow one, - 15 43345 as heretofore defined, as recovery of the deformation is prevented hy the constraining means. However, certain advantages accrue from the use of a controlled slow heating rate to reach the elevated temperature. One advantage is that damage to the article caused hy the force exerted against the constraining means during fast heating as the article attempts to recover is avoided or minimized because stresses occasioned hy the onset of recovery are substantially diminished. Secondly, it is possible to precondition alloys in this way that are only marginally suited to purely thermal or mechanical preconditioning. In view of the fact that stress induced martensite forms locally, it is also within the scope of this invention to impart to an article an elevated A0 by mechanical preconditioning and then cool the article to below its normal M , and deform it again giving it a dual A . The second A can he advanced hy thermal S S preconditioning to a temperature below that of the k imparted B by mechanical preconditioning.

Although the constraining means can he removed at the holding· temperature, two advantages flow from the additional step of cooling the deformed article to a lower temperature prior to such removal. The first is that cooling, for example, to the Mg-Mj range or below, may reduce the work needed to remove the constraining means. Secondly, hy cooling the article under constraint from the holding temperature to a lower temperature, an additional increment of heat recoverable strain can be imparted to the article. After the constraining means has been removed, - 16 this increment of strain is usually recovered during a subsequent fast heating step over the temperature interval defined by the temperature at uhich the constraining means is released and the holding temperature. This additional increment of strain has its own Ag temperature. In other words, the article has a 1st A below the A_ (2nd A ) imparted by mechanical preconditioning. As a result, a two stage heat recovery can be obtained.

As explained in our British Patent Specification No. 1,490,182, some metallic compositions also respond better to thermal and mechanical preconditioning if aged while in the austenitic state in that a higher portion of the retained strain is heat recoverable. However, if the mechanical preconditioning conditions are the same, the Ag temperature imparted to an unaged sample Is often somewhat higher than that of an aged sample of the same composition. For those beta phase alloys of copper containing varying amounts of zinc, aluminum, silicon, manganese and combinations thereof having an He temperature below room temperature aging at from 5O°C to 125°C for a time ranging from 5 minutes to 3 or 4 hours is usually adequate. For other compositions, the time and temperature that will produce the optimum results may vary but is readily ascertained by comparing the amount of heat recoverable strain retained by samples of the same composition aged under different conditions.

The end use to which the article is put will determine Ite recovered and recoverable configuration. The deformation - 17 4 3 3 4 5· force applied to the preconditioned article can he any of a 1 variety of types including bending, twisting, compressing and expansion forces and may employ any convenient constraining means. In this way, articles that recover from an 1 to an I shape and vie® versa can be obtained. Articles that lengthen or shorten are also possible. Hollow articles, particularly cylindrical ones, that expand to a larger diameter or that contract to a smaller diameter are readily made by the process of this invention. As a result of the fact that mechanical preconditioning occurs in the area of the stress it is possible to precondition a portion only of the article. This allows a series of deformations to be built into the article which can recover at different temperatures.

The following Examples illustrate the Invention: BTOtEEB 1 A 38 mm s 5 mm s 0.75 mm strip of brass containing 64*6 wt. 0 Cu -34.4 wt. 0 Zn -1.0 wt. 0 Si was betatized at 800oCj then water quenched. After this treatment, the M was a s +2°C and the strip was pseudoelastio at room temperature, that is, the Δβ and A^ were below room temperature.

The strip was bent into a loop at room temperature (outer fibre strains 70) and clamped for one hour. Upon releasi the loop remained bent (retained outer fibre strain -^-50). When heated to 200°C, the strip became straight again.

BZAHEES 2 . Α 14-όει length of 0.9 mm diameter wire made up of - 18 4 3 3 4 5 wt. % Cu - 26 wt. % Zn - 4 wt. % Al was betati2ed at 700°C for three minutes, then water quenched. After this treatment, the wire was pseudoelastic at room temperature and had an at -3°C. s the sample was bent 86 as to have an outer fibre strain of 4.3% and constrained in this configuration at room temperature. From time to time, the constraining means was released, the retained strain measured, then the wire was returned to its constraint. Retained strain increased as follows: Days Retained Strain % 0 1 1.4 193 2.8 252 2.9 After the last measurement, the bent wire was immersed in oil at 200°C. It straightened immediately. This example demonstrates the effect on the retained strain of prolonging the holding time.

Example 3 Samples were cut from 0.76 mm sheets of the alloy compositions listed below. The strips were betatized at 800°C and water quenched. All were pseudoelastic at room temperature, as their low Mg temperatures would suggest. The samples were bent and constrained at room temperature so as to cause an outer fibre strain of 4.25%. The samples and constraining fixtures - 19 43345 were transferred to a bath at 200°C and held for 72 hours. Next, the constrained samples were cooled fo room temperature. Virtaally no springback occurred as the samples were removed from the constraining means. The samples were then rapidly heated. Both the heat recoverable strain and the temperature range over which it occurred are listed in the table below: Composition % bv weidht s Elevated ERSAs 74Cu l8Zn 50 7A1 1i4n -40°C 0.5% 375°C 500°C 76Cu 12Zn 8A1 4Mn -44°C 2.3% 375°C 525°C 77.5CU 9.5Zn 9A1 4Mn —40°C 2.75% 350°C 525°C 15 77.75CU S.25Zn 9A1 5Mn -28°C 2.3% 300°C 500°C 79.1Cu 5.9Zn 10A1 5Mn -40°C 3% 350°C 525°C 79Cu 4Zn 20 10A1 7Mn -40°C 2.2% 350°C 525°C 77.5CU 7.5Zn 9A1 6Mn -5Q°C 1.6% 375°C 525°C 78.25CU 5.75Zn 0°C 1.7% 400°C 525°C 9A1 7Mn This example demonstrates that the A temperature imparted to the alloy is not dependant upon the temperature at which the preconditioning is accomplished.

Example 4 In that a number of variables are important to successful t mechanical preconditioning, an experiment was designed to test several variables simultaneously. Five variables were tested at each of two levels, thus the experimental design was 2 factional. The variables were: Cooling Holding Holding Betatization Rate Strain Temp, Time high level 650°C - 5 min. Air Cool 7.10% 125°C 150 min. low level 575°C - 5 min. Water quench 4.53% 50°C 15 min. and age 5O°C~5 min.

The experimental design was exercised using four alloys: Weight Percent Cu Al Mn Air Cooled WQ + 5 min. 50°C 79.2 10.0 10.8 -10C -32C 78.9 10.0 11.1 -41 -45 79.04 9.86 11.1 -30 -47 79.07 10.13 10.8 -14 -32 Samples were prepared by air-melting the compositions above, coasting and rolling the 0.76 mm sheet. Strips were cut from the sheet, and bstatized by heating 5 minutes at 575°C or 650°C. Next, the samples were v/ater quenched and aged 5 minutes at 50°C or air cool. All the samples were cooled to -60°C then deformed and constrained either 4.53 or 7.1% by bending the samples around a mandrel and placing them in a clamping fixture. The samples and their constraining fixtures were transferred to baths at 50°C or 125°C and held for 15 minutes or 150 minutes. After the holding process, the samples and constraining fixtures were cooled to -30°C, the constraints were removed and the retained strain measured. The unconstrained samples were transferred to a bath at 0°C, and again the retained strain was measured. This procedure was repeated with baths at 20°C, 50°C, 1OO°C, 200°C and 400°C.

The resulting strain measurements were analyzed to determine the magnitude of the main effects and interactions with respect to the ranges over which the variables were exercised.

The strain which was heat recoverable in the temperature range above 50°C was taken as a measure of performance.

Statistical analysis indicated significance for the main effect of strain average 1.95%, and hold temperature, average 1.65%.

The other main effects and interactions were not significant in this experiment.

Within this experimental design, the best conditions were 7.1% strain at a holding temperature of 125°C. This gave an average of 3.81% heat recoverable strain above 50°C.

Example 5 An alloy containing 64 wt % copper, 35 wt. % zinc and 1 wt. % silicon was studied. This alloy has an s temperature of -40°C.

Specimens were betatized for 5 minutes at 860°C, quenched into water at 20°C, and then aged for different times in the metastable beta phase, which in this series of experiments was performed at 50°C. After insertion in the tensile loading device (approximately 5 minutes to set up at ambient temperature, the specimens were cooled to -65°C 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 spontaneous expansion if one occurred. The constrained specimen was placed in water at +40°C, which provides a very fast heating rate, and was held at that temperature for different times before re-cooling to below the Specimens came free of the constraint during cooling with a slight expansion compared with the original set after deformation. The constraint was removed from the apparatus so that specimens, now in their preconditioned state, could heat recover freely when reheated at a fast rate in a furnace set at 600°C.

The Afl temperatures and heat-recoverable strains were measured as a function of the two main variables, aging time at 50°C before deformation and the time held under constraint at 40°C.

Results of mechanical preconditioning are shown in Table 1. For each aging time at 50°C some specimens have also been fast heated directly after deformation at -65°C, in order to compare the effect of mechanical preconditioning on the 4 3 345 Ag temperature.

Table 1 shows clearly the trend that the 2nd Afl temperature was raised ag the holding time at 40°C was increased and in many cases exceeded the temperature of 40°C. On the other hand, the total heat-recoverable strain (i.e. 1st A •r S to Af) was reduced with increased holding time at 40°C, and this less in recovery occurred mainly in that portion of heat-recoverable strain between the 2nd A and A-.

X 3 3 4 5 TABLE I Aging Pre-cond. Strain Time Holding Time Per Cent at 40°C Afl Temp. °C 1st 2nd Recovery above 2nd A per cent 8 etrain Total Recovery per cent strain No Precond. 7.05 -50 - - 6.50 5 mine 10 eece 6.90 -43 -4 5.65 6.80 at R.T. 30 gees 7.10 -37 31 4.15 5.65 1 min. 6.90 -40 19 4.80 5.90 5 min. 7.65 -37 59 2.90 3.95 10 min. 6.95 -17 23 2.80 3.55 1 hr. 7.10 -45 19 3.10 4.00 No. Precond. 7.25 -33 - - 6.95 45 mine 10 sees at 50°C 30 sees 6.75 -49 -9 5.30 6.55 6.35 -52 4 4.40 5.85 1 min. 7.10 -43 23 4.45 5.70 5 min. 7.35 -40 20 5.60 7.00 10 min. 7.20 -51 · 19 3.65 5.15 1 hr. 7.55 -44 54 2.65 4.20 No Precond. 7.00 -32 - - 6.75 3 hrs. 10 secs 7.25 -41 -4 5.75 7.00 at 50°C 30 secs 7.20 -32 15 4.15 5.65 1 min. 7.05 -30 19 5.65 6.85 5 min. 6.85 -47 13 4.80 6.20 10 min. 7.20 -32 29 5.65 6.65 1 hr. 7.30 -37 38 4.15 5.25 5 hrs. 7.15 -44 44 5.60 6.75 16 hrs. 7.50 -39 80 3.75 5.25 No Precond. 7.20 -27 - - 6.70 24 hrs. 10 secs 7.05 -37 -4 5.85 6.55 at 50°C 30 secs 7.25 -42 -5 5.80 7.25 1 min. 7.45 -43 0 5.70 6.95 5 min. 7.50 -35 24 5.75 6.70 10 min. 7.50 -42 35 5.85 7.25 1 hr. 7.80 -34 29 4.70 5.80 5 hrs. 7.40 -34 35 5.05 5.95 16 hrs. 7.15 -47 69 2.90 4.70 No Precond. 7.10 -33 - - 6.80 1 wk 10 min. 7.00 -28 33 5.60 6.45 at 50°0 1 hr. 7.25 -37 47 5.20 6.20 5 hrs. 7.45 -37 40 5.15 6.70 16 hrs. 7.55 -40 33 5.60 6.70 4 3 3 4 5 Increasing the aging time at 50°C„ in metastable beta phase, greatly improved the overall heat-recoverable strains (HRS’s) bat had only a slight effect in reducing the 2nd Αθ temperature.

The effect of storage at room temperature was also examined. After the mechanical preconditioning treatment specimens were cooled and the constraint removed, as previously. Instead of directly heating at a fast rate, the specimens were allowed to warm to room temperature (20°C + 2} at which temperature they.were stored for up to three weeks. After storage, specimens were replaced into the testing rig, and heated directly from room remperature to above the A^ temperature.

As an example, one specimen was aged 1 week at 50°C and held in restraint for 16 hours at 40°C (the last result in Table 1). The efficiency of heat recovery when directly heated from the after releasing the constraint was 74%. This value fell to 57.4% after storage for two days, 47.8% after one week, and 46.4% after three weeks at 20°C. The second Αθ temperature remained constant at about 35°C.

EXAMPLE 6 I An alloy containing 63.5 wt. % copper, 35.5 wt. % zinc ani 1.0 wt. % aluminum was studied. Experimental conditions for mechanical preconditioning of this alloy were exactly the same as described in Example 5, except the deformation temperature which was -50°C. The alloy had an Μθ approximately -25°C. Specimens were again aged in metastable beta phase at 50°C, and held under constraint at 40°C. 433 45 For specimens aged 3 hours at 50°C, and immediately fast-heated after deformation at -50°C, 1st A_ = -13°C, but no 2nd Ag was observed, HRS=7.20% (94% efficiency). Results of a specimen aged 3 hours at 50°C and mechanically preconditioned are shown in Table II. Compared with the previous example of a copper-zinc-sillicon alloy, the rise in 2nd Ag temperature is not as high in this alloy.

TABLE II Aging Time Pre-Cond. Holding Time Strain per cent A temp. °C. Recovery above 2nd Ag percent strain Total Recovery percent strain 1st 2nd 3 hrs 10 secs 7.75 -35 -1 6.60 7.10 at 50 °C 30 secs 7.60 -13 40 5.45 6.40 ' 1 min 7.95 -32 -5 5.60 5.90 5 min 8.40 -19 14 6.80 7.45 10 min 7.65 -24 17 6.00 6.60 1 hr 8.10 -15 34 5.05 6.45 5 hr 7.60 -22 20 5.55 6.45 16 hr 8.00 -24 25 5.90 6.55 EXAMPLE 7 An alloy containing 65.75 wt. % copper, 32.25 wt. % zinc and 2.00 wt. % aluminum having an M of approximately -25°C was studied.

This alloy was treated in the same way as the previous alloy, and was aged at 50°C prior to deformation and held at 40°C 4334s under constraint in the mechanical preconditioning treatment. Table 3 shows the results for this alloy when aged for 3 hours at 50°C.

Results for an unconditioned sample of this alloy, aged 3 hours at 50°C and immediately fast heated after deformation at -50°C were: 1st A = -35°C, no 2nd A . HRS = 7.10 (98% efficiency), & s .

As shown in Table III below the 2nd Αθ temperatures in this aLloy were hot’raised as much as the previous alloycontaining 1% aluminum, but correspondingly the heat recoverable' strains are very high.

TABLE III Aging Pre Con, Time Holding Time at 40°C Strain per cent Aq Temp. C, 1st 2nd Recovery Total Recovery above 2nd percent strain As percent strain 3 hrs 10 secs 6.60 -47 -10 5.85 6.30 at 50°C 30 secs 7.50 -40 -8 6.85 7.35 1 min 6.85 -19 15 5.75 6.50 5 min 7.10 -34 -9 5.95 6.60 10 min 7.10 -22 11 5.45 6.75 1 hr 7.70 -24 21 5.25 7.30 5 hr 7.75 -19 8 5.65 6.65 16 hr 7.65 -25 19 6.40 7.20 It should be pointed out that the aluminum-containing alloy of this example and that of Example 6 could not readily be - 28 43345 treated to have a raised Ag temperature hy thermal preconditioning, as it was not possible in practice to prevent heat recovery during slow heating to the preconditioning temperature.

The same alloy was also aged in the beta phase at 5 1OO6C and held at 40°C, and aged at 50°C and held under constraint at 80°C. The results from these treatments are shown in Table IV, for specimens aged 3 hours at the appropriate temperature and held for different times in the stress-induced martensite state.

TABLE IV Aging Time Pre Con Holding Pre Con Holding Time Strain per cent A Temp C Recovery above 2nd As per cent strain Total Recovery percent strain 1st 2nd 3 hrs 40 10 mins 7.15 -40 -6 5.65 6.50 at 1OO°C 1 hr 7.70 -33 -2 5.75 6.60 5 hrs 6.10 -28 23 4.35 5.35 16 hrs 7.35 -29 20 5.70 6.65 3 hrs 80 10 min 7.80 -33 43 4.85 6.10 at 50°C 1 hr 6.75 -32 53 3.40 5.10 5 hr 8.25 -26 102 1.90 3.30 The overall effect of the higher aging temperature is to reduce the raised Ag temperature and increase heat recoverable strains.

The increase in preconditioning temperature from 40°C to 80°C has a much greater effect than the aging temperature - 29 (t ΰ □ “4 «J on the raised Αθ temperature. As shown in Table IV increasing the holding time at 80°C from 10 minutes to 5 hours raises the preconditioned (2nd) A from 43°C (i.e. less than the s holding temperature). Heat recovery is correspondingly reduced as the 2nd Αθ temperature increases.

Example 8 An alloy containing S2.2 wt % copper, 37.3 wt % zinc and 0.5 wt. 9½ aluminum having an Μθ of -33°C and an alloy containing 67.5 wt. % copper/ 29.5 wt. % zinc and 3.0 wt, % aluminum having an M of -30°C were studied. These alloys were s treated in the manner as described for the other copper-zincaluminum alloys in Examples 6 and 7. Results of mechanical preconditioning after aging in the beta-phase for three hours at 50°C, and holding under constraint for different times at 40°C, are shown in Table 5. Under the same experimental conditions the heat recoverable strains between the 2nd Αθ temperature ahd A^ are greater in the 3% 'aluminum alloy than in the alloy with 0.5% aluminum. - 30 4 3 3 4 5 TABLE V Alloy Aging Time Pre Cond holding time (40°C, Strain A Temp C Recovery Total per 8 above 2nd Recovery cent 1st 2nd Α» per percent cent strain strain 62.2% 3 hrs .10 mins 8.20 -41 24 3.15 4.10 Cu at 50°C 1 hr 8.35 -39 34 3.80 4.80 5 5 hr 7.90 . -44 12 4.90 5.95 16 hr 8.15 -47 29 4.25 5.30 67.5% 3 hrs 10 mins 6.65 -27 8 5.75 6.40 Cu at 50°C 1 hr 7.25 -40 24 5.35 6.60 5 hr 7.15 -33 11 6.05 6.60 10 16 hr 7.60 -21 26 5.25 6.60

Claims (24)

1. CLAIMSζΙ . A method for treating an article made from memory metal composition which is capable of undergoing a reversible transformation between austenitic and martensitic states, the method comprising holding the article in a deformed configuration under constraint at a temperature above M_(as hereinβ before defined) for a time sufficient to cause at least a portion of the .deformation to be retained when the constraint is removed,

2. A method as claimed in claim 1, wherein the article is held under constraint at a temperature below M^(as hereinbefore defined)

3. A method as claimed in claim 1 or claim 2, wherein the article is initially deformed while in the austenitic state.

4. A method as claimed in claim 1 or claim 2, wherein the article is initially deformed at a temperature below the holding temperature and its temperature is then raised to the holding temperature while it is still under constraint.

5. A method as claimed in claim 4, wherein the article is initially deformed at about Μθ.

6. A method as claimed in claim 4, wherein the article is initially deformed at a temperature within the M g range (as hereinbefore defined)

7. A method as claimed in claim 4, wherein the article is initially deformed at a temperature below the Μθ-Μ^ range.

8. A method as claimed in any one of claims 1 to 7, wherein after completion of the treatment, the constraint is removed and wherein the temperature of the article is lowered belotv M prior to removal of the constraint. 32 4 33 45

9. A method as claimed in claim 8, wherein the temperature is lowered to within the Μ 0 -Μ^ range prior to. removal of constraint.

10. A method as claimed in claim 8, wherein the article 5 is cooled to below the M 0 -M f range prior to removal of the constraint.

11. A method as claimed in any one of the preceding claims in which the article is initially deformed while in the martensitic state, and wherein it is slowly heated to the 10 holding temperature.

12. A method as claimed in any one of claims 1 to 11, wherein, prior to its deformation, the article is held at a teaperature above M fl while in the austenitic state for a time sufficient to reduce the loss of reversibility 15 between the martensitic and austenitic states and to improve its amenability to treatment by the method.

13. A method as claimed in any one of claims 1 to 12, wherein the metallic composition is an alloy that forms an electron conpound. 20

14. A method as claimed in claim 12, wherein the alloy is a body centered cubic type analogous to beta-brass having a ratio of about 3 valence electrons to 2 atoms,

15. A method as claimed in any one of claims 1 to 12, wherein the metallic composition as a beta-phase alloy comprising 25 copper and zinc or copper anialuminum. - 33 & 3 3 4 5

16. „ A method as claimed in. claim 15, wherein the metal is a beta-phase alloy comprising by weight 60-850 copper, up to 40$ ainc and/or up to 140 aluminium; 0-50 silicon, and/or 0-150 manganese.

17. A method as claimed in claim 15 or claim 16, wherein the alloy comprises copper zinc and silicon.

18. A method aB claimed in claim 15 or claim 16, wherein the alloy comprises copper, zinc and aluminium.

19. 4 method as claimed in claim 15 or claim 16, wherein the alloy comprises copper, zinc, aluminium and manganese.

20. A method as claimed in claim 15 or claim 16, wherein the alloy comprises copper aluminium and manganese.

21. A method as claimed in claim 15 or claim 16, wherein th© alloy is as specified in any one of the Examples herein.

22. A method as claimed in claim 1, conducted substantially as described in any one of the Examples herein.

23. An article which has been treated by a method as claimed in any one of claims 1 to 22.

24. An article as claimed in claim 23, which is heatrecoverable.