EP0095798A1

EP0095798A1 - Process for thermally treating heat recoverable metallic articles and articles thereby obtained

Info

Publication number: EP0095798A1
Application number: EP83200677A
Authority: EP
Inventors: Luc Delaey; Jan Van Humbeeck
Original assignee: KU Leuven Research and Development
Current assignee: KU Leuven Research and Development
Priority date: 1982-05-13
Filing date: 1983-05-11
Publication date: 1983-12-07
Also published as: ES522400A0; EP0095798B1; DE3370828D1; ES8402880A1

Abstract

The invention relates to a process for heat treating heat recoverable articles with an A_s at room temperature or above. The articles are cooled from the Beta-phase to above M_s and held there for a sufficient time to stabilize the transformation temperatures. Heat recoverable articles are described comprising Cu-Zn-AI alloys, eventually with additions of cobalt or boron as well as thermoresponsive actuators including said articles.

Description

The invention relates to a process for thermally treating heat recoverable metallic articles with the purpose of improving their heat recoverability as defined below, e.g. by maintaining said property stable in time.
Heat recoverable articles are known which consist of an alloy containing e.g. copper, zinc and aluminium as described e.g. in the U.S. patent No. 3.783.037.
Heat recoverability is to be understood hereinafter as to comprise pseudo-elasticity, shape memory, reversible shape memory, good mechanical vibration damping and change in electrical conductivity which properties all relate to the transformation of an ordered Beta-crystal structure as metallic compound to a martensitic crystal structure. The temperatures which define the change of the crystal structure when following the transformation cycle are known as As, A_f, M_s and M_f.
It is known that the A -temperatures tend to change when the articles are maintained at a temperature in the martensitic state, which temperature is situated relatively close to this transformation temperature. However, varying A s and M _s are very inconvenient in applications where a high reliability of the operation of a device is linked to the crossing of a temperature level which depends on these transformation temperatures of the heat recoverable article. For instance, a substantial change of A is not s permissive in the operation of a fire safety device, an alarm device in a chemical process which is continuously exposed to a high temperature or for controlling a window opening in a greenhouse which is adjusted at an optimal temperature.
In some ranges of composition of copper-zinc-aluminium alloys, particularly with more than 5 % of aluminium - e.g. with about 6 % - and with 21 % zinc - the A _s temperature can increase so much after a certain time of holding the article at room temperature that the transformation from martensite to the beta crystal structure (on further heating) can be traversed by a recrystallization reaction. Cu-Zn-Al alloys and alloys similar thereto or related therewith are therefore not always usable as such with respect to their properties of heat recoverability.
In this application the alloy compositions useful for the invention will be given in weight percentages, wherein the sum of all metallic components are calculated as a 100 % composition and wherein additions as some or other non-metallic phase such as oxides are not considered. For Cu-Zn-Al alloys, substituting elements of the metallic phase in the composition will be regarded as a substitution of copper because aluminium and zinc are the main elements which determine the characteristic transformation temperatures M_s, M_f, As and A_f.
The alloys, applicable according to the invention, have an As temperature which generally, but not necessarily, corresponds to the M _s temperature or which only differs by a few °C from it and which moreover is above 0°C and preferably even above room temperature. Below these temperatures, the diffusion rate of vacancies in the lattice is sufficiently low to reduce the change of A_s as is intended by the invention.
The process according to the invention is especially applicable in those circumstances where during the thermal treatment no equilibrium phase of alpha or gamma type nor a transition phase such as bainite is formed, which would lead to an equilibrium phase. In other words the thermal treatment will be executed in such a way that these equilibrium or transition phases are not formed. However the presence of aluminium-containing precipitates of intermetallic compounds, such as e.g. cobalt, which have a high grain boundary plane energy, seem to be able to favourably influence the stability of the heat recoverability over a longer time.
In a treatment which is described by Schofield and Miodownik (Metals Technology - April 1980 p.167-173), it is the main purpose to stabilize at the same time A and M_s on keeping the alloy at a temperature above M s which alloy was previously quenched to this temperature above M and without having been submitted to a deformation. When staying at this temperature (for an alloy Cu-Zn-Al with 4 % Al) the ordering parameter S of the Beta-lattice changes. Hence all characteristic temperatures A_s, A_f, M_sand M_f change thereby together.
The present invention however, aims at keeping A at a s substantially unchanged level, even after a long stay of the heat recoverable articles in martensitic condition at A or just below A_s. The stay in these conditions is generally and also hereafter called ripening. In practice a very slow heating has often the effect of ripening. The tendency however, to raise the temperature A by ripe- s ning does not exist for the M and M_f-temperatures. For alloys with a higher A , e.g. at 150°C, the effect of ripening is more difficult to master, but the invention can usefully be applied for ripening temperatures between e.g. 50°C and 120°C.
The invention consists in reducing the number of lattice vacancies in the beta structure by a factor of at least hundred before its transformation to martensite and even to bring them to a negligible level (in the order of 10^-7 to 10^-9 ). With a concentration of vacancies which is too high, either a change in the parameter S of long range ordering in the martensitic stage is reached or a pinning of lattice vacancies with nuclei and with grain boundary planes in the martensite is reached. These martensite grain boundary plates are the boundaries of the different martensite plates or twin plates which form in the martensitic structure. The boundary plates are not grain boundaries as such, but the boundaries of areas (subgrains) which are in a fixed orientation relationship with respect to each other and in relation to the crystallographic parameters.
The invention relates to a process for treating heat recoverable articles with the temperatures A_s, A_f, M_s, M as features determining the recoverability and which articles consist of a metallic compound composed of e.g. an alloy comprising copper, zinc and aluminium and which have an A _s temperature above 0°C by transformation from a martensite to a beta crystal structure.
According to this process the articles are cooled down from a disordered beta-crystal structure at elevated temperature at least to a lower temperature where an ordered beta superlattice may exist without an alpha or gamma equilibrium phase or a transition phase, such as bainite, which may lead to an equilibrium phase being formed. These articles are kept for a sufficiently long time in the beta-crystal structure above the M _s temperature to stabilise the temperatures M_s, M_f, As, A_f at their equilibrium level, after which the articles are cooled down to the martensitic state below M . The term equilibrium level is meant herein to refer to the situation where the transformations always occur at the same respective temperatures upon repetitive application of the heating and cooling cycles, eg. after deformation. According to the invention the concentration of lattice vacancies is reduced and an increase in A _s temperature after ripening is counteracted (whereas also the other transformation temperatures M_sand M_f are kept constant).
In a first process according to the invention the reduction of the number of lattice vacancies is obtained by quenching the material of the articles from the disordered higher temperature beta-crystal structure to a first temperature range T in the beta structure, to keep to articles at these temperatures for a time interval t₁, to cool the material down to a second temperature range T₂, to maintain this temperature for a second time interval t₂₂ and subsequently to cool the articles down to the martensitic state below M_f.
The limits imposed on these time and temperature parameters are relative, i.e. the imposed treatment time may be shorter according as the temperature becomes higher.
In some cases the treatment times t₁ or t₂ may be so short that cooling in the air or in the furnace are sufficiently slow within a given time interval, to pass through the appropriate temperature ranges to obtain the desired stability of the A _s temperature. This also depends on the alloy composition. The temperature T₁ is for example 50°C higher than T₂ and lower than the alpha or gamma pre- cipitation or the beta-recrystallization limits, and lies (depending on alloy and treatment time) preferably between 150°C and 500°C. The treatment time is preferably as short as possible, e.g. from a few seconds to 30 minutes, preferably between 10 seconds and 10 minutes. The temperature T₂ is higher than the M temperature and smaller than or equal to 0.7 T_{c DO3}, whereby T_{c DO3} is in absolute scale the critical temperature at which the D03 superlattice is formed from the B₂ superlattice. The treatment time t₂ must satisfy a minimum criterion and may extend from 1 minute to a few hours, preferably from 5 minutes to 2 hours. Since a great number of vacancies are absorbed in the grain boundaries, especially the treatment time t₁ may be strongly limited when an alloy with a fine grain structure is used, for example when the average grain size is inferior to 200 µm.
The alloys to which these methods are applicable include the ternary Cu-Zn-Al alloys which have an A _s temperature above 0°C and preferably above 20°C because at lower temperatures the diffusion speed becomes so small that a change of A due to ripening will be hardly noticible.
Table 1 shows by way of example target compositions within which the invention can be applied :
In a second process according to the invention the article is kept at a temperature T for a predetermined time as in the first process. Instead of carrying out a treatment at T_l, a content of 0.01 % to 2 % and preferably less than 1 % and around 0.4 % of copper can be substituted by a subsidiary element such as cobalt or by cobalt in combination with another metal. Then under suitable annealing conditions precipitates with high grain boundary plane energy are produced which have an average size smaller than 10 µm and preferably smaller than 5 µm. They are insoluble below the temperatures at which the equilibrium phases alfa and gamma are dissolved in the disordered beta-crystal structure. The element cobalt which forms a metallic compound together with aluminium may also be substituted by the elements palladium and platinum. These elements may even be partially, and for at most 50 %, be replaced by titanium, chromium or nickel or by a combination thereof. This mixture of elements will be designated, hereinafter, as cobalt, insofar as they produce similar effects of grain refinement and uniformly distributed precipitates.
The boundaries of the grains and of the precipitate with high grain boundary plane energy probably absorb a great number of lattice vacancies. A similar effect is obtained in an alternative way by the addition of boron up to 0.1 %, preferably below 0.05 % and in particular between 0.01 % and 0.03 %. If, for example, owing to an extra annealing operation, these alloys have an average grain size of more than 200 µm, then a treatment time t₁ is preferred at T₁.
In a third process according to the invention a heat recoverable article as in the first process is quenched from the high-temperature beta-crystal structure to a temperature T₃ where the metallic lattice is in the beta superlattice, but quenching may take place in a medium which has a temperature in the area of T₂. The treatment time t₃ at this temperature is set at a minimum for obtaining a uniform temperature T₃ throughout the article after which the temperature is preferably returned to T₁ as quickly as possible and the treatment, as described in the first process, is applied.
In a fourth process according to the invention the heat recoverable article, similar to that in the third process, is initially quenched to a lower temperature, this time at a temperature at which transformation to martensite takes place, preferably even below M_f. Heating to a temperature at least equal to T₁ must then follow immediately. This quenching treatment is applicable to some alloys, for example to an alloy containing 70 % copper, 24 % zinc and 4 % aluminium. As for other alloys, for example composed of 73 % copper, 21 % zinc and 6 % aluminium, the possibility of using shape memory characteristics is strongly reduced except in combination with certain production methods, such as hot drawing or extrusion. In some cases, an additional annealing treatment in the beta-crystal structure is necessary. In other cases, the shape memory is recoverable by rapid heating and holding at a temperature above T_Ifor a very short time.
The third and fourth process according to the invention also have practical significance to alloys of which the time required to form a precipitate from an equilibrium phase such as alfa, gamma or bainite is very short. In a lower-temperature cooling medium, the tip of the T T T curve can be more easily avoided because the cooling speed is higher. Afterwards the treatment time t_l can be increased without danger of precipitation at T₁.
In the description of the different processes according to the invention no mechanical deformation of the article was mentioned neither in a beta-crystal structure nor at lower temperature or in martensite. Cold deformation produces a rise in A _s temperature, which disappears again after a number of thermal and deformation cycles. After these treatment cycles which may for example be carried out some 20 times, a reversible shape memory effect is produced. The article behaves as an undeformed material. The heat recoverability capacity can easily be quantified by measuring for example the electrical resis- tivity.
With the different processes according to the invention the overall average vacancy density in the martensite or beta-crystal structure is reduced by preferential absorption of vacancies in some places such as the grain boundaries.
The scope of protection also covers heat recoverable articles consisting of an alloy which contains e.g. copper, zinc and aluminium to such a proportion that at higher temperature a disordered beta-crystal structure exists and at lower temperature an ordered beta-crystal structure in the absence of precipitate of an equilibrium phase or of a transition phase which may lead to an equilibrium phase, so that at a still lower temperature, but above 0°C, the alloy consists of martensite with a reduced concentration of lattice vacancies.
Furthermore, the structure may contain a precipitate with high grain boundary plane energy such as an aluminium-cobalt compound, so that the average grain size is smaller than 200 µm. This kind of precipitate formation may also occur in the presence of boron. Alternatively or in addition thereto, an increased dislocation density may be achieved through hot deformation in the disordered beta-crystal structure.
The invention will be further clarified with reference to the adjoined drawings in which

Figure 1 shows the influence of the different heat treatments on the transformation temperatures ;
Figure 2 shows the composition range in the copper, zinc and aluminium diagram within which the invention is applicable ;
Figure 3 is a scheme of a suitable cooling curve : temperature decrease (ordinate) as a function of time (abscisa) ;
Figure 4 shows an alternative cooling curve ;
Figure 5 is a graph representing an example of the influence on ΔA_s according to a second process according to the invention.

Figure 1 shows in a schematic way for an article made of an alloy containing 73.5 % copper, 20.5 % zinc and 6 % aluminium how the hysteresis loop, which is a characteristic of heat recoverability, changes when running through the transformation cycle of the beta-crystal structure towards martensite as a function of temperature. The figure is relative to the extent that the different loops were drawn separate from each other to avoid confusion through overlaps. The characteristic of heat recoverability was measured as the change in electrical resistivity R (in ordinate) with temperature. Analogously also another parameter can be used. A type indication of the hysteresis loop 1 with the characteristic temperatures is shown in frame 2. A rod made of the alloy mentioned above is, after fabrication annealed for a first time at 750°C for 15 minutes and subsequently immediately quenched and kept for 20 minutes at 10°C above M _s (= 60°C) prior to being further cooled in the air. The hysteresis loop recorded hereafter is shown by the line sections 3 and 4.
An identical rod which after this treatment was subjected to a 5 % deformation in the martensite phase and heated again to above A_f has a first thermal cycle according to the line sections 5 and 6. It can be noted that A has s been shifted upwards as a consequence of the deformation in martensite. The following cycles again approach the line sections 6 and 4, in other words, A again drops to s its former level (and also A_f decreases strongly).
An identical rod which is directly quenched to a temperature of±10°C above M and which is kept there for a long time (two hours) before being cooled down in the air to the martensitic state, then goes through a cycle according to the line sections 6 and 4. In case of 3 % deformation in the martensitic state the loop deforms according to the line sections 7 and 4 to return to the line sections 6 and 4 after a few cycles.
If, however, the process according to the invention is applied, for example the first process as described above on a slightly modified composition, then an equilibrium cycle is obtained existing of the line sections 8 and 9 whereby the deformation of the martensitic phase broadens the hysteresis loop towards the line sections 9 and 10. After repeating the cycle a few times, the loop will again coincide with the line sections 8 and 9. This avoids a strong shifting of A . s
Figure 2 shows as an example the composition range of the copper, zinc, aluminium diagram where the invention is very well applicable. The compositions lie between the points 11, 12, 13 and 14 shown in Table 1 and can be completed with the already discussed subsidiary elements such as for example cobalt. A limited amount of accidental subsidiary elements is thereby not excluded. The transformation temperature may then shift with the composition. The connecting line between the points 11 and 14 corresponds with an A _s transformation temperature of about 0°C and that between the points 13 and 12 with a transformation temperature A of about 190°C.
This diagram is based on the A _s temperatures and has been derived from "Metallwissenschaft und Technik", 31st year, issue no.12, December 1977, page 1326, where a similar diagram is based on the M _s temperatures. However, the analysis accuracy of these brass alloys is more problematic than the accuracy with which the transformation temperature can be measured. For the phenomena further described in this Application the temperature measurement is a determining factor after a standardized heat treatment. Indeed, the transformation temperature takes automa- tically account of the accidental presence of subsidiary elements or undesired precipitate formation.
Figure 3a shows the temperature evolution in time for an article which is submitted to a stepwise heat treatment such as described above in the first process according to the invention. The point of departure is an annealing operation at high temperature in the beta-crystal structure which, for most alloys is conducted at 750°C for a minimum duration of e.g. 5 minutes and preferably 15 minutes.
After quenching, the article is kept in point 15 of the graph at temperature T for the time t to be cooled further down towards T for a time t₂, according to line 16, prior to cooling it further in a conventional way to below M_f according to the line 17.
A first test rod of a ternary alloy of 73.5 % copper, 20.5 % zinc and 6.0 % aluminium was annealed at 750°C for 15 minutes, quenched in water at 80°C, kept for two hours at this temperature and then cooled until full transformation into martensite. When passed through a thermal cycle, a check rod showed an M of 60°C and an A of 62°C. The s s check rod was then divided into pieces and stored in a ripening test at different temperatures for different durations. At 25°C and after 7 days a ΔA_s = 3°C was measured. At 60°C and after 1 day the ΔA_s = 10°C, after 7 days it was 17°C.
A second test rod, which after having been cooled down was kept at 250°C for 5 minutes and subsequently further cooled as the first test rod, (hence after further quenching to 80°C and maintaining at this temperature for 2 hours and then cooling down to under M_f),shows under comparable conditions in the ripening test a ΔA_s which s is less than half that of the first test rod. The evolution of the transformation temperatures was measured on the basis of the electrical resistivity to avoid parasitic effects of mechanical deformation.
As in the second process according to the invention it is also possible to quench an alloy, with eg. a precipitate of an aluminium-cobalt compound and with an average grain size of less than 200 µm, directly down to the temperature T₂ as shown by the dotted line 18.
Figure 3b shows the temperature evolution of a thermal treatment corresponding with the third process according to the invention. After an annealing treatment at 750°C for 15 minutes, the treated article is quenched to a tem- ^{perature T} ₃ above the M temperature and significantly below T₁. This is shown by the line section 19 of the cooling curve 20. The temperature T₃ is selected between the uppermost limit shown by the dotted line 25 and the bottom limit 26.
The corresponding time t₃ is preferably limited to the minimum level to obtain a uniform temperature in the article. The main purpose of the preliminary cooling is to obtain a greater treatment efficiency for thicker objects. The reduction in lattice vacancies is mainly obtained through a combination of the subsequent treatment at temperature T₁ for a time t₁ and at T₂ for a time t₂.
Figure 4 shows the cooling curve 21 of an article corresponding with the fourth process according to the invention. The quenching treatment is conducted at a temperature below M_f, after which it is heated to a temperature T₁ in a beta-crystal structure for a time t₁, followed by a second treatment time t₂ after cooling to a lower temperature T₂ also in the beta-crystal structure. The further cooling to below M may be conducted without special precautionary measures.
As an alternative, a slowed-down cooling as from T₁ according to the dotted line 22 can be applied, for example in a furnace ; so that the treatment time in the T₂ temperature range is equivalent to the proposed time t₂.
This fourth method has been advantageously applied after extrusion or hot rolling alloys into elongated articles such as wire and profiles. The heat deformation provides a high concentration of lattice defects. It is assumed that through the intermediate treatments at T₁ and T₂ more lattice vacancies can be absorbed in dislocation
clusters and in the grain boundaries with the consequence that hence the concentration of lattice vacancies in martensite is considerably reduced.
Although quenching below the M_f temperature is appropriate, a quenching temperature between M_s and M_f is also applicable. The quenching treatment is applied until above M for those alloys whereby the heat recoverability of the article would be jeopardized or endangered.
The treatment temperatures T₁ and T₂ and the treatment times t₁ and t₂ are preferably optimized on the basis of a limited test on a collection of samples.
The following test run in Table II is useful for determining the parameters for an individual alloy with the composition range according to Figure 2.
The transformation temperature A of all samples is then s immediately determined by means of resistivity measurements. All samples are returned to the martensitic state all for the same duration, e.g. two days at the same temperature, e.g. 2°C - 3°C below the determined A . The A_s s temperature is measured again. In this manner the different A can be determined. Circumstances causing s disturbing precipitation or other inconveniences are avoided.
The treatment parameters (such as e.g. treatment temperatures and stress) at temperatures T₁ and T₂ during the cooling process are therefore dependent upon the M _s temperature, the composition of the alloy out of which the article is composed and the absolute temperature at which for this composition the vacancies are reduced to a lower level.
For a cooling profile corresponding with the second process according to the invention, the influence of the ripening on A_s is shown in Figure 5. Articles made of an alloy of 73.5 % copper, 20.5 % zinc and 6 % aluminium are compared with articles of an alloy of 73.1 % copper, 20.5 % zinc and 6 % aluminium, 0.39 % cobalt and 0.024 % titanium. Owing to the influence of the annealing conditions on the transformation temperatures in the presence of e.g. cobalt, either in combination with titanium or not, the annealing took place each time at 750°C for 15 minutes after which quenching took place down to 80°C (T₂) for 20 minutes (t₂). In case of further cooling to martensite, an A s temperature of circa 60°C was found in both cases on the basis of measurements of the electrical resistivity. Subsequently successive measurements of A were conducted after various ripening times in the martensitic state at 60°C. The line 23 shows a value of ΔA_s without the addition of cobalt - titanium. The line 24 shows the influence of ΔA_s for articles of an alloy containing additions of cobalt and titanium after the thermal treatment produced a grain size of less than 200 µm.
In another example, an alloy comprising 73.5 % Cu, 20.5 % Zn and 6 % Al (M ≃ 60°C) (in the form of a plate strip with a thickness of 1 mm) was betatized for 15 minutes at 750°C and quenched into water at 80°C. It was maintained at 80°C for two hours and then cooled in the air to room temperature. The sample was heated again to 55°C (≃ A ) for some time and the Δ A was determined :
Similar strip samples with same composition, shape and dimensions as above, were betatized at the same conditions and subsequently quenched down into hot oil at 250°C and held there for five minutes whereafter they were further quenched down to 80°C, and maintained there for two hours. Upon further cooling down in the air to room temperature and reheating for a certain time to 55°C resp. 45°C, the following A A -values were found :
The heat treatment according to the invention results thus in a substantial decrease in ΔA_s.
The invention is not limited to the Cu-Zn-Al containing alloys mentioned hereinbefore. It is also applicable e.g. to alloys which contain (apart from unavoidable impurities) 4 - 40 % zinc, 1 - 12 % aluminium, 0 - 8 % manganese, 0 - 4 % nickel, 0,005 - 1 % boron and the balance copper. The zinc content is then preferably 5 - 32 % and the aluminium content 3 - 10 %.
In practice the heat treatments on the shape memory alloys according to the invention enable them be used as actuator for temperature control. A change in temperature can thus be identified and signalized by the articles of the invention between a lower temperature which is lower than or equal to A s of the alloy and a higher temperature which is (preferably) at least equal to Af of the alloy. The change in shape or tendency to such change which occurs at this change in temperature then forms the signal which permits identification of the change in temperature. The difference between the higher and the lower temperature can thereby be smaller than 50°C and the lower temperature can be at room temperature or above.
According to the invention it is now possible to store the thermo-responsive actuator for a longer time at the lower temperature without creating a substantial upward shift in the A _s temperature. Apparatuses or devices comprising said actuators are of course within the contemplation of the invention. These actuators then comprise heat treated articles described above as means which enable them to re- producebly change in shape or a to tend to such change upon crossing a predetermined temperature range with the actuator.

Claims

1. A process of treating heat recoverable metallic articles with temperatures A_s, A_f, M_s, M_f as characteristics of recoverability, wherein the transition temperature A _s of the articles is higher than 0°C, and wherein the articles, having a disordered beta-crystal structure at elevated temperature, are cooled down to at least a lower temperature where an ordered beta superlattice crystal structure may exist without forming an alpha or gamma equilibrium phase or a transition phase, such as bainite, which may lead to an equilibrium phase, characterized in that these articles are maintained in the ordered beta-crystal structure above the M _s temperature at least for a sufficiently long period to stabilize the temperatures M_s, M_f, A_s, and A_f at their equilibrium level after which these articles are cooled down to their martensitic state below the M_f.

2. A process according to claim 1, characterized in that the concentration of lattice vacancies in the beta lattice structure is reduced by a factor of at least hundred and this reduced concentration is maintained in the martensitic state.

3. A process according to claim 1 or 2 characterized in that the article is quenched from an elevated temperature beta-crystal structure to a first temperature range (T₁), held there during a first time interval (t_l) and immediately thereafter cooled down to a second temperature range (T₂) and held there during a second time interval (t₂), after which it is further cooled down to the temperature where martensite is formed.

4. A process according to claim 3 characterized in that the first temperature range T₁ is lower than the recrystallization limit.

5. A process according to claim 3 characterized in that the first temperature range T₁ is lower than 500°C.

6. A process according to claim 3 characterized in that the first time interval t₁ is smaller than 30 minutes.

7. A process according to claim 6 characterized in that the first time interval t is between ten seconds and ten minutes.

8. A process according to claim 3 characterized in that the second temperature range (T₂) is between the M_s temperature and 0.7 T_{c DO3}, wherein ^T _{c DO3} in absolute scale is the critical transition tempera- ture of a D03 superlattice towards the B2 superlattice.

9. A process according to claim 3 characterized in that the second time interval t is at least five minutes.

10. A process according to claim 3 characterized in that the metal articles are quenched from a disordered beta-crystal structure at elevated temperature to the level of the temperature range T₃ and are held in this temperature range for a limited time interval t₃ after which they are heated again to the first temperature range T₁ and again quenched to the second temperature range T₂ where they are held for a period of time prior to being further cooled down to the martensitic state.

11. A process according to claim 3, characterized in that the metal articles are quenched from the disordered beta-crystal structure at elevated temperature to below the M _s temperature and are maintained at this temperature for a limited period of time after which they are again heated to the first temperature range T_l, and successively after having been held at this temperature for a time interval t_l, again quenched to this second temperature range T₂ where they are held for a period of time t₂ and subsequently cooled down to full transformation to a martensitic state.

12. A process according to claims 1 or 2 characterized in that the articles in disordered beta-crystal structure at elevated temperature are submitted to an annealing treatment for at least five minutes.

13. A process according to claims 1 or 2 characterized in that the articles in disordered beta-crystal structure at elevated temperature are thermally deformed.

14. A process according to claim 13 characterized in that the thermal deformation comprises in an extrusion treatment.

15. A heat recoverable article comprising an alloy which at elevated temperature possesses a disordered beta-crystal structure and at a lower temperature an ordered beta-crystal structure in the absence of a precipitate of an equilibrium phase or of a transition phase, which may lead to an equilibrium phase characterized in that at a still lower temperature but higher than 0°C the alloy consists of martensite with a reduced concentration of lattice vacancies.

16. An article according to claim 15 characterized in that the alloy has a composition in the ternary copper-zinc-aluminium diagram within the limit points (11) 64 % Cu , 35 % Zn, 1 % Al ; (12) 74 % Cu, 21 % Zn, 5 % Al ; (13) 87.5 % Cu, 0 % Zn, 12,5 % Al ; (14) 86 % Cu, 0 % Zn, 14 % Al.

17. An article according to claims 15 or 16 characterized in that in the martensitic state a deposit with high grain boundary plane energy is present.

18. An article according to claims 15, 16 or 17 characterized in that the alloy contains between 0.01 % and 2 % cobalt.

19. An article according to claim 18 characterized in that the cobalt content is less than to 1 %.

20. An article according to claims 15, 16 or 17 characterized in that the alloy contains between 0.01 % and 0.1 % boron.

21. An article according to claim 20 characterized in that the alloy contains less than 0.05 % boron.

22. An article according to any of claims 18-21, characterized in that the alloy has an average grain size smaller than 200 µm.

23. An article according to claims 15, 16 and 17 characterized in that as a consequence of plastic deformation of the disordered beta-crystal structure a big concentration of dislocations is present.

24. An article according to claim 15 characterized in that it comprises 4 - 40 % zinc, 1 - 12 % aluminium, 0 - 8 % manganese, 0 - 4 % nickel and the balance copper.

25. An article according to claim 24 characterized in that it comprises 5 - 32 % zinc and 3 - 10 % aluminium.

26. Thermoresponsive actuator comprising means for identifying and signalizing a change in temperature which means include heat recoverable articles according to any of the claims 15- 25, characterized in that they are able to reproducibly change in shape or to tend to such change by crossing a temperature range from below the A of the article, which is at room temperature or above, and up to above its A_f.