[0001] 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.
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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
s and M
f change thereby together.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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).
[0014] 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 t
1, to cool the material down to a second temperature range T
2, to maintain this temperature for a second time interval t
22 and subsequently to cool the articles down to the martensitic state below M
f.
[0015] 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.
[0016] In some cases the treatment times t
1 or t
2 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 T
1 is for example 50°C higher than T
2 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 T
2 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 B
2 superlattice. The treatment time t
2 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 t
1 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.
[0017] 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.
[0018] Table 1 shows by way of example target compositions within which the invention can
be applied :

[0019] 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.
[0020] 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 t
1 is preferred at T
1.
[0021] 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
T
3 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 T
2. The treatment time t
3 at this temperature is set at a minimum for obtaining a uniform temperature T
3 throughout the article after which the temperature is preferably returned to T
1 as quickly as possible and the treatment, as described in the first process, is applied.
[0022] 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 T
1 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
I for a very short time.
[0023] 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 T
1.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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 ΔAs according to a second process according to the invention.
[0029] 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.
[0030] 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).
[0031] 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.
[0032] 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
[0033] 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.
[0034] 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.
[0035] 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.
[0036] 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 t
2, according to line 16, prior to cooling it further in a conventional way to below
M
f according to the line 17.
[0037] 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.
[0038] 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.
[0039] 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 T
2 as shown by the dotted line 18.
[0040] 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 T3 above the M temperature and significantly below T
1. This is shown by the line section 19 of the cooling curve 20. The temperature T
3 is selected between the uppermost limit shown by the dotted line 25 and the bottom
limit 26.
[0041] The corresponding time t
3 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 T
1 for a time t
1 and at T
2 for a time t
2.
[0042] 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 T
1 in a beta-crystal structure for a time t
1, followed by a second treatment time t
2 after cooling to a lower temperature T
2 also in the beta-crystal structure. The further cooling to below M may be conducted
without special precautionary measures.
[0043] As an alternative, a slowed-down cooling as from T
1 according to the dotted line 22 can be applied, for example in a furnace ; so that
the treatment time in the T
2 temperature range is equivalent to the proposed time t
2.
[0044] 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 T
1 and T
2 more lattice vacancies can be absorbed in dislocation
[0045] clusters and in the grain boundaries with the consequence that hence the concentration
of lattice vacancies in martensite is considerably reduced.
[0046] 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.
[0047] The treatment temperatures T
1 and T
2 and the treatment times t
1 and t
2 are preferably optimized on the basis of a limited test on a collection of samples.
[0048] 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.

[0049] 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.
[0050] The treatment parameters (such as e.g. treatment temperatures and stress) at temperatures
T
1 and T
2 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.
[0051] 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 (T
2) for 20 minutes (t
2). 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.
[0052] 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 :

[0053] 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 :

[0054] The heat treatment according to the invention results thus in a substantial decrease
in ΔA
s.
[0055] 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 %.
[0056] 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.
[0057] 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.
1. A process of treating heat recoverable metallic articles with temperatures As, Af, Ms, Mf 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
Ms, Mf, As, and Af at their equilibrium level after which these articles are cooled down to their martensitic
state below the Mf.
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 (tl) 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 Ms temperature and 0.7 Tc DO3, wherein Tc 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 Tl, and successively after having been held at this temperature for a time interval
tl, 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 Af.