FR2618163A1 - Process for education of an article made of alloy with shape memory, making it possible to ensure the precision and the stability of the reversible memory effect - Google Patents

Abstract

Process for the education of an article made of alloy with martensite transformation with shape memory, making it possible to obtain the precision and the stability of the reversible memory effect, the said article having been put in its initial shape by starting with an alloy blank and then homogenised in beta phase. The said article is subjected repeatedly to a succession of education cycles during which: - an increasing mechanical stress sigma is applied to it at a constant temperature T1 which is higher than the transformation point Af, resulting in a deformation epsilon max in the superelastic region up to a maximum value sigma max, which is lower than the elastic limit of the material in the austenitic state at the temperature T1, - the stress sigma is released as far as a zero value, the article recovering appreciably its initial shape apart from a small residual deformation epsilon rho 1 (T1), - the placing under stress sigma max is repeated, followed by a return to zero value, the article recovering substantially its initial shape apart from a residual deformation epsilon rho 2(T1)> epsilon rho 1(T1), - the placing under stress sigma max is thus repeated, followed by a return to zero value during n cycles, at least until the residual deformation epsilon rho n(T1) reaches a limiting value, -the article is then heated to a temperature T2 which is at least 50 DEG C higher than Af.

Description

METHOD FOR EDUCATING A MEMORY ALLOY OBJECT
OF FORM, TO ENSURE PRECISION
AND STABILITY OF THE REVERSIBLE MEMORY EFFECT
TECHNICAL FIELD OF THE INVENTION
The invention relates to a method of educating a shape memory alloy object to ensure high accuracy and stability of the reversible memory effect.

It applies to all shape-memory alloys with martensite-austenite processing, such as copper-based alloys and nickel-titanium alloys.

STATE OF THE PRIOR ART
The shape memory alloys, discovered in 1938, are alloys capable of presenting a martensitic thermoelastic transformation. It is this structural transformation that is at the origin of the properties of these alloys: rubber effect, superelastic effect, simple memory effect, reversible memory effect.

The high temperature crystallographic structure is called austenite and the low temperature crystallographic structure is called martensite. The start and end temperatures of martensitic transformation, before education, are noted by convention Ms and
Mf cooling, As and Af heating. These alloys which have multiple examples of use in simple memory effect have already been presented in a large number of publications.

On the other hand, the reversible memory effect, which is obtained after a treatment called "education treatment", has only been described in a small number of publications, in which the shape memory states can not be used. only for specific applications, and where only the overall state of shape change can be used.

European Patent Application EP-0161952-A2 does indeed disclose a method for obtaining from a shape memory alloy object the reversible memory effect, wherein the temperature is varied between the austenitic state and the martensitic state, and simultaneously one imposes a plastic deformation up to 20% in one of these states, or, successively in both states. The method proposed in this patent is different from that used in the present invention, which acts only on one or the other of these parameters, and which we will now describe.

The method, object of the invention, can be advantageously used with any shape memory alloy object to obtain a precise reversible memory effect. It is recalled that an object has the reversible memory effect when it has a so-called low temperature form different from its so-called high temperature form. The transition from one form to another takes place simply by changing the temperature. The term "obtaining a precise reversible memory effect" is understood to mean the memorization, as a result of the education treatment, of low and high temperature forms corresponding precisely to the shapes that it was desired to obtain. to obtain a stable reversible memory effect: stable vis-à-vis the holding time at the temperatures of use, and stable vis-à-vis the number of cycles between the low and high temperature forms.

It is known in fact that, for example, the CuZnAl shape memory alloy objects can lose their properties if they are heated to temperatures above 1000.degree.

OBJECT OF THE INVENTION
The subject of the present invention is an education process or treatment and the application of this education treatment to any object made of a shape memory metal alloy with a view to conferring on it a precise and stable reversible memory effect, such as defined above.

It is a process of repeating cycles called "education cycles", which may be mechanical or thermomechanical cycles.

A mechanical cycle refers to a cycle in which a variable mechanical stress is applied at a constant temperature. A thermomechanical cycle designates a cycle in which a constant mechanical stress and temperature variations are applied.

When the proper education of the alloy is complete, this process may include a complementary stage of heat treatment.

The invention will be better understood on reading the detailed description and on the observation of the drawings below, in which
FIG. 1 represents the stress-strain curve associated with the first embodiment of the education cycle, at constant temperature;
FIG. 2 represents the deformation-temperature curve associated with the second mode of implementation, with constant mechanical stress and variable temperature.
FIG. 3 represents the deformation-temperature curve corresponding to the reversible memory effect, obtained according to the invention
FIG. 4 schematically represents the apparatus used for implementing the education method.
- Figure 5 represents, for the second embodiment of the invention, the deformationmax obtained according to the stress applied in the case of torsional deformation.

 - Figure 6 represents for the same case as in Figure 5, the residual deformation & p as a function of omax stress after bringing the object to temperature T2.

DESCRIPTION OF THE INVENTION
The method described in the present invention can be applied to any known metal alloy with shape memory.

Among the alloys to which this process can be applied include, by way of example, and without limitation, copper-based alloys such as Copper-Manganese-Aluminum, Copper-Aluminum-Nickel,
Copper-Zinc-Aluminum; Copper-Zinc-Aluminum alloys can contain 4 to 12% by weight of Aluminum and 15 to 30% of Zinc; They may also contain addition elements such as nickel in proportions of up to 2% and possibly at least one element known for its grain-size refining properties (titanium, zirconium, magnesium), Nickel alloys
Titanium of composition close to the equiatomic composition, plus addition elements such as iron, copper, cobalt, aluminum.

After the initial shaping operation from an ingot or a billet, for example, the object must undergo a homogenization heat treatment in the p phase. This treatment is well known to those skilled in the art.

The method of educating a shape memory alloy object, object of the present invention, can be implemented according to two particular modes.

According to the first mode, the object is subjected repetitively to a succession of cycles of education during which
it is applied, at a constant temperature T1, higher than the transformation point Af, an increasing mechanical stress Uc causing an amax deformation in the superelastic domain, up to a maximum value max, lower than the elastic limit of the material to the austenitic state at temperature T1,
the stress is relaxed to a zero value, the object substantially covering its initial shape, unlike a residual weak deformation Ep1 (T1).

 the constraint 6rmax is repeated, followed by a zero value return, the object substantially covering its initial shape, unlike a residual deformation tp2 (T1)> Epl (T1).

 - The stress is thus repeated max, followed by a zero value return for n cycles, at least until the residual strain Epn (Tl) reaches a limit value.

the object is then brought to a higher temperature T2
at Af of at least 500C.

According to the second mode of implementation, the object is subjected so
repetitive to a succession of cycles of education during which
- we apply to the object a constant stress qUmax, to a
T1 temperature higher than the transformation point Af, and chosen
so that the constraint Crmax remains in the domain of
elastic deformation.

- the object is cooled to a temperature below the point
Mf, while keeping the constraint constant,
and leaving the object free to deform during the
cooling down to a value of 1.

the object is heated up to the temperature Tl.
significantly overlaps its original shape unlike a weak
residual deformation p1 ().

- cooling is repeated at a lower temperature than
Mf, while keeping the stress constant and leaving the object
free to deform to a value # 2> 1. then warm it up
at T1. The residual strain increases and takes a value p2 (v)> Ep1 (.

this cycle is repeated n times, at least until the residual deformation reaches a limit value & n (G), and that
the deformation n reaches a limit value tmax.

We will now examine, in detail, each of these two modes of implementation, with reference to Figures 1 to 3.

According to the first mode of implementation, the educational process
shape memory uses the superelastic effect. The effect
superelastic is schematically represented in Figure 1 which shows the deformation obtained when applying a constraint
on a shape memory alloy object initially in the austenitic state, therefore at a constant temperature T1 greater than Af.

There are three stages: the first stage marked 1, corresponds to the elastic deformation of the austenite. Beyond a constraint denoted #d, there occurs, with increasing stress, a significant deformation of the object, due to the formation of martensite induced by
the constraint. This is the noted stage 2 which corresponds to the so-called effect
"Superelastic". The constraint is increased to a value
max maximum fixed at a value below the elastic limit
material in the austenitic state at the corresponding temperature T1
at a maximum value of the strainlmax. When you release
stress during the stage noted 3, there is reversion of the deformation
by reversion of the martensite formed, then elastic return.
back to the original form is not perfect, there is still a
residual strain called p (Tl), although # max is less
at the elastic limit. This is due to the fact that the formation of
martensite induced stress is not rigorously
reversible when releasing the constraint; there remains martensite
retained at a temperature above Af, and it appears a
deformation that is assimilated to a true plastic deformation.

This first educational treatment consists of repeating, on a certain
number of cycles that we will specify a little further, the deformation
in the superelastic domain. Each cycle therefore consists of deforming
the object at temperature T1> Af up to the max value, by
application of the constraintecrmax. The constraint is then brought back
at a value of zero. The object will return to its original shape
unlike a small residual strain # p (Tl). The
DeformationEp (Tl) increases with the number of cycles and then stabilizes.

To obtain a given max deformation, the value of the constraint # max to be applied will be even stronger than the difference between
the T1 temperature at which the education treatment is performed
and the temperature Af is important. I should choose
this temperature judiciously. Indeed, if we perform the
education treatment at a temperature too much higher than Af and
if the value of Qmax approaches the elastic limit of the object
in the austenitic state, a true plastic deformation is induced
too much, which leads to a bad behavior of the effect
reversible memory that we obtain. On the other hand, we must not
perform the education treatment at a temperature T1 too close to Af. In this case, the proportion of martensite retained will induce too much residual deformation. In practice, this first type of education cycle should preferably be performed at a temperature T1 greater than 5 to 500C at the temperature Af. In conclusion, it must be emphasized that this cycle of education is remarkable in that it is carried out at a constant temperature.

According to the second mode of implementation, this education treatment can be achieved by the repetition of a second type of education cycle with constant stress and variable temperature. The first step of this second type of education cycle is to apply a max stress to the object, at a temperature T1> Af. The temperature T1 must be high enough for the constraint
max remains below the stress d defined using Figure 1. It must therefore be only elastic deformation.

The second step is to cool the alloy to a temperature below Mf, while keeping the stress constant.

The equipment used to carry out this cycle must be designed in such a way as to allow the object to deform during cooling. Indeed, under the action of the applied stress, the formation of martensite during cooling is accompanied by a change of shape. Figure 2 shows the evolution of the shape of the object as a function of temperature for two successive cycles.

At the temperature Tl <Mf the alloy has deformed to a value called 1. The last stage of the cycle consists in heating the object to the temperature T1. The object will return to its original shape unlike a small residual deformation. The new high temperature form is marked p (#). The successive values of (1, ,, e2> ..) during the successive cycles (1, 2, ..) will increase gradually (E2> 1) and stabilize at a value Emax which depends, linearly, on the constraint # max applied. In fig. 2, the corresponding curves are shown. After having carried out a number n of sufficient cycles of education one can relax the constraint. The number of cycles will be specified a little further. There will remain, at the temperature T1, a residual deformationp (Tl).

This residual deformation is due to a true plastic deformation and the presence of martensite retained at temperature T1.

The temperature T1 may be substantially greater than the temperature
Af, to the extent that there is no risk of aging problem of the shape memory alloy (recrystallization partial loss of memory). In the case of an alloy of the type
CuZnAl, this T1 temperature should not exceed 1000C.

In conclusion, it must be emphasized that the second type of education cycle is remarkable in that it is carried out at constant stress, and requires only temperature variations. The cooling and heating speeds during the education cycle can be slow (a few degrees / minute) or fast (exposure of the object to a hot or cold fluid).

Whatever the type of education cycle implemented, the method comprises a step of bringing the alloy to a temperature
T2 either during the education process (case of the second variant) or after the education treatment (case of the first variant). This temperature T2 must be at least equal and, if possible, greater than the maximum temperature of use at which the object endowed with the reversible memory effect could be worn during use and will preferably be greater than the temperature Af at least 500C. During this rise in temperature, the object must be free to deform. The residual strain Ep (T1) will decrease during this rise in temperature by reversion of the martensite which was held at the temperature
T1> Af and reach a stable value & p. It is this value that defines the high temperature form of the alloy endowed with the reversible shape memory. Another objective of this temperature rise is to increase the stability of the reversible memory effect obtained during subsequent uses of the object.

The rise rate at the temperature T2 and the holding time at this temperature are not critical parameters unless there is a risk of aging of the shape memory alloy object. This can be for example the case for a copper-zinc-aluminum type alloy if the temperature reaches or exceeds 1000C. In this case it is preferable to carry out the rise and fall in temperature at speeds greater than a few degrees per minute and to limit as much as possible the holding time to the temperature T2. These speeds must be all the faster and the holding time is shorter as the T2 temperature is high.

It may be noted that if, in the implementation of the second variant of the education method, the temperature T1 is at least equal to the temperature T2 that has just been defined, the stage of the final heat treatment at T2 is no longer necessary, since it was, in fact, carried out during constant stress cycling.

Figure 3 shows a curve giving the deformation as a function of the temperature of an object which has been conferred by implementation of the invention the reversible memory effect.

The initial shape of the object, following its shaping operation, the above homogenization treatment, and before implementation of the education process, corresponds to the state of zero deformation. p and (MARS) are respectively the high and low forms
p temperatures that the object has memorized by using the education method.

This process is remarkable in that it allows to master with great precision, by a judicious choice of the parameters imposed during the education cycles, the high and low temperature forms.

The high temperature form p is a function of the maximum stress
p applied during education cycles. Low temperature form
MART) MART) is determined by the maximum deformation max achieved during the education cycles. An education performance is defined by the ratio (MART) / (max). This return is different according to the type of education cycle implemented (variant nol or n02). The best performance will be obtained by the second embodiment of the invention.

It is advantageously possible, if the second type of cycle is used, to limit the deformation during cooling. This will make it possible to set the value of (max) to a very precise value and thus to obtain a better precision on the low temperature form that will be memorized. In this case, the return on education is improved. The limitation of the deformation may be effected by external mechanical means, such as a stop, or may be inherent in the change of shape of the object itself. This is the case for example for a spring brought to contiguous turns.

For an object to memorize the low and high temperature forms, we can limit ourselves to repeating the cycles of education described a few times: up to a dozen times, for example. However, to obtain a very stable reversible memory effect, it is necessary to repeat these cycles a much larger number of times, for example a few tens to a few hundred times. It may advantageously be between 10 and 100, for one or other of the two embodiments of the invention.

EXAMPLES OF IMPLEMENTATION OF THE INVENTION
The method of the present invention can be used with any metal shape memory alloy object, regardless of its initial shape and the mode of deformation, to impart to this object the reversible shape memory effect, since an apparatus has been devised which makes it possible to implement the method described. The object may for example be, without limitation, a spring, a deformed blade bending, a wire or a torsionally deformed strip.

Figure 4 schematically shows an apparatus that we used for torsional deformation.

The object is immersed in a liquid bath 8. The element 4 makes it possible to apply the torsion torque, or simply to deform the object in torsion, if the first mode of education is implemented. This may be for example an engine or a system using pulleys and weights. The rod 5 is the movable jaw, the fixed jaw is marked 7 and the object (strip or wire) corresponds to the mark 6.

For a band deformed in tors <n, it is easy, by using as a first approximation the formulas established in the elastic domain, to make the correspondence between the angle of rotation and the deformation of the band in percentage and between the torque of applied torsion and stress. It is thus possible to use the deformation and stress quantities independently of the dimensions of the band.

The numerical values we are going to give relate to an alloy
Copper-zinc-aluminum having a temperature Af of 300C.

The maximum deformation that can be induced during the education cycles can reach a dozen percent. However, it is preferable to limit oneself to values of less than ten percent. For a strip 1 mm thick, 4 mm wide and 32 mm long, a deformation of 10% corresponds to a rotation of 190 degrees.

The first mode of education was implemented at a fixed temperature T1, between 40 and 60 C. At the end of the education treatment, the band is brought to a temperature T2 of 1000C and immediately cooled.

The educational output is 50%. This means that to obtain for example a low temperature form corresponding to a rotation of 50 degrees with respect to the initial shape, it is necessary to induce during the education cycles a rotation of 100 degrees.

If the second type of education cycle is implemented, the
Figure 5 gives, for a torsionally deformed band, the emax deformation obtained as a function of the applied ormax stress.

After the rise to the temperature T2, the plastic deformation p, depending on. the constraint G max applied is represented on the
Figure 6.

In this case, the return on education is 82%. If, for example, this second type of education cycle is carried out with a stress of 20 MPa, the maximum-max distortion will be 6.4% (fig.5). The
The stored low temperature form will thus correspond, with respect to the initial form, to a deformation of 5.2 70 (6.4 x 0.82) and the high temperature form to a deformation of 0.4% (FIG.

Numerical values are only given here as examples
limiting for a torsionally deformed band. By practicing the present invention, those skilled in the art will be able to easily obtain from any shape memory alloy object, whatever the nature, the reversible storage of two well-defined low and high temperature forms.

ADVANTAGES PROCURES PAB THE INVENTION
The method, which is the subject of the invention, can advantageously be used to confer on a shape memory alloy the reversible memory effect. In each of the two education cycles described, only one of the two temperature or stress parameters is varied, the other remaining constant, which constitutes the main difference with respect to the method described in EP 161952. This increases substantially ease of implementation and speed of education processing, which is very valuable for industrial use.

 It is also very easy to evaluate the value of the temperature / stress parameters to be used during the education cycles to obtain the memorization of the shapes, high and low temperature, well determined, precise, in accordance with what one wishes to contain. , and perfectly reproducible on a large series of objects.

These forms are stable over time: thanks to the heat treatment at T2 temperature, the high and low temperature forms no longer tend to evolve during use.

The quality of the reversible memory effect results in particular from the fact that, contrary to the prior art, the process only involves a residual plastic deformation of low value (greater than one percent) instead of exerting mechanical stresses. important internals, by plastic deformation.

Claims (12)

1 - Process for educating an object made of a transformation alloy  martensitic shape memory, to obtain precision  and the stability of the reversible memory effect, said object having  been put into its original form from an alloy blank,  then homogenized, in phase ss, characterized in that said object  is subjected repeatedly to a succession of cycles  of education during which  - it is applied, at a constant temperature T1, superior  at the transformation point Af, an increasing mechanical stress causing deformation t max in the superelastic domain,  up to a maximum value QGmax, less than the yield stress  material in the austenitic state at temperature T1,  - the constraint is released to zero, the object  substantially covering its original shape unlike a  low residual strain Epl (Tl).  unlike a residual strain p2 (T1)> gpl (T1).  at zero value, the object substantially covering its original shape  - the max stress is repeated, followed by a return  the residual strain Epn (Tl) reaches a limit value.  return to zero value for n cycles, at least until  - we repeat and put under stress Cmax, followed by a  at Af of at least 500C.  the object is then brought to a higher temperature T2 2 - Process according to claim 1, characterized in that T1 is  greater than 5 to 500C at Af. 3 - Process according to claim 1, characterized in that T2 is  at least equal and preferably greater than the temperature  maximum use of the object, and below the temperature  recrystallization of the alloy. 4 - Process for educating a transformation alloy object  martensitic shape memory, to obtain precision  and the stability of the reversible memory effect, said object having  been put into its original form from an alloy blank,  then homogenized, in phase, characterized in that said object  is subjected repeatedly to a succession of cycles  of education during which  - we apply to the object a stress constantemax, to a  temperature T1 higher than the transformation point Af, and chosen  in such a way that the contraintermax remains in the domain of  elastic deformation.  low residual deformation pl (Q).  significantly overlaps its original shape unlike a  the object is heated up to the temperature T1. The object  cooling, to a value 61  constant, and leaving the object free to deform during  Mf transformation, while maintaining the max stress  - the object is cooled to a temperature below the point  that the deformation reaches a limit value max.  residual strain reaches a limit value -), and  this cycle is repeated n times, at least until the  a value Ep2 ()> # p1 #) -  it is warmed to T1. Residual deformation increases and takes  Mf, while keeping the stress constant and leaving the object free to deform to a value 8 2> & 1 then  - cooling is repeated at a lower temperature than 5 - Process according to claim 4, characterized in that, during the cooling stage, the deformation # max of  the object. 6 - Process according to claim 4 or 5, characterized in that, after  the n cycling, the object is brought to a higher temperature T2  at Af of at least 500C. 7 - Process according to claim 6, characterized in that T2 is  at least equal and preferably greater than the temperature  maximum use of the object and below the temperature  recrystallization of the alloy.  at least 500C, the final treatment at T2 is not carried out.  characterized in that if the temperature T1 is greater than Af  8 -. Process according to any one of Claims 4, 6 or 7  9 - Process according to claim 1 or 4, characterized in that the  n number of cycles is at least 10 and preferably included  between 10 and 100. 10 - Process according to claim 1 or 4 characterized in that the  memorized high-temperature form is a function of the constraint  6-maX applied during n cycles of education.  maximumeamax during n education cycles.  memorized low temperature form is determined by deformation  11 - Process according to claim 1 or 4, characterized in that the 12 - Application of the process according to any one of the claims  1 to 11 to objects in copper-zinc shape memory alloys  aluminum-nickel, with a Zn content of 15 to 30%, in Al of  4 to 12%, Ni up to 2%, and optional addition of an agent  grain refining.