CA1077991A

CA1077991A - Memory metal article

Info

Publication number: CA1077991A
Application number: CA295,469A
Authority: CA
Inventors: Robin J. T. Clabburn
Original assignee: Raychem Ltd
Current assignee: Raychem Ltd
Priority date: 1977-01-24
Filing date: 1978-01-23
Publication date: 1980-05-20
Also published as: FR2378101A1; DE2802571C2; JPS6141983B2; JPS53108012A; DE2802571A1; FR2378101B1; GB1600000A; US4149911A

Abstract

ABSTRACT

The effective transition temperature from martensite to austenite in a dimensionally heat-recoverable memory metal member can be temporarily raised by applying a stress and maintaining the applied stress at a tempera-ture below the raised transition temperature so that the memory metal remains substantially in the martensitic state. When it is desired to utilise the member the stress is removed at a temperature above the original transition temperature. The applied stress is advantageously maintained by a disburdenable keeper.
In an especially preferred embodiment the keeper comprises an aluminium tube positioned within a band or tube made from a brass memory alloy, the combined article being suitable as a connector for hydraulic pipes and the like.

Description

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This invention relates to memory metal~ and article~, especially heat-recoverable article~, made there~rom.
As i~ now known, certain alloys, commonly called memory alloys, can be used to make heat-recoverable articles, that i~ to ~ay articles which have been de-formed ~rom an original coniiguration and which retain their de~ormed con~iguration until they are heated, Nhen they reoo~er towards their original configur~tion.
Thess alloy~ exhibit a so-oalled marten~itic trans-~ormation irom a low temperature ~orm to a high tempera-ture iorm and it i8 this trans~ormation which produce~
the memory eiiect. ~mongst such alloys there may be mentioned, ~or e~ample, variou~ alloys o~ titanium and nickel which are described, for example in U.S.
Patent~ Nos. 3,174,851, 3,351,463, 3,753,700, 3,759,552, British Patent~ Nos. 1,327,441 and 1,3~7~442 and NA~A
Publication SP 110, "55-~it~nol-~he ~lloy with a Memory, etc.~ ~U.S. Government Printing O~ice, Wa~hlngton, D.C. 1972). The property oi hoat-recoverability has ; 20 not, however, been ~olely confined to ~uch titanium-~ickel alloys~ Thus, for example, variouY beta-brass alloy~ have been demonstrated to exhibit this property , in, e~g. N. Nakaniehi et al, ScriPta Motallur~ioa 5, 433-440 (~ergamon Pre89 1971) and ~uch materials may be doped to lo~er thoir tran0ition temperature~ to crgogenlo regimes by known technique~. Similarl~ 304 107799~

stainless steels have becn shown to enjoy such characteristics E. Enami et al, id, at pp. 663-68.
As mentioned above, these alloys exhibit a memory effect on passing from a low temperature, e.g.
martensitic, form to a high temperature, e.g. austenitic, form. This transformation occurs over a small tempera-ture range and, because of hysteresis, the position of the temperature range usually varies depending on whether the alloy is being heated or cooled. Thus, on heating, transformation occurs over the range A - Af, where As and Af are the temperature at which formation of austenite begins and is completed, respectively, and, on cooling, transformation occurs over the range of from Ms ~ Mf, where Ms and Mf are the temperatures at which formation of martensite begins and is completed, res-pectively. The transformation from one form to the other may be followed by measuring one of a number of physical properties of the alloys, as well as from the shape change.
For example, the electrical resistivity of such an alloy shows an anomaly as the transformation takes place, a typical plot of resistivity vs. temperature being shown below.

Mf ~ As ~ s ~ Af lY
Temperature , 9gl For many alloys M~ and A~ are at approxi~ately the same temperature, that iæ to say the tran~ormation begin~ at about the same temperature independent o~
whether the allo~ i8 being hea~ed or cooled. ~owe~er, in certain commercial applications it is d~slrable that As Yhould be significantly higher than M9, ~or the iollowing rea~on.
Many articles made ~rom suoh alloy~ are provided to users in the de~o~med, i.e. heat-recoverable, shape and are thue in the low temperature martensitic state.
For example, the hydraulic couplings described and claimed in Briti~h P~tent ~o. 1,327,441 are sold in the radially e~panded shape, the customer placing them about the hydraulic pipes to be Joined and allowing them to warm above tne transitlo~ temperature, where-upo~ they shrink and ~orm the de~ired connectlon.
Be~ause it i~ ~ecesaary ~or the couplings to remain in the au~tenitic state du-ing uee (~or e~ample, to avoid po ~lble re-e~pan~ion o~ the coupling# and loo~ening o~ the Joint~ and al80 beCaU8e oi the superior strength propertie~ in the austenitic ~tate), the M8 ~ the material must be ohosen to be lower than the lowe~t temperature ~hich the coupling may encow~ter in use.
B~cause A8 i8 approximately the ~ame as M8, this u~ually ~ean~ that the coupli~gs mu~t be ~tored and supplied in a coolant such a~ liquid nitrogen to prevent premature ' ~ .

. 1077991 co~ver~ion to au~tenite before use.
~ his is inconvenient and, for this raason, attempts have been made artificially to rai~e A8, without a correspond~ng ri~e in ~ or at least one heating cycle, 80 that the couplings may be ~tored and supplied at ambient temperature and cau~ed to recover by heatlng.
This increa~e in A8 has been achieved succes~iully ior certain allo~s, especially ~-bra~ alloy3, by so-called "pr~conditionin~ method3.
For e~ample, in aceordance with German Offenlegung-~schrift 2,603,878, ~uch alloye may be thermally preconditioned by the following method. ~he alloy i~
cooled ~rom it~ auetenite ~tate to a tempera-ture lo~r than Mi, lt i3 then heated very 810wly to a temperature, ~ay Ap, at ~hich it would normally exist wholly in the austanitic ~tate, i.e. above A~. However, becau~e heating iB very ~low, trans~ormation from ~arten~ite to austenite does not occur a~d, ii the alloy i~ cooled aga~n to below Mf and then rapidly heated, tranetormation to austenite i~ ~ound to begin at about Ap, i.e. the temperature to which it was slowly heated~
Another method of effectively raising A~ for at lea~t one heating cycle le by mechanical preconditioning, ln accordance with German Oifenlegungsschrift 2,603,911.
In this method the alloy i~ ~aintained in a defo~med configuration above N~ ~or such a ti~e that ~hen the , ~, . ~ .

- 6 ~

de~orming ~tre~ i8 removed at lea3t part o~ -the induced ~train i~ retained. When the alloy i~ then heated it ia found that some of thi~ retained ~train i~ thermally recoverable and, more important9 that transformation to au~tenite begins at a temperature Ap which i8 ~ignificantly higher than A~. In this case Ap i~ not the same temperature a~ that at which the alloy was maintai~ed in a deformed state, but generally increa~e~
as the holding time i8 increased.
It ha~ also been iound po~ible to precondition an alloy by both o~ the above processes so as to impart t~o artificially rai~ed values of ~, thereb~ allowing a two;~tage reoovery.
The above methods have proved very succe~eiul in preconditioning certain allog~ and have opened up ne~
~ields o~ application ~or memory metals. However, the e~fect~ are not 80 gre~t ~or other alloy~ and, in any caes, preconditioning i~ a ~eparate ~tep and rel~tivel~ time consuming.
Accordingly, thare i~ ~till a need ~or a ~imple e~f~cti~e method by~hlch memory metal articles may be stored at, ~ay, room temperature, even thou~h the~r M~
remai~ below any temperature likely to be encountered in use.
The pre~ent invention provide~ a method of making a connection ueing a devico compri~in~ a heat-re¢overable . ~ .

1(~7799~

memory metal member, wherei~ a ~trese i9 applied to the member in it~ heat-recoverable ~orm 80 as temporarily to increase the ~ temperature o~ the memory metal from Tl to T2 and, whilst mai~taining a stres~ the 5 article i8 stored at ~uch a temperature le~ than T2, preferably between M~ and T2, ~or e~ample betveen Tl and T2, and at such an applied stross that the memory metal ~ub~tantially remain~ i~ it~ martensiti¢ ~tate (~o that, wore the applied ~tress to be removed, would revert ~ub~tantially to Tl), and wherein lmmediately be~ore making the connection tho ~tre~e ie removed at a temperature greater than Tl so that the member recover~ to effe¢t connectionc The present invention also provides a method of making a heat-reco~erable memory metal member ~hich compri~es applying a etress to the member in it~ de-~ormed heat-recoverable stata so as temporarily to increase the A~ value irom Tl to T2 and, whilst main-taining a ~tre~s, storing the article at ~uch a tempera-ture le~s tha~ T2, preferably betwe0n M9 and T2, fore~ample between ~1 and T2, and at such a maintai~ed applied ~tres~ that tho memory metal ~ubstantially remain~ in ite marteneitio etate~
~he applied str~s i~ con~enie~tly that which is uaed to deform the memory metal ~rom its ori~inal con-~iguration to ite do~orm~d heat-recoverable con~igurationO

:

Thus, in a preferred embodiment the present invention provides a method of making a heat-recoverable article which compri~es imparting a thermally recoverable de-formation to a memory metal member at a temperature below the normal tran~ition temperature of the metal, the applied deiorming ~tre~ being al~o sufficient to raise the transition temperature o~ the memory ~etal above the temperature at which the article i~ to be stored, maintaining the applied ~tre~s and storing the article at a temperature le~ than the rai#ed transition temperature, preferably at a temperature between the normal tran~ition temperature and the rai~ed tran~ition temperature.
It w~ll be appreciated, howe~er, that the present invention provide~ ~or the application of stress to a pre-~ormed heat-reooverable article and al~o that the ~tress applied during storage may be ~reater or less than that used in the lnitial deformation, provided that, in acoordance with the ~undamental concept o~
the prese~t invention, the rai~ed A~ value during ~torage i~ at all times ~reater than the temperature of ~torage.
In many ad~antageou~ embodiment~ of th~ present inv~ntion the memory metal member will be hollow and will be provided with at least one opening ~or rec~iving a ~ub~trate to which a connection i~ to be .:

~07'79gl made; it may, for e~ample be a continuous or split band or tube. For convenience, the invention will ~rom now on be de~cribed with reference to heat-~hrinkable hollow coupling devices such as bands, rings and tube~, but it will be appreciated that the heat-recoverable memory metal member may take other form~
and may also, ~or example, be an actuating element in a compoeite connecting device compri~ing one or more other parts.
The stre~s applled to such a ¢oupling member to rai~e A~ from Tl to T2 may convenientl~ be applied by mean~ o~ a keeper, which may be resilient, po~itioned withi~ the coupling and which applies the stres~, ~or example, by it~ own re~ilie~ce and/or by the re~ilience o~ the coupling it~eli. In ~ome case~ the keeper ~ay be the mandrel which i8 conventionally used to de~orm the ooupling irom its original undeformed state to its de~ormed, heat-recoverable ~tate. Eowever, in pre~erred embodiments, it i8 a ~eparate member which may be applied during or immediately a~ter mandrel expan~ion and whioh i9 readily dieplaceable or, e~peoially destructible or de~ormable, In certain pre~erred embodiment~, ~or example, the keeper may be positioned within the di~continuity o~
an expanded ~plit rin~ or tube and, i~ desired, i~
knocked out to allow recovery. In other embodiments .

.

1~779g~L

the keeper may be made from a fusible or heat-softenable material ~o that recovery can be ef~ected by heating at a temperature above ~1 to weaken the keeper. In yet other embodiment~ the keeper may be made in~uf~iciently strong to withstand the recovery ~orces ge~erated durlng the transformation to austenite in which case recovery can be effected simply b~ heating to T~ (or to ~hatever raised As value i~ being maintained by the applied stress) or above.
Other iorm~ of keeper, 9uch for example as chemically degradable, e.g. ~oluble, keepers, ~ill be apparent to those ~killed in the art. It will al90 be appreoiated that in ~ome cases the keeper ma~ itself iorm a useful part of the connecting device, for e~ample a~ a fueible ~older in~ert or a~ a ~ubstrate-compatible in~ert of the type deecribed and claimed in German Applications OS 2,448,160; OS 2,615,572; OS 2,615,723;
0~ 2,615,683 and 0$ 2,615,685.
~he pre~ent invention i~ ba~ed upon the known ~act that, ~or mo~t alloys, A~ increasee with increasing 3tres~. For e~ample ~or ni¢kel-titanium alloys, the ~s value inorea~ee by about 1C ~or every 1,000 p.s.i.
applied stre~. As the elastic limit oi theee alloy~
is about 70,000 p.~.i. thi~ means that the A~ ~alue can quit~ readily be increa~ed by, ior e~ample, 50C.
~hus, i~ the "normal" M8 (and A~) of ~uch an alloy lie~

1~7799~

at -20C, it is po~ible in accordance with the present invention to ~tore a heat-recoverable article made irom such an allo~ at room temperature at an applied ~tress of 50,000 p.s.i. so that i-ts raised ~s value i~
about ~30Co Recovery can, when desired, be ef~ected by removing the ~tress, i.e. displacing or destroying a keeper, whereupon A9 reverts to -20C, or by heating to ~bove +30C 80 that the keeper melts or is destroyed or oru~hed by the recovery of the coupling.
De~pite tho iact that, as mentioned aboYe, it has been known ~or 80me time that A~ increases with increaeing stre~s, it has never before been proposed to u3e thi~ iact and, whilst it has been propo~ed, for e~ample in Briti~h Patent ~o. 1,327,441 to position a keeper withln a heat-reco~erable metal coupling (but not to use a readily di~placeable or dî~po~able keeper to this end), the object oi such keepers was ~o that the coupling~ could be stored in their e~panded iorm at temperatures above Ai and so that when the coupling/
keeper a~embly wa~ once again i~mersed in liquid nitrogen the coupling would expand slightly to iaoilitate removal o~ the keeper. [It ~hould be noted thab operating ln accordance with Briti~h Patent ~o. 1,327,441 it is necessary to re-immerse the a~e~bly in liquid nitrogen in order once again to bring the couplin~ to ite martensitic iorm before it i8 applied to the eubstrate~]

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~ . ' , ' ' ' ~.

~799~
- 12 _ Furthermore, whil~t in the mechanical preconditioning method o~ ~erman OS 2~603,911~ a memory metal member is hsld under stre~s at a temperature above M~ in order to increa~e its A~ value ~or the ~irst cycle of hea$ing, it will be noted that the ~8 value does not revert to its orlginal value on removal o~ the restraining ~orce.
Thi~ is becau~e the method is, to 30me e~te~t~ dependent upon a conversion to au~tenite 80 a~ to allow an i~o-thermal and localised conversion o~ au~tenite to pseudo-ela~tic martensite as the memory metal member attempt~ to recover against the constralning stress, some of the elastic strain therebg imparted being retained when the deforming stress iB removed. Thi~ retained strain is partially recoverable when the memory metal i8 heated to ~p, its temporarily increased ~8 value.
Similarly, it will be noted that in this mechanical preconditioni~g method, the memory alloy is held at a temperature abo~e M9 ~or a con~iderable perlod o~ time in order to obtain the retained strain which i8 conne¢ted with the raised A~, tho A8 value being increased a~ the length o~ the treatment is increa~ed.
At at least some ~tage in this period the memory metal is maintained under etre~s at a temperature greater than the rai~ed AB value at that time (even though, in some cases, that temperature may be less than the ~inal A9 value obtained by the preconditioning method). This therefore ensures an at least partial conversion to austenite, ~or the reasons discus~ed above~ and in order to realise the total dimenæional memory it may be necessary to cool below ~ before reheatingO
In accordance ~ith the present invention, however, the ~ valu~ is immediately temporarily increa~ed by the ap~lication o~ ~tress and at no stage during the ~torage period i8 the temperature greater than the rai~ed As value~ T~u~, the memory metal rem~in~ in ite ~artensitic ~tato throughout storage and no cooling etep is necessary to realise the dimensional recovery.
~8 soon a~ the applied stres3 i8 removed at a tempera-tur~ above Tl, the orig~nal A~ ~alue, then recovery will occur.
~hus, whil~t variou~ heat and ~tress treatments o~ memory alloys have been propo~ed, none of the~e have utili~ed the novel and rundamental concept o~ the pre~eut invention, namely to increase normal A8 by appiying ~tre~ whil~t at the same time keeping the alloy ~ub~tantially in its martencitic iorm and thereby both preventing permanent or temporary loss oi thermally-recoverable strai~ and en~urlng that the alloy will recover as ~oon as the strese i~ remo~ed at or above the normal A~ temperature.
It follow~ that no one has be~ore conceived the readlly de~tructible, de~ormable or displaceable ' , , :
' ~077991 keepers which are advantageously u~ed in the practice o~ the present invention and, in it~ broadest ~orm, the present invention pro~ides a heat-recoverable memory metal member which ie held in ~tres~ in a pre-domlnantly marte~sitic ~tate, i.e. below ~2~ by a di~burdonable keeper. By a "disburdenable keeper" therei8 herein meant one which can be removed or ohanged immediately before or at the onset of recovery 80 a8 to free the memory metal member ~rom the d0forming etress without the need to reduc~ the temperature o~
the keeper/memory metal member assembly to below Tl, Tl being the ~8 oi the memory metal of the heat-re-coverable member in the absence of the applied stres~.
Attention iB drawn in this reepect to Patent ~pplication Serial No. , ~iled t~da~, inventor~ R. J. T. Clabburn and R. J. Pe~ne¢k, claimipg priority from Briti~h Patent Application No. 2833/77, ~iled 24th January, 1977, and to Patent ~pplication Serial No. , filed today, inventor M. D. Gazeley, claiming priority irom Briti~h P~tent Application No. 8408/77 ~iled 28th Pebruary, 19770 The~e appllea-tions, the dl~oloeuree oi whi¢h are lncorporated herein by this re~erence, de~cribe in detail such disburdenablç
keepers and suitable material~ from which they may be made.
In one e~pecially preferred iorm o~ the present ~77991 invention the di~burdenable keeper i8 one which is atrong enough to provide the applied stress during storage but i8 in~ufficiently strong to withstand the recovery ~orcea o~ the memory metal member. Such a keeper may accordingly be de~ormed by the simple expedient of heating the article to the raised tran~ition temperature (~2)- For example, ~uch an article can be made in accordance with the teachin~s of British Patent ~o~ 1,488,~9~, in the ~orm of a tubular aluminium in~ert acting a~ a disburdeD~ble keeper positioned inside a radially heat-recoverable band or tube made ~rom a memory metal auch a~ a ~-brass. This article ~ay be u~ed to connect ~ubstrates auch aa hydrauli¢ pipelinea.
Su~h an article i8 economically interesti~g because the M~/Q~ di~ference ~or certain bra~ alloys i9 quite large and be¢au~e the~ are relatively cheap. One dra~back u~til now ha~ bee~ that, de~pite the relatively lar~e temperature difference betweon Me and A8, it haa been ~ecessary, in order to keep M~ belo~ the minimum - temperature likely to be encountered in operation, to use alloya ~or which A~ is only Just above ambient temperature, This has sometime~ led to premature recovery of the articles during storage, rendering them us~le8~. In acoordance with the pre~ent invention, however, it is pos~ible to rai~e A~ au~iciently to 1(~77991 remove thi~ proble~ without at the same time raising M~ to an u~acceptable le~el.
One pr~erred ~orm o~ article in accordance with the present invention will now be described in more detail, by way o~ example only, with re*erence to the accompanying drawings, in which ~igure 1 illustrates a connector, and ~igure 2 illustrates the u~e of the connector to join two pipe~.
Referring now to the drawings~ there i8 ehown a connector compri~ing a radially heat-shrinkable tube 1 made ~rom a memory metal positioned about an insert eleeve 2 which acts as a keepar to maintain an applied stress on the tube 1. In a preferred embodiment the tube 1 i~ made ~rom a bra~ allog and the in~ert sle~e

2 iB made ~rom a metal such a~ aluminium. In accordance with the pre~ent invention the in~ert ~leeve 2 i~
su~iciently ~trong that it maintain~ an applied stress on the tube 1 90 long as the tube 1 remains in the marten~itic ~tate, but, on the other hand, i~ not ~trong enough to resist the recovery ~orces o~ the tube 1 when the temperature is raised to above T2.
~9 shown in ~igure 2, there~ore, the device may be u~ed a~ a compo~ite oonnecting device in accordance with Briti~h Patent No. 1,488,393. In the arrangement shown, two pipes 3 and 4 are brought into abutting relationship iO779gl by locating them around a spacer member 5 which is provided with an annular collar 6 against which the ends o~ the pipes 3 and 4 abut. This assembly i9 positioned in~ide the connector shown in Fig~re 1 and, when the temperature is raised to T2 or above, the memory metal tube 1 shrinks radially and ~orcee the insert ~leeve 2 into ~irm connection with both pipe~.
Of cour~e, it will be appreciated that, in accordance with British Patent No. 1,488,39~, the insert sleeve 2 may be made irom a gall-prone material or another material which is compatible with the material o~ the pipe3 3 and 4~ Alternatively, it ma~ be provided on it~ inner ~ur~ace with teeth or other projecting members which bite into the pipes 3 and 4 on recovery.
I~ an ~pecially advantageous embodiment, the memory metal tube 1 is made ~rom a preconditioned bra~s alloy which ha~ an M~ well below room temperature, for example, as low as -40C. With such an alloy the normal preconditioned A~ will be at, or slightly above, ambient temperature. In the absence o~ the use of the pre~ent invention there would be a danger that the device shown i~ ~igure 1 would become sufficiently warm a6 to cause temperature reoovery during ~torage.
However, because oi the applied stress maintained by the in~ert ~leeve2, the ~8 may be raised to a level which is ~u~iciently high to avoid this danger.

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iO77~91 The connector may be made by various method~. ~or example, the heat-recoverable tube 1 may be deformed and a stress in accordance with the present i~vention may be applied by a mandrel, which iæ subsequently removed and replaced by the keeper ~leev~ 2.
Alternatively, the heat-recoverable tube 1 and the insert sleeve 2 may, in some ca~e~, be de~ormed simultaneously to produce the connector shown in Figure 1.
The present in~ention t~ere~ore provides a novel and important method o~ transiently increasing the recovery temperature o~ heat-reco~erable memory metal articles by a treatment in which a stres~ i8 applied to increase the A8 value o~ the memory metal and the ~tress i8 maintained at a temperature below the increa~ed A9 ~o that there i~ no attempt by the article to recover against the applied stre~. When the heat-recoverable article i8 to be used the applied stress is re~o~ed, whereupon the ~ reverts substantially to Tl.
Whil~t the present invention is e~peciall~ ad~anta-geouely applied to nickel-titanium and bra3~ alloye, aa discu~ed above, it will al80 be euitable for many other memory alloys, especially those in which A8 i~
increased ~ignificantly b~ applied stres~. Suitable ~tres~ee and temperatures to be u~ed ior such alloys in accordance with the present invention will, determinable 1~77991 by routine e~periment u~i~g, *or e~ample, the electrical, cr~stallographic and other methods known to those skilled in the art.
Finally, in Figure~ ~ and 4 there are shown typical hy~tere~i~ loops ~or a nickel titan~um alloy and a preconditioned bra~s alloy, respectivel~.
A~ can be seen ~rom Figure ~, the hysteresis loop ~or a nickel titanium alloy i~ rather narrow and, there~ore, the article~ in accordance with the pre~ent invention will in general be stored at a temperature within the shaded range between Tl and T2.
On the other hand, a~ i~ shown in ~igure 4, the original hystere3i~ loop ~or a preconditioned bras8 alloy ~uch, ~or example, as a quaternary copper/
alumi~ium/zinc/mangane~e alloy o~ the type described a~d claim~d in Belgian Patent No. 838,197 iæ relatively wide and therefore the articles oi the pre~ent invention may, 1~ de3ired, be stored at a temperature within the shaded area, i.e. at a temperature lower than Tl.
Qther ~ariations and modi~ications i~ acoordance with the present in~ention will be apparent to those ~killed ln the artO

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Claims

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

1. A method of making a heat-recoverable memory metal member which comprises applying a stress to the member in its deformed dimensionally heat-unstable state so as temporarily to increase the AS value of the memory metal from Tl to T2, and whilst maintaining an applied stress, storing the article at such a temperature less than T2 and at such a maintained applied stress that the memory metal remains substantially in the martensitic state.

2. A method as claimed in claim 1, wherein the applied stress is applied at the same time as the stress applied to deform the memory metal member to its dimensionally heat-unstable state.

3. A method as claimed in claim 1, wherein the applied stress is applied after the memory metal member has been deformed to its dimensionally heat-unstable state.

4. A method as claimed in claim 1, wherein a disburden-able keeper is used to provide the applied stress.

5. A method as claimed in claim 4, wherein the keeper is destructible or deformable.

6. A method as claimed in claim 5, wherein the keeper is made from a fusible material.

7. A method as claimed in claim 5, wherein the keeper is made from a chemically degradable material.

8. A method as claimed in claim 5, wherein the keeper is insufficiently strong to withstand the recovery forces generated on heating the memory metal member to T2.

9. A method as claimed in claim 1, wherein the memory metal member is a radially expanded band or tube.

10. A method as claimed in claim 9, wherein a keeper in the form of a band or tube is positioned inside the memory metal member.

11. A method as claimed in claim 1, wherein the memory metal member is a hollow member having a discontinuity in its cross-section in the plane of recovery.

12. A method as claimed in claim 11, wherein a keeper is positioned inside said discontinuity.

13. A method as claimed in claim 1, wherein the memory metal member is stored at a temperature between MS
and T2.

14. A method as claimed in claim 1, wherein the memory metal member is stored at a temperature between T1 and T2.

15. A method of making a heat-recoverable article which comprises imparting a thermally recoverable de-formation to a memory metal member at a temperature below the normal transition temperature of the metal, the applied deforming stress also being sufficient to raise the transition temperature of the memory metal above the temperature at which the article is to be stored, maintaining the applied stress and storing the article at a temperature less than the raised transition temperature.

16. A method as claimed in claim 15, wherein the applied stress is maintained using a disburdenable keeper.

17. A method as claimed in claim 16, wherein the keeper is insufficiently strong to withstand the recovery forces generated on heating the article to the raised transition temperature.

18. A method as claimed in claim 17, wherein the memory metal member is a radially recoverable band or tube and the keeper is a deformable band or tube positioned inside it.

19. A method as claimed in claim 15, wherein the memory metal is a nickel-titanium alloy.

20. A method as claimed in claim 15, wherein the memory metal is a brass.

21. A heat-recoverable article which comprises a dimensionally heat-recoverable memory metal member held in stress in a predominantly marteneitic state with a raised transition temperature by a disburdenable keeper.

22. A heat-recoverable article as claimed in claim 21, wherein the memory metal member is a band or tube made from a brass and the keeper is a band or tube of aluminium positioned within it.

23. A method of making a connection using a device comprising a memory metal member, wherein a stress is applied to the member in its dimensionally heat-recoverable form so as temporarily to increase the AS
temperature of the memory metal from T1 to T2 and, whilst maintaining a stress the article is stored at a temperature less than T2, the memory metal remaining substantially in the martensitic state during storage, and wherein immediately before connection the stress is removed at a temperature greater than T1 to that the member recovers to make the connection.

24. A method as claimed in claim 23, wherein the stress is applied by a disburdenable keeper.

25. A method as claimed in claim 24, wherein the stress is removed by heating the article to T2 so that the memory metal member recovers and deforms the keeper.

26. A method as claimed in claim 23, wherein the connection is made between two generally tubular substrates, the memory metal being in the form of a band or tube sized to fit about said substrates.