US3748108A - Thermally activated spring with improved thermal properties - Google Patents

Abstract

A THERMALLY ACTIVATED SPRING OF MARTENSITE TRANSFORMATION MATERIAL WHICH HAS IMPROVED THERMAL DIFFUSIVITY AND METHODS OF MAKING SAME. MARTENSITE TRANSFORMATION MATERIALS, SUCH AS NITI AND AUCD, ARE ABLE TO "REMEMBER" A GIVEN HIGH TEMPERATURE PHASE SHAPE AND MAY BE USED AS THERMALLY ACTIVATED SPRINGS. SINCE THESE MATERIALS ARE POOR HEAT CONDUCTORS, THEIR THERMAL DIFFUSIVITY MAY BE ENHANCE BY LAYERING THEM WITH A MATERIAL HAVING A MUCH HIGHER THERMAL DIFFUSIVITY, SUCH AS COPPER OR SILVER. ANOTHER METHOD OF ACHIEVING IMPROVED THERMAL DIFFUSIVITY IS TO USE A CORE OF COPPER OR SILVER LAYERED WITH THE MARTENSITE TRANSFORMATION MATERIAL.

Description

F. ROTHWARF ET AL 3,748,108

July 24, 1973 THERMALLY ACTIVATED SPRING WITH IMPRQVED THERMAL PROPERTIES 2 Sheets-Shet 1 Filed March 7, 1970 x -1 TAMB=23C Q-NO P|LATE FIGJ.

8 IO v (Cm) OvPLATING NITINOL PLATING mm 1 O T NM Y m ETN 7 E 1 N R L. o K W c. A E D EUM Qa RA Y B lws m 6 m T. A lmm w 4 0 G L .l. MF. 15m N O 3,748,108 THERMALLY ACTIVATED SPRING WITH E'IPROVED THERMAL PROPERTIES Frederick Rothwarf and Paul D. Flynn, Philadelphia, Pa.,

assignors to the United States of America as represented by the Secretary of the Army Filed Apr. 7, 1970, Ser. No. 26,367 Int. Cl. 1532b 15/00 US. Cl. 29195.5 11 Claims ABSTRACT OF THE DISCLOSURE A thermally activated spring of martensite transformation material which has improved thermal diffusivity and methods of making same. Martensite transformation materials, such as NiTi and AuCd, are able to remember a given high temperature phase shape and may be used as thermally activated springs. Since these materials are poor heat conductors, their thermal diffusivity may be enhanced by layering them with a material having a much higher thermal diffusivity, such as copper or silver. Another method of achieving improved thermal diifusivity is to use a core of copper or silver layered with the martensite transformation material.

The invention described herein may be manufactured, used and licensed by or for the Government for governmental purposes without the payment to us of any royalty thereon.

This invention relates to thermally activated springs and methods of enhancing the effective thermal diffusivity of same.

Many intermetallic compounds exhibit some rather interesting elastic properties associated with certain phase transitions known as martensite transformations. These alloys exhibit this transformation upon cooling from a body centered cubic high temperature phase (h.t.p.) to a low temperature phase (l.t.p.) of lower symmetry. The l.t.p. is usually a twinned structure whose twins are very mobile under the application of stress. A specimen may be deformed plastically entirely by movement of twin boundaries. The effective elastic moduli of such materials can be modified significantly by these twinning effects. Interesting memory properties are also exhibited by these alloys. In studies of the intermetallic compound NiTi with stoichiometric composition; it was found that if an initially straight wire was deformed at room temperature by coiling it into a helix, and it was then heated to about 65 C., the wire rapidly straightened out and returned to its original shape. The demonstration could be repeated an indefinite number of times. Thus it was found that if a sample is constrained to a given shape while being given a special anneal in the high temperature phase, this shape will be retained and subsequently remembered even though the sample is severely plastically deformed in the low temperature phase. This memory is established by the fact that the sample will regain its original high temperature phase shape when it is heated above the characteristic martensite transformation temperature, M There is also a large change in the elastic constants that accompanies the phase change. For example, in the NiTi stoichiometric material the effective Youngs modulus, E, has been found to increase by a factor of approximately 3 to 4 on heating. Such a large change in Youngs modulus makes such materials very attractive for the construction of special thermally activated springs.

There are three modes of operation in such springs. For certain applications it is useful to have a given load undergo displacement on heating. A simple example would be a cantilever spring of martensite transformation material. A given load will be maintained at a certain displacement :nited States Patent 0 Patented July 24, 1973 in the low temperature phase. When heated to the high temperature phase, Youngs modulus will increase and the displacement will decrease. Thus the load will undergo a change in position upon heating. The displacement cycle achieved upon heating and then cooling the spring from its high temperature phase to its low temperature phase is consistently reproducible. Similarly, if the rod were constrained to a constant displacement, a large increase in force is possible upon heating to the high temperature phase. In general, a given device will operate between the two extreme cases discussed above, that is, an increase in force and a decrease in displacement will occur upon heating.

Springs such as we have been discussing can be activated in several ways: by infrared or solar radiation; electrical heating; or thermal conduction of heat down the length of the spring. In general, alloys which undergo a martensite transformation have fair electrical conduction properties but very poor thermal conduction properties. In the first two of the above means of activation, the heat is supplied somewhat uniformly along the length of the spring and more than enough energy is usually available to raise the sample temperature above its transformation temperature. Thus, such springs have found considerable application in devices which utilize these means of activation. Problems have occurred, however, in the application of these springs to situations where they are activated by thermal conduction. In these cases one is usually concerned with transferring heat down a spring from a heated end and one deals with a situation where only a limited amount of energy may be available. The heated end may be connected directly to some source of energy or may be connected to a conduction rod which brings the heat from some remote source. It has been found that the low thermal conductivity of martensite transformation. materials leads to great difliculties in employing them for operations that require a fast thermal response. In applications that require thermal conduction to activate a spring it is possible that the total length of material in a given spring will not be transformed unless a very high temperature pulse is applied to the heated end. It has been found in steady state temperature distribution experiments with NiTi rods that drops of approximately 240 C. may occur over a length of only 4 centimeters. These characteristics make it clear that martensite transformation materials would be undesirable for applications in which they would be activated by thermal conduction.

In accordance with the present invention, it has been discovered that if a martensite transformation material is layered with a material of high thermal diffusivity, such as copper or silver, the thermal characteristics of the composite rod are greatly improved. For a copper-tomartensite volume ratio, it has also been determined that a martensite material having a tubular shape or a sandwich configuration with a copper or silver core will not only improve the thermal properties of the martensite material, but would have better mechanical properties.

It is accordingly an object of this invention to provide a new type of thermally activated spring.

It is another object of this invention to provide a new composite material for use in a thermally activated spring.

It is still another object of this invention to provide composite martensite transformation materials with a high effective thermal diffusivity.

It is a further object of this invention to provide a novel spring for use as a component in small caliber fuzes.

Other objects of the invention will be pointed out in the following description and claims when read in conjunction with the accmompanying drawings in which:

FIG. 1 is a graph representing the steady state temperature distribution along a NiTi rod with a length, L, of 13 cm., and a diameter, d, of 96mil, when heated to 270 C.

at one end, in air with an ambient temperature of 23 C., for vairous thicknesses of copper plating.

FIG. 2 shows force vs. displacement curves for canti lever bending of a copper-plated Nitinol rod (95 mil diameter, 13 cm. long) with various thicknesses of copper plating. Measurements were made at a temperature of 23 C., with the Nitinol partially transformed to its low temperature phase, since the transformation is not complete until 65 C.

FIG. 3 shows force vs. displacement curves for rods similar to those in FIG. 2, except that the measurements were made with the fixed end of the rod heated to 270 C., and the rod in air with an ambient temperature of 23 C.

FIG. 4 represents the ratio of the effective flexural rigidity, EI, in the h.t.p. to that in the l.t.p. vs. the thickness of plating for composite rods, Where E is'Youngs modulus and I is the moment of inertia of the cross-section with respect to the neutral axis of bending.

The preferred martensite transformation material for use in this invention is the martensite alloy nickel titanium. Much documentation of the physical properties of this material already exists and has been accomplished by a group at the Naval Ordnance Laboratory (NOL). The alloys near the stoichiometric composition have been given the trade name Nitinol (Nickel Titanium Naval Ordnance Laboratory) in recognition of the extensive research done by the NOL people on this material. This material will be referred to by the name Nitinol throughout the remainder of this specification. Nitinol, however, is not the only material which may be used in the present invention. In some applications it might be desirable to have a different martensite transformation temperature, M than that available with Nitionl. The compound AuCd is another preferred martensite transformation material for use in this invention. It is possible to vary M by adding a third element to a given alloy or by chOOsing other related binary alloys. For example, the M of AuCd alloys can be lowered significantly by the addition of silver. Also, M can be varied over a wide range of temperatures for a series of CsCl-type equiatomic phases in binary alloys of transition elements such as TiFe, TiCo, TiNi, ZrRu, and ZrPd. Many other binary alloys exist that exhibit martensite transformations. These include CuAl, CuSn, CuZn, AuCu, AuMn, MnAs, MnBi, AgCd, and AgZn. Such materials have a variety of M temperatures which vary with deviations from stoichiometry. Thus it seems possible to choose a material with an M suitable for an given application.

The effect of copper plating on the heat conduction of a Nitinol rod can be seen readily in FIG. 1. The lowest curve in FIG. 1 shows the steady state temperature distribution in a solid Nitinol rod. It is clear from these curves that the heat conduction of a Nitinol rod is very poor but can be improved significantly by copper plating. It should be noted that the flexural rigidity of the composite rod is altered by the copper plating. For cantilever bending, FIG. 2 shows force vs. displacement curves for copper-plated Nitinol rods in the low temperature phase with various thicknesses of copper plating. FIG. 3 shows similar curves for copper-plated Nitinol rods in the high temperature phase. It can be seen that the copper plating may increase appreciably the flexural rigidity or stiffness of the composite rod. As the copper plating becomes thicker, it is clear from these figures that the initial slopes of the low temperature curves, FIG. 2, approach those of the high temperature curves, FIG. 3. By plotting the ratio of the initial slopes for h.t.p. and l.t.p. of the force vs. displacement curves, i.e., (El) /(El) for a given thickness of copper plate against the amount of plating, as is done in FIG. 4, one can see how this ratio for the copper-plated Nitinol rod approaches that of a pure copper rod. Thus, FIGS. 1 and 4 show that, while the diffusivity is increased by copper-plating, the deflection or force changes which accompany the transformation of the copper/Nitinol composite are reduced. Nevertheless, under certain stringent design constraints one might wish to sacrifice some of the memory properties for a more rapid thermal response which can be had with copper plating.

The use of a tubular sample of martensite material filled with a copper core for heat conduction or a strip of copper layered symmetrically on both sides with martensite material would extend the martensite material memory characteristics to larger values of the copper-tomartensite volume ratios. This follows from the wellknown principle of applied elasticity, namely, that for structural members in bending, the outer fibers carry most of the load. Curve C in FIG. 4 shows that increases with increasing thicknesses of the Nitinol wall.

In a solid beam of circular cross-section, with diameter, d, in bending, the moment of inertia, I, about the neutral axis is given by:

In the case of a hollow beam,

1 flw where d is the inner diameter and d is the outer diameter. The bending of a composite beam consisting of two concentric materials is described by the effective fiexural rigidity, EI, given by:

where E and I are Youngs modulus and the moment of inertia of the inner or core material and E and I are the corresponding parameters for the outer or plated material. Therefoe,

15 awn The effect on the fiexual rigidity by plating with a material of high diffusivity can be examined by factoring Equation 4 218 fOllOWSt EI =E whereas, the effect of coring can be examined by rewriting Equation 4 as follows:-

a a- 1 4 EI-EZ 64 i E2 12) (6) Thus, when plating i EIZEl 64. (7

i.e., the effective EI will be greater than that of the martensite core material and the characteristics of the plated material can dominate the mechanical behaviour of the system if the plating is too thick. On the other hand, when coring,

2 IE EI E1 64 (8) depending on whether E /E gl.

of thermally conductive material, or vice versa and analogous results would accrue.

A preferred embodiment of this invention would be to form a strip of copper and sandwich this strip with Nitinol. This embodiment would differ from the conventional bimetallic strip in that the memory effects mentioned above would be the driving force for mechanical action rather than differences in thermal expansion between the metals. Furthermore, the symmetrical disposition of the martensite material would preclude any thermal expansion elfects.

Utility for a spring of martensite transformation material with enhanced thermal diffusivity may be found in electromechanical fuses, switches and timing devices. Particularly, such a spring might find application in a small caliber fuse in which cumbersome time mechanisms would be impractical. In order to obtain the energy necessary to activate a helical spring made in accordance with this invention, at least two methods are possible. In one method, the aerodynamic heating of the projectile itself while in flight is used. Another is the use of a resonance effect which might pump enough energy into the projectile interior during the portion of the flight when ultrasonic velocities are attained. In both of these methods, the heat for activation of the spring is supplied via conduction down the length of the spring. The use of Nitinol alone to construct such a spring is of only marginal usefulness. With proper geometric conditions and with the thermal diffusivity enhanced in accordance with this invention, it is possible to obtain the desired force and/ or displacement to initiate a certain train of mechanical events within some specified time.

It should be understood that any material which undergoes a martensite transformation may be used as the martensite transformation material in this invention, and any material with a high thermal diffusivity and preferably a low modulus of elasticity may be used as the thermal-property enhancing material. Furthermore, any known method may be used to layer the copper on the martensite transformation material, as, for instance, coating or mechanical bonding. Any known method also may be used to layer martensite transformation material around a copper or silver core. The term rod as used in the specification and claims may refer to any rodor wireshape either straight or curved into a helix or in any other configuration that might occur to a person skilled in the art. The term strip may also refer to any substantially long, flat piece either straight or curved.

We wish it to be understood that we do not desire to be limited to the exact details of construction and geometry shown and described, for obvious modifications will occur to a person skilled in the art.

We claim:

1. A thermally activated spring like memory device comprising martensite transformation material which undergoes a phase transformation upon heating and cooling between a high temperature phase and a low temperature phase providing a large difference in the elastic properties of said high temperature phase as compared to said low temperature phase, the combination therewith of the improvement comprising:

a base material layered to and in contacting relation with said martensite transformation material, said base material being a thermally conductive material having a higher thermal diffusivity than said martensite transformation material and enhancing thermal diffusivity of said martensite transformation material, said martensite transformation material being selected from the group consisting of NiTi, AuCd, TiFe, TiCo, TiNi, ZrRu, ZrPd, CuAl, CuSn, CuZn, AuCu, AuMn, MnAs, MnBi, AgCd, and AgCn, and said thermally conductive material being selected from the group consisting of copper and silver. 2. A thermally activated spring like memory device as recited in claim 1, wherein:

said martensite transformation material is in the shape of a strip; and said thermally conductive material is layered in a sandwich configuration upon said strip. 3. A thermally activated spring like memory device as recited in claim 1, wherein:

said martensite transformation material is in the shape of a rod; and said thermally conductive material is disposed concentrically about said rod. 4. A thermally activated spring like memory device as recited in claim 1, wherein:

said martensite transformation material is NiTi. 5. A thermally activated spring like memory device as recited in claim 1, wherein:

said martensite transformation material is AuCd. 6. A thermally activated spring like memory device as recited in claim 1, wherein:

said thermally conductive material is in the shape of a rod; and

said martensite transformation material is disposed concentrically about said rod. 7. A thermally activated spring like memory device as recited in claim 6, wherein:

said martensite transformation material is NiTi. 8. A thermally activated spring like memory device as recited in claim 6, wherein:

said martensite transformation material is AuCd. 9. A spring like memory device as recited in claim 1, wherein:

said thermally conductive material is in the shape of a strip, and said martensite transformation material is layered in a sandwich configuration upon said strip. 10. A thermally activated spring like memory device as recited in claim 1, wherein:

said martensite transformation material is NiTi. 11. A thermally activated spring like memory device as recited in claim 1, wherein:

said martensite transformation material is AuCd.

References Cited UNITED STATES PATENTS 1,799,689 4/1931 Jones 29-1955 2,090,312 8/ 1937 Sawyer 29-195 .5 2,144,915 1/ 1939 Derby 29-1955 2,254,484 9/1941 Hutchins 29195.5 2,327,500 8/1943 Chace 29-195.5 2,341,858 2/ 1944 Dubilier 29-1955 3,219,423 11/1965 Sears 29-1955 3,454,373 7/1969 Ornstein 29195.5

HYLAND BIZOT, Primary Examiner

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