T. W. Shield

Transient heat transfer effects on the pseudoelastic behavior of shape-memory wires

Abstract Experimental results of a displacement-controlled elongation of a shape-memory wire of nickel-titanium are presented. It is observed that the hysteretic strain-stress curves depend strongly on the strain rates at which the wire is extended. A theoretical model is proposed to explain this phenomenon. This model couples the fully time-dependent heat transfer in the wire to its quasi-static mechanical behavior through the temperature dependence of the transformation stress of the alloy. It accounts quantitatively for experimentally observed changes in the pseudoelastic hysteresis. The model presented here is different from others proposed in the literature, as it does not make use of a kinetic relation and accounts for the observed changes in the pseudoelastic hysteresis without parameter fitting. The results show that a model consisting of a single moving austenite-martensite interface is sufficient to predict the response of the wire over several decades of strain rate.

Ferromagnetic shape memory in the NiMnGa system

Strain versus field measurements for a ferromagnetic shape memory alloy in the NiMnGa system demonstrate the largest magnetostrictive strains to date of nearly 1.3%. These strains are achieved in the martensitic state through field-induced variant rearrangement. An experimental apparatus is described that provides biaxial magnetic fields and uniaxial compressive prestress with temperature control while recording microstructural changes with optical microscopy. The magnetostrictive response is found to be sensitive to the initial state induced by stress-biasing the martensitic variant structure, and exhibits rate effects related to twin boundary mobility. Experiments performed with constant stress demonstrate work output capacity. Experimental results are interpreted by using a theory based on minimization of a micromagnetic energy functional that includes applied field, stress, and demagnetization energies. It is found that the theory provides a good qualitative description of material behavior, but significantly overpredicts the amount of strain produced. Issues concerning the martensitic magnetic anisotropy and variant nucleation are discussed with regard to this discrepancy.

Orientation dependence of the pseudoelastic behavior of single crystals of CuAlNi in tension

Uniaxial tension experiments were performed on single crystals of Cu-13.95 wt% Al-3.93 wt% Ni. Three specimens were prepared with tension axes in directions that were chosen based on Schmid law calculations using the 96 possible Austenite-Martensite (AM) interface orientations in this alloy. Specimen number one was chosen to have a tensile axis of [2.43,1,0] which results in a very near minimum value for its predicted tension transformation stress. Specimen number two was oriented 15 degrees from [111] direction and has a [1,1,1.73] tensile axis direction. The third specimen has the [111] direction as its tensile axis, which is the direction of maximum tensile transformation stress. A strong relationship is found between the mechanical behavior of the specimens in tension and their observed microstructure. Specimen one exhibits an extremely flat stress plateau during transformation and almost no hysteresis. The microstructure observed in this specimen consists of two nearly perpendicular AM interfaces that interact to form an X-structure that results in a purely uniaxial deformation. This microstructure is completely reversible and seems to present no restriction on the motion of either interface. Specimen two was observed to have only a single AM interface after transformation. This interface appears to preclude the formation of any other interfaces. Specimen three required five times the normal stress of that needed to transform specimen one. This specimen also exhibited a large amount of hysteresis. The microstructure observed consisted of two A M interface systems that meet to form wedges. Because the interfaces must end at the wedge apex, the formation of the wedges resulted in a kinematic coupling between the two AM interface systems. The amount of coupling between the interfaces in the microstructure correlates to the amount of hysteresis observed.

Microstructure in the cubic to monoclinic transition in titanium-nickel shape memory alloys

Abstract Microstructure in the cubic to monoclinic transition in the technologically important Ti–Ni shape memory alloys is considered. Using a geometrically nonlinear theory of martensitic transformations, the twinned martensite, austenite–martensite, wedge, triangle, and diamond microstructures are studied. Specifically, compound, type I, and type II twins are possible for any choice of the lattice parameters; while, non-standard twins may exist with special lattice parameters only. In addition, 192 habit planes are found for a particular Ti–Ni alloy, but only 24 have been unambiguously observed in experiments. Further, the twinned wedge is possible in this alloy, but the triangle and diamond are not. These latter three are special microstructures, which provide a mechanism through which a specimen can easily transform and are possible only for alloys with specific lattice parameters. A complete enumeration of the various microstructures is given, and algorithms are presented so that the calculations can be repeated with different lattice parameters.

Enhanced reversibility and unusual microstructure of a phase-transforming material

Materials undergoing reversible solid-to-solid martensitic phase transformations are desirable for applications in medical sensors and actuators, eco-friendly refrigerators and energy conversion devices. The ability to pass back and forth through the phase transformation many times without degradation of properties (termed ‘reversibility’) is critical for these applications. Materials tuned to satisfy a certain geometric compatibility condition have been shown to exhibit high reversibility, measured by low hysteresis and small migration of transformation temperature under cycling. Recently, stronger compatibility conditions called the ‘cofactor conditions’ have been proposed theoretically to achieve even better reversibility. Here we report the enhanced reversibility and unusual microstructure of the first martensitic material, Zn45Au30Cu25, that closely satisfies the cofactor conditions. We observe four striking properties of this material. (1) Despite a transformation strain of 8%, the transformation temperature shifts less than 0.5 °C after more than 16,000 thermal cycles. For comparison, the transformation temperature of the ubiquitous NiTi alloy shifts up to 20 °C in the first 20 cycles. (2) The hysteresis remains approximately 2 °C during this cycling. For comparison, the hysteresis of the NiTi alloy is up to 70 °C (refs 9, 12). (3) The alloy exhibits an unusual riverine microstructure of martensite not seen in other martensites. (4) Unlike that of typical polycrystal martensites, its microstructure changes drastically in consecutive transformation cycles, whereas macroscopic properties such as transformation temperature and latent heat are nearly reproducible. These results promise a concrete strategy for seeking ultra-reliable martensitic materials.

Shape memory and ferromagnetic shape memory effects in single-crystal Ni2mnGa thin films

Epitaxial Ni2MnGa and Ni2Mn1.2Ga0.8 thin films have been grown by molecular beam epitaxy on GaAs (001) substrates with Sc0.3Er0.7As interlayers. Structural characterization of as-grown films confirms the epitaxially stabilized single crystal structure of the films and indicates that the films grow pseudomorphically on GaAs (001) substrates in a tetragonal structure (a=b=5.65 A, c=6.18 A). The films are ferromagnetic at room temperature with coercivity of ∼50 Oe, saturation magnetization of ∼250 emu/cm3, and weak in-plane magnetic anisotropy. The Curie temperature of the films is found to be ∼340 K. However, while the films were attached to the substrate martensitic phase transformations were not observed. In order to observe martensitic phase transformations, free-standing Ni2MnGa bridges and cantilevers were fabricated using front and back side photolithography together with a combination of dry and wet etching. After removal of the substrate, the free-standing bridges and cantilevers showed a unique tem...

Symmetry and microstructure in martensites

Abstract The shape memory effect and superelastic behaviour occur by the nucleation and growth of microstructure during a thermoelastic martensitic transformation. A constrained geometrically nonlinear theory is used to construct the twinned martensite, austenite-martensite, wedge, triangle, and diamond microstructures. The latter three microstructures are special microstructures, and they are of interest because they provide a mechanism through which the specimen can easily undergo the transformation. In this paper, it is shown that all of the twinning elements can be determined for any given transition, and that the twins are related by a symmetry transformation. Additionally, it is shown that there exists a symmetry amongst solutions to the habit plane equation with different variants, and this symmetry is exploited to show that the special microstructures are possible only for materials with special lattice parameters. This symmetry amongst the solutions provides a method by which all possible microst...

The Buckling of an Elastic Layer Bonded to an Elastic Substrate in Plane Strain

The results are compared to an approximate solution that models the layer using beam theory. This comparison shows that the beam theory model is adequate until the buckling strain exceeds three percent, which occurs for modulus ratios less than 100. In these cases the beam theory predicts a larger buckling strain than the exact solution. In all cases the wavelength of the buckled shape is accurately predicted by the beam model. A buckling experiment is described and a discussion of buckling induced delamination is given

Mean Stress Effects in Biaxial Fatigue of Inconel 718

Biaxial fatigue tests were conducted on Inconel 718 thin-walled tubular specimens to quantify the effect of mean stress. The specimens were loaded in combined tension and torsion in strain control at room temperature. Fatigue lives ranged from 3000 to 15,000 cycles depending on the mean stress. These data were correlated with a parameter based on the maximum plastic shear strain amplitude, normal strain amplitude and mean normal stress on the plane of maximum shear strain amplitude. This parameter was combined with the Coffin-Manson equation for estimating fatigue lives. Observations of the cracking behavior show that mean stress affects the rate of crack growth and distribution of cracks.

Experimental study of the deformation near a notch tip in copper and copper–beryllium single crystals

Abstract The plastic deformation around a notch tip within ductile single crystal material was investigated experimentally. Moire microscopy was used to measure the strain field on the surface of bending samples of Orientation II with a notch on the (0 1 0) plane and its tip along the [ 1 0 1] direction. The results of tests conducted on copper and copper–beryllium single crystals are compared to analytical solutions, numerical calculations, and prior experiments on samples of Orientation I with a notch on the (1 0 1) plane and its tip along the [1 0 1 ] direction. Distinct sectors with sharp sector boundaries are observed in experiments as were predicted analytically by Rice (Mech. Mater. 6 (1987) 301). However, Rice predicted that both Orientations I and II would give rise to the same sectors. It is found that experimental results for these two orientations differ and neither set of results agree with the analytical solution. In both orientations, the sector boundaries do not exclusively correspond to angles at which slip and kink can occur in these crystallographic orientations. The experiments also lead to the conclusion that some sectors in the deformation field remain elastic even after large amounts of deformation have occurred elsewhere. Based on the optical observations and strain measurements, a stress field is presented for Orientation II.