http://www.aem.umn.edu/~james/research/Publications_Files/PDF/83.pdf

Phase transformation and magnetic anisotropy of an iron-palladium ferromagnetic shape-memory alloy

The tweed microstructure associated with fcc-fct martensitic transformation in Fe-Pd alloys containing about 30 at. pct Pd was investigated by transmission electron microscopy and the selected area diffraction. It was observed that the tweed contrast consisting of {011}/ striations and accompanying the diffuse streaks became distinct from a temperature nearly 100 K above the Ms with lowering temperature. The tweed contrast was found to be induced by the static displacement of atoms, δ{011}/[01ī], associated with the formation of disc-shaped martensite nuclei on the {110} planes of the austenite. The tweed microstructure persistently remained in the fct martensite in the early stage of the transformation, where the degree of tetragonality is small and the c-axis directions of the small fct martensite variants are tilted against each other, giving rise to the tweed pattern. A similar tweed contrast observed in the initial stage of ordering in a stoichiometric Fe-Pd alloy was also explained by a strain contrast effect arising from an elastic shear strain induced by the nucleation of short range order particles with tetragonal distortion.

Tweed structures associated with Fcc-Fct transformations in Fe-Pd alloys

Ti-18Zr-14Nb, Ti-18Zr-13Nb-1Ta, Ti-18Zr-12Nb-2Ta and Ti-18Zr-11Nb-3Ta (at%) shape memory alloys were obtained by vacuum arc remelting and subjected to different thermomechanical treatment (TMT) schedules. The transmission electron microscopy analysis revealed that a TMT, which consisted of cold rolling with a true logarithmic strain of e = 0.3 and post-deformation annealing at 600 °C (30 min), led to a subgrained structure formation with a size of structural elements of 200–300 nm and to a room temperature superelastic behavior. The X-Ray diffraction (XRD) analysis showed that a gradual replacement of Nb content by its Ta equivalent decreases the impact of alloying elements on the β-phase stabilization phenomenon and suppresses the ω-phase formation: at room temperature, Ti-18Zr-14Nb and Ti-18Zr-13Nb-1Ta alloys contain mainly parent β-phase, while Ti-18Zr-12Nb-2Ta and Ti-18Zr-11Nb-3Ta alloys contain considerable amount of α”-martensite. The crystallographic resource of recovery strain for all the alloys exceeds 5% and reaches 6%, for the Ti-18Zr-11Nb-3Ta alloy. The fatigue life of the Ti-18Zr-13Nb-1Ta alloy is almost twice as high as that of the Ti-18Zr-14Nb alloy. During multi-cycle mechanical testing, the following regularity appeared for all the alloys tested: during 10–15 first loading-unloading cycles, the mechanisms of plastic deformation and stress-induced martensite formation and stabilization prevail, while after that, the β⇄α” martensitic transformation mechanisms become dominant, and the alloys’ behavior turns from elasto-plastic into superelastic.

Ternary Ti-Zr-Nb and quaternary Ti-Zr-Nb-Ta shape memory alloys for biomedical applications: Structural features and cyclic mechanical properties

Synchrotron x-ray diffraction (XRD) measurements, nuclear forward scattering (NFS) measurements, and density functional theory (DFT) calculations were performed on L1_2-ordered Pd_3Fe. Measurements were performed at 300 K at pressures up to 33 GPa, and at 7 GPa at temperatures up to 650 K. The NFS revealed a collapse of the ^(57)Fe magnetic moment between 8.9 and 12.3 GPa at 300 K, coinciding with a transition in bulk modulus found by XRD. Heating the sample under a pressure of 7 GPa showed negligible thermal expansion from 300 to 523 K, demonstrating Invar behavior. Zero-temperature DFT calculations identified a ferromagnetic ground state and showed several antiferromagnetic states had comparable energies at pressures above 20 GPa.

Pressure-Induced Invar Behavior in Pd3Fe.

Precise X-ray diffraction study has been carried out in the vicinity of fcc-fct transition in Fe-Pd alloys in order to investigate the pretransitional phenomena at martensite transformations. Using a high-resolution X-ray spectrometer with fine temperature control unit, the temperature variation of the profiles of Bragg reflections were measured. A new intermediate phase was discovered in a narrow temperature range between the austenite (fcc) and the martensite (fct) phase. Its structure is characterized by a “two-tetragonal-mixed” phase, and interpreted as a coherent mixture of two tetragonal lattices with different tetragonalities. The origin of the stability of this intermediate phase has been investigated. It may be interpreted in terms of the previously postulated “crest-riding-periodon” phase.

Shuji Harada

Papers

Change in young's modulus of thermoelastic martensite Fe-Pd alloys