Crystal structures and textures in the hot-forged Ni-Mn-Ga shape memory alloys

Three ferromagnetic shape-memory alloys with the chemical compositions of Ni₅₃Mn₂₅Ga₂₂, Ni₄₈Mn₃₀Ga₂₂, and Ni₄₈Mn₂₅Ga₂₂Co₅ were prepared by the induction-melting and hot-forging process. The crystal structures were investigated by the neutron powder diffraction technique, showing that Ni₅₃Mn₂₅Ga₂₂ and Ni₄₈Mn₂₅Ga₂₂Co₅ have a tetragonal, 14/mmm martensitic structure at room temperature, while Ni₄₈Mn₃₀Ga₂₂ has a cubic, L2₁ austenitic structure at room temperature. The development of textures in the hot-forged samples shows the in-plane plastic flow anisotropy from the measured pole figures by means of the neutron diffraction technique. Significant texture changes were observed for the Ni₄₈Mn₂₅Ga₂₂Co₅ alloy after room temperature deformation, which is due to the deformation-induced rearrangements of martensitic variants. An excellent shape-memory effect (SME) with a recovery ratio of 74 pct was reported in this Ni₄₈Mn₂₅Ga₂₂Co₅ polycrystalline alloy after annealing above the martensitic transformation temperature, and the “shape-memory” influence also occurs in the distributions of grain orientations.

Crystallographic features during martensitic transformation in Ni-Mn-Ga ferromagnetic shape memory alloys

The martensitic transformation crystallography in two Ni ₅₃Mn₂₅Ga₂₂ (at. %) ferromagnetic shape memory alloys (FSMAs) was investigated by means of misorientation calculation and pole figure analysis based on the orientation of the martensitic lamellae obtained from electron backscattered diffraction (EBSD) measurements. In the alloy that was first annealed at 1073K for 4h, and then cooled to 473K at ~4K/min and held for 30min, followed by cooling to room temperature at ~10K/min, there are only two kinds of differently orientated martensitic lamellae with a misorientation angle of ~82° distributed alternatively in each initial austenite grain. There is a compound twinning orientation relationship between the two lamellae. The prevalent orientation relationship between austenite and martensite is Kurdjumov-Sachs (K-S) relationship with (111)_A//(10I)_M, [1-10]_a//[11-1]_m. In the alloy that was annealed at 1173K for 4h followed by furnace cooling, nanoscale twins inside the martensitic lamellae were observed and the orientation relationships both between the nanotwins within one lamella and between the nanotwins in two neighboring lamellae were determined. The results presented in this paper will enrich the crystallographic data of the FSMAs and offer useful information for the development of novel FSMAs with optimal performances.

Determination of crystal structure and crystallographic characteristics in Ni-Mn-Ga ferromagnetic shape memory alloys

Ni-Mn-Ga ferromagnetic shape memory alloys (FSMAs) have received great attention during the past decade due to their giant magnetic shape memory effect and fast dynamic response. The crystal structure and crystallographic features of two Ni-Mn-Ga alloys were precisely determined in this study. Neutron diffraction measurements show that Ni48Mn30Ga22 has a Heusler austenitic structure at room temperature; its crystal structure changes into a seven-layered martensitic structure when cooled to 243K. Ni53Mn25Ga22 has an I4/mmm martensitic structure at room temperature. Electron backscattered diffraction (EBSD) analyses reveal that there are only two martensitic variants with a misorientation of ~82° around <110> axis in each initial austenite grain in Ni53Mn25Ga22. The investigation on crystal structure and crystallographic features will shed light on the development of high-performance FSMAs with optimal properties.

Structure and phase stability of NiMnGa ferromagnetic shape memory alloys by experimental and ab-initio techniques

This paper summarizes some of our recent results on crystal structure, microstructure, orientation relationship between martensitic variants and crystallographic features of martensitic transformation in Ni-Mn-Ga FSMAs. It was shown that Ni53Mn25Ga22 has a tetragonal I4/mmm martensitic structure at room temperature. The neighboring martensitic variants in Ni53Mn25Ga22 have a compound twinning relationship with the twinning elements K1={112}, K2={11-2}, η1=<11-1>, η2=<111>, P={1-10} and s=0.379. The ratio of the relative amounts of twins within the same initial austenite grain is ~1.70. The main orientation relationship between austenite and martensite is Kurdjumov-Sachs (K-S) relationship. Based on the crystallographic phenomenological theory, the calculated habit plane is {0.690 -0.102 0.716}A (5.95° from {101}A), and the magnitude, direction and shear angle of the macroscopic transformation shear are 0.121, <-0.709 0.105 0.698>A (6.04° from <-101>A) and 6.88°, respectively.

Fabrication of Ti14Nb4Sn alloys for bone tissue engineering applications

In this paper, porous Ti14Nb4Sn alloys were fabricated using a space holder sintering method, resulting in a porosity of ~70%. Scanning electron microscopy (SEM) analyses revealed a combination of both macropore and micropore structures. The fabricated titanium alloy scaffolds exhibited a similar structure to that of natural bone, which is expected to improve bone implant longevity. Bacterial cells of Pseudomonas aeruginosa ATCC 9027 were employed for the in vitro test.