Shape Memory Effects And Biphases Toughness Of NiMnGa-based High-temperature Shape-memory Alloys

Now the mainly researched high-temperature shape-memory alloys are as follow: CuAl-based (CuAlAg,CuAlPd),NiAl-based (NiAlFe,NiAlMn) and NiTi-based (NiTiHf/Zr,NiTiPd/Au). At present, NiTiPd alloy shows the best integrated properties.Its maximum shape memory strain is 5.5% after proper thermo-mechanical treatment. But Pd element is very expensive. The content of Pd in NiTiPd alloy for attaining high martensitic transformation temperature is almost 50% in weight. So it is too expensive for the practical application of NiTiPd alloy, and the research of low cost high temperature shape memory alloys possessing the comparable properties as NiTiPd is one of the challengeable works.Previous researches revealed that NiMnGa alloys show great potentials as practical high temperature shape memory alloys with their high martensitic transformation temperatures, outstanding shape memory effects and good thermal stability. However, the high brittleness of polycrystal NiMnGa alloys is a big problem preventing their development. In our research, the introduction of a ductile second phase by adding the fourth element was proved to be an effective way improving the ductility of NiMnGa polycrystals. At the same time, the alloys exhibit moderate shape memory effects. Particularly, in this thesis, Co or Cu was added to Ni₅₆Mn₂₅Ga₁₉ alloy by replaced Ni, Mn or Ni and Mn at the same time, and their crystal structures, microstructures, martensitic transformation behaviors, mechanical and shape memory characteristics were studied by optical micrograph, XRD, DSC, SEM, EPMA, tensile and bend tests.The results show that when Mn or Ni and Mn are replaced by Co, the alloys exhibit dual phases containing tetragonal martensite phase andγphase with fcc structure, and the amounts ofγphase increase with increasing Co content. When Co< 8at.% for replacing Ni or Cu < 2at. % for replacing Mn, the alloys exhibit a single martensite phase, and theγsecond phase will be appeared with further increasing Co/Cu content. DSC investigations revealed that the martensitic transformation temperatures rise with the increase of Cu content for replacing Mn until theγphase was formed at Cu=2at.%.When Cu>2at.%,the martensitic transformation temperatures keep constant around 530℃.On the other hand, when the adding forth element is Co, the martensitic transformation temperatures decrease with the increasing of Co content. The varieties of phase transformation behaviors were resulted from the integrated actions of the size factor, electron concentration and the precipitation ofγphase.The ductility of NiMnGa alloys could be improved greatly when Co/Cu replacing Mn or Ni and Mn. Some alloys can be hot rolled to plates with 0.5 mm thickness, and their mechanical properties and shape memory effects were studied by tensile tests for the first time. The results revealed that the tensile stress and elongation of Ni₅₆Mn₂₁Co₄Ga₁₉ alloy are 491MPa and 8.2%, respectively. The shape memory recoverable strain is 2.1% at residual strain of 4.3%. The tensile stress and elongation of Ni₅₃Mn₂₂Co₆Ga₁₉ alloy are 483MPa and 5.5%, respectively, and the recoverable strain is 3.2% upon residual strain of 4.8%. However, when the adding forth element is Cu, Ni₅₆Mn₂₁Cu₄Ga₁₉ alloy exhibits insufficient ductility. With further increasing Cu content, Ni₅₆Mn₁₇Cu₈Ga₁₉ alloy exhibits tensile strength and elongation of 424MPa and 3.8%, respectively. However, its shape memory effect disappears.The fracture surfaces and microcracks of plates after tensile tests are investigated by SEM. The results show that the fracture surfaces of Ni₅₆Mn₂₅Ga₁₉ alloy exhibit typical brittle character with intergranular pattern. But the fracture surfaces of Ni₅₆Mn₁₇Co₈Ga₁₉ alloy exhibit characteristics of ductile rupture due to the formation of dimples. Additionally, microcrack observations revealed that many microcracks germinate and propagate along the boundaries between different orientations of martensitic variants. When the microcracks expand to the boundaries between the martensite and the second phase, the microcracks get weak/disappear, or directly cross the second phase. All these manners will certainly increase the energy needed for the further propagating of the microcracks, so as to increase the toughness and ductility of NiMnXGa alloys.