Yunqing Ma

Martensitic transformation, ductility, and shape-memory effect of polycrystalline Ni56Mn25-xFexGa19 alloys

Abstract Buttons of high-temperature shape-memory alloy Ni56Mn25–xFexGa19 were prepared by arc melting under argon atmosphere. The specimens were hot rolled to 1 mm plates, and their martensitic transformation behaviors, ductilities, and shape-memory effects were investigated through differential scanning calorimetry and bending tests. It was found that the brittleness of polycrystalline Ni2MnGa was ameliorated through introduction of a ductile phase by Fe addition. Ni56Mn17Fe8Ga19 alloy shows great potential as a practical high-temperature shape-memory alloy with martensitc transformation start temperature above 200 °C. It can bear strain of 6% before fracture in bending tests and shows a maximum reversible strain of 3.5%.

Grain Refinement and Mechanical Properties of a Metastable Austenitic Fe-Cr-Ni-Mn Alloy

A lot of works for developing the structural nano-materials have been performed all over the world in recent years. Severe deformation techniques like HPT, ECPA and ARB have been applied to different materials such as Al, Cu, Ti and several steels. Such techniques greatly reduced the grain size and improved the yield and tensile strengths. However, the elongation of the materials is greatly decreased due to the small amount of work hardening, and these techniques do not seem suitable for the mass production. Therefore, this study has been carried out as a fundamental research for developing austenitic steels with high strength and good elongation using a conventional rolling and annealing processes. Fe-0.1%C-10%Cr-5%Ni-8%Mn alloy was melted, homogenized, hot rolled, and cold rolled at room temperature to transform γ austenite to α ’ martensite. After that, the specimens were annealed just above its reverse transformation finish temperature (Af) to obtain the fine reversed austenite grains. The grain size of the metastable austenitic steel was successfully refined to less than 200nm by repeating rolling and annealing processes. The resultant nanocrystalline material shows not only high strength but also large elongation because the work hardening ability is enhanced by the strain-induced martensitic transformation during the tensile test.

Experimental investigations of the high-temperature phase relationship and ordering transition in Ni-Mn-Ga ternary system

The phase equilibria at 900 °C and B2/L21 order-disorder transition in the Ni-Mn-Ga ternary system were investigated by analyzing the equilibrated alloys and diffusion couples using a combination of techniques. It was confirmed that a bcc single phase region exists in a wide composition range at 900 °C, and the critical temperatures of B2/L21 order-disorder transition were determined in Ni-50 at.% section, which exhibits a maximal ordering transition temperature of 796 at Mn content of 25 at.%. The obtained results will be helpful for the preparation and annealing of Ni-Mn-Ga alloys in the specific temperature.

Heusler Type CoNiGa Alloys with High Martensitic Transformation Temperature

A strong need exists to develop new kinds of high-temperature shape-memory alloys. In this study, two series of CoNiGa alloys with different compositions have been studied to investigate their potentials as high-temperature shape-memory alloys, with regard to their microstructure, crystal structure, and martensitic transformation behavior. Optical observations and X-ray diffractions confirmed that single martensite phase was present for low cobalt samples, and dual phases containing martensite and γ phase were present for high cobalt samples. It was also found that CoNiGa alloys in this study exhibit austenitic transformation temperatures higher than 340°C, showing their great potentials for developing as high-temperature shape-memory alloys.

Effect of Trace Boron on Microstructure and Mechanical Properties of Ti-9V-3Al-3Cr-3Zr-35Mo Alloy

The effects of trace boron on microstructure and mechanical properties of β type Ti-9V-3Al-3Cr-3Zr-3.5Mo (wt. %) alloy have been investigated in this study. Upon the addition of 0.02 wt. % boron, the grain size of the B-modified alloy was almost four times smaller than that of the B-free alloy. Accordingly, the tensile strength and elongation of B-modified alloy increased from 712 MPa and 14.6 % to 813 MPa and 17.9 %, respectively, mainly due to the effect of grain refinement.

Effect of the Addition of Gd on Ni53Mn22Co6Ga19 High-Temperature Shape Memory Alloy

The rare earth element Gd is added to Ni53Mn22Co6Ga19 high-temperature shape memory alloy to refine the grain size and adjust the distribution of γ phase, and their microstructure, martensitic transformation behaviors, mechanical and shape memory properties were investigated. The results show that the grain size is obviously decreased and the γ phase tends to segregate at grain boundaries with increasing Gd content. Small amounts of Gd-rich phase were formed with 0.1 at.% Gd addition. The martensitic transformation temperature abruptly increases with 0.1 at.% Gd addition, then almost keeps constant with further increasing Gd content. The addition of 0.1 at.% Gd is proved to be beneficial to both tensile stress and strain before fracture, but negative to the shape-memory effect.

Shape memory effect and magnetic properties of Co-Fe ferromagnetic shape memory alloys

In 1996, after Ullakko et al. first reported a measurement of 0.2% strain along the [001] direction of a Ni2MnGa single crystal when subjected to a magnetic field of 8KOe at 265K, the researches of ferromagnetic shape memory alloys became popular. The reversible strain induced by magnetic-field in Ni2MnGa has been proved to be nearly 10% in a magnetic field of less than 1T, which is much higher than that of the rare-earth giant magnetostrictive alloys. But the high brittleness of Ni2MnGa hinders its practical application. Therefore, the development of ferromagnetic shape memory alloys with good ductility is of primary importance. In this paper, the microstructure, martensitic transformation temperature, shape memory effect, as well as magnetic and mechanical properties of Co-Fe alloys were investigated by optical observation, X-ray diffraction, DSC, bending test and vibrating sample magnetometer. It was confirmed that the shape memory effect in Co-Fe alloys is associated with the fcc/hcp martensitic transformation. Moreover, Co-Fe alloys exhibit high saturation magnetization with the values of above 170 emu/g, which are much higher than that of Ni2MnGa (66 emu/g). So the driving force under magnetic field will be large for Co-Fe alloys. Additionally, Co-Fe alloys possess good ductility for practical application with tensile elongation higher than 17.5%. All these results indicate that Co-Fe alloys are promising candidates for developing as ferromagnetic shape memory alloys.