Fabricating Ni–Mn–Ga Microtubes by Diffusion of Mn and Ga Into Ni Tubes
Tubes of the ferromagnetic shape-memory alloy Ni–Mn–Ga of composition near the Ni2MnGa Heusler phase can be used, alone or combined in structures, in magnetic actuators or magnetic refrigerators. However, fabrication of Ni–Mn–Ga tubes with sub-millimeter diameter by classical cold or hot drawing methods is hampered by the brittleness of the alloy. Here, we demonstrate a new process, where Ni–Mn–Ga tubes are fabricated by interdiffusion of Mn and Ga into drawn, ductile Ni tubes with 500 and 760 μm inner and outer diameters. After interdiffusion and homogenization of Mn and Ga at 1000 °C for 24–36 h, Ni–Mn–Ga tubes with ~300 and ~900 μm inner and outer diameters were obtained with homogeneous radial composition distribution, independently of the diffusion sequences (i.e., Mn and Ga diffused sequentially or simultaneously). Longitudinal composition was uniform over lengths of ~1 mm, but variable over longer length due to incomplete process control. For two of the three diffusion sequences, a sizeable (20–80 μm) region exhibiting Kirkendall pores formed at the outer surface of the tubes. Magnetization values as high as ~60 emu/g were measured, which is comparable to the magnetization of the Ni2MnGa Heusler phase. X-ray diffraction on the tube with the highest magnetization confirmed the room-temperature structure as cubic austenite.