Microstructure And Martensitic Phase Transformation Of NiTiHfCu Alloy Thin Films And Ribbons

Ni-Ti-Hf-Cu alloy thin films and have ribbons been fabricated by D.C. magnetron sputtering and melt-spinning techniques, respectively. Surface characteristics, microstructure, martensitic phase transformation behaviors and its effects of both Ni-Ti-Hf-Cu high temperature shape memory thin films and ribbons have been systematically investigated by means of SEM, AFM, DSC, XRD and TEM.The results show that the parameters of magnetron sputtering have remarkable effects on the chemical composition, the thickness and the surface morphologies of Ni-Ti-Hf-Cu alloy thin films. The contents of Ni is decreased and the contents of Ti and Hf increased with increasing Ar gas pressure, but the contents of Cu keep stable. The contents of Ni is increased and the contents of Hf is decreased with increasing sputtering time, but the contents of Hf and Cu have not been obviously changed. The sputtering power has no obvious effect on the contents of elements in the thin films. With the increas of sputtering time and sputtering power, the thickness of Ni-Ti-Hf-Cu alloy thin films increases.However the increas of Ar gas pressure has no remarkable effects on the thickness of thin films. As sputtering power is increased, the roughness and grain size of thin films are increased. With the increas of Ar gas pressure, the roughness and grain size of thin films firstly is increased and then decreased. The as-deposited Ni-Ti-Hf-Cu alloy thin films are amorphous. After crystallization treatment, this phase of thin films are mian martensitic composed of a few of (Ti, Hf, Cu)2Ni phases. It is should be noted that the martensitic crystal construction of alloy thin films and ribbons is still single oblique B19′construction, and the addition of Cu has no obviously effect on the martensitic crystal lattice constant. With the raising crystallization temperature, the crystal grains is reform, thereby, leading to the occurrence of perferential orientation.The results of DSC show that the Ni-Ti-Hf-Cu alloy thin films and ribbons undergo the martensitic transformation and the reverse martensitic transformation during the precess of cooling and heating. When the content of Cu less than 3at.%, the occurrence of B19′besides R transformation is happened. Both B19′ and R phase transformation hysteresis of ribbons are decreased due to the addition of Cu, the phase transformation temperature decreased by (Ti, Hf, Cu)2Ni phase separate out.The results obtained from TEM, they are show the crystallization of Ni-Ti-Hf-Cu alloy thin films and ribbons martensite variants exhibit selfaccommoda-tion configuration with the spear-like and masaic-like morphologies and martensite variants are found to be (011) Type I twin related. The interior part of martensite is comprised of parallel and tiny (001) compound twin belt-like substructure.