Abstract
	Study of hydrogen effect on microstructure and mechanical properties of NiTi alloy was performed. The material used was Ni-51.56 At. % Ti in wire form with diameter of 2.3 mm.  Tensile testing was realised at room temperature on the wire specimens of NiTi polycrystalline alloy in state as-received and after heat treatment at 850 °C for one hour in two different atmospheres: in flowing argon and in flowing hydrogen. Deformation under tension of the specimens in martensitic state proceeded via martensite variant reorientation and the second yield point RP was determined. Under tension, a flat stress-plateau occured for specimens non-hydrogenated, while no flat stress-plateau and quickly strain hardening were observed for specimens hydrogenated. Martensitic microstructure was studied using light (OM), scanning electron (SEM) and atomic force microscopies (AFM). Based on stress-strain curves, microstructure and fractographic observation, the following conclusions about hydrogen effect on the NiTi alloy were drawn: decrease of ultimate tensile strength in hydrogenated samples was related with slight change of fracture patterns from high ductile fracture with deep dimples to ductile pattern with flatter dimples and microcracks. Furthermore, it seems that the presence of hydrogen affected the martensitic transformation and the residual austenite presence in NiTi alloy after heat treatment. No doubt, hydrogen had an effect on the mobility of dislocations and the move of twin boundaries during the deformation that manifested by change of stress-strain curve. More experimental testing and observation are needed to confirm the submitted conclusions, including the evaluation of hydrogen content in specimens and the study using transmission electron microscopy (TEM).