Powder metallurgical processing of Ni–Ti–Zr alloys undergoing martensitic transformation—part II

Ni48–49.5–Ti37–38.5–Zr13.5 at.% alloys obtained by conventional sintering have been studied by scanning electron microscopy (SEM) and high temperature differential scanning calorimetry (DSC). SEM confirms the results of X-ray investigation. ψ, φ, σ and ξ phases were observed reliably while no γ-phase was found. A new peculiarity that was found by SEM is the long-range spatial inhomogeneties of Zr, which appeared after sintering at high temperatures. After sintering at temperatures above 1000 °C fine precipitates were also found. The nature and origin of it is discussed. Morphology, distribution and amount of pores formed during sintering were also studied by SEM. Together with density analysis this allows to elaborate in detail the mechanisms of sintering at different temperatures. Sintering processes occur at different intensities for Ti–Ni and Zr–Ni pairs. At temperatures above 780 °C, Zr–Ni pairs sinter in the presence of a liquid phase stimulating the mechanism of a transient liquid phase in Ti–Ni pairs. Spontaneous liquid phase sintering in Ti–Ni starts above 880 °C. Long spatial distribution of Zr appears as a result of solid state redistribution of Zr and Ti atoms between the (Zr, Ni) and (Ti, Ni) regions.