An in situ Study of NiTi Powder Sintering Using Neutron Diffraction

Abstract

This study investigates phase transformation and mechanical properties of porous NiTi alloys using two different powder compacts (i.e., Ni/Ti and Ni/TiH2) by a conventional press-and-sinter means. The compacted powder mixtures were sintered in vacuum at a final temperature of 1373 K. The phase evolution was performed by in situ neutron diffraction upon sintering and cooling. The predominant phase identified in all the produced porous NiTi alloys after being sintered at 1373 K is B2 NiTi phase with the presence of other minor phases. It is found that dehydrogenation of TiH2 significantly affects the sintering behavior and resultant microstructure. In comparison to the Ni/Ti compact, dehydrogenation occurring in the Ni/TiH2 compact leads to less densification, yet higher chemical homogenization, after high temperature sintering but not in the case of low temperature sintering. Moreover, there is a direct evidence of the eutectoid decomposition of NiTi at ca. 847 and 823 K for Ni/Ti and Ni/TiH2, respectively, during furnace cooling. The static and cyclic stress-strain behaviors of the porous NiTi alloys made from the Ni/Ti and Ni/TiH2 compacts were also investigated. As compared with the Ni/Ti sintered samples, OPEN ACCESSMetals 2015, 5 531 the samplessintered from the Ni/TiH2 compact exhibited a much higher porosity, a higher close-to-total porosity, a larger pore size and lower tensile and compressive fracture strength.

Metals Challenged by Neutron and Synchrotron Radiation

“…Such in situ phase evolutions are presented by Chen et al upon vacuum sintering of Ni-Ti powder compacts [4]. The authors present neutron diffraction patterns of both Ti-Ni and TiH 2 -Ni mixtures taken every 34 s upon a heating ramp to 1373 K, unraveling the appearance and evolution of intermediate and final product phases.…”

“…Already two of the examples presented in Section 2.1 emphasize the importance of titanium and its alloys [4,5]. Much focus on alloys designed for extreme operating conditions is on the titanium aluminide intermetallics, complemented by a nice feature of a Ti-Zr solid solution.…”

Synthesis of As-sintered 60NiTi Parts with a High Open Porosity Level

“…Based on the results published previously on the less Ni-rich NiTi alloys, an increase in the amount of liquid increases the porosity and vice versa 19,21 . In this research, critical processing parameters such as pressing pressure, sintering temperature, heating rate and sintering holding time were investigated in a systematic way to produce parts with approximately 30 Vol.% open porosity.…”

In situ Investigation of Titanium Powder Microwave Sintering by Synchrotron Radiation Computed Tomography

“…However, studies adopting conventional testing techniques have yet to observe microwave sintering in situ because of the experimental limitations of conventional testing techniques, such as optical microscopy, transmission electron microscopy, and scanning electron microscopy. Therefore, in situ and non-destructive testing techniques [8], such as synchrotron radiation computed tomography (SR-CT) [9], should be developed.…”