An in situ Study of NiTi Powder Sintering Using Neutron Diffraction

Gang Chen, Klaus-Dieter Liss, Peng Cao
2015 Metals  
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
more » ... 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 ACCESS Metals 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. Introduction NiTi alloys have excellent properties including unique shape memory effect (SME), superelasticity, good biocompatibility and great energy absorption, which have been attracting attention from multiple areas such as medical devices, energy absorbers, actuators and mechanical couplings [1, 2] . Powder metallurgy (PM) is a simple, energy-saving and widely used route to produce NiTi alloys [3] . Additionally, powder sintering is an effective technique to produce various porous structures, which are beneficial to bone tissue ingrowth and also provide an effective way of reducing stiffness of the implant [4] . Elemental powder sintering to fabricate porous NiTi alloys has been tremendously successful recently [4] [5] [6] [7] [8] [9] [10] . Interestingly, TiH2 powder was frequently used in NiTi powder sintering in previous studies [4,10-17] due to its cleansing effect of dehydrogenation, which lowers oxygen content and potentially promotes chemical homogenization and densification [18, 19] . There is no doubt that the use of TiH2 favors final phase homogenization after high temperature sintering in the previous reports [4,10-17]. However, our most recent results [10, 17, 20, 21] and the report from Robertson and Schaffer [14] disclosed a discouraging densification and a much larger porosity when using TiH2 powder. As such, the use of such powder cannot guarantee densification promotion in all NiTi studies, although it does show densification in some other alloys, e.g., pure Ti, Ti-6Al-4V, Ti-5Al-2.5Fe and TiAl [19, [22] [23] [24] [25] [26] [27] . This might be caused by other factors simultaneously affecting the sintering process and thus the densification. These factors include TiH2 particle size in Refs. [11, 12, 28, 29] and the binders used in the reports [4, 16] . Our recent results [17, 20, 21] also pointed out that it is the dehydrogenation of TiH2 powder that increased the porosity of sample and then hindered its densification, when compared with that using similar particle size of Ti powder. The process of TiH2 dehydrogenation has been studied for many years [17, 19, 20, 25, 27, [30] [31] [32] [33] [34] [35] [36] . However, most of the studies are conducted in either argon or air atmosphere [15, 19, 32, 33, 35] . With respect to the atmosphere, the dehydrogenation usually takes place in the temperature range from 523 to 973 K (250 to 700 °C), which possibly causes the concern of TiH2 oxidation. On the other hand, some studies, e.g., Refs. [31, 34] , were performed in vacuum, effectively avoiding the oxidation issue. In spite of this, the diffraction instrument used is laboratory low-intensity X-ray diffraction systems [34] , which normally require several minutes to one hour to achieve a complete scan for phase analysis and the achieved data is normally semi-accurate. Such "long"-time scanning properly leads to delayed or missing information. These technical limitations can be tackled with high-energy neutron diffraction under vacuum, which is able to penetrate bulk metals, and this type of diffraction has been successfully employed for in situ studies for sintering mechanism and reactions [20, 36] . The beam intensities allow information from bulk material to be followed on short time scales (less than 60 s), while undergoing an in situ heating/cooling cycle to observe phase transformations. Furthermore, due to the strong incoherent
doi:10.3390/met5020530 fatcat:jceojyijvna4hltmbcf5mgwyzi