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
]. 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
]. Interestingly, TiH2
powder was frequently used in NiTi powder sintering in previous studies [4
] due to its cleansing effect of dehydrogenation, which lowers oxygen content and potentially promotes chemical homogenization and densification [18
]. There is no doubt that the use of TiH2
favors final phase homogenization after high temperature sintering in the previous reports [4
]. However, our most recent results [10
] 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
]. 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
] and the binders used in the reports [4
]. Our recent results [17
] 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
]. However, most of the studies are conducted in either argon or air atmosphere [15
]. 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
], 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
]. 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 neutron scattering from hydrogen, neutron diffraction can also track the development of hydrogen concentration during dehydrogenation [20
Since dehydrogenation of TiH2
involving in the reaction procedure of powder sintering, this reactive process is thought to be more intricate and different from the case of Ni/Ti blend. To the best of our knowledge, no report has elaborated the reactive sintering mechanism using Ni/TiH2
blend involving dehydrogenation of TiH2
and the mechanism investigation of TiH2
decomposition under vacuum. Bearing in mind, it is of great importance to investigate the combination of dehydrogenation of TiH2
and newly born Ti and Ni sintering hereafter and the comparative study of mechanical properties of as-fabricated NiTi alloys using Ni/Ti and Ni/TiH2
powder blends. In this study, it is the first time to observe and study the combined phase transformation processes of dehydrogenation of TiH2
and the subsequent reactions between new-born Ti and Ni particles using in situ
neutron diffraction under vacuum as a comparison of the Ni/Ti blend. Further, the systematic mechanical comparison was investigated in terms of pore size, porosity, pore shape and pore size distribution. Therefore, this study is an additional and supplemental report to our recent results in Refs. [17
2. Experimental Section
The mean particle size of Ti, TiH2 and Ni raw powders used in this study was 32.2, 24.6 and 16.4 µm, respectively. Powder mixtures of Ni/Ti and Ni/TiH2 were gently mixed in a ball mill for 10 h. Both powder mixtures had a nominal composition of 51 at.% Ni and 49 at.% Ti.
After mixing, powder mixtures were pressed into cylindrical discs of 12 mm diameter with three heights (i.e.
, 4, 10 and 20 mm for microstructural characterization, neutron diffraction measurement and compression test, respectively) and tensile testing bars (15 mm in gauge length and 2 mm in thickness) in a single-action steel die under 250 MPa pressure. Stearic acid lubricant was slightly applied to the compaction die wall. Subsequently, the 4- and 20-mm-thick green compacts and tensile bars were sintered in a vacuum furnace at 3 × 10−3
Pa, while the 10-mm-thick green compacts were sintered in a high temperature vacuum furnace (5 × 10−4
Pa) equipped on the WOMBAT for in situ
neutron diffraction measurements. The WOMBAT is a high-intensity diffractometer at the Australian Nuclear Science and Technology Organization (ANSTO), which uses monochromatic neutrons and is equipped with a two-dimensional area detector [37
]. The basic technical information of WOMBAT is detailed in Refs. [20
]. The sintering profile with a heating rate of 5 K/min will be shown in Section 3.2
. The heating process was designed into two stages where the first stage is for dehydrogenation of TiH2
powders, while the second one is to perform final sintering at a temperature of 1373 K (1100 °C) for 2 h, followed by furnace cooling.
A free Rietveld program MAUD
was chosen to analyze the full powder-diffraction pattern using the Rietveld method, which is to obtain quantitative values of the phase fractions throughout the in situ
]. To determine the phase fractions, each 1-D diffraction pattern was subsequently fed into the Rietveld analysis as a function of time. The analysis was began with a well-fitted analysis file in MAUD
, which was then used for recursive fitting of the following data files. The batch running was repeated several times with different starting values and constraints to start the iterating process until there was a consistently good fitting throughout the entire run.
Open porosity and sintered density were measured by the Archimedes method as specified in the ASTM B962-08 standard. Pore size distribution analysis was conducted using a pore-size distribution analyzer (GaoQ PDSA-20) using the bubble-point method as per the ASTM F316-03 standard [38
]. Microstructures of the as-sintered compacts were observed using an environmental scanning electron microscope (ESEM, FEI Quanta 200F, FEI, Houston, TX, USA) equipped with an energy dispersive X-ray spectrometer (EDX, Oxford Instruments, Oxfordshire, UK). Phase constituents were determined using X-ray diffraction (XRD, Bruker D2 Phaser, Bruker, Karlsruhe, Germany). Differential scanning calorimetry (DSC, Netzsch 404 F3, Netzsch, Selb, Germany) was used to determine the various reactions of compacts during sintering with a heating rate of 5 K/min under flowing argon gas.
The tensile properties of the as-sintered NiTi tensile bars were measured on an Instron 3367 universal machine with a cross-head speed of 0.5 mm/min at ambient temperature. The tensile bars were tensioned approaching to its fracture strength. The compressive properties of the 20-mm-thick samples after 1373 K sintering were measured on an MTS 810 universal machine with a load rate of 0.6 kN/s at room temperature. An alignment cage ensured the parallelism of all samples during testing. The ends of compression cylindrical samples (machined into 10.5-mm diameter and 15-mm height) were polished and smoothed using sand papers, and finally the ends were greased before compression tests. Cyclic experiments were performed to study possible deformation and superelasticity. The cylindrical samples were first compressed until a significant deflection of the linear elastic deformation portion on the stress-strain curve was obtained or the stress level approached to its fracture strength. After that they were unloaded to zero stress and the subsequent cycle followed.