Effect of sintering temperature on the microstructure and mechanical properties of Ti50Ni50 and Ti47Ni47Al6 intermetallic alloys
Introduction
Near equiatomic TiNi intermetallic alloys, known as shape memory alloys, not only present high melting point and good tensile strength, but also exceptionally good ductility even at room temperature [1]. Al addition substituting for Ti or Ni in near equiatomic TiNi alloys can significantly increase the mechanical properties at the room and high temperature [2], [3], [4]. The density of TiNiAl alloys is about 6.3 g/cm3, 20% lower than that of nickel-based, iron-based, cobalt-based superalloys, so, this high specific strength TiNiAl alloys are potential structure materials to replace traditional superalloys for reducing the weight of the components and improving the efficiency.
TiNiAl alloys are generally prepared by casting methods under the vacuum or Ar atmosphere and the melting temperature can be over 2000 °C, resulting in the high production cost. Compared with the casting method, powder metallurgy technology is an effective near-net-shape forming process, which can gain homogeneous microstructure with little segregation of the alloy elements at the lower sintering temperature [5]. In recent study, the microstructure and mechanical properties of TiNiAl alloys with equal Ni/Ti ratio prepared by hollow cathode plasma sintering have been investigated; the sintered Ti47Ni47Al6 alloy have high porosity, which deteriorates the mechanical properties dramatically [6].
A series of complicated reactions occur during the sintering process, which have a great influence on the density and homogeneity of sintered TiNi alloys. Ti-rich liquid phase formation and an associated thermal explosion are possible above 942 °C, contributing to the increase of the porosity in samples sintered above this temperature [7], [8], [9]. Two-step sintering method, which consists of a hold step at around 900 °C followed by higher temperature sintering (i.e., 1100–1150 °C), has been adopted to eliminate the Ti-rich eutectic melting event and improve the sintered density [8], [9]. As sintering parameters, including heat rate and sintering temperature and time, are significant factors in determining the microstructure of the alloys, the effect of sintering temperature on the microstructure and mechanical properties of Ti50Ni50 and Ti47Ni47Al6 alloys sintered by a two-step sintering method was studied in this paper.
Section snippets
Sample preparation
Titanium (99.0%, purity), nickel (99.9%, purity) and aluminium (99.0%, purity) powders with the average size of less than 45 μm, 7 μm, 48 μm, respectively, were employed to prepare Ti50Ni50 and Ti47Ni47Al6 intermetallic alloys. The powders were weighed and put into a jar containing inert gas, then mixing was done with QM-1SP4 planetary ball mill. The ball milling parameters were as follows: ball-to-power mass ratio of 5:1, milling time of 1 h and rotation speed of 200 r/min. The powder was then
Thermal analysis
Fig. 1 illustrates DSC curves for Ti50Ni50 and Ti47Ni47Al6 powders heated at 20 k/min to 700 °C. Only one endothermic peak with an onset of 317 °C appears, indicating the Curie temperature transformation in the pure Ni powders within Ti50Ni50 mixed powders [10]. For Ti47Ni47Al6 powders, a large exothermic peak starting at 642 °C suggests the thermal explosion of Ni and Al, which is ignited by the eutectic reaction (Al + Al3Ni → L) [11], [12]. Zhu and Abbaschian indicated the similar formation
Conclusions
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The microstructure of Ti50Ni50 sintered at 1080 °C consisted of NiTi matrix, strengthening phases Ti2Ni, Ni3Ti and some pores. The porosity and the amount of Ti2Ni of Ti47Ni47Al6 alloy increased, while the amount of Ni3Ti decreased. As the sintering temperature increased to 1180 °C, Ni4Ti3(Al) appeared and distributed mainly along grain boundaries, and the porosity and the size of pores decreased. With the temperature further increasing to 1230 °C, the porosity changed insignificantly, but some
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