Investigation of the mechanisms of reactive sintering and combustion synthesis of NiTi using differential scanning calorimetry and microstructural analysis
Introduction
The fabrication of the Ti–50 at.%Ni intermetallic compound through sintering of pure Ni and Ti powder mixtures has been the subject of several research investigations, some of which date back over 20 years [1], [2], [3]. A number of studies have focused on the production of high-density NiTi [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. More recently an increased interest in the use of NiTi for biomedical applications has lead to investigations of porous NiTi [12], [13], [14]. The need for high levels of porosity in biomedical areas has also led to an increase in the use of combustion synthesis or self-propagating high-temperature synthesis (SHS) to fabricate porous NiTi [15], [16], [17], [18], [19], [20].
The sintering methods used to produce NiTi have included “normal” pressureless sintering in a vacuum or inert Ar gas [2], [3], [5], [7], [12], [13], [14], [15], [17], [20] and sintering under pressure (i.e. hot isostatic pressing or HIPing) [8], [9], [10], [11], [21]. A few recent studies have looked at alternative sintering processes using the application of pressure and current [22], the use of reducing additives [23], [24] and a two-step sintering procedure [6], [25].
Refs. [15], [17], [20] are examples where the thermal explosion (TE) mode of SHS takes place during sintering. TE mode occurs when an exothermic reaction between Ni and Ti, to form NiTi, is initiated by heating during the sintering process (i.e. typically above 950 °C). Other studies represent the plane-wave propagation (PWP) mode of SHS [16], [18], [19]. In this approach an exothermic reaction is initiated at one end of the sample using a local heat source such as electric current. This generates a self-propagating high-temperature reaction through the remaining sample. Therefore, PWP differs from sintering in that only a part of the sample is heated high enough to initiate the exothermic reaction. In many cases PWP samples are also pre-heated to temperatures in the range 200–500 °C prior to the ignition process.
From the literature, the thermal explosion mode of combustion synthesis (TE-SHS) and reactive sintering can be identified as two distinct sintering processes. Unlike TE-SHS, where a strong exothermic reaction occurs during sintering, in reactive sintering, microstructural evolution is more gradual with the progressive formation of intermediate phases including NiTi2, NiTi and Ni3Ti during heating.
Evidence in the literature [2], [3], [5], [7], [12], [13], [14], [15], [17], [20] indicates that the TE-SHS sintering mode results in higher levels of porosity and a higher degree of NiTi formation. Conversely, reactive sintering can lead to higher densities but lower fractions of NiTi formation. Consequently, obtaining high-density, high-volume fraction NiTi during sintering (without the use of external pressure such as HIPing) remains a challenge. Overcoming this challenge requires a more fundamental understanding of the mechanisms of reactive and TE-SHS sintering. In particular, despite the importance of phase formation during the sintering of Ni and Ti to form NiTi it is still not very well understood, and detailed experimental analysis during sintering has been limited. Therefore, a primary purpose of this investigation was to develop an experimental technique using differential scanning calorimetry (DSC) capable of elucidating phase formation during sintering of a Ni + Ti powder compact in order to increase the understanding of the sintering mechanisms in NiTi, including how they impact on density and NiTi conversion.
Section snippets
Experimental procedure
Titanium powder from Alfa Aesar was sieved into the mesh range −170 to +200 (75–90 μm). This powder was mixed with INCO 123 nickel powder (average particle size of 10 μm) to a composition of 50 at.% (i.e. 54.6 wt.%) Ni and subsequently milled for 2 h in a jar containing inert gas. The powders were then uniaxially die-compacted using a force of 1000 kg. With a die pin diameter of 4.80 mm this results in a compaction pressure of 905 MPa. The use of relatively coarse Ti powders has two primary advantages:
Results
The overall DSC trace for the example of heating condition (ii) with a 30 min hold at 900 °C is shown in Fig. 1 in the temperature range of 800–1050 °C. During the initial heating segment up to 900 °C there is a gradual but modest exothermic shift in the DSC trace above about 825 °C. During the isothermal hold at 900 °C the DSC trace decays back to the zero baseline position. Upon the second heating segment up to 1020 °C, the trace obtains a slightly endothermic position until a small endothermic peak
Microstructural evolution during heating condition (i)
The DSC and microstructural results can be understood in reference to the conceptual model of Fig. 9. The Ni–Ti microstructure can be represented by a unit cell consisting of a Ti particle embedded in a Ni matrix. Fig. 9a depicts the microstructure present after a specific hold time at 900 °C. During heating to 900 °C the (α-Ti) transforms to (β-Ti) and the Ti2Ni, NiTi and Ni3Ti intermetallics form and grow. As the figure suggests, and the real micrograph of Fig. 5 indicates, the predominant
Summary and conclusions
During initial heating and holding at 900 °C of a pure Ni + pure Ti powder mixture, the α-Ti transforms to β-Ti and the gradual growth of Ti2Ni, NiTi and Ni3Ti intermetallics occurs at the Ni/Ti contact points. The Ti particles, which transform to (β-Ti) at higher temperatures, quickly obtain a Ni content close to their maximum solubility at 900 °C. Conversely, the Ni particles obtain a Ti content of about 2 at.% Ti, well below the maximum solubility of Ti in Ni at 900 °C. Therefore, diffusion of Ni
Acknowledgements
The authors would like to thank Emerging Materials Knowledge (EMK), CVRD-INCO Ltd., and the National Sciences and Engineering Research Council (NSERC) of Canada for their support of this work.
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