The effect of Si additions on the sintering and sintered microstructure and mechanical properties of Ti–3Ni alloy

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Abstract

Thermodynamic predictions suggest that silicon has the potential to be a potent sintering aid for Ti–Ni alloys because small additions of Si lower the solidus of Ti–Ni alloys appreciably (>200 °C by 1 wt.% Si). A systematic study has been made of the effect of Si on the sintering of a Ti–3Ni alloy at 1300 °C. The sintered density increased from 91.8% theoretical density (TD) to 99.2%TD with increasing Si from 0% to 2%. Microstructural examination reveals that coarse particles and/or continuous networks of Ti5Si3 form along grain boundaries when the addition of Si exceeds 1%. The grain boundary Ti5Si3 phase leads to predominantly intergranular fracture and therefore a sharp decrease in ductility concomitant with increased tensile strengths. The optimum addition of Si is proposed to be ≤1%. Dilatometry experiments reveal different shrinkage behaviours with respect to different Si contents. Interrupted differential scanning calorimetry (DSC) experiments and corresponding X-ray diffraction (XRD) analyses clarify the sequence of phase formation during heating. The results provide a useful basis for powder metallurgy (PM) Ti alloy design with Si.

Highlights

Silicon is a potent sintering aid for Ti–Ni alloys revealed by predictions and confirmed by experiments. ► The addition of Si should be limited to ≤1% to avoid coarse Ti5Si3 phase and ensure good ductility. ► Liquid forms during heating at ∼988 °C due to reactions between Si and Ni and Ni and Ti. ► Silicon can be a unique addition to PM Ti alloys for significantly improved mechanical properties.

Introduction

Nickel is a fast diffuser in Ti; its diffusion rate is about two orders of magnitude faster than the self-diffusion of Ti in the beta region [1]. Empirical rules of diffusion suggest that the addition of a faster diffusing species as a solute enhances the self-diffusion rates of both the solvent and the solute atoms [2]. Ni is thus a desired alloying addition to Ti alloys for solid state sintering. A small number of researchers, beginning with Kroll [3], have sintered Ti–Ni alloys with the Ni content being varied from 2% to 12% (in wt.% throughout) [3], [4], [5], [6], [7], [8]. It was suggested by Gonser [9] that the Ni content should be limited to 6–8%Ni for acceptable ductility. Recent work by the current authors on the sintering of Ti–7Ni alloy indicate that as-sintered Ti–7Ni show poor ductility with tensile elongation <2% due to the formation of the brittle eutectoid Ti2Ni (the eutectic point occurs at ∼7% Ni) [10]. To substantially reduce the eutectoid formation for improved ductility, the selection of a largely hypo-eutectoid Ti–Ni alloy is necessary. The Ti–3Ni alloy was therefore chosen based on some preliminary experiments.

Powder metallurgy (PM) by conventional cold compaction and sintering offers an attractive route to the near-net shape or preform fabrication of titanium (Ti) and its alloys for cost reduction and/or improved constitutional and microstructural capabilities [11]. However, preliminary experiments conducted have indicated that Ti–3Ni can only be sintered to ∼92% theoretical density (TD) at 1300 °C after 120 min (−100 mesh Ti powder; compaction pressure: 400 MPa). To attain full densification, it is necessary to explore the use of a third alloying element as a sintering aid. Silicon has a much lower density than Ti (2.33 g/cm3 for Si vs. 4.54 g/cm3 for Ti) and is inexpensive. In addition, small additions of Si (≤0.5%) have been introduced to a number of commercial Ti alloys (such as TIMETAL 62S, TIMETAL 1100, IMI 685) to improve their creep and oxidation resistance [12], [13], [14]. Intriguingly, preliminary thermodynamic predictions have revealed that Si has the potential to be a potent sintering aid for Ti–Ni alloys because small additions of Si are predicted to significantly lower the solidus of Ti–Ni alloys.

This work investigates the effect of Si additions on the sintered density, microstructure and mechanical properties of Ti–3Ni, beginning with a detailed thermodynamic assessment. It is shown that small additions of Si can make a significant difference. The results provide a useful basis for the exploitation of Si for powder metallurgy Ti alloy design.

Section snippets

Experimental procedure

Hydride–dehydride Ti powder (Sumitomo −100 mesh, 99.5% purity with O  0.25%; N  0.03%; Cl  0.04%; Fe  0.02% and C  0.02%, all in wt.%), elemental Ni powder (≤45 μm, 99.5% purity, CERAC Inc.) and Si powder (≤45 μm, 99.5% purity, CERAC Inc.), were used. Blends of Ti–3Ni containing Si additions from 0% to 4% were prepared in a Turbula mixer for 30 min. The blends were compacted uniaxially in a floating die into cylinders (10 mm in both diameter and height) at 400 MPa for sintering experiments, and into

Thermodynamic predictions of the effect of Si

Fig. 1a shows the Thermo-Calc predictions of the solid–liquid equilibrium mole fractions for Ti–3Ni–(0–4)Si over the temperatures of interest. In the context of sintering, we are mainly interested in the variations of the solidus temperature arising from the presence of Si. The predictions reveal that Si shows high potency for lowering the solidus of Ti–3Ni. The solidus temperature declines from 1462 °C to 1248 °C by 1%Si and further to ∼1000 °C when the addition of Si is increased to 2%. As a

Enhanced sintering of Ti–3Ni alloy by Si

The sintering temperature used (1300 °C) is well below the solidus of Ti–3Ni (1462 °C). Accordingly, the isothermal sintering of Ti–3Ni at 1300 °C should occur in the solid state. The low sintered density obtained after 120 min at 1300 °C (91.8%TD) is consistent with solid state sintering. As noted previously, Ni is a fast diffuser in Ti; it enhances the self-diffusion rate of Ti. An experimental study of the sintering of Ti–2.4Ni (2 at.% Ni) and Ti–6Ni (5 at.% Ni) alloys confirmed that Ni reduces

Summary

Si is an effective sintering aid for Ti–3Ni alloy. An addition of 1%Si increases the sintered density of Ti–3Ni at 1300 °C from 91.8% TD to 96.4% TD. The sintered density increases with increasing Si content and peaks at 99.2% TD with 2% Si.

The optimum Si content should be limited to 1%, at which both the ultimate tensile strength and yield strength of as-sintered Ti–3Ni–1Si are increased by ∼20% while the tensile elongation remains almost unchanged compared with as-sintered Ti–3Ni. However,

Acknowledgements

The work was funded by the Australian Research Council (ARC) through the Centre of Excellence for Design in Light Metals and an ARC Discovery grant (Australian Postdoctoral Fellowship). Constructive comments and useful suggestions from the reviewers are acknowledged.

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