Behaviour of novel low-cost blended elemental Ti–5Fe-xAl alloys fabricated via powder metallurgy

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Highlights

  • Ti–5Fe-xAl alloys were developed via the blending elemental powder metallurgy approach.

  • The solid solution strengthening of Al was studied.

  • Ti–5Fe-xAl alloys become progressively stronger and harder with the addition of Al.

  • The thermodynamics of sintering and the microstructural features depend on the Al content.

Abstract

Ti alloys, generally made via wrought metallurgy, are commonly used as biomedical materials. The manufacturing of such alloys via powder metallurgy offers the possibility to reduce the cost as well as to develop innovative compositions not otherwise achievable. The aim of this study is to understand the effect that the progressive addition of Al has on the physical and mechanical behaviour of the low-cost powder metallurgy Ti–5Fe alloy for structural biomedical implants. Specifically, Ti–5Fe-xAl (x = 1–6 w.%) alloys were developed combining blending elemental and cold pressing plus vacuum sintering to further limit the manufacturing costs as Al is lighter and cheaper than Ti. This investigation demonstrates that the amount of Al added significantly changes the thermodynamics of the sintering process and induces microstructural modifications such as grain refinement. These effects jointly with the Al solid solution strengthening leads to progressively stronger and harder (but less ductile) α+β Ti alloys characterised by the typical α+β lamellar microstructure with mechanical behaviour suitable for a variety of structural biomedical implants.

Introduction

Titanium (Ti) and Ti alloys are excellent structural materials for a wide variety of applications in the aerospace, automotive, chemical, shipbuilding, and biomedical fields due to their unique set of material properties (Cui et al., 2011). Among Ti alloys, Ti–6Al–4V is the most widely used commercial Ti alloy, which according to Froes (Froes et al., 2001) accounts for approximately 75% of the global industrial consumption of Ti-based materials. Despite the widespread use of this α+β alloy in numerous applications, the high cost of vanadium as β stabilising alloying element and its cytotoxicity are limiting factors for the applicability of Ti alloys. To address these, a main trend is to replace expensive β stabilisers with cheaper and biocompatible elements still able to stabilise the high temperature BCC β-Ti phase at room temperature to manufacture α+β alloys.

Although casting remains the primary manufacturing route in Ti metallurgy, powder metallurgy (PM) methods are becoming particularly important in industry for the development of cost-affordable wrought-equivalent as well as low-cost Ti alloys (Esteban et al., 2011). Therefore, as reported by Froes et al. (2004), a trend to tackle the challenge of reducing the cost of Ti is the use of alternative manufacturing techniques, including PM which allows to minimise or eliminate drawbacks characteristic of casting such as the high reactivity of titanium with oxygen and the manufacturing tools, the problems associated with the evaporation and segregation of heavy alloying elements, and common casting defects like blowholes and inclusion (Hayama et al., 2014). The key advantages of PM compared to conventional casting are the much lower fabrication costs, the ability to create net-shape components with reduced machining operations (limiting machining costs and generation of expensive scrap), and good surface finishing (Froes et al., 2004).

The use of non-toxic, abundant and biocompatible iron (Fe) as a β stabiliser is a promising approach for the development of new low-cost α+β Ti alloys with the typical lamellar structure due to the high diffusivity of Fe in both the α-Ti and β-Ti phases (Nakajima et al., 1996), which improves the sinterability of Ti alloys (Amherd Hidalgo et al., 2017). Since PM methods are solid state processes which do not involve heating the material to its melting point, segregation of heavy elements (with respect to Ti) such Fe is not a concern which, conversely, is an issue when Ti–Fe alloys are fabricated by casting (Lin et al., 1999). Estaban et al. (Esteban et al., 2011) analysed the processing of low-cost binary Ti–7Fe alloys via press and sinter using difference sources of Fe and identified that finer alloying element particle sizes are preferred to prevent the formation of Kirkendall porosity. Bolzoni et al. (2014a) used different commercial Fe-bearing powders like 430SS to develop low-cost Ti alloys achieving mechanical properties comparable to those of Ti–6Al–4V for an equivalent addition rate of 5 wt% of Fe. Chen et al. (2011) studied the effect of the cooling conditions on the formation of the α-Ti phase in binary Ti–Fe alloys highlighting that the eutectoid reaction of β → α+TiFe does not occur during cooling from sintering in the β field (i.e. 1150 °C).

Most commercial Ti alloys do also contemplate a α stabiliser alloying element in their composition where in the majority of the cases Al is the primary α stabiliser intentionally added to stabilise the HCP α-Ti phase (Koike et al., 2000) as reduction of the density, enhancement of the strength, and increase of the solubility limit of β stabilisers in the α phase are achieved (Jinwen et al., 2014).

Although not completely exploited industrially, ternary Ti alloys made using Al as a α stabiliser and Fe as a β stabiliser have the potential to become ideal structural materials for many applications by combining the best material properties of the two binary Ti–Al and Ti–Fe systems (Kostov et al., 2008). For example, the mechanical properties of the 2nd generation biomedical wrought Ti–5Al-2.5Fe alloy (one of the few commercial Ti alloy including Fe as alloying element) are comparable to those of the Ti–6Al–4V alloy in terms of strength (800–900 MPa) and Young's modulus (110 GPa) (Long and Rack, 1998). Siqueira et al. (2009) studied the microstructural evolution of the Ti–5Al-2.5Fe alloy produced via the blended elemental approach achieving relative density values of 96% and confirming that the use of small alloying elements particle sizes is beneficial to prevent Kirkendall porosity. Jia et al. (2018) investigated the feasibility of manufacturing the Ti–5Al-2.5Fe alloy using reactive induction sintering to further reduce the manufacturing cost proving that blended elemental Ti alloys containing Fe and Al can be successfully produced via PM.

From literature, the combined addition of Fe and Al has previously been assessed, primarily to manufacture the wrought-equivalent Ti–5Al-2.5Fe alloy. No extensive understanding is available on the effect of the addition of Al to binary Ti–Fe alloys produced via the blended elemental PM approach, especially the low-cost Ti–5Fe alloy. However, Sjafrizal et al. (2020) studied the effect of the addition of Fe to the Ti–6Al alloy produced via powder metallurgy obtaining higher strength for higher Fe additions. The main scientific aim of the present study is therefore to investigate the effects and the strengthening mechanisms of the addition of a progressively higher amount of Al on the behaviour of ternary Ti–5Fe-xAl alloys (where x = 1–6 wt%) manufactured by means of a cost-effective PM method to limit the manufacturing costs and assess whether their mechanical behaviour is suitable for structural biomedical implants.

Section snippets

Experimental procedure

The base material for the investigation was a hydride-dehydride pure Ti powder with purity 99.4% and particle size smaller than 75 μm. For the alloying elements, spherical Fe powder with purity 99.0% and particle size lower than 10 μm and atomised Al powder with a purity of 99.7% and maximum particle size of 45 μm were utilised. Fig. 1 shows SEM micrographs of the morphology of the raw powders which are, respectively, irregular and spherical for Ti and Fe and Al as consequence of their

Results

The values of the relative green and sintered density are shown in Fig. 2 where it can be seen that the green density of all the samples is similar (~94.0 ± 1.5%) with slight variations but not a well-defined general trend. Sintering induces the isotropic shrinkage of the sample and results in an increase of the relative density. It can be noticed that the relative density increases for an Al content of up to 3 wt% (reaching 98.7%) and then decreases to 96.1% for the Ti–5Fe–6Al alloy. It is

Discussion

The Ti–5Fe-xAl samples have similar green density values regardless of the Al content. The little differences are thought to be due to the intrinsic variability of the compaction stage and inaccuracy of the manual measurement of the dimensions of the samples rather than a particular effect of the addition of the Al powder as no defined trend is found. It is well recognised that during pressing powders with different morphologies, the compressibility of the compact could be affected. However, in

Conclusions

This study demonstrates the viability of the production of blended elemental powder metallurgy ternary Ti–5Fe-xAl alloys via the cost-effective press and sinter approach. The progressive addition of spherical Al powder particles to the blended elemental Ti–5Fe alloy does not significantly affect the compressibility of the material but has a remarkable effect on the sintering thermodynamics. Consequently, the green density remains fairly constant whilst the density of the sintered alloys

Author statement

YA: Data curation, Writing - original draft. BM: Formal analysis, Investigation. SR: Methodology, Investigation. FY: Methodology. LB: Conceptualization, Funding acquisition, Methodology, Project administration, Supervision, Writing - review & editing.

Data availability

All metadata pertaining to this work will be made available on request.

Declaration of competing interest

The authors declare no competing financial interest.

Acknowledgements

The authors would like to acknowledge the financial support from New Zealand Ministry of Business, Innovation and Employment (MBIE) through the TiTeNZ (Titanium Technologies New Zealand) UOWX1402 research contract.

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