Effect of particle size and manufacturing technique on the properties of the PM Ti-5Fe alloy
Introduction
Titanium (Ti) is a strong and light transition non-ferrous metal that exhibits excellent corrosion resistance properties. Ti alloys have several advantages over both other non-ferrous and ferrous materials, including high specific strength, high heat resistance, and outstanding mechanical properties. The maximum working temperature for Ti alloys is approximately 550 °C [1]. Over the past decades, commercially pure (CP) titanium and Ti alloys have been increasingly employed in aerospace, aviation, shipbuilding, and chemical applications [2,3]. Moreover, the biocompatibility of Ti makes Ti-based materials attractive candidates for various biomedical applications [4] and for the development of new metallic biomaterials [5]. Generally, Ti alloys are classified into the categories of α alloys, β alloys [6], and α + β alloys [1]. Dual-phase α + β Ti alloys are generally employed in applications that require good toughness, high strength, excellent fatigue behaviour, and corrosion resistance. The relative volume fraction of the α and β phases, as well as the phase morphology, mainly depends on the composition of the alloy, the manufacturing method and the process conditions [7]. In Ti alloys, the β phase is stabilised by alloying elements that form substitutional solutions and they are classified as: β-isomorphic (Mo, V, Ta, Nb), β-pseudo-isomorphic (Rh, Re, Ru, W, Ir, Os), and β-eutectoid (Fe, Cr, Co, Mn, Ni, Cu, Au, Ag) [8,9]. Even though among all β-eutectoid stabilisers Fe has the highest β-stabilising strength, its use as an alloying element in wrought Ti has generally been avoided, mainly because of the segregation of Fe during the casting process as a result of its relatively high density. Moreover, the Ti-Fe phase diagram indicates the formation of TiFe intermetallic compounds at 1085 °C [10]. Thus, the formation of the eutectic liquid should be avoided as the presence of intermetallics leads to embrittlement of Ti-Fe based alloys. The attention of many researchers has lately focused on the development of low-cost Ti alloys using elemental Fe as a β-stabiliser as Fe is an abundant non-toxic alloying element that can be added to titanium to produce α + β alloys with the typical lamellar structure [[11], [12], [13]]. Since Fe is the cheapest metallic element available, any addition of this metal to the composition of the alloy lowers the cost of the final material. However, the addition of Fe as β-stabiliser has to be carefully controlled if manufacturing of low-cost materials that exhibit mechanical properties suitable for specific engineering applications has to be considered [[14], [15], [16]].
Powder metallurgy (PM) is an effective process for the fabrication of commercial Ti alloys that overcomes the aforementioned limitations of using Fe as alloying element for Ti. The intrinsic solid-state nature of the PM approach permits to prevent the sedimentation of Fe as well as limit the formation kinetics of the TiFe intermetallic compounds [12] because the alloy does not reach the molten state. Apart being cost-effective, PM processing yields the advantage of a high degree of freedom in the selection of microstructural design and alloy composition [17,18]. Specifically, the blended elemental (BE) approach allows designing Fe-bearing low-cost Ti alloys by simply mixing Fe and Ti particles [19]. For example, Wei et al. [17] reported that the addition of Fe enhances the sintering behaviour of Ti alloys, and the most significant sintering shrinkage is achieved upon heating from a temperature of 950 °C to 1200 °C. The accelerated mobility of Ti atoms as a result of rapid diffusion of Fe is a factor explaining the improved sinterability of Ti-Fe alloys [17,20,21]. Chen et al. [22] investigated the influence of the parameters of the cooling process (holding temperatures of 740 °C, 640 °C, 550 °C) on the formation of α phase and the mechanical properties of Ti-3Fe, Ti-5Fe, and Ti-7Fe alloys produced by vacuum sintering at 1150 °C. The samples with higher values of hardness and tensile strength were obtained at holding temperatures of 740 °C and 640 °C. This can be explained by the presence of acicular α precipitates inside the β grains and the higher amounts of stabilised β phase. The strength of Fe as a β-stabiliser is demonstrated by the suppression of the eutectoid reaction at a temperature of 595 °C, as evidenced by the absence of TiFe intermetallic compounds even at slow cooling rates [22].
The present work therefore studies a cost effective way of fabricating the PM Ti5Fe alloy considering two processing routes: pressing plus vacuum sintering, and hot forging in the β-phase field. The work is carried out as a comparative study to understand the effect of the Fe particle size on the physical and mechanical properties on the fabricated alloy.
Section snippets
Starting materials
The morphology, particle size, and chemical composition of the starting powders are shown in Fig. 1 and Table 1, respectively. The CP Ti powder displays an angular shape, which is the result of the hydride/de-hydride (HDH) process. The source of Fe as alloying element to create the Fe-bearing low-cost Ti-5Fe alloy are a Fe carbonyl powder, which was prepared by chemical decomposition of purified pentacarbonyl Fe (labelled as Fec), and a milled powder obtained via milling of electro-refined iron
Physical properties, microstructure and phase identification
The relative green density is 93.8% and 92.5% for Ti-5Fec and Ti-5Fem respectively. The difference in the green density is due to the different particle size of the starting Fe powder particles and their morphology as smaller particles are expected to fit better in between the larger particles resulting in higher packing and, thus, higher green density. As warm pressing was used, both the Ti-5Fec and the Ti-5Fem alloys have higher green density than that of other cold pressed and sintered PM Ti
Conclusions
This study investigated the effect of the processing method and Fe particle size on the microstructure and mechanical properties of the PM Ti-5Fe alloy. Both the processing method and the particle size have an effect, which can be summarised as:
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The particle size of the starting Fe powder has a significant effect on the relative density of the Ti-5Fe alloy, which is maintained even after post-processing via hot forging in the β field.
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Chemically homogeneous materials are always achieved
Acknowledgment
The authors want 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.
Declaration of Competing Interest
The authors declare no competing financial interest.
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