The potential for superlattice precipitate reinforcement in titanium-iron alloys
In the pursuit of novel structural metallic alloys, the concept of alloys featuring ordered B2 precipitates within a BCC matrix offers a promising possibility due to the potential synergistic benefits of a matrix with superlattice precipitates. The titanium-iron system is selected for this purpose, particularly as both are abundant elements, which should address cost and sustainability concerns. Titanium is an ideal base element owing to its high specific strength and solubility for a range of transition metals, though the BCC phase is metastable at lower temperatures. Additions of Fe serve to stabilise this phase and allow for formation of the desired superlattice phase precipitates. However, the lattice misfit between these phases is large in the binary system, which limits precipitate-matrix coherency, and may prevent full exploitation of the superlattice benefits. In principle, the lattice misfit may be tuned through compositional modification. Consideration must also be given to alloying that ensures sufficient environmental resistance and phase stability. This work explores the possibility of viable titanium-iron-based superlattice precipitate reinforced alloys through the introduction of ternary additions. The phase equilibria in Ti-Fe-X systems were investigated, particularly in the vicinity of the BCC + B2 two-phase field. In Chapter 4, the inclusion of Cu is shown to lead to a BCC + B2 two-phase field that is relatively large in compositional extent, particularly at higher temperatures. Such a result should allow considerable additions of Cu to titanium-iron-based BCC+B2 alloys, which could be particularly beneficial for the mechanical properties. In Chapter 5, the BCC+B2 two-phase field in the Ti-Fe-Co system is found to be limited in compositional extent by a Ti-Co intermetallic phase that has particularly high Fe solubility, significantly limiting the potential addition of Co in BCC+B2 alloys. Investigation of the phase equilibria in the Ti-Fe-Al system in Chapter 6 shows that the intermetallic Ti3Al phase is present in all alloys with an Al content of greater than 5 at.% at 800 °C and below. However, at 1000 °C, the majority of the alloys were in a stable BCC+B2 two-phase field, as desired. Lattice parameter measurements demonstrate that Al significantly reduces the lattice misfit between BCC and B2 phases, whilst Cu increases the lattice parameters of both phases simultaneously, leading to negligible effect on the overall lattice misfit. The effect of Co on the lattice misfit is inconclusive owing to limited experimental investigation of alloys containing a BCC+B2 microstructure. The inherent metastability of the BCC phase in the Ti-Fe-Al alloys leads to a range of nanoscale modulated structures that form by apparent decomposition of the BCC phase, resulting in hierarchical microstructures, and very high hardness. These effects are explored in Chapter 7. The findings reveal opportunities for exploring alternative design concepts, and highlight the versatility of these systems as the basis for novel high-strength metallic alloys.
Lattice misfit, Phase Equilibria, Superalloys, Superlattice precipitates, Titanium alloys
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