Perspectives on Titanium Science and Technology
Introduction
Recent compilations [1], [2], [3] provide exhaustive descriptions of the science, technology and application of titanium. Over the past 20 years, titanium and titanium alloy production practices have matured more rapidly than perhaps any structural material in the history of metallurgy. Fig. 1 provides examples of the growth in the use of titanium in widely differing applications, and with a range of engineering demands, that in many cases impact human safety and well-being. These highly successful applications have been largely guided by enlightened empiricism covering the practical aspects of Ti ingot melting and processing into mill products, and secondary fabrication. Still, significant variation in material performance requires including an appropriate degree of conservatism where design practices are concerned. The focus on eliminating production defects starting from extraction and melting practice to machining and joining in order to improve the performance and reliability of titanium alloys and to avoid unexpected failures has been unusually high and has produced good results. We note that the major barrier to a still broader application range of Ti and Ti alloy continues to be cost. One large element of the cost of Ti is the necessity to reduce oxides or chlorides of Ti to the metallic form. The current industrial reduction process, devised long ago by Kroll, is energy-intensive and is a batch process. Currently, there are several efforts underway to use alternative reduction methods to produce either metallic Ti or alloyed Ti by co-reduction of mixed chlorides. Several of these methods have been shown to be feasible at the laboratory scale, but scale-up to production quantities is proving more challenging than originally thought.
However, significant gaps in physics-based models that capture the multitude of various phenomena that govern these engineering processes continue to exist even as models evolve. We have attempted to capture in this paper several aspects of the evolving science of titanium, related to its physical metallurgy. We believe that these developments will continue to challenge and inform efforts towards a greater computational and simulation capability supporting the processing–structure–property paradigm of this material system.
Titanium and its alloys derive many of their properties from the metal’s allotropic modifications (Fig. 2a) at 1155 K from the low temperature hexagonal close packed (hcp) (alpha) phase to the high temperature, body centred cubic (bcc) (beta) phase. The ability to manipulate these properties is dependent on the effect of alloying on the stability and physical and mechanical behaviour of these two phases both individually and in a variety of microstructural permutations and combinations. Thus, single-phase alpha alloys are extensively used in applications that are not particularly demanding in terms of strength but focus more on the attractive corrosion resistance of titanium (Fig. 1a). Alternatively, two-phase alpha + beta alloys offer a range of combinations of strength, toughness and high temperature properties that make them attractive in wide ranging aerospace and other products demanding high specific properties to temperatures of ∼873 K (Fig. 1b). The metallurgy of beta alloys enables the development of compositions and processing routes that can satisfy diverse requirements of very high strength with adequate toughness and fatigue resistance required in airframe applications (Fig. 1c) or meeting requirements of low modulus and biocompatibility with shape memory response and fatigue strength for use in biomedical applications (Fig. 1d).
Section snippets
Phase equilibria and phase transformations
Various classes of phase diagrams (Fig. 2b) may be used to illustrate the effect of alloying on titanium: those that show increasing alpha or beta stability with alloying addition or those in which alpha and beta stability remain neutral. Fig. 3 superimposes alpha and beta phase stability for common alloying additions. Within the beta stabilizer category, refractory metals such as Mo and W cause an immiscibility gap in the beta phase, leading to a monotectoid reaction, while the beta
Processing, structure, texture and microtexture
Thermomechanical processing of titanium alloys involves deformation in the alpha, alpha + beta or beta phase fields depending on the alloying content (Fig. 13). The interaction between the flow process and microstructure evolution has been studied extensively [32], [33], [34]. A complex interplay exists between deformation and recrystallization textures of the individual phases, transformation textures arising out of the beta-to-alpha and the reverse alpha-to-beta transformation, and the scale of
Alpha titanium
The most recent determination of elastic moduli of alpha titanium appears to be that of Ogi et al. [46] and these are shown in Fig. 19. The Young’s modulus in the “c” direction is higher than that in the “a” direction. The anomalous behaviour just below the transformation temperature to the beta phase is attributed to premonitory effects associated with the softening of elastic constants that enable the transformation to proceed via the Burgers mechanism. Elastic constants obtained from first
Summary
In the past ∼20 years, Ti alloys have become increasingly important structural materials for high-value, weight-sensitive products. The successes of applying Ti alloys have largely been the result of pragmatic engineering as opposed to bottom-up scientific discovery and application. Nevertheless, Ti alloys are being successfully used in aircraft, aircraft engines and rocket engines, among other products. As product realization cycles become shorter, the benefit of using modelling and simulation
Acknowledgements
One of the authors (DB) is grateful for the support of the Department of Materials Engineering, University of North Texas, and in particular Rajarshi Banerjee, during a substantial part of the writing of this paper.
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