2005 November • JOM 69 Titanium Alloys Research Summary Major advances continue to be made in enhancing patient care while at the same time attempting to slow ever-rising health costs. Among the most innovative of these advances are minimally invasive surgical techniques, which allow patients to undergo life-saving and quality-of-life enhancing surgery with minimized risk and substantially reduced hospital stays. Recently this approach was introduced for orthopedic procedures (e.g., during total hip replacement surgery). In this instance, the implantable devices will bear the same loads and will therefore be subject to higher stress. This paper provides a brief overview of several potential approaches for developing new advanced titanium alloys and processes that should provide substantial benefit for this application in minimally invasive devices. INTRODUCTION Historically, the need for enhanced performance has typically been met through the introduction of new alloys. Often, however, rather simple alloy modifications can result in substantial benefits at a fraction of the expenditure required to design and introduce a new alloy to the marketplace. To some degree, this has been done in the orthopedic com- munity through the wider application of low-iron, grade 4 commercial-purity (CP) titanium. Here, strength is added by increasing the oxygen content of grade 2 CP titanium while maintaining or reducing the residual iron content. Jablowski et al. 1 have recently suggested that this same concept may be applied to other titanium alloys, particularly the recently developed low-modulus meta- stable alloys. Figure 1 illustrates this effect, showing the influence of increased oxygen content on several biomedical alloy mill products in the mill-annealed Advanced Titanium Alloys and Processes for Minimally Invasive Surgery H.J. Rack and Javaid Qazi condition. It should be noted that these results represent the average of yield strength data collected for each oxygen content and ignore minor variances in processing parameters such as rolling temperature, mill anneal temperature, and final bar size, and represent more than 2,000 data sets. As expected, the yield strength increases with increasing oxygen content. Further examination of the influence of oxygen on the yield strength of Ti- 35Nb-7Zr-5Ta is also warranted. At oxygen levels of 0.16% to ~0.38%, Ti- 35Nb-7Zr-5Ta has lower yield strength than all of the alloys other than grade 2 CP titanium and Ti-15Mo, a metastable beta alloy. With increasing oxygen levels of 0.38% to 0.62%, the range in yield strength for Ti-35Nb-7Zr-5Ta is equivalent to the yield strength range of the alpha + beta (Ti-6Al-4V ELI, Ti-6Al-4V, and Ti-6Al-7Nb) and the Ti-12Mo-6Zr-2Fe metastable beta alloy. Finally, at oxygen levels above ~0.62%, the yield strength of Ti-35Nb-7Zr-5Ta exceeds that of all of the other alloys. Not only does this demonstrate that a broad range of yield strengths are achievable for Ti-35Nb-7Zr-5Ta simply by increasing or decreasing the ingot oxygen content, the data also suggests that further efforts aimed at examining the effect of intersti- tial content on the mechanical properties of other biomedical-grade alloys are also justified. THERMAL PROCESSING Contrary to CP titanium and, to some degree, alpha-beta titanium alloys, meta- stable beta alloys offer opportunities to achieve a wide range of strengths, ductilities, and fracture toughnesses. For example, following solution treat- Figure 1. Dependence of average yield strength on the ingot oxygen content for titanium and titanium alloys. 1 ■ ■ ■ ■ ■■ ■ 0.10 0.00 0.20 100 0 200 300 400 500 600 700 800 900 1,000 1,100 Ti-35Nb-7Zr-5Ta Ti-12Mo-6Zr-2Fe Ti-6Al-7Nb Ti-6Al-4V Ti-6Al-4V ELI Ti-CP Gr 2 Ti-15Mo 1,200 0.30 0.40 Ingot Oxygen Content (wt.%) Average 0.2% Yield (MPa) ◆ — Ti-6Al-4V ELI (ASTM F 136); N > 800 ■ — Ti-6Al-7Nb (ASTM F 1295); N > 650 ▲ — Ti-12Mo-6Zr-2Fe (ASTM F 1813); N > 250 ● — Ti-6Al-4V (ASTM F 1472); N > 200 ■ — Ti-15Mo (ASTM F 2066); N > 60 ◆ — Ti CP Gr 2 (ASTM F 67); N > 100 ● — Ti-35Nb-7Zr-5Ta (F 04.12.23); N > 50 0.50 0.60 0.70 0.80 ◆ ◆ ◆ ◆ ● ● ● ● ● ■ ■ ■ ● ● ◆ ● ● ● ● ● ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ▲ ▲ ▲ ▲ ▲ ▲