A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 1 1 5 Fig. 1 — Length scale of salient microstructural features in α/β titanium alloys. Microtexture is a feature of titaniumalloymicrostructures that has received considerable renewed attention in the past several years as it impacts materials processing, properties, and inspectability of titanium alloys. This article describes the microstructural evolution processes that result in microtexture formation, while Part II of this series will further elaborate on the effects of microtexture on deformation behavior. Titanium alloys feature an excellent blend of high strength, low density, and good corrosion resistance. It is the fatigue performance, however, that helped titanium find its way into fracture-critical rotating hardware in gas turbine engines. Following the 1989 crash of a DC-10 in Sioux City, Iowa, due to the presence of a melt-related anomaly known as “hard α,” the titanium industry set out to refine melt practices and improve nondestructive inspection methods in order to prevent such an event from occurring again. Fast forward nearly three decades and we can say with confidence that those involved significantly advanced the state of the art in titanium melt practices. Following the event, the Federal Aviation Administration (FAA) convened the Jet Engine Titanium Quality Committee (JETQC), which still exists today. This group is responsible for ensuring that the premium quality titanium that finds its way into gas turbine take equiaxed or elongated morphology, and secondary α or “transformed β,” which can take on either basketweave or colony morphology. As discussed in this article, the mechanism by which MTRs develop results in crystallographically aligned primary α particles, so the solution heat treatment temperature is often selected to be high enough in the α/β phase field that the primary α particles are isolated from one another and separated by the transformed β. While this strategy is generally effective, under certain conditions the secondary α can adopt a similar orientation to its adjacent primary α[3-5]. The causes and implications of such an event are also discussed. The goal of this article series is to bring awareness to this microstructural feature and highlight what the community currently knows about microtexture, draw attention to some gaps in understanding, and highlight opportunities for future research. MICROTEXTURE EVOLUTION The evolution of microtextured regions in near-α and α/β titanium alloys such as Ti-6242 and Ti-6Al-4V is related to industrial thermomechanical processes used to convert large cast ingots (~800-1000 mm in diameter) into semi-finished mill products such as billets, slabs, and plates. Typically, ingots are synthesized via techniques such engines is safe and free from melt-related anomalies. The entire industry shares its experience with such anomalies in the interest of aviation safety. In fact, the industry has cleaned up titanium so much that it is rare to find hard α during the now-routine ultrasonic inspections performed on billets and forgings. With no other hard second phases, inclusions, or pores (when properly processed), fatigue cracks tend to initiate at microstructural weak links. In the case of titanium alloys, these are known as microtextured regions (MTRs) or macrozones. These terms are used interchangeably to describe a feature of the microstructure that is actually an aggregation of the lower length scale microstructural features, specifically when those features share a common crystallographic orientation. These features have long been known to cause issues related to dwell fatigue of near-α alloys[1]. However, it has only recently come to light that the industry workhorse Ti-6Al-4V alloy may also be susceptible under certain combinations of stress, temperature, microstructure, and composition[2]. The salient microstructural features, which span about seven orders of magnitude, are depicted in Fig. 1. Titanium alloys used for rotating components are most often used in the so-called bimodal condition, which consists of a mixture of primary α particles that may
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