Thermomechanical processing (TMP) is associated with two major requirements: (i) to produce usable shapes through primary working (ingot breakdown) and secondary mill operations (hot rolling or forging) and (ii) to optimize mechanical properties through microstructure control during the different stages of the thermomechanical process. This paper reviews the thermomechanical processing of beta titanium alloys in general and high temperature deformation mechanisms, microstructure control during TMP, and final mechanical properties in particular.

Thermomechanical processing of beta titanium alloys—an overview

Beta titanium alloys were recognized as a distinct materials class in the 1950s, and following the introduction of Ti-13V-11Cr-3Al in the early 1960s, intensive research occurred for decades thereafter. By the 1980s, dozens of compositions had been explored and sufficient work had been accomplished to warrant the first major conference in 1983. Metallurgists of the time recognized beta alloys as highly versatile and capable of remarkable property development at much lower component weights than steels, coupled with excellent corrosion resistance. Although alloys such as Ti-15V-3Al-3Sn-3Cr, Ti-10V-2Fe-3Al and Ti-3AI-8V-6Cr-4Mo-4Zr (Beta C) were commercialized into well-known airframe systems by the 1980s, Ti-13V-11Cr-3Al was largely discarded following extensive employment on the SR-71 Blackbird. The 1990s saw the implementation of specialty beta alloys such as Beta 21S and Alloy C, in large part for their chemical and oxidation resistance. It was also predicted that by the 1990s, cost would be the major limitation on expansion into new applications. This turned out to be true and is part of the reason for some stagnation in commercialization of new such compositions over the past two decades, despite a good understanding of the relationships among chemistry, processing, and performance and some very attractive offerings. Since then, only a single additional metastable beta alloy, Ti-5Al-5V-5Mo-3Cr-0.5Fe, has been commercialized in aerospace, although low volumes of other chemistries have found a place in the biomedical implant market. This article examines the evolution of this important class of materials and the current status in airframe applications. It speculates on challenges for expanding their use.

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State of the Art in Beta Titanium Alloys for Airframe Applications

Effects of ω phase precipitation on β→α, α′′ transformations in a metastable β titanium alloy have been studied by means of transmission electron microscopy. Isothermal holding at 300°C for 100 min after the solution-treatment at 1000°C yielded homogeneously distributed spherical ω phase particles of ∼10 nm in diameter. With raising the holding temperature, ω phase particles of the same orientation variant precipitated as groups. At temperatures above 350°C, α phase laths nucleated at the ω/β interfaces and grew into both the matrix and ω particles with consuming ω phase particles. Thus, the α/ω Takemoto and the ω/β Burgers relationships were held simultaneously, and the orientation of α laths nucleating at an ω/β interface was determined uniquely. In the specimens quenched directly from β single-phase region, α′′ martensite plates nucleated preferentially at β grain boundaries. The α′′ formation, however, was accelerated by a short time holding at temperatures between 600 and 300°C, but was largely retarded by the progress of ω phase precipitation.

Effects of ω-phase precipitation on β→α, α′′ transformations in a metastable β titanium alloy

Lightweighting of automobiles by use of novel low-cost, high strength-to-weight ratio structural materials can reduce the consumption of fossil fuels and in turn CO2 emission. Working towards this goal we achieved high strength in a low cost β-titanium alloy, Ti-1Al-8V-5Fe (Ti185), by hierarchical nanostructure consisting of homogenous distribution of micron-scale and nanoscale α-phase precipitates within the β-phase matrix. The sequence of phase transformation leading to this hierarchical nanostructure is explored using electron microscopy and atom probe tomography. Our results suggest that the high number density of nanoscale α-phase precipitates in the β-phase matrix is due to ω assisted nucleation of α resulting in high tensile strength, greater than any current commercial titanium alloy. Thus hierarchical nanostructured Ti185 serves as an excellent candidate for replacing costlier titanium alloys and other structural alloys for cost-effective lightweighting applications.

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A low-cost hierarchical nanostructured beta-titanium alloy with high strength

Titanium and titanium alloys are excellent candidates for aerospace applications due to their high strength to weight ratio and excellent corrosion resistance; titanium and its alloys are immune to almost every medium to which they would be exposed in an aerospace environment. Titanium usage is, however, strongly limited by its higher cost relative to competing materials, primarily aluminum alloys and steels. Hence the advantages to using titanium must be balanced against its added cost. The titanium alloys used for aerospace applications, some of the characteristics of these alloys, the rationale for utilizing them, and some specific applications of different types of actual usage will be discussed herein. This is an extension of References 1 and 2, which reviewed applicaiions of β alloys. These references will provide more details on applications of the β alloys, while the α and α/β alloys are added herein.

Titanium for aerospace: Rationale and applications

In the /3-Ti alloy Ti-10V-2Fe-3Al a variety ofα-andω-aged microstructures with different yield stresses was established by combinations of forging and heat treatment. Tensile tests have shown that plastic deformation and fracture are strongly influenced by the morphology, size, and volume fraction of the different types of a-phase (primary a, secondaryα, grain boundaryα), as well as by the-phase. A detailed microscopical study revealed several deformation and fracture modes. It appears that at several sites stress and strain concentrations and subsequent void nucleation can occur and that the quantitative combinations of the differentα-types determine which sites are active. The dominant deformation mode for the (α +gb) solution treated andα-aged conditions was a strain localization in theα-aged matrix leading to voids at the interface between aged matrix and primary a-phase. In case of theβ-solution treated andα-aged microstructures the grain boundaryα leads to a strain localization in the softα-film and to void nucleation at grain boundary triple points at low macroscopic strains. Based on the above mechanisms it is discussed in detail how varying size, volume fraction, and morphology of theα-phase affect the ductility. The embrittling effect ofω-particles can be largely reduced by a grain refinement.

Microstructure, tensile deformation, and fracture in aged ti 10V-2Fe-3Al

Tensile properties and Fatigue Crack Propagation (FCP) rates have been measured in laboratory air for a wide variety of different Ti-10-2-3 microstructures, all having nominal yield strengths of 1240 MPa. These microstructural variations included both recrystallized and unrecrystallized conditions, as well as varying amounts of primary α, both α- and Ω-aged conditions, and also several hot forming histories. With one exception, microstructure was found to have very little or no influence on FCP. Not only was the rate of FCP unaffected, but moreover, the macroscopic fracture surface roughness was largely unaltered. The one exception was a notable one: the Ω-aged condition. Strengthening through the precipitation of Ω instead of α was found to retard FCP significantly; no significant change in macroscopic fracture surface roughness was found, however. The retardation was attributed to changes in slip character and reversibility.

T. W. Duerig

Papers

Microstructure, tensile deformation, and fracture in aged ti 10V-2Fe-3Al

Microstructural influences on fatigue crack propagation in Ti-10V-2Fe-3Al