JMEPEG (1996) 5:335-352 9 International Bulk Deformation of Ti-6.8Mo-4.5Fe-1.5AI (Timetal LCB*) Alloy L Weiss, R. Srinivasan, M. Saqib, N. Stefansson, A.G. Jackson, and S.R. LeClair Recently, a low-cost near-[3 titanium alloy (Timetal LCB Ti-6.8Mo-4.5Fe-I.5AI wt%) containing iron and molybdenum has been developed. This alloy is cold formable in the [3 microstructure and can be aged to high strengths by precipitating the r phase. Due to its combination of cold formability and high strength, the alloy is a potential replacement for steel components in the automotive industry. The current study was undertaken to evaluate the cold bulk forming characteristics of Timetal LCB for use in lightweight automotive applications. Room-temperature compression tests conducted over a strain-rate range of 0.01 to 5/s indicate that the bulk cold compression of the alloy is affected by two factors: the micro- structure and the length-to-diameter aspect ratio of the specimen. In the aged condition, when the micro- structure has r particles distributed along flow lines in the ~phase matrix, the alloy has the propensity for shear failure when deformed in compression in a direction parallel to the flow lines. In the solution-heat-treated condition, the microstructure consists of [3 grains with athermal r phase. In this condition, the alloy can be cold compressed to 75 % reduction in height using specimens with aspect ratio of 1.125, but fails by shear for a larger aspect ratio of 1.5. Plastic deformation of the material occurs in- itially by single slip in most grains, but changes to multiple slip at true plastic strains larger than about 0.15. At a slow strain rate, the deformation is uniform, and the material work hardens continuously. At high strain rates, shear bands develop, and the localized deformation and temperature rise due to defor- mation heating leads to flow softening during compression. Although there is a considerable rise in tem- perature (200 to 500 *C) during deformation, precipitation of the r phase was not observed. Keywords I beta titanium alloys, compression testing, LCB titanium, shear bands, work hardening and softening 1. Introduction TITAN1UM alloys are of growing interest for automotive appli- cations due to the requirements placed on automobile manufac- turers to increase the fuel economy of cars. Titanium alloys have long been used in the aerospace industry because of their high specific strength and stiffness, where cost is a secondary issue in comparison to weight. Similarly, cost is secondary to performance in racing cars, and titanium alloys have been suc- cessfully used for springs and engine valves. In commercial automotive applications--passenger cars and light trucks--ti- tanium alloys must compete with existing materials not only on the basis of weight or performance, but also on the basis of cost. Therefore, both material and processing costs must be mini- mized. The introduction of a new material poses a number of ques- tions and challenges. Can the material outperform currently used materials (i.e., steel)? Can this material be processed by the same methods currently used for steel? If not, what other processing options are available? Are these alternative meth- ods cost effective, and will the parts made with the new mate- rial be affordable? The answer, in the case of titanium alloy components, is that they have the potential to perform better *TIMET, Henderson, NV I. Weiss, R. Srinivasan, M. Saqib, and N. Stefansson, Mechanical and Materials Engineering Department, Wright State University, Day- ton, OH 45435, USA; A.G. Jackson and S.R. LeClair, Wright Labo- ratory-Materials Directorate, WL/MLIM, Wright-Patterson AFB, OH 45433, USA. than steel components in many automotive applications, but are expensive in terms of material costs. The development of low-cost titanium alloys and processing technologies is impor- tant if titanium alloys are to be used extensively in the automo- tive industry. The room-temperature structure of titanium alloys consists of all alpha phase (~ hexagonal close-packed, or hcp, struc- ture), or all beta phase ([3 body-centered cubic, or bcc, struc- ture), or a mixture of alpha and beta phases (or + 1~). If the amount of elements such as vanadium and molybdenum, re- ferred to as [3-stabilizing elements, is greater than a critical amount, the structure at room temperature consists entirely of the 13 phase (Ref 1). Some alloys with [3-stabilizing elements less than this critical amount can be cooled rapidly from above the o~ + [3 to [3-transus temperature to retain a metastable struc- ture of 13 phase at room temperature. These alloys are called the near-[3 alloys. Because the near-[3 alloys have a metastable microstructure, the strength of these alloys can be controlled through heat treatment and precipitation of the a phase (Ref 2). However, due to the microstructural instability of these alloys, their use is limited to below about 250 ~ (480 ~ (Ref 3). Recently, a new low-cost [3-titanium alloy has been devel- oped. Known as Timetal LCB (Ti-4.5Fe-6.8Mo-I.5AI, in weight percent), the alloy uses the [3-stabilizing element mo- lybdenum, added in the form of a ferro-moly compound. In the solution-heat-treated and quenched condition, the alloy has a [3-phase (bcc) structure at room temperature. It thus possesses excellent workability compared to alloys containing the ct phase (hcp). Aging the alloy at a temperature 30 to 100 ~ (55 to 180 ~ below the transus temperature causes the ct phase to precipitate. In this condition, the alloy has a yield strength greater than 900 MPa (130 ksi) and a tensile elongation of about 18% (Ref 4, 5), which compares favorably with high- strength steels. Journal of Materials Engineering and Performance Volume 5(3) June 1996----335