Hot Compression of TC8M-1: Constitutive Equations, Processing Map, and Microstructure Evolution
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INTRODUCTION
Bulk deformation, e.g., forging, rolling, and extrusion, usually involves hot working process at elevated temperatures, where the isothermal hot compression test is often used as a simulation test because of its similarity to the actual environment.[1,2] Characterization of plastic flow behavior, microstructure evolution, or even texture evolution is of vital importance for the optimization with regard to both workability and microstructure development.[3,4] The process of hot compression comprises stable deformation, including dynamic recrystallization (DRX), dynamic recovery (DRV), and super-plasticity (related to grain boundary sliding (GBS) and atomistic diffusion), and flow instabilities, including void formation, shear banding, and micro/macro-cracks.[3] Based on the dynamic materials model (DMM) proposed by Prasad and Sasidhara,[5] the processing map superimposed by the power dissipation map, which identifies the deformation mechanism, and the instability map, which illustrates the safe and unsafe regions during hot working, has been widely used to optimize the processing parameters, including strain, strain rate, and temperature, for the precise control of the microstructure.[6,7] A basic phenomenon that the flow stress usually decreases with the
KE YUE, Ph.D. Student, ZHIYONG CHEN, Associate Professor, and JIANRONG LIU and QINGJIANG WANG, Professors, are with the Division of Titanium Alloys, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. Contact e-mail: [email protected] BO FANG and LIJUN DOU, Senior Engineers, are with the AVIC Shenyang Liming Aero-engine (Group) Corporation, LTD, Shenyang 110016, P.R. China. Manuscript submitted July 8, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
increase of deformation temperature and the decrease of the strain rate exists in a variety of titanium alloys.[2,6,8] With a balance between work hardening and softening under DRX and/or DRV, the true stress–true strain curves reach a steady state eventually.[9] In some materials, a behavior of yielding drop during initial strain was observed on the curves,[10,11] which was rationalized by the rapid generation of mobile dislocations from grain boundary sources, leading to hot deformation proceeding from the grain boundary region inward.[12] In titanium alloys,[4,7] many researchers found that the deformation mechanism is usually dominated by DRX in the upper a/b phase region and by DRV in the b phase region. On the contrary, K. Hua et al.[13] found that for Ti-5553, DRV is the dominant deformation mechanism at low temperature, while DRX dominates during deformation at higher temperature. This may be related to the stacking fault energy (SFE) considering the different structures of alpha and beta phases. A. A. Hameda and L. Blaz´[14] mentioned that DRX may operate during hot deformation in metals and alloys with low SFE, whereas DRV is the dominant softening process with high SFE. TC8M-1 is a promising material for moderate-temperature components in aeronautical applicat
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