Mechanism of Martensitic to Equiaxed Microstructure Evolution during Hot Deformation of a Near-Alpha Ti Alloy
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INTRODUCTION
AMONG the various materials which are candidates for high-temperature applications in compressor of turbojet engines, near-alpha titanium alloys are the best choices for their high strength-to-weight ratio, good corrosion resistance, and higher temperature capability.[1] A series of hot working and heat treatment steps are usually performed on melted ingot to prepare titanium mill products. In order to reach a homogenized microstructure, ingot breakdown is usually being done within temperature ranges where beta is the stable phase.[2] Alpha laths/platelets with a high aspect ratio are formed within each beta grain during cooling. It is well accepted that in titanium alloys phase transformation obeyed Burger orientation relationship between beta (b) and alpha (a) phases.[3] In other words, it is expected that during the phase transformation of these alloys, the close-packed plane of the parent phase would be parallel to that of the transformed product (i.e., {110}bi(0001)a and h111ib k h1120ia ). Twelve different orientation variants of a can nucleate from each
SEYED AMIR ARSALAN SHAMS and SHAMSODDIN MIRDAMADI are with the School of Metallurgy and Materials Engineering, Iran University of Science and Technology (IUST), Tehran 16846-13114, Iran. SEYED MAHDI ABBASI is with the Metallic Materials Research Center (MMRC), Maleke Ashtar University of Technology, Tehran 16788-15611, Iran. DAEHWAN KIM and CHONG SOO LEE are with the Graduate Institute of Ferrous Technology (GIFT), Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea. Contact e-mail: [email protected] Manuscript submitted July 27, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
beta grain. Cooling rates define the thickness of the transformed product; i.e., martensite is achieved by high cooling rates, while lamellar is the product of slow cooling rates. The equiaxed alpha microstructure has higher ductility and strength at room temperature[4] with good super-plasticity at higher temperatures.[5] Equiaxed alpha particles will be produced, if secondary hot working is applied to initial lamellar or martensitic microstructure below beta transus (temperature at which alpha + beta M beta). Different mechanisms have been proposed to reach a spheroidized alpha.[6–8] Globularization, a two-step mechanism, which consists of boundary splitting and termination migration, is active when the initial microstructure is lamellar. Boundary splitting starts from an internal boundary through the thickness. This boundary may be derived from the formation of a recovered substructure[6] or intense localized shear during hot deformation.[7] Lamella kinking is also one of the causes of globularization.[8] Sub-grain formation brings about an unstable dihedral angle (90 deg) in alpha lamella. Diffusion of beta as an inter-lamellar phase into the boundaries will make a reduction in dihedral angle and finally results in a groove in alpha lamellae. Termination migration is the process in which mass is transferred from the curved to the
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