Abstract
This work identifies microstructural conversion mechanisms during hot deformation (at temperatures ranging from 750 °C to 1050 °C and strain rates ranging from 10−3 s−1 to 1 s−1) of a Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti-17) alloy with a lamellar starting microstructure and establishes constitutive formulae for predicting the microstructural evolution using finite-element analysis. In the α phase, lamellae kinking is the dominant mode in the higher strain rate region and dynamic globularization frequently occurs at higher temperatures. In the β phase, continuous dynamic recrystallization is the dominant mode below the transition temperature, Tβ (880~890 °C). Dynamic recovery tends to be more active at conditions of lower strain rates and higher temperatures. At temperatures above Tβ, continuous dynamic recrystallization of the β phase frequently occurs, especially in the lower strain rate region. A set of constitutive equations modeling the microstructural evolution and processing map characteristic are established by optimizing the experimental data and were later implemented in the DEFORM-3D software package. There is a satisfactory agreement between the experimental and simulated results, indicating that the established series of constitutive models can be used to reliably predict the properties of a Ti-17 alloy after forging in the (α+β) region.
Highlights
Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti-17) alloy, a near-β-type (α+β) alloy, was first developed by GE Aviation [1]
Unlike discontinuous dynamic recrystallization (DDRX), CDRX occurs in a manner in which subgrains with low-angle boundaries are formed; these subgrains subsequently evolve into grains with a large fraction of high angle boundaries upon increasing the plastic strain
The microstructure was analyzed with a field emission scanning electron microscope (FE-SEM) fitted with an electron back-scattering diffraction (EBSD) analyzer equipped with the HKL Channel 5 software
Summary
Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti-17) alloy, a near-β-type (α+β) alloy, was first developed by GE Aviation [1]. During the hot deformation (low temperatures ranging from 750 °C to 900 °C) of a Ti-6Al-4 V alloy having a lamellar (α+β) starting microstructure, deformation proceeds with dynamic globularization, prior β-boundary cracking, lamellae kinking, and adiabatic. The nucleation sites for globularization are at the kinks in the lamellae as well as at some of the prior β grain boundaries This type of microstructural change involving a reduction in the aspect ratio of the plates is interpreted in terms of geometric dynamic recrystallization, as proposed by McQueen et al [8,9]. The present work focuses on the microstructural predictions (dynamic globularization behavior and change in grain size) and modeling of a processing map characteristic for the forging of a Ti-17 alloy with a lamellar (α+β) starting microstructure. Constitutive formulae were established on the basis of the experimental results and implemented in the finite-element method (FEM) software (DEFORM-3D, v.10.2)
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