New grain formation during warm deformation of ferritic stainless steel
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I.
INTRODUCTION
HOT deformation of various metallic materials with low to moderate stacking fault energy (SFE) leads to the evolution of new grain structure, that is, the occurrence of discontinuous dynamic recrystallization (DRX). Its fundamental features, such as plastic flow behaviors and the evolution of DRX grains and their substructures, have been studied in detail.[1,2,3] On the other hand, the evolution of new grains with high-angle boundaries sometimes takes place in higher SFE materials even during cold or warm deformation.[4–7] In this case, the structural changes do not result from DRX and are usually associated with the appearance of high-angle dislocation boundaries, such as microbands or shear bands, which are accompanied with a decrease in the strain hardening.[8–14] There is a limited amount of work on the microstructure evolution of bcc metals during deformation at ambient to moderate temperatures.[4,7–9] Dimek and Blicharski[9] studied the structural changes of a ferritic steel with deformation at ambient temperature and found that the substructures composed of dislocation-rich layers, i.e., the dense dislocation walls (DDWs), were formed in homogeneous cellular structures at an early stage of deformation, and microbands were developed in the initial grain interiors with further straining. Rybin et al.[8] originally termed such structures, including microbands and DDWs, the fragmented structure. The aim of the present work is to study the changes in heterogeneous microstructures with deformation at moderate temperatures and particularly the dynamic processes of new fine grain evolution taking place in a ferritic steel with high SFE. The characteristics of heterogeneous dislocation substructures and the mechanisms operating during such dynamic processes are discussed in detail.
ANDREY BELYAKOV, Research Associate, and TAKU SAKAI, Professor, are with the Department of Mechanical and Control Engineering, the University of Electro-Communications, Chofu, Tokyo 182, Japan. RUSTAM KAIBYSHEV, Professor, is with the Institute for Metals Superplasticity Problems, Khalturina 39, Ufa 450001, Russia. Manuscript submitted June 24, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
EXPERIMENTAL PROCEDURE
The material tested was a hot-rolled ferritic stainless steel, having the following chemical compositions: C0.13, Cr25, Ti0.9, and balance Fe (all in mass pct). Specimens 10 mm in diameter and 12 mm in height were machined parallel to the rolling direction and then annealed at 1523 K for 1 hour, leading to the evolution of an average grain size of 250 mm. Compression tests with no lubricant were conducted at 873 K (0.5 Tm) under strain rates of 1024 to 1022 s21 using Instron 1185 and Schenck RMS-100 universal testing machines. For the observation of deformed structures, the specimens tested were quenched by water jet immediately after warm deformation. Some specimens were cut and polished parallel to the compression axis before testing and were then compressed for the observation of deformation relief.
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