Deformation Behavior of a Coarse-Grained Mg-8Al-1.5Ca-0.2Sr Magnesium Alloy at Elevated Temperatures
- PDF / 3,388,810 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 71 Downloads / 255 Views
JMEPEG (2018) 27:905–914 https://doi.org/10.1007/s11665-018-3155-7
Deformation Behavior of a Coarse-Grained Mg-8Al-1.5Ca0.2Sr Magnesium Alloy at Elevated Temperatures Yan Lou and Xiao Liu (Submitted December 24, 2016; in revised form October 28, 2017; published online January 19, 2018) The compression tests were carried out on a coarse-grained Mg-8Al-1.5Ca-0.2Sr magnesium alloy samples at temperatures from 300 to 450 °C and strain rates from 0.001 to 10 s21. The flow stress curves were analyzed using the double-differentiation method, and double minima were detected on the flow curves. The first set of minima is shown to identify the critical strain for twinning, while the second set indicates the critical strain for the initiation of dynamic recrystallization (DRX). Twin variant selection was numerically identified by comprehensive analysis of the Schmid factors for different deformation modes and the accommodation strains imposed on neighboring grains. It was found that twinning is initiated before DRX. Dynamic recrystallization volume increases with strain rate at a given deformation temperature. At high strain rate, various twin variants are activated to accommodate deformation, leading to the formation of twin intersections and high DRX volume. Fully dynamic recrystallized structure can be obtained at both high and low strain rates due to the high mobility of the grain and twin boundaries at the temperature of 400 °C. Keywords
deformation, dynamic recrystallization, grain boundary sliding, magnesium alloy, twinning
1. Introduction It is well known that twinning plays a very important role during deformation of Mg alloys. It is very sensitive to temperature and strain rate (Ref 1). Twins are rarely observed in Mg at temperatures higher than 350 °C and strain rates less than 0.01 s1 (Ref 2-4). However, some researchers recently found that relative high strain rate may lead to twinning above 350 °C. Using optical microscopy, Ishikawa et al. (Ref 5) observed twins in AZ91 at 400 °C and at a high strain rate of 30 s1. Twin bands were observed by Lou et al. (Ref 6) in AZ80 even at 450 °C by TEM at a strain rate of 10 s1. Three types of twins were often detected in Mg alloys, namely extension twin, contraction twin and double twin (Ref 7-11). Extension twinning is an important mechanism of plastic deformation in magnesium alloys. It is able to produce extension along the c-axis during in-plane compression of the textured material. Contraction twinning is the contraction along the c-axes leading to contraction twinning. Double twinning has a particular feature of twin modes with the {1012} twinning in the interior of the primary twin and the preferential alignment of primary twins. Variant selection involves many factors during the formation of mechanical twins in polycrystalline magnesium alloys. The simplest rule is that the twin variants with the highest critical resolved shear stress (CRSS) on their habit planes and along their shear directions will be selected first, according to the values of the macroscopic Sch
Data Loading...
