Superplasticity evaluation in an extruded Mg–8.5Li alloy

Abstract

With the remarkable feature of a combination of ultra lightness and improved formability at room temperature, magnesium–lithium (Mg–Li) base alloys are deserving increased attention among the high-performance Mg alloys for either academic or technical interest [1, 2]. It has been reported that Mg–Li base alloys in the eutectic composition range, with their microstructure consisting of two phases of hexagonal a (Mg-rich phase) and centered cubic b (Li-rich phase), can produce superplasticity under appropriate thermo-mechanical processing and testing conditions [3–7]. But to date, only few of them were found to show large elongation at high strain rates more than 10 s –1 [8]. Similar to aluminum alloys and other metal-based materials, developing high strain rate superplasticity in magnesium alloys would be of benefit for their commercial forming applications [9]. An attempt to obtain superplasticity in a Mg–8.5Li alloy prepared by stir casting and high ratio extrusion has been made in this work and the results will be introduced herewith. The composition of the studied binary Mg–Li alloy was selected as Mg–8.5Li (wt. %) which is within the typical eutectic range and has been proved to make a (a + b) twophase mixture microstructure [10, 11]. The alloy was cast by a stirring route followed by high ratio (100:1) extrusion at 623 K. Round tension specimens with a 2.5 mm diameter and a 15 mm gage length were directly made from the extruded rods of 4 mm in diameter. Tension test specimens were deformed at initial strain rates ranging from 9 · 10 to 9 · 10 sand at 573 K, 623 K and 673 K, respectively. Microstructure of the specimens before and after deformation was examined by optical microscopy. Superplastic characteristics of the Mg–8.5Li alloy at the examined temperatures are shown in Fig. 1. As seen from Fig. 1 (a), the flow stress of the Mg–8.5Li alloy increases as the strain rate increases or as the testing temperature decreases. The strain rate sensitivity exponent, m value, is about 0.40 at 623 ~ 673 K at strain rates ranging from 10 to 10 s, and decreases to 0.20 at the strain rate higher than 10 s .The variation of total elongation in the Mg–8.5Li alloy with strain rate is illustrated at Fig. 1 (b). At the same strain rate, the Mg–8.5Li alloy exhibits higher an elongation value at 623 K that at 573 K and 673 K. A maximum elongation of more than 500% is obtained at the strain rate of 1 · 10 s at 623 K, indicating that the studied Mg–8.5Li alloy has the potential to produce superplasticity at higher strain rate. Typical deformed specimens along with the tension specimen before test are shown in Fig. 2. The superplastic behaviors reported to date in the Mg–Li based alloys depend much on the fabrication and processing history and the resulting microstructure of the alloys, and almost all involve a rolling process [3–6]. The Mg-8.5– 9%Li laminates prepared by a foil metallurgy route exhibited a larger elongation of 450% ~ 460% at initial S. L. Dong School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China