Elsevier

Materials & Design

Volume 90, 15 January 2016, Pages 157-164
Materials & Design

Microstructure and deformation behaviors of two Mg–Li dual-phase alloys with an increasing tensile speed

https://doi.org/10.1016/j.matdes.2015.10.111Get rights and content

Highlights

  • Mechanical properties of Mg–8.02Li–0.210Y alloy were more sensitive to the tensile speed.

  • Fracture occurred mainly in β-Li phase.

  • Long-stripe-like α-Mg grains hindered the fracture and led to higher elongations.

  • Serrated effect in Mg–8.02Li–0.210Y alloy related to slips and α-Mg grains

Abstract

Two duplex Mg–Li alloys, Mg–8.02Li–0.210Y and Mg–8.43Li–0.353Ymm (Y-rich mischmetal) alloys, were prepared by casting method with a permanent metal model. Microstructures, mechanical properties and deformation behaviors at tensile rates from 1.28 × 10 4 s 1 to 1.28 × 10 3 s 1 were evaluated. The results showed that when increasing the tensile speed, the strength increased with a decreasing elongation. The strength and ductility of Mg–8.02Li–0.210Y alloy were more sensitive to the tensile speed. Fracture occurred mainly in the β-Li phase, and the α-Mg phase participated into fracture at relatively high tensile speeds, which led to the strength increase of both alloys with the increment of tensile speed. Long-stripe-like α-Mg grains in Mg–8.02Li–0.210Y alloy hindered the fracture and resulted in a higher elongation. Mg–8.02Li–0.210Y alloy presented a serrated effect at a relatively high initial rate of 1.28 × 10 3 s 1. This was proposed to be resulted from the interaction between slips and α-Mg grains, which was quite different from the serrated effect explained by the dynamic strain aging (DSA) mechanism.

Introduction

Due to their low density and good performances, Mg alloys are drawing increasing attentions in both academic and engineering areas, since they provide potentials weight reducing and energy economy in transportation, and also prosperities in applications in electronic areas [1], [2]. When Li, a metal with an ultra-low density of 0.534 g·cm 3, is added to Mg and Mg alloys, Mg–Li alloys are achieved as the lightest metallic engineering materials with high specific strength and high rigidity. Mg–Li alloys with Li content lower than 5.3 wt.% and higher than 10.7 wt.% are of single α-Mg (hexagonal-closed-packed, HCP) and β-Li (body-center-cubic, BCC) phases, respectively [3]. Li addition between 5.3 wt.% and 10.7 wt.% makes Mg–Li alloys to be dual phase alloys with both α-Mg and β-Li phases [3]. The addition of Li decreases the axial ratio (c/a) of the hexagonal Mg lattice, and also enables Mg–Li alloys, with or without β-Li phase, to be more ductile than pure Mg and other Mg alloys [4].

In the latest years, alloying and mechanical procedures are conducted to further improve the mechanical properties of Mg–Li alloys [5], [6], [7], [8], [9], [10], and some of these alloys are enhanced even to be superplastic [8]. Recently, researches on Mg–Li alloys are also focused on newly summoned icosahedral quasicrystal phase to use as strengthening materials and various properties [11], [12], [13], [14]. Among these researches, the Y element is used as one of the alloying elements and the duplex Mg–Li–Y alloys present to possess high plasticity. However, most of the present works on duplex Mg–Li alloys focus on the Y content of more than 1 wt.% [8], [9], [10]. As a rare earth (RE) element, Y is of high price because of the lack of resource and the high cost to acquire. It is latently beneficial to study Mg–Li dual phase alloys with a low fraction of Y element. Ymm (Y-rich mischmetal) is of lower price than Y element itself because of the lack of purifying as well as the lowering of Y content.

In the engineering area, a high processing speed means a high producing efficiency. For materials, ductility is related to the processing speed, similar to tensile speed in tensile tests. Usually, alloys present a high ductility in relatively low tensile speed. It was reported that duplex Mg–8Li–1Y alloy can show superplasticity even at a relatively high tensile speed [8]. Also, Mg–Li dual phase alloys are considered to be potentially processed at ambient temperatures to shorten the cost of processing because of the saving of fuel for heating [15], [16], [17]. However, the deformation and fracture behaviors of Mg–Li dual phase alloys are not fully uncovered. This hinders the improvement of mechanical properties of duplex Mg–Li alloys, especially at ambient temperatures.

On the other hand, it is shown that several Mg alloys [18], [19], including two Mg–Li alloys of single α-Mg phase [20], [21] and a duplex Mg–5Li–2Zn alloy with a small fraction of β-Li phase [22], incline to present a serrated strain–stress curve, also called the Portevin-Le Chatelier (PLC) effect, which is believed to be negative to engineering applications of these alloys. This work seeks to study the effects of initial tensile rates, from 1.28 × 10 4 s 1 to 1.28 × 10 3 s 1, on microstructures, mechanical and deformation properties of two Mg–Li dual phase alloys with small amount of Y or Ymm.

Section snippets

Experimental procedures

One of the studied alloys was a Ymm containing alloy, Mg–8.43Li–0.353Ymm alloy, once used in a previous study [10]. The studied alloys were prepared from raw materials of commercial pure Mg (99.8 wt.%), pure Li (99.9 wt.%), and Mg–19.3 wt.% Y or Mg–18.87 wt.% Ymm master alloys by the same method in the previous literature [10], and the composition of Y-riched mischmetch can be seen in the literature [10]. Raw metals, in a pure graphite crucible, were melted at 1023 K, and a LiCl–KCl mixture was used

Microstructures

Fig. 1 demonstrates XRD pattern of Mg–8.02Li–0.210Y alloy. It is shown that this alloy is made up of α-Mg and β-Li phases of HCP and BCC lattice structures, respectively, as well as Mg–8.43Li–0.353Ymm alloy shown in the literature [10]. This situation is in consistence with the results of the Mg–Li binary phase diagram [3], which illustrates that Mg–Li alloys with a Li content of 5.3–10.7 wt.% present the existence of both α-Mg and β-Li phases.

OM images of both Mg–Li dual phase alloys are

Microstructure

In this work, Y element and Y-rich mischmetal (Ymm) are used as alloying elements to improve microstructures and mechanical properties of Mg–Li dual phase alloys, which can also be seen in previous literatures [8], [9], [10]. As is shown in Fig. 2, the addition of 0.353 wt.% Ymm seems to be more effective for the grain refinement than the addition of Y element with the fraction of 0.210 wt.%. This is deduced from two aspects. One is that some α-Mg grains in Mg–8.02Li–0.210Y alloy are much higher

Conclusions

Samples of Mg–Li dual phase alloys, Mg–8.02Li–0.210Y and Mg–8.43Li–0.353Ymm (Y-rich mischmetal) alloys, were prepared from casting ingots and conducted to tensile tests at initial rates from 1.28 × 10 4 s 1 up to 1.28 × 10 3 s 1, and microstructures were evaluated. Conclusions were drawn and listed as below.

  • 1)

    Compared with the Y-containing alloy, Mg–8.43Li–0.353Ymm alloy presented a higher fraction of β-Li phase with a lower strength and a low elongation. With the increment of tensile speed, strengths

Acknowledgments

This work is financially supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (20921002), the National Science and Technology Program of China (2013DFA71070, 2014DFG52810), the National Natural Science Foundation of China (51401005) and the foundations from Chongqing Science and Technology Commission (CSTC2013JCYJC60001, CSTC2014KJRC-QNRC50002, CSTC2014JCYJJQ50002).

References (27)

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