Modification of Microstructure and Properties of Extruded Mg-Li-Al Alloys of α and α+ β Phase Composition using ECAP Processing

Abstract

Two magnesium based alloys containing 4.5 wt% Li and 1.5 wt% Al (alloy 1) and 9 wt% Li and 1.5 wt% Al (alloy 2) were cast under argon atmosphere and hot extruded at 350• C. Microstructure of alloy 1 consisted of hexagonal α phase of average grain size 20 µm and small aluminum rich precipitates being the most probably AlLi2Mg phase. Alloy 2 in the extruded form consisted of lamellas of α + β phases of thickness 5-20 µm and length above 100 µm. Significant grain refinement down to about 2 µm was observed in one-phase hexagonal (hcp) alloy 1 after one pass of ECAP processing with helical component. Two-phase (hcp + bcc) alloy 2 showed higher non-homogeneity after the first equal channel angular pressing pass due to easier deformation of softer bcc phase, while both, α and β phases exhibited low angle grain boundaries. The hardness and the yield strength of the alloys were higher for alloy 1 (68 HV and 205 MPa, respectively) than those of alloy 2 (61 HV and 175 MPa). Subsequent equal channel angular pressing passes were performed at lower extrusion stress. The hardness of both alloys did not change significantly after subsequent equal channel angular pressing passes and revealed tendency to decrease. Two-phase alloy showed superplastic properties already after one equal channel angular pressing pass at 160• C with grain growth after superplastic tensile testing. Single phase hcp alloy did not show such properties after 1 pass, but after a few equal channel angular pressing passes it could be superplastically formed.

Thermal Assessment of Modified Ultra-Light Magnesium-Lithium Alloys

“…EDS results of analysed intermetallic compounds revealed a high concentration of Al, Sr and Mg though Mg may come from the matrix. The observed intermetallic compounds have most probably resulted from the grain refinements addition in the form of Mg 17 Sr 2 , Mg 2 Sr or Al 4 Sr phase as suggested in [23][24][25], however, it must be noted that melting temperatures of proposed phases are 606°C, 680°C and 1040°C, respectively. Moreover, according to the Mg-Sr binary system [22] and suggestions in [23], the Mg 2 Sr phase is observed Mg alloys with a higher concentration of Sr (more than 1.5%).…”

“…The addition of 0.2wt.% TiBor and 0.2wt.% AlSr10 master alloys cause decreases in nucleation temperature and solidus temperature to 579.32°C and 520°C, respectively. It was found that thermalderivative analysis of modified Mg-9Li-1.5Al alloys reveals an exothermic peak that probably becomes from nucleation of intermetallic compound as η(LiAl) phase [25]; however, more studies must be done. The addition of 0.2wt.% TiBor and 0.2wt.% AlSr10 master alloys decrease a crystallisation range and transition in solid state from 130°C to 105°C.…”

Effect of grain refinements on the microstructure and thermal behaviour of Mg–Li–Al alloy

“…Darker particles have most probably resulted from the aluminium addition in the form of g(LiAl) phase as suggested in Fig. 2 Optical microstructure of as-cast and after modification of Mg-9Li-1.5Al alloy: a Mg-9Li-1.5Al, b Mg-9Li-1.5Al?0.2Zr, c Mg-9Li-1.5Al?0.2TiBor, d Mg-9Li-1.5Al?0.2Sr, e Mg-9Li-1.5Al?0.2TiBor?0.2Sr [8,9,19,20]. SEM analysis of marked circles shows high concentration of Al, indicating that it is g(LiAl) phase.…”

Development of Microstructure, Mechanical and Wear Characteristics of the Brass Alloy Processed by Ecap

“…The microstructure of the initial annealed alloy (Fig. 2a) consists of wide major regions of equiaxed α phase (lighter) and minor amount of β phase distributed on the grain boundaries (darker) (Dutkiewicz et al, 2009). The microstructural percentage of these two phases has been determined to be almost 70% for α phase and 30% for β phase.…”