SUPERPLASTIC DEFORMATION OF TWO PHASE MgLiAl ALLOY AFTER TCAP PRESSING

Jan Dutkiewicz,Paweł Pałka,Ondrej Hilser,Stanislav Rusz,Grzegorz Boczkal,Dariusz Kuc

doi:10.12776/ams.v23i3.987

Abstract

Magnesium based alloy containing 9 wt. % Li, 1,5 wt. % Al, composed of  (hcp + bcc) phases was cast under argon atmosphere and extruded at 350oC as bars of cross section 12x12 mm. Twist Channel Angular Pressing TCAP deformation was applied at a temperature of 160oC using BC rotation of samples in the following passes. TCAP tool consisted of helical part in horizontal area of the channel with an angle of lead γ = 30° to simulate back pressure and thus to increase the extrusion force. The initial grain size of the hexagonal  phase estimated at 30 m decreased in the following passes down to 6 m and that of bcc  phase decreased after TCAP from initial 12 m, down to 5 m. TEM studies after TCAP passes showed higher dislocation density in the  region than in the  phase. Crystallographic relationship (001)  || (110)  indicates parallel positioning of slip planes of both phases due to activation of slip. Electron diffraction indicates increase of grain misorientation with a number of TCAP passes. Stress/strain curves measured at temperature of 200oC show superplastic forming after 1st and 3rd TCAP passes with better superplastic properties with increasing number of passes. Values of strain rate sensitivity coefficient m were calculated at 0.31 after 1 TCAP pass using flow stress values from step strain increase curves. The sample after 3 TCAP passes showed increase of m value up to 0.47 for the strain rate range 10-5 – 5 x 10-4 s-1. Obtained values are slightly lower than those reported in the literature, however increase of a number of TCAP passes has a positive effect on superplastic properties due to fine grains and increase of their misorientation.

Highlights

Severe plastic deformation using ECAP causes substantial grain refinement and improves mechanical properties of magnesium base and other metals [1,2,3,4]
From the microstructure studies performed after superplastic deformation of two phase alloys reported in [12] it was found that grain boundary sliding (GBS) is the dominant deformation mechanism
Twist Channel Angular Pressing (TCAP) method was chosen since in [13] a high effectiveness of the TCAP process is confirmed by the lower number of passes in comparison with the classical ECAP

Summary

Introduction

Severe plastic deformation using ECAP causes substantial grain refinement and improves mechanical properties of magnesium base and other metals [1,2,3,4]. Superplastic deformation was observed in two phase Mg 8% Li 2% Zn alloy after grain refining using rolling with rather nonhomogeneous grain distribution from 1-2 m to several m [9]. It allowed to obtain tensile elongation from 100-400 % at temperatures 423523K even at moderate grain refinement. It was confirmed in rolled Mg 9% Li 1,8% Al 1,6% Zn alloy where above 500% superplastic deformation was observed in two phase alloys [10]. A small aluminum addition was applied to MgLi alloys in order to increase their mechanical properties as reported in [14]

Methods

Results

Conclusion