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Effect of Various SPD Techniques on Structure and Superplastic Deformation of Two Phase MgLiAl Alloy
Jan Dutkiewicz,Piotr Bobrowski,Stanislav Rusz,Ondrej Hilser,Tomasz A. Tański,Wojciech Borek,Marek Łagoda,Paweł Ostachowski,Paweł Pałka,Grzegorz Boczkal,Dariusz Kuc,Tomasz Mikuszewski 대한금속·재료학회 2018 METALS AND MATERIALS International Vol.24 No.5
MgLiAl alloy containing 9 wt% Li and 1.5% Al composed of hexagonal α and bcc β phases was cast under protecting atmosphereand hot extruded. Various methods of severe plastic deformation were applied to study their effect on structure and grainrefi nement. Rods were subjected to 1–3 passes of Twist Channel Angular Pressing TCAP (with helical component), cycliccompression to total strain ε = 5 using MAXStrain Gleeble equipment, both performed at temperature interval 160–200 °Cand, as third SPD method, KOBO type extrusion at RT. The TCAP pass resulted in grain refi nement of α phase from 30 μmdown to about 2 μm and that of β phase from 12 to 5 μm. Maxstrain cycling 10 × up to ε = 5 led to much fi ner grain size of300 nm. KOBO method performed at RT caused average grain size refi nement of α and β phases down to about 1 μm. Hardnessof alloy decreased slightly with increasing number of TCAP passes due to increase of small void density. It was higherafter MAXStrain cycling and after KOBO extrusion. TEM studies after TCAP passes showed higher dislocation density inthe β region than in the α phase. Crystallographic relationship (001) α|| (110) β indicated parallel positioning of slip planesof both phases. Electron diff raction technique confi rmed increase of grain misorientation with number of TCAP passes. Stress/strain curves recorded at temperature 200 °C showed superplastic forming after 1st and 3rd TCAP passes with bettersuperplastic properties due to higher elongation with increasing number of passes. Values of strain rate sensitivity coeffi cientm were calculated at 0.29 after 3rd TCAP pass for strain rate range 10 −5 to 5 × 10 −3 s −1 . Deformation by MAXStrain cyclingcaused much more effective grain refi nement with fi ne microtwins in α phase. Superplastic deformation was also observed inalloy deformed by KOBO method, however the value of m = 0.21 was obtained at lower temperature of deformation equal to160 °C and deformation rate in the range 10 −5 to 5 × 10 −3 . Tensile samples deformed superplastically showed grain growthand void formation caused by grain boundary slip. Summarizing, all methods applied resulted in suffi cient grain refi nementto obtain the effect of superplastic deformation for alloys of two phase α + β structure.
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