Compressive Creep Behaviour of Cast Magnesium Under Stresses Above the Yield Strength and The Resultant Texture Evolution

Abstract

Creep behaviour of commercial purity cast magnesium with preferred orientation was examined in compression at 100, 125 and 150°C and 35 and 50 MPa. The as-cast structure is composed of large columnar grains in which prismatic poles are parallel to the longitudinal direction. Cylindrical compression specimens were prepared from the cast material in the way that columnar grains were almost perpendicular to the compressive loading direction. Steady-state creep rate increased from -2 xl x 1 0-7/s to 5 x 10-7/s at 35 MPa and from 5.7 x 1O--7/s to 2 x 10- 6/s at 50 MPa with increasing temperature from 100 to 150 °C. The apparent activation energy, Qc, was determined to be 34 kJ/mol at 50 MPa and 29kJ/mol at 35 MPa. The stresses used were above the yield strength of the cast Mg and instantaneous plastic deformation occurred at creep loading. It was noticed that creep under the aforementioned conditions changed the initial grain orientation. Such evolution of orientation was stronger at higher temperatures. Some evidence of grain boundary serrations was also observed after creep and these were attributed to creep-deformation due to strain-induced grain boundary migration.