Effect of calcium additions on the creep behavior of magnesium die-cast alloy ZA85

Abstract

Magnesium alloys are typically used for weight saving in a wide range of technical applications, but the most common alloys are limited to low temperatures. Magnesium die-cast alloys with zinc (ZA85, 8 wt pct Zn, 5 wt pct A1) as the major alloy addition show an acceptable die castability and creep resistance, and more recent investigations revealed that their creep strength can be significantly improved by adding small amounts of Ca. In this work, the creep behavior of ZA85 and alloys containing 0.3 and 0.9 wt pct Ca in die-cast and preannealed conditions was investigated. The correlation of creep test results with microstructural investigations showed that dislocation-based creep is the rate controlling mechanism for the high-temperature deformation in the entire stress regime from 20 to 100 MPa, and the formation and growth of small incoherent precipitates controls the creep behavior. For the creep tests at stresses above 40 MPa, it has been found that the creep behavior can be described phenomenologically on the basis of a threshold creep model. The addition of Ca was found to lead to an increase in threshold stress, and it is argued that the segregation of Ca to the precipitate/matrix interface leads to a stronger dislocation core relaxation at this interface, thereby increasing the creep resistance of the Ca containing alloys.