Creep and rupture properties of a squeeze-cast Mg-Al-Ca alloy

Abstract

A squeeze-cast Mg-Al-Ca alloy (MRI153) was creep tested at 150, 175, and 200 °C under applied stresses in the range of 30 to 120 MPa. The creep curves were characterized by an extended tertiary stage in which the creep rate increases progressively with the creep strain. Microstructural examinations revealed the precipitation and coarsening of new particles during creep. The stress dependence of the minimum creep rate suggests a transition from power-law creep at low stresses to power-law breakdown at high stresses. Creep rupture of this alloy occurred as a result of cavitation damage at dendritic grain boundaries, with the creep rupture time and the minimum creep rate following the empirical Monkman-Grant equation. A comparison is made between the creep and rupture properties of MRI153 and those of a squeeze-cast Mg-Al alloy (AZ91).

Creep and Fracture Behavior of Peak-Aged Mg-11Y-5Gd-2Zn-0.5Zr (wt pct)

“…9, and the creep damage tolerance parameter, λ, could be determined from the slope. Note that the data of MRI153 [5] [Mg-9Al-0.7Zn-1Ca-0.1Sr (wt.%)] and WE54-T6 were also included for comparison, which will be discussed latter. The λ values of the studied WGZ1152-T6 alloy ranged from 1.4 to 2.5 for most test conditions.…”

“…[15,33] Other alloys that exhibit this behavior are 316L austenitic steel, [14] 1Cr-1Mo-0.25V bainitic steel, [14] 0.5Cr-0.5Mo-0.25V ferritic steel, [34] Mg-Al-Ca alloy [5] and AZ91hp [35] . Normal tertiary creep occurs when there is an effective reduction in cross-sectional area either because of necking or internal void formation, such as grain boundary cavitation and cracking.…”

“…Normal tertiary creep occurs when there is an effective reduction in cross-sectional area either because of necking or internal void formation, such as grain boundary cavitation and cracking. [32] The extended tertiary creep is generally believed to be a consequence of precipitate coarsening [5,[34][35][36] or strain-induced instability of the dislocation substructure [15,33] . As described in the previous section, the fine β′ phase, of diameter between 5-40nm in the initial condition (Fig.…”

“…Another microstructural feature which may have contributed to the high activation energy for creep was the second-phase particles, especially those present at the grain boundaries. [5,41,52,53] These particles can act as effective obstacles to matrix dislocation motion both within the grains and at the grain boundaries.…”

“…[1,3] Most commercially used Mg alloys for automotive applications are Mg-Al based alloys, such as AZ (Mg-Al-Zn) series and AM (Mg-Al-Mn) series, and they can't be used for major powertrain applications due to their poor creep resistance at temperatures above 398K. [4,5] These powertrain applications work at higher temperatures. For instance, automatic transmission cases can operate at up to 448K, engine blocks up to 473K, and engine pistons up to 573K.…”