Developing a Viable Process Window for Novel Alloys Based on the Mg-Zn-Nd-Y System

In recent years there has been an increased demand for light-weight alloys from the aerospace and automotive industries. One particular challenge is producing cost-effective alloys for automotive applications. In response to this demand, Magnesium Elektron North America Inc. have produced three novel alloys based on the Mg-Zn-Nd-Y system. Although these alloys cost less to produce than their aerospace counterparts, it is necessary for them to provide good levels of formability for wrought applications, while retaining a high strength to weight ratio. The three alloys have been tested with hot axisymmetric compression. A matrix of tests was carried out between 425 and 450 ºC with strain rates ranging from 0.5 to 5/s. The narrow range of the test conditions was due to the narrow process window exhibited by the alloys. Tests outside of this range failed due to incipient melting at higher temperatures or brittle fracture at lower temperatures. The limited ductility of the alloys was due to a significant volume fraction of a eutectic phase that existed at grain boundaries. The phase was identified with differential scanning calorimetry (DSC) and transmission electron microscopy to be the W-phase. The W-phase is deleterious to the tensile properties of Mg-Zn-RE alloys. The Zn/RE ratio was identified as being a controlling factor in the formation of the brittle eutectic compound. Changes in the alloy composition have been recommended in order to expand the process window. The deformed microstructures exhibited extensive dynamic recrystallisation (DRX) when examined with electron backscattered diffraction (EBSD). The dominant active mechanism of DRX was believed to be a function of the Zener-Hollomon parameter. Continuous dynamic rotation recrystallisation (CDRRX) was the only mechanism of DRX to significantly weaken the otherwise strong basal texture that developed after deformation. A complementary study showed that the alloys age harden with the beta(1) and beta(1) prime precipitates being identified as possible strengthening phases.