Abstract:
The microstructural evolution under various conditions (as-cast, homogenized, and hot rolled) and the deformation mechanisms at four distinct temperatures (250°C, 300°C, 350°C, and 400°C) and two strain rates (1x10-4 and 5x10-4 s-1) of the Mg-1Zn-1.5Y alloy were investigated with the addition of Gd (1%, 2%, and 6 wt%) to the alloy system. The chemical composition was analyzed using XRF, confirming the desired alloy composition with negligible deviation. Optical microscopy was employed to observe dendritic arm presence, measure grain size, assess precipitation conditions, and identify secondary phases within the matrix. Dendritic arms were evident in the as-cast condition, with density increasing due to Gd addition. Additionally, a trend of grain size reduction was observed with Gd inclusion in all conditions. Notably, hot rolling significantly reduced grain size through dynamic recrystallization (DRX). XRD, SEM, EDS, DSC, and CALPHAD analysis were utilized to determine different phases present in the alloy matrix, phase morphology, chemical composition, phase formation temperature, and phase evolution and volume fraction with temperature for the three alloys. The 14H-LPSO phase was consistently found within the α-Mg matrix of all compositions, with volume fraction increasing as Gd content rises. However, morphological variations were identified due to compositional changes, with lamellar and blocky LPSO phases forming in alloy A (1% Gd) and B (2% Gd), and blocky LPSO in alloy-C (6% Gd). Furthermore, in CALPHAD analysis Mg5(Gd,Y) was observed in alloy-A and C, while alloy-B contained the W-phase (Mg3Zn3(Y,Gd)2), which remained stable till the elevated temperatures around 600°C. However, above 450°C, the 14H-LPSO phase was found to start dissolving within the α-Mg matrix. Tensile testing at four different temperatures and two strain rates was conducted to assess changes in strength, ductility, and deformation mechanisms due to Gd addition. An optimal combination of strength and ductility was observed in alloy-B (2% Gd) at high temperature (400°C) and both strain rates, attributed to the presence of a bimodal 14H-LPSO phase (mix of lamellar and blocky LPSO) and the W-phase at elevated temperatures. Conversely, alloy-C demonstrated promising performance below 350°C, possibly due to the maximum volume fraction of the blocky LPSO phase. To determine the deformation mechanisms, activation energy, strain exponent (n) values, and strain rate sensitivity (m) of the alloys were calculated. Dispersed strengthening mechanism was predominant at lower temperatures (250°C and 300°C), transitioning to climb-controlled creep, glide-controlled creep, and grain-boundary sliding at higher temperatures (350°C and 400°C).