Precipitation in Uniaxially Stressed Mg-Nd Alloys During Creep Testing
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recipitation-hardened Mg-based alloys containing rare earth elements (e.g., Nd, La, Ce) are recognized as potential candidates for automobile parts suited for high temperature 423 K to 573 K (150 C to 300 C) applications.[1,2] For example, quaternary Mg-Nd-Zn-Zr alloys are reported to exhibit favorable creep properties at 523 K (250 C).[2] Focusing on model binary Mg-RE alloys, a creepbased study [at 90 MPa and 450 K (177 C)] on highpressure die casted Mg-La, Mg-Ce, and Mg–Nd alloys revealed that Nd additions improve creep resistance considerably via dynamic precipitation within the a-Mg matrix during creep testing.[1] Importantly, that study[1] demonstrated the coupled effects of temperature and applied stress on the precipitation behavior. Precipitation in Mg-Nd alloys resulting from isothermal aging (effect of temperature) is fairly well understood, particularly the early to middle stages in the precipitation sequence: GP zones, b¢¢ (DO19), b¢ (orthorhombic), b1 (fcc), b (tetragonal), and be (tetragonal).[3,4] However, it is unclear if a constant applied uniaxial stress (i.e., creep) has any effect on the precipitation behavior in the same Mg-Nd alloys. Therefore, this work examines the role of applied stress on precipitation by comparing Mg-Nd DEEP CHOUDHURI, Postdoctoral Research Associate, NILESH DENDGE, Graduate Student, SOUMYA NAG, Research Assistant Professor, and RAJARSHI BANERJEE, Professor, are with the Center for Advanced Research and Technology, Department of Materials Science and Engineering, University of North Texas, Denton, TX 76207. Contact e-mail: [email protected] MARK A. GIBSON, Principal Research Scientist, is with the CAST CRC and CSIRO Process Science & Engineering, Clayton South MDC, Private Bag 33, Clayton 3169, VIC, Australia. Manuscript submitted March 9, 2013. Article published online May 16, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
samples subjected to the following two conditions: (i) after high-pressure die casting (HPDC) + creep testing (90 MPa/450 K (177C)/100 hours) and (ii) after HPDC + isothermally aging [450 K (177C)/100 hours]. Mg-2.6 wt pct (or 0.5 at. pct) Nd (here on, Mg-Nd) alloy was prepared with the HPDC technique.[5] Backscattered electron images indicated that HPDC results in a dendritic microstructure (Figure 1(a) and inset showing low magnification image of the same area). Cylindrical dog bone-shaped tensile bars [inset Figure 1(a)] with ~5.6 mm gage diameter were cast using a 250 tonne Toshiba cold-chamber HPDC. The tensile bars were crept nominally at 90 MPa uniaxial tensile stress in an oil bath maintained at 450 K (177 C).[1] Typical creep behavior of Mg-Nd is presented via a strain vs time plot in Figure 1(b). The creep experiment was terminated after 100 hours, i.e., near the middle of the steady state creep regime [indicated with an arrow in Figure 1(b)]. For comparison, a tensile bar was aged separately at 450 K (177 C) for 100 hours without any applied stress. The time duration (100 hours) chosen in these experiments allowed sufficient time
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