Effects of heat treatments on Microstructure and mechanical properties of Mg–4Y–4Sm–0.5Zr alloy

Abstract

Microstructure and mechanical properties of Mg–4Y–4Sm–0.5Zr alloy during heat treatments were investigated. The eutectic phase dissolved into the matrix and there was no evident grain growth after solutionized at 798 K for 8 h. The alloy showed very fine scale precipitates in the grains and along the grain boundaries after ageing at 473 K for 16 h. And the ultimate tensile strength of 348 MPa, yield strength of 217 MPa and elongation of 6.9% were attained in this under-aged state. The elongation of alloy greatly decreased, and the ultimate tensile strength decreased respectively with increasing the ageing time, for increasing amount and coalescence of the precipitates along the grain boundaries. The mechanical properties of the alloy could hold up to 473 K, and decreased steeply at temperatures higher than 523 K, for the softening of the precipitates and activation of non-basal slip systems. Keywords Mg–Y–Sm alloy Heat treatment Microstructure Mechanical properties 1 Introduction Magnesium is the lightest of all metals used as the basis for constructional alloys. The characteristic properties of magnesium alloys, like low density, high specific strength/stiffness, good damping capacity, diecastability and recycling potential make it very attractive for applications in transportation [1] . It has been demonstrated that rare earth metals (RE) are the most effective elements to improve the strength properties of magnesium especially at elevated temperatures [2,3] . The strength of these Mg–RE alloys is achieved essentially via precipitation strengthening [4,5] . The typical solution-plus-ageing heat treatment of Mg–RE alloys can produce a dense fine scale precipitates that effectively enhanced the mechanical properties of alloys [6,7] . A recently developed Mg–4Y–4Sm–0.5Zr alloy exhibits more attractive properties than the widely used WE series alloys [8] . Varies heat treatments were carried out on this Mg–4Y–4Sm–0.5Zr alloy to further increase the mechanical properties in the present work. Microstructure and mechanical properties of Mg–4Y–4Sm–0.5Zr alloy during heat treatments were investigated. Optimization of heat treatment parameters of the alloy was determined in this study. 2 Experimental method The Mg–4Y–4Sm–0.5Zr alloy was prepared by melting in an electrical resistance furnace in steel crucible under protect gas consisting of SF 6 (1 vol.%) and CO 2 (balance) in order to prevent burning of the melts. Pure Mg was used as starting materials, during melting, Y and Sm were added as master alloys, Mg–25% Y and Mg–25% Sm, prepared beforehand. The alloy was cast into steel moulds. The chemical composition of the alloy analyzed by inductively coupled plasma (ICP) is listed in Table 1 . The alloy was solutionized at 798 K for 4 h, 8 h and 12 h under the SO 2 atmosphere followed by quenching in cold water. After solution treatment, the alloy was isothermally aged at 448 K, 473 K and 498 K for different times in the oil-bath. The ageing response of the alloy was checked by hardness measurement. Hardness was measured with a Vickers-hardness tester under a load of 30 kg. The room temperature tensile properties were tested on Zwick/Roell testing machine, and the high temperature tensile tests were carried out on Shimadzu machine at an initial strain rate of 10 −3 s −1 at different temperatures from 423 K to 623 K. The tensile specimens have a gauge length of 10 mm, a gauge width of 3.5 mm and a gauge thickness of 2 mm. The microstructures of alloy were analyzed by light microscope (OM, LEICA MEF4M) and scanning electron microscope (FEI SIRION 200) equipped with an Oxford INCA energy dispersive X-ray spectrometer. The specimens were mechanically polished and etched with a dilute solution of 4% nitric acid in alcohol. The average grain size was measured by the linear intercept method. Thermal analyses were taken using DTA1600 under argon atmosphere with a heating rate of 10 K/min. Characterization of precipitates was performed in a JEOL-2010 TEM and operating at 200 kV. The thin foils for TEM investigation were prepared by thinning small plates cut from specimens, grounding to a thickness of 0.05 mm. Then discs of 3 mm in diameter were punched from plates, and ion milled using the gatan Model 691 Precision Ion Polishing System until a hole appears. 3 Experimental results 3.1 Microstructure 3.1.1 Microstructure of as-cast alloy Fig. 1 shows OM and TEM micrographs of as-cast Mg–4Y–4Sm–0.5Zr alloy. The average grain size of as-cast alloy is about 50 μm. The TEM observation shows the morphology of the eutectic phase formed mostly in the triple junction of grain boundaries. It has been reported that the eutectic phase in Mg–4Y–4Sm–0.5Zr alloy has the Mg 5 (Sm 0.6 Y 0.4 ) stoichiometry with a FCC crystal structure ( a = 2.326 nm) [8] . Fig. 2 shows the DTA results of the as-cast Mg–4Y–4Sm–0.5Zr alloy during the heating process. There were two endothermic peaks appeared during the process. The first peak appeared at about 817 K, and this peak may correspond to the melting temperature of the eutectic phase. The other peak appeared at about 920 K, and this peak may be related to the melting of the alloy. 3.1.2 Microstructure of solutionized alloy Fig. 3 indicates the influence of solution heat treatments on the microstructure of the alloy. The average grain size of the alloy solutionized at 798 K for 4 h, 8 h and 12 h was 53 μm, 61 μm and 140 μm, respectively. There were still some eutectic phases remained at grain boundaries after solutionized for 4 h, as shown in Fig. 4 (a). The eutectic phases almost dissolved into the matrix and the average grain size was just a little larger than that of the as-cast alloy after solutionized for 8 h. And the grain size became extremely larger after solutionized...