| Effect of Y and Nd content on the microstructures and mechanical properties of Mg-6Zn-1Mn-4Sn(ZMT614) alloy were systematically investigated by means of optical microscopy(OM), scanning electron microscopy(SEM) equipped with energy dispersive spectronmetry(EDS), X-ray diffraction(XRD), differential scanning calorimetry(DSC), electron back scanning electron microscopy(EBSD), transmission electron microscopy(TEM) and tensile properties testing. This paper mainly focus on the microstructural evolution, morphology characteristics of second phases, room temperature and high temperature tensile properties of Mg-6Zn-1Mn-4Sn-x Y(ZMT614-x Y,x=0.1、0.5、1.0,wt.%) and Mg-6Zn-1Mn-4Sn-x Nd(ZMT614-x Nd,x=0.5、1.0、1.5,wt.%) alloys.The as-cast ZMT614 alloy exhibit continuous net-work dendritic microstructures, which are comprised of α-Mg dendrites and intermetallic compounds at interdendritic regions. Its phase compositions are α-Mg, Mn, Mg7Zn3 and Mg2 Sn phases. With addition of Y, a new ternary Mg Sn Y phase is formed and the dendritic structure is generally refined. The as-cast shows discontinuous net-work dendritic microstructures ZMT614-Y alloys, which phase compositions are α-Mg, Mn, Mg7Zn3 Mg2 Sn and Mg Sn Y phases. After homogenization(420℃/12h), the Mg7Zn3 eutectic compounds dissolve into the matrix, the blocky Mg2 Sn and Mg Sn Y phases are remained in the matrix. The as-cast microstructures get refined and Mg Sn Nd ternary phase formed with addition of Nd. The phase compositions of Nd-containing alloys are α-Mg, Mn, Mg7Zn3, Mg2 Sn and Mg Sn Y phases. The coarse Mg2 Sn particles and Mg Sn Y rods are remained after homogenization(420℃/12h).The extrusion temperatures make significant effects on the microstructures and mechanical properties of as-extruded ZMT614-Y and ZMT614-Nd alloys. During the extrusion process, dynamic recrystallization(DRX) occurs, and the grains get refined. The orientation of DRXed grains is randomly. In addition, the remained eutectic compounds are broken into small particles and distribute as streamlines parallel to the extrusion direction. For the Y-containing alloys, when the content of Y is 0.1-0.5wt.%, the strength and elongation increase as the Y content increases. However, the strength and elongation become decrease when the content reaches the 1.0wt.%. For the Nd-containing alloys, when the addition of Nd increases, the strength gradually increases while the elongation decreases. As the extrusion temperatures increase from 360℃ to 420℃, the texture intensity decreases and the average grain size increases, so the tensile properties decrease. The ultimate tensile strength(UTS), yield strength(YS) and elongation of as-extruded ZMT614-0.5Y alloy decrease from 368MPa、276MPa and 17.2% to 335MPa、246MPa and 11.4%, respectively. The UTS, YS and elongation of as-extruded ZMT614-1.5Nd alloy decrease from 361MPa、273MPa and 14.0% to 330MPa、245MPa and 11.4%,respectively.For the 360℃ extruded alloys, the optimized solution and T6 treatments are(440℃/2h) and(90℃/24h+180℃/8h). For the 420℃ extruded alloys, the optimized T5 treatment is(180℃/12h). Many extremely fine spheres and rods disperse on the basal plane of the matrix when pre-aged at 90℃ for 24 h. High number density of fine ’1b rods and ’2b plates exist in the matrix of T6 treated ZMT614-0.5Y alloy. High number density of fine rod-shaped’1b phase, disc-shaped and T-shaped ’2b phases disperse in the matrix of T6 treated ZMT614-1.5Nd alloy. It is speculated that the coherent’1b phase is formed prior to ’2b phase. These ’1b rods are believed to act as heterogeneous nucleation sites for the’2b plates during the aging process, promoting the T-shaped’2b phase formed. The orientation relationship of T-shaped ’2b is ’2[0001]b∥[0001]a,’2(2110)b∥(1100)a. However, much lower number density of ’1b rods are formed in the T5 treatment.Due to grains grows obviously and high number density’1b rods precipitated in the T6 treatment, the strength increases while elongation decreases obviously. The UTS, YS and elongation of T6 treated ZMT614-0.5Y alloy are 376 MPa, 371 MPa and 7.73%, respectively. The T6 treated ZMT614-1.5Nd alloy shows the best strength and moderate elongation, i.e., a UTS of 391 MPa, a YS of 382 MPa and an elongation of 5.50%. After T5 treatment, the strength increases obviously and elongation decreases slightly are mainly attributed to the fine grains and many ’1b rods. The UTS, YS and elongation of T5 treated ZMT614-0.5Y alloy are 343 MPa, 302 MPa and 10.6%, respectively. The UTS, YS and elongation of T5 treated ZMT614-1.5Nd alloy are 364 MPa, 339 MPa and 10.1%, respectively.According to the thermodynamics calculations and experimental results, the Mg Sn Y and Mg Sn Nd phases have these characteristics: 1) the formation temperatures of Mg Sn Y and Mg Sn Nd phases are higher than that of other phases, the Mg Sn Y and Mg Sn Nd phase are preferentially formed during the solidification. The formation of these phases consumes Y and Nd, so the formation of other RE-containing phases is suppressed. 2) They are high temperature phases and undissolved in the homogenization and solution treatments. 3) They have low symmetry crystal structures.The hot tensile temperatures exert significant effects on the mechanical properties and fracture mechanism of as-aged ZMT614-Y and ZMT614-Nd alloys. When the tensile temperatures increase from 150℃ to 300℃, the high temperature strength decreases and elongation increases. The fracture surface of as-extruded and T5 treated alloys is covered with small dimples, tearing edges, second phase particles and voids, showing a characteristic of ductile fracture. However, the fracture surface characteristics of T6 treated alloys changed as the tensile temperatures increase. When the tensile temperatures are 150℃ and 200℃, the fracture surface consists of small dimples, tearing edges and second phase particles, showing a characteristic of transgranular fracture. The fracture surface is covered with small dimples, tearing edges, second phase particles and cleavage planes, showing a characteristic of transgranular and intergranular mixed fracture when tensile at 250℃. When the tensile temperature is 300℃, the fracture surface is covered with tearing edges, second phase particles and cleavage planes, showing a characteristic of intergranular fracture. |
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