Fabrication of fine-grained,high strength and toughness Mg alloy by extrusion−shearing process

A novel extrusion-shearing(ES) composite process was designed to fabricate fine-grained, high strength and tough magnesium alloy. The structural parameters of an ES die were optimized by conducting an orthogonal simulation experiment using finite element software Deform-3D, and Mg-3 Zn-0.6 Ca-0.6 Zr(ZXK310) alloy was processed using the ES die. The results show that the optimized structural parameters of ES die are extrusion angle(α) of 90°, extrusion section height(h) of 15 mm and inner fillet radius(r) of 10 mm. After ES at an extrusion temperature and a die temperature of 350 °C, ZXK310 alloy exhibited good ES forming ability, and obvious dynamic recrystallization occurred in the forming area. The grain size decreased from 1.42 μm of extrusion area to 0.85 μm of the forming area. Owing to the pinning of second phase and formation of ultrafine grains, the tensile strength, yield strength and elongation of alloy reached 362 MPa, 289 MPa and 21.7%, respectively.

Effect of Cu on microstructure,mechanical properties,and texture evolution of ZK60 alloy fabricated by hot extrusion−shearing process

As-cast Mg-6Zn-xCu-0.6Zr(x=0,0.5,1.0,wt.%)alloys were fabricated by permanent mold casting;then,the alloys were subjected to homogenization heat treatment and extrusion-shearing(ES)process.The microstructure and mechanical properties of the alloys were evaluated by OM,SEM/EDS,XRD,TEM,EBSD and tensile tests.The results show that the hard MgZnCu phase in Cu-added alloy can strengthen particle-stimulated nucleation(PSN)effect and hinder the migration of dynamic recrystallization(DRX)grain boundary at an elevated temperature during ES.The ZK60+0.5Cu alloy shows an optimal tensile strength–ductility combination(UTS of 396 MPa,YS of 313 MPa,andδ=20.3%)owing to strong grain boundary strengthening and improvement of Schmid factor for{0001}■basal slip.The aggregation of microvoids around the MgZnCu phase mainly accounts for the lower tensile elongation of ZK60+1.0Cu alloy compared with ZK60 alloy.

Effect of annealing on the microstructure and mechanical properties of Mg-2.5Zn-0.5Y alloy

The microstructure and mechanical properties of extruded Mg-2.5Zn-0.5Y alloy before and after annealing treatments were investigated. The as-extruded alloy exhibits a yield tensile strength （YTS） of 305.9 MPa and an ultimate tensile strength （UTS） of 354.8 MPa, whereas the elongation is only 4%. After annealing, the YTS and UTS decrease to 150 MPa and 240 MPa, respectively, and the elongation increases to 28%. Interestingly, the annealed alloy maintains an acceptable stress level even after a much higher ductility is achieved. These excellent mechanical properties stem from the combined effects of fine α-Mg dynamic recrystallization （DRX） grains and a homogeneously distributed icosahedral quasicrystalline phase （I-phase） in the α-Mg DRX grains. In particular, the superior ductility originates from the coherent interface of I-phase and α-Mg and from the formation of the secondary twin {101 1}-{101 2}（38°〈1 2 10〉） in the tension twin {1012}.

Hot tearing behaviors and in-situ thermal analysis of Mg-7Zn-xCu-0.6Zr alloys

Thermal analysis was used to investigate the microstructural evolution of Mg-7 Zn-x Cu-0.6 Zr alloys during solidification. The effect of Cu content（0, 1, 2 and 3, mass fraction, %） on the hot tearing behavior of the Mg-7 Zn-x Cu-0.6 Zr alloys was investigated with a constrained rod casting（CRC） apparatus, equipped with a load sensor and a data acquisition system. The thermal analysis results of Mg-7 Zn-x Cu-0.6 Zr alloy revealed that the alloy consisted of two distinct phases： α-Mg and Mg Zn2. Three distinct peaks were observed in the alloys with Cu addition, which were identified as α-Mg, Mg Zn Cu and Mg Zn2. In addition, the reaction temperature of α-Mg decreased and the reaction temperatures of Mg Zn2 and Mg Zn Cu increased as the Cu content increased. The experimental results of hot tearing demonstrated that the addition of Cu significantly reduced the hot tearing susceptibility（HTS） of Mg-7 Zn-x Cu-0.6 Zr alloys due to the higher eutectic temperature and the shorter solidification temperature region.

Hot tearing susceptibility of MgZn_(4.5)Y_xZr_(0.5) alloys and mechanism

The hot tearing susceptibility of MgZn_(4.5)Y_xZr_(0.5)(x = 0.5, 1, 2, 4, 6) alloys was evaluated using ClyneDavies' theoretical model and a constrained rod casting(CRC) apparatus equipped with a load cell and data acquisition system. The results obtained from these two approaches were in good agreement, illustrating that the hot tearing susceptibility of the investigated alloys is in the order of MgZn_(4.5)Y_xZr_(0.5) > MgZn_(4.5)Y_xZr_(0.5) > MgZn_(4.5)Y_xZr_(0.5) > MgZn_(4.5)Y_xZr_(0.5)> MgZn_(4.5)Y_xZr_(0.5). The microstructure and morphology of hot tearing regions were observed by means of X-ray diffraction and scanning electron microscopy. The function curves on variation of contraction force and temperature versus time were recorded by a computer during solidification of the MgZn_(4.5)Y_xZr_(0.5) alloys in a "T" type mold after A/D(Analog to digital) conversion. Results show that both the amount and morphology of the second phases have a great relationship with the hot tearing susceptibility. But the former plays a more important role on that of MgZn_(4.5)Y_xZr_(0.5) alloys. The role of low-melting point eutectic phases on the hot tearing susceptibility of MgZn_(4.5)Y_xZr_(0.5) alloys changes gradually with different Y contents. When Y ≤ 1wt.%, it is mainly a damaging effect on intergranular bonding force; when Y > 1wt.%, it is mainly an intergranular feeding effect on formed separated dendrites.

Dynamic tensile and compressive properties of vacuum and ordinary die-casting AT72 magnesium alloy at high strain rates

To investigate dynamic tensile and compressive properties of AT72 magnesium alloys under high strain rates,the Split Hopkinson tensile Bar(SHTB)and the Split Hopkinson Pressure Bar apparatus was used at ambient temperature.The specimens after dynamic tension and dynamic compression were analyzed by scanning electron microscope(SEM)and metallographic microscope.Dynamic mechanical properties and fracture mechanism were discussed in this paper.The results show that at approximate tensile strain rate,the dynamic tensile strength of ordinary die-casting AT72 and vacuum die-casting AT72 is very similar,but the dynamic failure strain is of great difference.AT72 magnesium alloys under dynamic compressive yield continuously,and have the slight positive strain rate effect.The dynamic compressive behaviors of ordinary die-casting AT72 and vacuum die-casting AT72 are nearly the same.The dynamic tensile fracture mechanism of both vacuum die-casting AT72 and ordinary die-casting AT72 is inter-granular fracture.The dynamic compressive fracture mechanism of both vacuum die-casting AT72 and ordinary die-casting AT72 is quasi-cleavage intra-crystalline failure.The pores have an important effect on dynamic properties of die casting magnesium alloys.

Hot tearing susceptibility of Mg-xZn-2Y alloys

Effects of Zn content (0, 0.5%, 1.5% and 4.5%) on the hot tearing characteristics of Mg?2%Y alloy were studied in aconstrained rod casting (CRC) apparatus attached with a load cell and data acquisition system. The experimental results indicate thatthe hot tearing susceptibility (HTS) is affected by the content of Zn. The Zn-free base alloy shows the lowest HTS. The HTS ofMg?xZn?2Y alloys increases with increasing Zn content, reaches the maximum at 1.5% Zn, and then decreases with further Znaddition. The high HTS observed in the alloy with 1.5% Zn is attributed to its high force release rate and large force drop duringsolidification. The hot cracks of casting are initiated and propagate along the dendritic or grain boundaries. The predictions of HTS ofMg?xZn?2Y alloys using ProCAST software are in good agreement with the results obtained by experimental measurements.

Solidification pathways and hot tearing susceptibility of MgZn_xY_4Zr_(0.5) alloys

Hot tearing is known as one of the most serious solidification defects commonly encountered during solidification. It is very important to study the solidification path of alloys. In the work, thermal analysis with cooling curve was used for the investigation of microstructure evolution with different Zn contents during solidification process of MgZn_xY_4Zr_(0.5) alloys. Thermal analysis results of MgY_4Zr_(0.5) alloys revealed one distinct phase precipitation: α-Mg. Three different phase peaks were detected in the Zn-containing alloys: α-Mg, Z-phase(Mg_(12)YZn) and W-phase(Mg_3 Y_2Zn_3). In addition, for the present MgZn_xY_4Zr_(0.5) alloys, the freezing ranges of these alloys from large to small were: MgZn_(1.5)Y_4Zr_(0.5)>MgZn)(3.0) Y)4Zr_(0.5)>MgZn0.5 Y4 Zr0.5>MgY_4Zr_(0.5). The effect of different contents of Zn(0, 0.5, 1.5, 3.0 wt.%) on hot tearing behavior of MgY_4Zr_(0.5) alloy was investigated using a constrained rod casting(CRC) apparatus equipped with a load cell and data acquisition system. The experimental results show that the addition of Zn element significantly increases hot tearing susceptibility(HTS) of the MgY_4Zr_(0.5) alloy due to its extended freezing range. Some free dendrite-like bumps and ruptured liquid films on the fracture surfaces were observed in all the fracture surfaces. These phenomena proved the fact that the hot tearing formation was caused by interdendritic separation due to lack of feeding at the end of solidification.

Enhanced strengthening by two-step progressive solution and aging treatment in AM50-4%(Zn,Y) magnesium alloy

AM50-4%(Zn,Y)alloy with a Zn/Y mole ratio of6:1was subjected to thermal analysis,and the results were used for designing a two-step progressive solution treatment process.The effects of solution and aging treatments on the microstructure and mechanical properties of the AM50-4%(Zn,Y)alloy were investigated using OM,XRD,SEM/EDS,TEM,tensile test and hardness test.The experimental results demonstrated that the two-step progressive solution treatment could make theΦandβphases sufficiently dissolve into the matrix which possessed higher supersaturated degree of the dissolved solute compared with the one-step solution treatment.This resulted in a certain enhancement of the precipitation strengthening effect during the subsequent aging process.The precipitation of theФphase had a greater impact on the comprehensive mechanical properties of the alloy thanβphase precipitation when the aging treatment was performed at180℃.The peak aging strength of the AM50-4%(Zn,Y)alloy which was subjected to the two-step progressive solution treatment process(345℃for16h and375℃for6h)was obtained after the aging treatment at180℃for12h.

Microstructure,mechanical properties and first-principle analysis of vacuum die-cast Mg-7Al alloy with Sn addition

The influences of Sn element and heat treat-ment on the microstructure and mechanical properties of Mg-7Al alloy prepared by vacuum die cast(VDC)were studied.The results indicate thatα-Mg dendrite and Mg_(17)Al_(12) are refined and new Mg_(2)Sn phase is formed in Mg-7Al alloy with 2 wt%Sn addition.The ultimate tensile strength(σ_(b))of Mg-7Al alloy increases from 205 to 258 MPa(i.e.,a 25%increase).The mechanical properties of Mg-7Al-2Sn(AT72)alloy are further improved through heat treatment because of the dense microstructures processed by VDC.In particular,the mechanical properties of AT72 alloy are greatly improved after solution treatment at 400℃for 20 h due to solution strengthening of Al and Sn elements and precipitation strengthening of Mg_(2)Sn phases.The precipitation strengthening effect of particle Mg_(2)Sn phases is more apparent due to the decomposition of Mg_(17)Al_(12) phases.In addition,from the view of microscopic theory,the effect of Sn on mechanical properties of Mg-7Al alloy was discussed by first-principle calculation.