Characterization of the mechanical properties changes in an Al–Zn–Mg alloy after a two-step ageing treatment at 70° and 135 °C

Fine-scale precipitation of the η′ phase and its precursors are essential for the mechanical properties of Al-4.6 wt%Zn-1.2 wt%Mg alloy. This paper deals with an investigation of precipitation in an industrial Al–Zn–Mg alloy at various stages of a conventional two-step ageing treatment at 70 °C and 135 °C. The effect of microstructure on the mechanical properties was performed using microhardness and tensile tests, together with optical, scanning and transmission electron microscopy. After ageing at 135 °C, corresponding to the maximum value of hardness, small η′ precipitates were observed in the alloy matrix. After two-step ageing at 70 °C plus at 135 °C, the volume fraction of this precipitate becomes higher. Consequently, the yield strength of the material increases and it maintains its ductility. This high precipitate density slows the dislocation movement and thus a higher stress was required for its bowing.     

Tensile strength improvement of an Mg–12Gd–3Y (wt%) alloy processed by hot extrusion and free forging

An Mg–12Gd–3Y (wt%) alloy was prepared by conventional casting method using permanent steel mold. Then this alloy was subjected to hot processing, involving hot extrusion and free forging. Tensile strength at room temperature can be improved, with the highest ultimate tensile strength (UTS) value of 390.2 MPa achieved by hot extrusion in comparison to that of as-cast alloy. Temperature dependence of tensile strength is distinguishable for the as-extruded alloy, while the relative stability in UTS values of the alloy after being freely forged should be ascribed to the inter-crossing among deformation bands located at various orientations and the accommodation effect of twining lamellas resulting from forging process on plastic deformation during tensile test at elevated temperatures. Further annealing after hot processing can only have adequate influence on the tensile strength of as-forged alloy. For the alloy freely forged and annealed at 523 K for 4 h, the highest UTS (441.1 MPa) at room temperature is found, which should be mainly related to an evolution from the original as-forged microstructure with subgrains to a more stable combination of large and refined grains through dynamic recrystallization during free forging, and the stress at offset yield YS (384.3 MPa) is also comparable to that relatively high value of 396.9 MPa after solution treatment and isothermal aging of the as-cast alloy.     

Effect of laser offsets on joint performance of laser penetration brazing for magnesium alloy and steel

The experiments of laser penetration brazing for magnesium alloy and steel were carried out, and the effect of laser offsets on mechanical properties and microstructure of the welds was investigated. In the range of 0.4–0.8 mm, with the increase of laser offset, the tensile strength of the joints increased firstly and reduced afterward. In particular, at 0.6 mm laser offset, the average tensile strength could reach a maximum of 185 MPa, which was attributed to the defect-free joining of magnesium alloy to steel. If laser offset exceeded the optimized range (0.5–0.7 mm), the defects of welding crack or incomplete fusion would occur at the interface. As it could be noticed from hardness distribution, a rapid increase in hardness was found near the interface, which had some relations with the coarse microstructure and high element concentration of the interface.     

Artificial ageing of Al-Si-Cu-Mg casting alloys

The T6 heat treatment is commonly used for gravity cast Al-Si-Cu-Mg alloys. The influence of the alloying elements Cu and Mg and the artificial ageing temperature on the age hardening response were investigated. Artificial ageing was conducted at 170 °C and 210 °C for various times for three alloys, Al-7Si-0.3Mg, Al-8Si-3Cu and Al-8Si-3Cu-0.5Mg, cast with three different solidification rates (secondary dendrite arm spacing of about 10, 25 and 50 μm). The coarseness of the microstructure has a small influence on the yield strength, as long as the solution treatment is adjusted to obtain complete dissolution and homogenisation. The peak yield strength of the Al-Si-Mg alloy is not as sensitive to the ageing temperature as the Al-Si-Cu and Al-Si-Cu-Mg alloys are. The ageing response of the Al-Si-Cu alloy is low and very slow. When 0.5 wt% Mg is added the ageing response increases drastically and a peak yield strength of 380 MPa is obtained after 20 h of ageing at 170 °C for the finest microstructure, but the elongation to fracture is decreased to 3%. The elongation to fracture decreases with ageing time in the underaged condition as the yield strength increases for all three alloys. A recovery in elongation to fracture of the Al-Si-Cu-Mg alloy on overageing is obtained for the finest microstructure, while the elongation remains low for the coarser microstructures. The quality index, Q = YS + K?, can be used to compare the quality of different Al-Si-Mg alloys. This is not true for Al-Si-Cu-Mg alloys, as K depends on the alloy composition. Overageing of the Al-Si-Mg alloy results in a decrease in quality compared to the underaged condition.     

Effects of trace Er addition on the microstructure and mechanical properties of Mg–Zn–Zr alloy

The effects of trace Er addition on the microstructure in Mg–9Zn–0.6Zr alloy during casting, homogenization, pre-heating, and hot extrusion processes were examined. The mechanical properties of alloys with and without Er were compared. The results showed that Er exhibited a lower solubility in solid magnesium and formed thermally stable Er- and Zn-bearing compounds. The Er-bearing alloy exhibited a considerably improved deformability, as well as a fine and uniform microstructure. Moreover, dynamic precipitation of fine MgZn₂ particles with a modified spherical morphology occurred during hot extrusion, resulting in a tensile yield strength of 313 MPa and a high elongation to failure value of 22%. Further aging of the Er-bearing alloy led to an increment of another 30 MPa in yield strength. In addition, Er markedly increased the thermal stability of the alloy structure.     

Effects of solution treatment on the microstructure and mechanical properties of Al–Cu–Mg–Ag alloy

The effects of solution treatment on the microstructure and mechanical properties of Al–Cu–Mg–Ag alloy were studied by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), differential scanning calorimeter (DSC), transmission electron microscopy (TEM) and tensile test, respectively. The results show that the mechanical property increases and then decreases with increasing the solution temperature. And the residual phases are dissolved into the matrix gradually, the number fraction of the precipitation and the size of recrystallized grains increase. Compared to the solution temperature, the solution holding time has less effect on the microstructure and the mechanical properties of Al–Cu–Mg–Ag alloy. The overburnt temperature of Al–Cu–Mg–Ag alloy is 525 °C. The yield strength and the elongation get the best when the alloy is solution treated at 515 °C for 1.5 h, is 504 MPa and 12.2% respectively. The fracture mechanism of the samples is ductile fracture.     

Microstructure features affecting mechanical properties and corrosion behavior of a hypoeutectic Al–Ni alloy

The aim of this article is to analyze the influence of microstructural parameters on the mechanical properties and corrosion behavior of a hypoeutectic Al–Ni alloy. Experimental results include secondary dendrite arm spacing, corrosion potential, current density, pitting potential, ultimate tensile strength and yield strength. It was found that cooling rates during solidification of about 0.6 °C/s and 8 °C/s can provide secondary dendritic spacings of 7 μm and 16 μm, respectively. Although the microstructure having their phases finely and homogeneously distributed was shown to induce better mechanical properties and higher pitting potential, its general corrosion resistance decreased when compared with the corresponding results of the coarser microstructure.     

Elevated-temperature plasticity and mechanical properties of a rare earth-modified Mg-Zn-Al alloy

The elevated-temperature plasticity and flow behavior of an Er-modified, heat-resistant ZA73 alloy was evaluated by thermal simulation. The results showed that the addition of Er to ZA73 alloy notably improves the deformability and higher strain rate and temperature favors hot deformation. Bars with sound surface quality were successfully extruded at 350 °C and a strain rate of ~ 0.1 s^− 1. Furthermore, dynamic precipitation of nano-sized spherical τ phase was found to occur uniformly in the α-Mg matrix during hot extrusion, which is considered helpful to both strength and plasticity enhancement. The yield strength and ultimate tensile strength of the as-extruded bars reached 240-265 MPa and 355-360 MPa, respectively, while maintaining a large elongation rate of 18-19.5%.     

Creep of Nextel™720/alumina–mullite ceramic composite at 1200 °C in air,argon, and steam

The tensile creep behavior of an oxide–oxide continuous fiber ceramic composite was investigated at 1200 °C in laboratory air, in steam and in argon. The composite consists of a porous alumina–mullite matrix reinforced with laminated, woven mullite/alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. The tensile stress–strain behavior was investigated and the tensile properties measured at 1200 °C. The elastic modulus was 74.5 GPa and the ultimate tensile strength was 153 MPa. Tensile creep behavior was examined for creep stresses in the 70–140 MPa range. Primary and secondary creep regimes were observed in all tests. Creep run-out (set to 100 h) was achieved in laboratory air for creep stress levels ?91 MPa. The presence of either steam or argon accelerated creep rates and reduced creep lifetimes. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Effects of thermal treatment on precipitate shape and mechanical properties of Mg–8Gd–4Y–Nd–Zr alloy

Precipitation reactions in a Mg–8Gd–4Y–Nd–Zr alloy have been investigated using TEM, HREM, hardness measurements and tensile testing. Globular β′ precipitates, which were different from the typical plate-shaped β′ precipitates usually observed in Mg–Gd-based alloys, were detected in the 160 °C/192 h-aged sample. Instead of dissolution and then precipitating as plate-shaped β′ precipitates, the formed globular β′ precipitates grew up when further aged at 215 °C, which resulted in the decrease in strength comparing with that of the 215 °C single-stage aged samples. Two-stage ageing treatments on the alloy demonstrated that ageing 192 h at 150 °C plus 16 h at 215 °C made the ultimate strength and the yield strength improved 17 MPa and 13 MPa, respectively.     

Microstructure and mechanical properties of a sand-cast Mg–Nd–Zn alloy

The microstructures and mechanical properties of a sand-cast Mg–Nd–Zn alloy in the as-cast, solution-treated and peak-aged conditions were investigated. The as-cast alloy was comprised of α magnesium matrix and Mg₁₂Nd eutectic compounds. The eutectic compounds dissolved into the matrix and small Zr-containing particles precipitated at grain interiors, due to the solution treatment. After the solution treatment, two kinds of cooling manner, either cooling in air or quenching in water, were employed. It was worth noting that some basal precipitates formed in the matrix during the in-air cooling process after solution treatment, which led to the succedent weak ageing hardening response and low strength in peak-aged condition. The hardness, yield strength, ultimate tensile strength and elongation at room temperature, of the samples in the T61 condition, were HV81, 191 MPa, 258 MPa and 4.2%, respectively. When tensile tested at high temperature, they exhibited serrated flow. Moreover, the casting surface of the tensile testing bar also had a great influence on its mechanical properties.     

Effects of neodymium rich rare earth elements on microstructure and mechanical properties of as cast AZ31 magnesium alloy

Abstract

The effects of neodymium rich rare earth elements [RE(Nd)] on microstructure and mechanical properties of as cast AZ31 magnesium alloy were investigated. The microstructures of as cast AZ31–xRE(Nd) alloys display a dendrite configuration, and the secondary dendrite spacing of the α-Mg phase was decreased with the increasing Nd content. The addition of RE(Nd) resulted in the formation of Al₂Nd and Mg₁₂Nd phases. Mechanical properties were improved significantly due to grain refinement and precipitation of intermetallic phases. When the amount of RE is 1·0 wt-%,The as cast AZ31 alloy reached its maximum tensile strength of 249 MPa at room temperature, yield strength of 169 MPa and elongation of 9·0%.     

Formability,mechanical and corrosive properties of Mg–Nd–Zn–Zr magnesium alloy seamless tubes

Some large Mg–3.0Nd–0.2Zn–0.4Zr (NZ30K) magnesium alloy seamless tubes were prepared by forward extrusion. The as-extruded tubes were cooled in the air or by spraying liquid N₂ after extrusion. The formability, mechanical and corrosive properties of the NZ30K magnesium alloy seamless tubes were investigated. The experimental results show that seamless NZ30K tubes with an outer diameter of 110 mm and inner diameter of 90 mm can be produced by forward extrusion and the tubes have good roundness, concentricity and straightness even without any straightening. The tensile results show that the maximum ultimate tensile strength, yield strength and elongation of the extruded tubes cooled in the air and by spraying liquid N₂ are 306.3 and 314.6 MPa, 250.4 and 270.3 MPa, 14.2% and 15.6%, respectively. The corrosion rates of the as-extruded tubes cooled in the air and by spraying liquid N₂ immersed in 5% NaCl solution for 3 days are 0.225 and 0.234 mg cm⁻² day⁻¹, respectively, which are a little inferior to the as-cast, T4 and T6 NZ30K alloys, but much lower than that of AZ91 alloy. Localized corrosion is suggested to be its corrosion pattern.     

Effect of isothermal extrusion parameters on mechanical properties of Al-Si eutectic alloy

In this paper, mechanical properties of a deformed Al-Si eutectic alloy processed by isothermal extrusion at temperature from 573 K to 773 K with reduction ratio from 25% to 85% were investigated at ambient temperature. The results showed that a banded structure composed of matrix region and accumulation region of second phase particles was formed and a few cracks were generated in particles and evolved to voids among particles. The tensile strength of test specimens ranged from 250 MPa to 400 MPa and was directly related with temperature from 623 K to 773 K. The elongation of test specimens ranged from 2.8% to 13.1%, and had a peak value at 673 K under each section reduction ratio. A reduction in elongation occurred at section reduction ratio larger than 75% because of particle bands splitting aluminum matrix severely. The effect of temperature on mechanical properties was more significant than that of section reduction ratio. Excellent balance between strength and ductility can be obtained by extrusion at temperature 623-723 K and section reduction ratio 40-70%.     

Effects of Al and Zn additions on mechanical properties and precipitation behaviors of Mg-Sn alloy system

For the Mg-Sn-Al alloys, the ultimate strength (UTS) of an extruded Mg-9wt.%Al-2wt.%Sn alloy reached 390 MPa. TEM observation indicated that plate-like Mg₁₇Al₁₂ precipitates having Burgers orientation relationship with the matrix are responsible for the strength. This alloy also exhibits an age hardening behavior: the peak hardness appears after 15-20 h of heat treatment at 473 K. On the other hand, the UTS of the Mg-Mg-Sn-Zn alloys are on the order of 300 MPa. The precipitates in these alloys are composed of the Mg₂Sn and MgZn₂ particles. It was found that these phases often precipitate together, suggesting that the MgZn₂ phase can act as a nucleation sites for Mg₂Sn.     

Microstructure, mechanical properties and electrical conductivity of industrial Cu-0.5%Cr alloy processed by severe plastic deformation

Microstructure, mechanical properties and electrical conductivity of industrial Cu-0.5% alloy subjected to equal channel angular pressing (ECAP) by route A and cold rolling with and without aging treatment were investigated. The lamellar grains in thickness of 100 nm were obtained after eight ECAP passes. They were not further pancake shaped, but fragmentary and obtained less sharp boundaries with more dislocations in addition to cold rolling. After aging at 450 °C for 1 h, high density of dislocations and some coarse grains were observable after ECAP and the additional cold rolling, respectively. The tensile tests show that tensile strength arrived at 460 MPa and 484 MPa after four and eight passes of ECAP, respectively, the corresponding tensile strength increased to 570 MPa and 579 MPa after the additional cold rolling. However, the electrical conductivity was not more than 35% IACS. It was proved that four passes of ECAP followed by 90% cold rolling and aging at 450 °C for 1 h offered a short process for Cu-0.5%Cr alloy to balance the paradox of high strength and electrical conductivity, under which the tensile strength 554 MPa, elongation to failure 22% and electrical conductivity 84% of IACS could be obtained. The high strength was explained by precipitation strengthening and fine grain strengthening.     

Effect of brazing temperature on tensile strength and microstructure for a stainless steel plate-fin structure

This paper presented a vacuum brazing technology for 304 stainless steel plate-fin structures with BNi2 filler metal. The effect of brazing temperature on tensile strength and microstructure has been investigated. The tensile strength is increased along with the increasing of brazing temperature. The microstructure is very complex and some Boride compounds are generated in the brazed joint. Full solid solution can be generated in the middle zone of joint when the brazing temperature is increased to 1100 °C. The brittle phases always exist in the fillet no matter how the brazing temperature changes, but the microstructure in fillet becomes more uniform and the tensile strength is increased with the brazing temperature increasing. In total, the brittle Boride compounds are decreased with the brazing temperature increase. Brazing with a filler metal thickness 105 μm and 25 min holding time, 1100 °C is the best suitable brazing temperature and a tensile strength of 82.1 MPa has been achieved for 304 stainless steel plate-fin structure.     

Effect of Al on the microstructure and mechanical properties of Mg–6Zn–2Sn–0.5Mn alloy

The microstructure and mechanical properties of Mg–6Zn–2Sn–0.5Mn–xAl (x?=?0, 1, 2, 3) alloy are investigated. The addition of Al leads to the refinement of grain size and the formation of Al₆Mn, Mg₃₂(Al,Zn)₄₉ also forms when the amount of Al is higher than 2?wt-%. Because of the addition of Al, the precipitates in the alloy after ageing treatment are refined. The alloy containing 1?wt-% Al shows good mechanical properties in the as-cast state which is attributed to the refined grains and low volume fraction of large second phases, it also shows high strength after ageing treatment resulted mainly from the homogeneously distributed fine precipitates, the yield strength, ultimate tensile strength and elongation are 183, 310?MPa and 11%, respectively.     

机械零部件用高强镁合金加工工艺与组织性能研究

针对Mg-Gd-Y合金塑性较差的问题,研究了固溶态和不同温度锻造加工态高强Mg-Gd-Y合金的组织与性能。结果表明,固溶态Mg-Gd合金的晶粒尺寸不均匀,平均尺寸约225μm;当锻造加工温度为440℃和410℃时,合金中第二相的数量较多,大量弥散分布的第二相的存在可以抑制动态再结晶的形成;随着锻造加工温度的降低,Mg-Gd合金的抗拉强度和屈服强度呈现逐渐升高的趋势,在锻造加工温度为470℃时,Mg-Gd合金的断后伸长率达到最大值19.2%,降低锻造加工温度至440℃和410℃时,断后伸长率反而有所降低;固溶态Mg-Gd合金的拉伸断口呈现脆性断裂的特征;锻造加工温度为500℃的拉伸断口呈现混合断裂特征,而锻造加工温度为410℃、440℃和470℃时Mg-Gd合金的断口都呈现为韧性断裂特征。