Microstructure and mechanical properties of squeeze cast and semi-solid cast Mg–Al alloys

Magnesium–aluminium castings produced by means of squeeze casting, new rheocasting and thixocasting have been investigated. These casting processes provide very different microstructures consisting of α-Mg and β-Mg₁₇Al₁₂. The shape and distribution of the brittle β-Mg₁₇Al₁₂ has a large influence on mechanical properties. Isolated particles of β-phase in squeeze cast components are less detrimental to ductility than the continuous β-phase network found in semi-solid processed parts. A heat treatment results in complete dissolution of β-Mg₁₇Al₁₂ and accounts for significant improvements of ductility and fracture toughness. Crack propagation in solution heat treated Mg–Al castings is associated with extensive twinning.     

Influence of grain refinement on some mechanical properties of non ferrous cast alloys

The influence of the grain refining method on some mechanical properties of non ferrous casting, by analyzing published data, is presented. Grain refining methods are grouped into: cooling rate control; nucleant agents addition; and mechanical agitation. Grain size decrease influence on the reported mechanical properties can be grouped into: decrease; improvement; and no observed influence. This analysis shows that there is a relationship between the grain refining method and the mechanical behavior of the studied alloys.     

Mn含量对铸造高铌TiAl合金组织和性能的影响

选择两种Mn含量不同的高铌TiAl合金Ti46Al8Nb2Mn0.2B和Ti46Al8Nb1.3Mn0.2B,通过热等静压(HIP)及后续热处理,结合组织和力学性能的分析,研究了Mn含量对高铌TiAl合金的组织和性能的影响.XRD和SEM背散射电子实验结果表明:Mn含量较高的Ti46Al8Nb2Mn0.2B合金,经热等静压及循环热处理,得到的双态组织较粗大,并且有少量脆性β相存在;Mn含量较低的Ti46Al8Nb1.3Mn0.2B合金,经热等静压后直接在双相区长时间保温处理,得到了细小的双态组织,并且完全消除了β相.室温拉伸实验表明,Mn含量的降低提高了Ti46Al8Nb1.3Mn0.2B合金的力学性能,其延伸率、屈服强度和断裂强度分别达到2.4%、548MPa和660MPa.断口形貌分析表明,室温下两种合金都属于解理断裂.     

Influence of Al-Si additions on mechanical properties and corrosion resistance of Mg-8Li dual-phase alloys

Sheet samples of Mg-8Li,Mg-8Li-3Al,Mg-8Li-3AlSi and Mg-8Li-5AlSi alloys were obtained by hot rolling.Optical microscope,microhardness tester,nanoindentor,X-ray diffractometer and electrochemical analyzer were adopted to investigate the microstructures,micro-mechanical properties and corrosion resistance.Roller was preheated to 150°C before rolling process,and rolling reduction designed was about20% per pass with a total rolling reduction of 84%.The rolled plates were annealed at 200°C for 120 min.The tensile tests were performed at room temperature.Experimental results showed that both the strength and corrosion resistance of theα+βdual-phase of Mg-Li alloy were significantly improved with adding Al-Si elements.The strength enhancement was attributed to the solid solution of Al into theα-Mg matrix and into theβ-Li matrix as well as to the precipitation strengthening of Mg2 Si particles.Besides,the dendrite grains ofα-Mg transformed to equiaxed ones with addition of Al into alloy Mg-Li.     

Automotive copper and magnesium containing cast aluminium alloys: Report on the correlation between Yttrium modified microstructure and mechanical properties

An experimental investigation is carried out in order to measure and thus report the effect of SDAS values on the mechanical properties of the Yttrium modified casting Al alloys. Microstructural features and the alloy composition determine the mechanical properties of the as-cast Al alloys. The solidification rate plays the main role on the microstructural characteristics including primary and secondary dendrites arm spacing (DAS and SDAS), also the fraction, size and distribution of defects, eutectic silicon and intermetallic phases. In this study, Yttrium is used as a chemical modifier to the cast Al alloys in an attempt to transform the acicular shape of the Si to the fibrous morphology. The investigation of the samples show the presence of the pores and voids in the microstructure of the as-cast Y modified alloy. The typical cast microstructure shows dendritic Al accompanied with eutectic Si and Al₂Cu, Al₁₅(Fe, Mn)₃Si₂ and other complex multi-element intermetallic. Based on the findings of the tensile tests, strength of the Y modified alloy declines with increasing the SDAS values.     

Elevated temperature mechanical properties and residual tensile properties of two cast superalloys and several nickel-base oxide dispersion strengthened alloys

The elevated temperature tensile, stress-rupture and creep properties and residual tensile properties after creep straining have been determined for two cast superalloys and several wrought Ni-16Cr-4Al-yttria oxide dispersion strengthened (ODS) alloys. The creep behavior of the ODS alloys is similar to that of previously studied ODS nickel alloys. In general, the longitudinal direction is stronger than the long transverse direction, and creep is at least partially due to a diffusional creep mechanism as dispersoid-free zones were observed after creep-rupture testing. The tensile properties of the nickel-base superalloy B-1900 and cobalt-base superalloy MAR-M509 are not degraded by prior elevated temperature creep straining (at least up to 1 pct) between 1144 and 1366 K. On the other hand, the room temperature tensile properties of ODS nickel-base alloys can be reduced by prior creep strains of 0.5 pct or less between 1144 and 1477 K, with the long transverse direction being more susceptible to degradation than the longitudinal direction.     

Grain coarsening in semi-solid state and tensile mechanical properties of thixoformed AZ91D-RE

For thixoforming to be possible,the microstructure of the starting material must be non-dendritic,which can be obtained by the strain induced melt activation(SIMA)route.Based on the SIMA route,as-cast AZ91D alloy with the addition of yttrium was deformed by cyclic closed-die forging(CCDF).Microstructure evolution of CCDF formed AZ91D-RE alloy during partial remelting were investigated.Furthermore,the mechanical properties of thixoformed AZ91D-RE magnesium alloy components were also studied.The results showed that prolonged holding time resulted in grain coarsening and the improvement in degree of spheroidization.The coarsening behaviour of solid grains in the semi-solid state obeyed Ostwald ripening mechanism.The coarsening rate constant of CCDF formed AZ91D-RE during partial remelting was 324 um3/s at 550℃.The value of yield strength,ultimate tensile strength and elongation to fracture of four-pass CCDF formed AZ91D-RE magnesium alloy were 214.9,290.5 MPa and 14%,respectively.Then the four-pass CCDF formed alloys were used for thixoforming.After holding at 550℃ for 5 min,the values of yield strength,ultimate tensile strength and elongation to fracture of thixoformed component were 189.6 MPa,274.6 MPa and 12%,respectively.However,prolonged holding time led to remarkable decrease in mechanical properties of thixoformed components.     

In-Situ observations of oxidation and phase stability in cast nickel-based intermetallic alloys

The effects of the as-cast microstructure on the oxidation characteristics of two Ni-Al-Cr alloys with either γ or γ′ primary solidification were investigated with an in-situ, time-resolved X-ray diffraction (TRXRD) technique using synchrotron radiation. The measurements, carried out during rapid heating and cooling, showed that a segregated microstructure in these cast alloys leads to the preferential formation of zirconium oxide before the formation of aluminum oxides is detected. The oxidation leads to a change in the phase stability and to the modification of surface microstructures. Computational thermodynamic models were used to explain the preferential formation of oxides in the as-cast microstructure.     

Microstructure and mechanical properties of Mg-Zn-Y-Nd-Zr alloys

The microstructure and tensile properties of the as-cast and solution treatment Mg-4.5Zn-1Y-xNd-0.5Zr (x=0, 1 wt.%, 2 wt.%, 3 wt.%) alloys were investigated. The results showed that the microstructure of Mg-4.5Zn-1Y-0.5Zr alloy consisted of α-Mg, Zn-Zr, W (Mg₃Y₂Zn₃) and I (Mg₃YZn₆) phases. With the addition of Nd, I-phase disappeared and Mg₃Y₂Zn₃ phase changed into Mg₃(Nd,Y)₂Zn₃ phase. When the content of Nd reached 3 wt.%, T phase, i.e., ternary Mg-Zn-Nd phase, formed. In addition, with the increase of Nd content in the alloys, the secondary dendritic arm spacing decreased, while the amount of intermetallic phases increased. For as-cast Mg-4.5Zn-1Y-xNd-0.5Zr alloys, after solution treatment, microsegregation was eliminated and the shape of eutectic structure of α-Mg+W transformed from lamellar into spherical. The tensile strength and elongation of Mg-4.5Zn-1Y- 3Nd-0.5Zr alloy were increased from 219.2 MPa and 11.0% to 247.5 MPa and 20.0%, respectively.     

硼含量对Ni-Cr-Mo合金热腐蚀性能的影响

采用真空液相烧结法制备了4种掺加不同B含量的Ni-Cr-Mo合金,研究了B对其组织与性能的影响。研究结果表明,B与Mo、Cr、Ni等合金元素在烧结时可以形成共晶液相,通过原位化学反应,生成Mo2NiB2、(Mo,Cr)2NiB2陶瓷相。热腐蚀性试验表明,形成的硼化物相具有较好的耐腐蚀性,能够有效提高Ni-Cr-Mo合金的耐热腐蚀性能。     

Microstructure and properties of cast ingots of Al-Zn-Mg-Cu alloys modified with Sc and Zr

The effect of combined additions of Sc and Zr on the microstructure and tensile properties of the direct chill (DC) cast ingots of developmental Al-Zn-Mg-Cu alloys has been evaluated in this work. The properties in both the longitudinal and transverse directions were determined in as-cast and cast-plus-heat-treated conditions, at room temperature (RT) and cryogenic temperature (CT). Extensive microstructural evaluation was carried out using optical microscopy and scanning electron microscopy, including orientation image microscopy (OIM) by the electron backscatter diffraction (EBSD) technique. The Sc-containing developmental cast alloys showed the tensile properties, which are much better than the properties of commercial cast Al alloys and are similar or even superior to the properties of 7075-T6 alloy forgings. The microstructural evolution, the strengthening mechanisms, the optimum content of the dispersoid-forming elements, and the processing-structure-property correlations are discussed.     

Structure and mechanical properties of Fe-Cr-Mo-C alloys with and without boron

A study of the structure and mechanical properties of Fe-Cr-Mo-C martensitic steels with and without boron addition has been carried out. Nonconventional heat treatments have subsequently been designed to improve the mechanical properties of these steels. Boron has been known to be a very potent element in increasing the hardenability of steel, but its effect on structure and mechanical properties of quenched and tempered martensitic steels has not been clear. The present results show that the as-quenched structures of both steels consist mainly of dislocated martensite. In the boron-free steel, there are more lath boundary retained austenite films. The boron-treated steel shows higher strengths at all tempering temperatures but with lower Charpy V-notch impact energies. Both steels show tempered martensite embrittlement when tempered at 350 °C for 1 h. The properties above 500 °C tempering are significantly different in the two steels. While the boron-free steel shows a continuous increase in toughness when tempered above 500 °C, the boron-treated steel suffers a second drop in toughness at 600 °C tempering. Transmission electron microscopy studies show that in the 600 °C tempered boron-treated steel large, more or less continuous cementite films are present at the lath boundaries, which are probably responsible for the embrittlement. The differences in mechanical properties at tempering temperatures above 500 °C are rationalized in terms of the effect of boron-vacancy interactions on the recovery and recrystallization behavior of these steels. Although boron seems to impair room temperature impact toughness at low strength levels, it does not affect this property at high strength levels. By simple nonconventinal heat treatments of the present alloys, martensitic steels may be produced with quite good strength-toughness properties which are much superior to those of existing commercial ultra-high strength steels. It is also shown that very good combinations of strength and toughness can be obtained with as-quenched martensitic steels.     

Influence of impurities on the properties of powder metallurgy and cast iron-manganese alloys

Translated from Poroshkovaya Metallurgiya, No. 6(306), pp. 56–61, June, 1988.     

Effect of alloying elements on the structure and properties of Al-Li-Cu cast alloys

The structure and mechanical and service properties of Al-Li-Cu alloys are determined, and the results obtained are used to find the effect of major and additional components on their properties and to design a high-strength heat-treatable cast alloy with a low density. After quenching and maximum-strength aging, this alloy has the following level of mechanical properties (casing in a metal mold): σ_u = 360–370 MPa and δ = 6.0–7.5%.     

Study on microstructure and mechanical properties of as-cast Mg-Sn-Nd alloys

Microstructure, tensile properties and compressive creep behaviors of Mg-(1.65-11.52) wt.% Sn-2 wt.% Nd alloys were studied in this paper. The microstructure of the as-cast Mg-Sn-Nd alloys consisted of dendritic α-Mg, Mg2Sn and Mg-Sn-Nd ternary phase containing rare earth element. The highest ultimate tensile strength of 140 MPa and percentage elongation after fracture of 9.7%, were achieved with a composition of Mg-8.23 wt.% Sn-2 wt.% Nd. The compressive creep resistance of Mg-8.23 wt.%Sn-2 wt.% Nd alloy w...