Effect of eutectic phase on damping and mechanical properties of as-cast Mg-Ni hypoeutectic alloys

Dynamic mechanical analysis (DMA) was applied to systematically investigate the low frequency damping properties of as-cast hypoeutectic Mg-Ni alloys. The results show that the as-cast hypoeutectic Mg-Ni alloys exhibit high damping capacities. The strain amplitude dependent damping curve has its own special characteristic, in which the damping is strongly related to the strain amplitude. The effect of the eutectic phase on damping and the mechanical properties of as-cast hypoeutectic Mg-Ni alloys were also discussed in detail.     

Effect of grain refinement on phase transformation behavior and mechanical properties of Cu-based alloy

A small amount of misch metal was added to Cu-Zn-Al alloy in order to study its effect on grain refinement, mechanical properties, phase transformation behavior and stabilization of martensite. It is found that the addition of misch metal is very effective for reducing the grain size. The coarse grains over 1 000 μm are refined to the size of 30 μm by the addition of 0.43% (mass fraction) misch metal. The grain size of thermo-mechanically treated alloys is barely affected by cold working. The fracture strength and ductility increase significantly with the increase of misch metal content when tensile test is carried out below M_f temperature. Also, the fracture strength is larger in the case of post-quench ageing treatment than that in the case of direct quench ageing treatment. The fracture mode is changed from transgranular brittle fracture to ductile fracture with void formation and coalescence by the addition of misch metal.     

The effect of Mn on the mechanical behavior of Al alloys

Manganese has been known to be an alloying element of Al alloys that contributes to uniform deformation. Recently, it was found that as the manganese content increases over 0.5 wt.% in such aluminum alloys as the 6000, and 7000 series alloys, both yield and ultimate tensile strength increase significantly without decreasing ductility. The added manganese forms a manganese dispersoid of Al₆Mn. This dispersoid has an incoherent structural relationship with respect to the matrix, FCC, in retarding the motion of dislocations that increase strength. Once the dislocation is blocked by the dispersoid, it tends to change the slip system by means of cross-slip. This cross-slip allows the deformation to maintain uniformly good ductility. TEM observation has proven the above mentioned activities of dislocation by analyzing the characters of the dislocations around and away from the dispersoids. Adding manganese to aluminum alloys not only enhances tensile strength but also significantly improves low-cycle fatigue resistance. Corrosion resistance is also measurable improved by the addition of manganese. After extrusion, the recrystallization is also retarded so that a very small grain size is maintained, contributing to an improvement in the mechanical properties. This article is based on a presentation made in the symposium “The 1^st KIM-JIM Joint Symposium: High Strength Ratio Aluminum Alloys”, held at Inha University, Inchon, Korea, October 22, 1999 under the auspices of The Korean Institute of Metals and Materials and The Japanese Institute of Metals.     

Effect of second phase precipitation behavior on mechanical properties of casting Al-Cu alloys

The effect of the second phase precipitation behavior on the mechanical properties and fracture behavior of the modified casting Al-Cu alloys was investigated. The tensile strength of the alloys increases firstly and then decreases due to the appearance of θ＇ precipitation phases, which increases firstly and then become coarser with the aging time increasing from 10 h to 20 h at 155 ℃. The strength of the alloys reaches the peak, resulting from ,Ω and θ＇ precipitation phases, and decreases due to ,Ω phases becoming coarser and θ＇ precipitation decreasing with the aging time increasing from 10 h to 20 h at 165 ℃. ,θ phase becoming coarser and θ＇ precipitation decreasing result in the strength of the alloys drastically decreasing after aging at 175 ℃ for 20 h. The ductility remains high level with increasing aging time at 155 ℃. The ductility irregularly changes as aging time prolongs at 165 ℃. The ductility is very low and at the same time gradually decreases with increasing aging time at 175 ℃. The Al-Cu alloy with a promising combination of tensile strength and ductility of about 474 MPa and 12.0% after aging at 165℃ for 10 h is due to a dense, uniform distribution of,Ω precipitation phases together with a heterogeneous distribution of θ＇ precipitations.     

Effects of phase composition on the mechanical properties and damping capacities of as-extruded Mg-Zn-Y-Zr alloys

The present work mainly investigated the microstructures, mechanical properties, and damping capacities of as-extruded Mg-Zn-Y-Zr alloys with varied phase composition. Alloys of MgZn₂, W-phases (Mg₃Y₂Zn₃), I-phases (Mg₃YZn₆), and X-phases (Mg₁₂YZn) were obtained by adjusting the Zn/Y ratio (in wt%). The crystallographic structure of the X-phase [long period stacking ordered (LPSO) phase] and the crystallographic relationship between the W-phase and the Mg matrix were determined. The strengthening effects of the phase composition on the alloys exhibited the following trend: W + LPSO > LPSO>W + I > MgZn₂. Variations in the phase composition resulted in almost consistent variations in the damping capacities of the alloys compared with their mechanical properties. The LPSO structural phase could enhance the mechanical properties and simultaneously maintain the good damping capacity of the alloys.     

变形条件对SiCp/2014Al复合材料力学行为和晶粒度的影响

研究了变形温度,应变速率和变形程度等热加工参数对ＳｉＣｐ／２０１４Ａｌ复合材料力学行为和晶粒度的影响。结果表明,该材料的流动应力对变形温度和应变速率比较敏感。变形每升高４０℃,流动应力降低５￣１０ＭＰａ,应变速率每提高一个数量级,流动应力增加１．３￣１．８们。提高应变整编有利于获得细晶组织。以足够的变形程度锻造以及适当提高锻后冷却速度均有利于获得细晶组织。根据实验结果确定了该材料的流动应力与热加工     

Room temperature mechanical behavior of silicon-doped TiAl alloys with grain sizes in the nano- and submicron-range

Nano- and submicron-grained intermetallic compounds consisting of γ-TiAl and ξ-Ti₅(Si,Al)₃ were produced by high energy milling and hot isostatic pressing. Owing to the pure and controlled processing conditions, the mechanical properties may be indubitably related to the microstructure. Both yield strength and hardness show a Hall–Petch-type dependence on grain size, resulting in extremely high flow and fracture stresses under compression of up to 3 GPa. With a reduction of grain size, the coefficient of strain hardening as well as the compressive fracture strain decrease and drop to zero for alloys with grain sizes of about 150 nm. Deformation at room temperature is accomplished by dislocation glide and mechanical twinning, with twinning attaining more and more importance as the grain size is further reduced. Diffusion-controlled deformation mechanisms can be ruled out even for intermetallics with crystallite sizes as small as 50 nm. A room temperature ductilization of intermetallic compounds by switching to a nanocrystalline microstructure seems to be rather unlikely.     

Thermal evolution and grain boundary phase transformations in severely deformed nanograined Al–Zn alloys

The structure, phase composition and thermal evolution of binary Al–Zn alloys were studied before and after high-pressure torsion (HPT) in Bridgman anvils. On heating of HPT-deformed samples from room temperature to 300 °C, Zn grains dissolved, and a relatively fine-grained (Al) equilibrium solid solution formed. Differential scanning calorimetry curves reveal two-stage melting in the Al–Zn alloys studied, i.e., the melting of the (Al) solid solution starts 10–25 °C below the bulk solidus line. The effect is more pronounced in fine-grained samples. It is explained by the presence of layers of liquid-like phase in the (Al) grain boundaries (GB) between bulk and GB solidus lines. The new metastable GB solidus line appears in the (Al) single-phase region of the Al–Zn phase diagram, it can be compared with the metastable solvus lines for the formation of GP zones and α phases in the (Al)+Zn two-phase area.     

Effect of cold work on the tensile properties of 6061, 2024, and 7075 Al alloys

Aluminum alloys 6061, 2024, and 7075 were heat treated to various tempers and then subjected to a range of plastic strain (stretching) in order to determine their strain limits. Tensile properties, conductivity, hardness, and grain size measurements were evaluated. The effects of the plastic strain on these properties are discussed and strain limits are suggested.     

Characteristics of rapidly solidified Al 7075-xwt.%Mn alloys

The present study demonstrates that the Mn addition to conventional Al 7075 can readily increase the strength level over 15% without sacrificing the tensile ductility. Up to 2.5wt.% of uniformly distributed Mn addition was successfully obtained with a rapid solidification technique. Both rod- and sphere-types of Mn-containing precipitates were observed in the RS/PM Al 7075-xMn alloys, and these precipitates were responsible for the increase in tensile strength.     

组织及相结构对Cu-Al-Be-B形状记忆合金阻尼与力学性能的影响

研究了Cu Al Be B形状记忆合金的成分、微观组织和相结构与阻尼和力学性能的关系 ,通过扫描电镜原位观察合金在单向拉伸应力作用下裂纹萌生和扩展的特征。结果表明 :应力诱发马氏体变体的转变使Cu Al Be B合金具有较高的阻尼性能 ;弹性各向异性和相变应变差引发晶界应力集中 ,导致合金在拉伸条件下发生晶间断裂 ;18R与 2H马氏体混合程度大时 ,会增加相变应变差引起的晶界应力集中和可能萌生的晶界裂纹源 ,大大减小合金的塑性和强度。     

Cavitation in superplastic 7075Al alloys prepared via friction stir processing

The effect of strain rate, temperature, and grain size on the cavitation of superplastically deformed 7075Al alloys prepared by friction stir processing (FSP) was systematically examined. The nucleation of cavities was generally observed to be associated with the grain triple junctions and coarse particles. While new cavities were continuously nucleated during deformation, the cavity density did not increase above a certain strain due to significant cavity coalescence and linkage. The density, size, and volume fraction of cavities increased with increasing initial strain rates from 1×10⁻² to 1×10⁻¹ s⁻¹. Increasing the temperature from 450 to 510 °C resulted in a decrease in the cavity density. However, the specimen deformed at the optimum superplasticity temperature of 480 °C exhibited the lowest cavity volume fraction and ratio of large-size cavities. The decrease in the grain size from 7.5 to 3.8 μm resulted in a significant decrease in the density, size, and volume fraction of cavities. The growth of cavities was controlled by plasticity, and the cavity growth rate parameter, η, decreased with decreasing grain size and strain rate. FSP aluminum alloy exhibited lower cavity level and higher critical strain compared to a thermo-mechanically processed one.     

The effect of vanadium and grain refiner additions on the nucleation of secondary phases in 1xxx Al alloys

High purity Al–0.3 wt% Fe–0.1 wt% Si alloys with different Si, V and grain refiner contents were melt spun to produce microstructures of submicron secondary phases entrained in a higher melting point Al matrix. On reheating, a dispersion of eutectic liquid droplets forms that represents an exaggerated version of the liquid puddles that solidify pinched-off between Al dendrite arms during conventional casting. The subsequent resolidification of the droplets, analysed using differential scanning calorimetry (DSC), allows the nucleation-controlled aspects of secondary phase selection to be studied. The droplets solidify as the metastable FeAl_m phase in ribbons containing ≃500 ppm V or ≃100 ppm V plus Al–Ti–B, Al–Ti–C or Al–B grain refiner. This phase contributes to the “fir-tree” surface defect in commercial sheet products. This work suggests that the combination of V and Al–Ti–B promotes FeAl_m in commercial ingots, and confirms that solidification rate and bulk Si content also influence phase content.     

Mn元素对高纯Mg-3Al合金晶粒尺寸的影响

研究Mn元素对高纯Mg-3Al(质量分数,%)合金晶粒尺寸的影响。结果表明:当合金中的Mn元素含量小于0.21%(质量分数)时,合金的晶粒尺寸变化不大;但当Mn元素含量大于0.21%时,Mn元素的存在则使合金的晶粒尺寸迅速增大;Mn元素的作用主要与影响熔体中含碳晶核的形核能力有关;当Mn元素的含量小于其溶解度时,Mn元素对含碳晶核的形核能力影响不大,但当Mn元素的含量达到或超过其溶解度时,Mn元素对含碳晶核的影响迅速增大,最终导致含碳晶核的形核能力降低,引起晶粒粗化。     

Mechanical properties of intermetallic inclusions in Al 7075 alloys by micropillar compression

The constituent particles (also called inclusions) play an important role in the deformation behavior of Al 7075 alloys. The majority of inclusions in Al 7075 alloys can be classified as Fe-bearing and Si-bearing inclusions. Among them Al₇Cu₂Fe and Mg₂Si are predominant. In this study, the mechanical properties of these inclusions and Al 7075 matrix were studied using micropillar compression testing. Micropillars were fabricated by focused ion beam (FIB) milling and were tested using a nanoindenter equipped with a flat tip. For the first time, the stress–strain behavior of these intermetallic particles was obtained experimentally, resulting in the measurement of the compressive failure and yield strength. The stress–strain behavior obtained from pillar compression show that both inclusions possess higher strength than the Al 7075 matrix.     

Revisiting the role of peritectics in grain refinement of Al alloys

The grain refining effect of four peritectic-forming solutes (Ti, V, Zr and Nb) as well as three eutectic-forming solutes (Cu, Mg and Si) on pure Al was investigated. Significant grain refinement is observed by the addition of peritectic-forming solutes, whereas the addition of eutectic-forming solutes only slightly decreases the grain size. The mechanisms underlying the grain refinement of these alloys were then studied by a new analytical methodology for assessing grain refinement that incorporates the effects of both alloy chemistry and nucleant potency. It is found that the low degree of grain refinement by the addition of eutectic-forming solutes is mainly attributed to the segregating power of solutes, i.e. the constitutional undercooling contribution. However, peritectic-forming solutes do not only cause grain refinement by their segregation power but, more importantly, they introduce copious potent nuclei into the melt and promote significant grain refinement via heterogeneous nucleation.     

后热处理对激光焊接7075铝合金显微组织与力学性能影响

采用激光自熔焊接技术制备7075高强铝合金接头,研究了后热处理对接头显微组织与拉伸性能的影响. 采用SEM与EBSD系统表征了后热处理对7075Al接头显微组织演变与拉伸断裂行为的影响. 结果表明,焊缝中心到母材依次存在等轴晶区域、柱状晶区域、胞状晶区域与母材轧制态区域,并且胞状晶区域与等轴晶区域为接头薄弱区域. 与未后热处理接头相比,后热处理接头平均抗拉强度达到475 MPa,同比提升约59%. 后热处理构筑的纳米沉淀相触发了第二相强化机制,这是提升接头抗拉强度的主要因素.     

Effect of Al and Sc on deformation behavior of FeCoNi multi-element alloys

The effects of Al and Sc on mechanical properties of FeCoNi multi-element alloys (MEAs) were investigated by compressive tests. The microstructures of FeCoNi MEAs with different contents of Al and Sc were characterized and the strengthening mechanisms were discussed. The results show that FeCoNi MEA with a low content of Al has a face-centered cubic (FCC) structure. The yield strength increases linearly with the increase of Al content, which is largely caused by solid solution hardening. Further addition of Sc can promote the formation of a new phase in (FeCoNi)_1−xAl_x MEAs. A minor addition of Sc can significantly increase the yield strengths of (FeCoNi)_1−xAl_x MEAs with a low Al content and improve the compressive plasticity of (FeCoNi)_1−xAl_x MEAs with a high Al content.     

固溶处理对ZK系镁合金力学及阻尼性能的影响

采用光学显微镜、X射线衍射仪、显微硬度计、拉伸试验以及动态热机械分析仪等研究了固溶处理对ZK系（ZK21,ZK40,ZK60）镁合金组织、力学及阻尼性能的影响。结果表明:经固溶处理后,ZK系合金的晶粒尺寸略有长大,第二相溶解、晶格畸变增加。晶界处脆性相的溶解产生的固溶强化效应导致固溶态合金的抗拉强度和显微硬度明显高于铸态。固溶处理后合金的与应变振幅无关阻尼下降、与应变振幅相关阻尼上升,且临界应变振幅明显增大。同一应变振幅下固溶态合金阻尼性能低于铸态;第二临界应变振幅(ε_cr2)增大使固溶态ZK系合金可以在更大应变振幅范围下使用。ZK系镁合金上述阻尼性能的变化可以用Granato-Lücke理论和塑性阻尼理论来解释。      

相组成对Cr-Nb合金高温氧化行为的影响

采用机械活金化 热压烧结法制备5种不同相组成的Cr-Nb合金,研究相组成对Cr-Nb合金在950～1200℃空气中氧化行为的影响。结果表明,Cr相能显著增加NbCr2合金950℃的抗氧化性能;而Nb相不利于合金高温抗氧化性的提高,甚至发生灾难性氧化。SEM和XRD分析显示,单相Cr-2.5Nb合金发生了Cr的外氧化,只形成单一的Cr2O3膜;而双相Cr-18.5Nb合金和单相Laves相NbCr2合金均发生Cr的外氧化和内氧化,形成两层结构的氧化膜。但随着氧化温度增加到1200℃,由于Cr2O3的挥发,导致Cr-Nb合金高温抗氧化性变差。因此,为满足实际高温应用要求,对富软第二相Cr或Nb的NbCr2基合金实施相应的表面防护是必须的。