Understanding formation of Mg-depletion zones in Al-Mg alloys under high pressure torsion

Redistribution of elements may take place in alloys during severe plastic deformation, which significantly alters the mechanical properties of the alloys. Therefore, comprehensive knowledge about deformationinduced redistribution of elements has to be established. In the present paper, the distribution of Mg in an Al-Mg alloy processed by high pressure torsion was examined using atom probe tomography(APT).With crystallographic information extracted by APT data analysis, this research reveals that the movement of dislocations plays an important role in the formation of Mg-depletion zones in the deformed microstructure.     

Interface properties of aluminum/steel friction-welded components

The study of the metallurgy of the interface of metal/metal friction-welded components is essential for understanding the quality of bonding. We have studied, through optical and electron microscopy, and tensile strength measurements, the bonding properties of Al and interstitial free steel and Al and stainless steel friction-welded components. The samples were produced by varying the friction time and rotational speed, friction pressure, upsetting pressure, and upsetting time constant at optimized values reported earlier. The bonding occurs over an intermetallic phase, which, when too thick, influences the bonding properties adversely. The thickness of the intermetallic interlayer depends linearly of on the square root of the friction time, indicating that the growth is caused by diffusion. The effect of oxidation on the bonding is also studied on samples prepared under argon atmosphere and normal atmosphere.     

On enhancing the mechanical properties of aluminum P/M alloys

Sintered aluminum alloys are an attractive material for the automobile industry, both because of the low specific gravity and high strength-to-weight ratio of aluminum itself, and the fabrication advantages associated with a powder metallurgy process. However, properties such as impact, stiffness, corrosion and wear resistance are often poor, thereby restricting the widespread use of these materials. Recent work by the authors has shown that hardness, wear resistance and tensile properties of a P/M Al–Cu–Mg ternary master alloy can be improved using a novel diffusion/supersolidus liquid phase sintering process. Improvements were due to in-situ microalloying during sintering, in particular, the influence of Ag and Sn. To complement this work, the present investigation addresses the response of a commercial alloy, AA2014, to the microalloying process. Results show that sintered densities for the commercial alloy were relatively unaffected by the presence of either Ag or Sn, and were superior to the ternary master alloy. Hardness and tensile properties were also improved relative to those obtained for the ternary, and were comparable to wrought 2014. Examination of final microstructure of Ag modified AA2014 using TEM showed the presence of Ω as the principal precipitate, but only after extended sintering times. This particular precipitate is believed to contribute to enhanced hardness. The apparent absence of Ω for short sintering times was due to the presence of silicon in the commercial product. However, the corrosion behavior of the P/M AA2014 was superior to the wrought product and thus the process is presented as a potential P/M alternative to using ingot metallurgy techniques for microalloying.     

Prediction of elastic properties of precipitation-hardened aluminum cast alloys

A methodology is proposed for predicting the elastic properties of precipitation-hardened alloys by combining different modeling techniques: the CALPHAD method, first-principles calculation, and elasticity models. The proposed procedure was applied to conventional aluminum cast alloys to predict their elastic moduli. The predicted Young’s moduli are in reasonable agreement with values reported in the literature, which verifies the potential applicability of the methodology to the development of high-stiffness aluminum cast alloys.     

Deformation mechanisms of a modified 316L austenitic steel subjected to high pressure torsion

The present work is assigned to the microstructural evolution of a modified 316L stainless steel during high pressure torsion (HPT) in a temperature range between −196 °C and 720 °C. The aspect of microstructural evolution is similar to that of materials with low stacking fault energy: at high deformation temperatures (T_def > 450 °C) the dominant deformation mechanism is dislocation glide whereas for medium temperatures (450 °C > T_def > 20 °C) mechanical twinning is observed. At very low deformation temperatures (20 °C > T_def > −196 °C) mechanical twinning is replaced by the deformation induced martensite transformation γ(fcc) → ?(hcp). Based on the present results, the formation mechanisms of nanocrystalline austenite are discussed.     

Diffusion bonding of Fe-28Al(Cr) alloy with low-carbon steel in vacuum

Fe-28Al(Cr) alloy and low-carbon steel were diffusion bonded in a vacuum of 10⁻⁴-10⁻⁵ Pa. The relationship of the bond parameters and shear strength at the interface was discussed. Microstructure characteristics and the reaction products at the interface were investigated by scanning electron microscopy (SEM) and X-ray diffractometry (XRD). The thickness of the diffusion reaction layer was measured with electron probe microanalysis (EPMA). The results indicated that controlling bonding temperature 1333 K for 3.6 ks, shear strength at the interface can be up to 112 MPa. Three kinds of reaction products were observed to have formed during the vacuum diffusion bonding, namely FeAl, Fe₃Al and α-Fe (Al) solid solution. The thickness (X) of the diffusion reaction layer increases with bonding time (t) according to a parabolic law X²=6.4×10³ exp(−104.1/RT)(t-t₀) (μm²).     

Enhancing the mechanical properties of low-carbon steel by a graded dislocation microstructure

In the present paper, the microstructures and mechanical properties of a low-carbon steel processed by graded pre-torsion (PTO) and homogeneous pre-tension (PTE), respectively, have been investigated. Experimental results demonstrate that both PTO and PTE can improve the strength of the low-carbon steel, but at a loss of ductility and toughness. However, a much better strength–ductility–toughness synergy is achieved in samples processed by graded PTO than that in samples subjected to PTE. This enhancement of comprehensive mechanical properties is due to the formation of a graded microstructure, that is, the dislocation-density increases gradually with decreasing the depth from the sample surface. This study provides a strategy for enhancing the mechanical properties of metallic materials by graded plastic deformation.     

Wetting of pure aluminum and selected alloys on polycrystalline alumina and sapphire

Using a modification of the dispensed drop method to measure true contact angles of readily oxidizing metals and alloys, the wettability of polycrystalline alumina and A-plane sapphire by pure aluminum and selected aluminum alloys was investigated. The experiments were performed under high vacuum in a horizontal tube furnace. The experimental setup produces a sessile drop free of its natural surface oxide layer minimizing flight time of the drop, and maintaining a drop impingement on the substrate.The experiments showed that there is no significant difference in the wettability of alumina and sapphire by aluminum as well as Al–11.5Si, Al–1Mg and Al–7Cu. On both substrates, aluminum shows a strong increase in contact angle well into the non-wetting regime just above the melting point. The wetting behavior of Al–7Cu on both substrates is slightly but significantly reduced in comparison to pure aluminum. The contact angles of Al–1Mg and Al–11.5Si remain rather constant between the respective liquidus temperatures of the alloys and 800 °C with θ (Al–1Mg) < θ (Al–11.5Si). Only Al–7Cu above 730 °C achieves the contact angle interval of 70–86° suggested to be most beneficial in terms of aluminum foam stabilization.     

铝/镀锌钢搅拌摩擦铆焊接头组织与力学性能

为实现铝钢之间的优质连接,采用搅拌摩擦铆焊新方法对6061铝合金和DP600镀锌钢进行搭接点焊,利用扫描电子显微镜、能谱仪及拉伸试验对接头的微观组织及力学性能进行了研究.结果表明:接头成形平整美观,中心没有匙孔;接头包含铆接区和扩散区,其中在铆接区铝合金以铝柱的形式嵌入到钢板的圆孔中,形成了一个"铝铆钉",底部有富铝的α固溶体偏聚,圆孔四周形成扩散区,铝和钢形成了冶金结合,依靠金属间化合物Fe Al3连接在一起;接头有3种断裂形式,在最佳工艺参数下接头的抗剪力达到8.2 k N;铝柱上断口的微观形貌是被拉长的韧窝,扩散区的断口由灰色基体和白色颗粒组成.     

Onset of fracture in high pressure die casting aluminum alloys

This paper deals with the development of a phenomenological criterion for the prediction of crack initiation in engineering structures made of the pressure die casting alloy Al-10Si-Mg-Mn. A custom-made biaxial testing device is employed to load a newly-designed flat specimen under various combinations of shear and normal loading. In a hybrid experimental-numerical approach, the crack initiation could be studied for stress triaxialities ranging from about 0.0 to 0.6. A phenomenological fracture criterion has been calibrated which predicts the onset of fracture based on the stress triaxiality and the maximum principal strain. The results from post-mortem metallographic analysis suggest that fracture of Si-particles leads to the macroscopic crack formation at stress triaxialities above 0.25, whereas Al-matrix instability failure along with particle-matrix delamination seems to initiate macroscopic cracks at stress triaxialities around 0.0.     

Phases,microstructures and properties of multi-component FeCoNi-based alloys

ABSTRACT

Disordered and ordered solid solution phases are identified in FeCoNi-based multi-component alloys. An ultimate strength of 2380?MPa was achieved in the dual face-centered cubic?+?body-centered cubic structural (FeCoNi)₆₀Al₁₅Cr₂₅ alloy, accompanied by a fracture strain of 38% and a hardness of 376.3?HV. After holding at 900°C for 100?h, oxidation resistance of the alloys follows by an increasing rank of (FeCoNi)₈₅Al₁₅?<?(FeCoNi)₇₅Cr₂₅?<?(FeCoNi)₆₀Al₁₅Cr₂₅. Moreover, the oxidation kinetic curves of the alloys follow a power-law dependence. (FeCoNi)₆₀Al₁₅Cr₂₅ alloy shows a good trade-off of mechanical properties and an excellent oxidation resistance at evaluated temperature, and can be a potential candidate for high temperature materials.     

Microstructure development and hardening during high pressure torsion of commercially pure aluminium: Strain reversal experiments and a dislocation based model

The effect of strain reversal on hardening due to high pressure torsion (HPT) was investigated using commercially pure aluminium. Hardening is lower for cyclic HPT (c-HPT) as compared to monotonic HPT (m-HPT). When using a cycle consisting of a rotation of 90° per half cycle, there is only a small increase in hardness if the total amount of turns is increased from 1 to 16. Single reversal HPT (sr-HPT) processing involves torsion in one direction followed by a (smaller) torsion in the opposite direction. It is shown that a small reversal of 0.25 turn (90°) reduces hardness drastically, and that decrease is most marked for the centre region. These behaviours and other effects are interpreted in terms of the average density of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs). A model is presented that describes the experimental results well. A key element of the model is the assumption that at the very high strains developed in severe plastic deformation processes such as HPT, the dislocation density reaches a saturation value. The model indicates that the strength/hardness is predominantly due to GNDs and SSDs.     

Effects of carbon content on the formation of nano/ultrafine grained low-carbon steel treated by martensite process

Martensite process consists of simple cold rolling and annealing of martensite structure. This process can produce a nano/ultrafine grained structure in carbon steels. The key to the process is to start from martensite structure. Carbon can hardly affect the morphology of lath martensite. In this study, the effects of carbon content on required rolling reduction and formation mechanisms of nano/ultrafine ferrite grains are investigated. For this purpose, two low-carbon steels containing 0.04 and 0.08 wt% carbon are processed. Rolling reduction ranging from 40 to 80% is used. Specimens are annealed at various temperatures from 450 to 700 °C. After annealing at 500 °C, ultrafine ferrite grains are obtained in the 80% cold-rolled specimen containing 0.08 wt% C. The formation of these ultrafine ferrite grains is interpreted here in terms of the relationship between the size of the laths and the blocks of the lath martensite and required rolling reduction for dividing them into sub-micron scale sections. The required rolling reduction increases with decreasing the carbon content of steel.     

Effect of temperature and strain rate on tensile behavior of ultrafine-grained aluminum alloys

Ultrafine-grained Al–4Y–4Ni and Al–4Y–4Ni–0.9Fe (at.%) alloys were synthesized by the consolidation of atomized powders and subsequent hot extrusion. The mechanical behavior of these two alloys has been studied by performing uniaxial tension tests ranging from room temperature to 350 °C. These alloys, with high volume fraction of second-phase particles, exhibited ambient temperature tensile strength ranging from 473 to 608 MPa and plastic elongation ranging from 6.7 to 9.6% at an initial strain rate of 1 × 10⁻³ s⁻¹. However, lower ductility was observed with decreasing strain rate at the intermediate temperature ranging from 150 to 250 °C for Al–Y–Ni–Fe alloys due to limited work hardening.     

The effect of annealing treatment on mechanical properties of aluminum clad steel sheet

In this paper, the effects of annealing temperature and time on mechanical properties and bond strength of aluminum clad steel sheet are evaluated. The results indicate that there exist an optimum annealing temperature and time for achieving a suitable formability and bonding strength between the clad layer and base metal. At this annealing time and temperature, the brittle intermetallic layer at the intimate interface of the layers is minimized.     

Production of aluminum-matrix carbon nanotube composite using high pressure torsion

In this study, an Al-based composite containing carbon nanotubes (CNTs) was fabricated using a process of severe plastic deformation through high pressure torsion (HPT). Neither heating nor sintering was required with the HPT process so that an in situ consolidation was successfully achieved at ambient temperature with 98% of the theoretical density. A significant increase in hardness was recorded through straining by the HPT process. When the composite was pulled in tension, the tensile strength of more than 200 MPa was attained with reasonable ductility. Transmission electron microscopy showed that the grain size was reduced to 100 nm and this was much smaller than the grain size without CNTs and the grain size reported on a bulk sample. High resolution electron microscopy revealed that CNTs were present at grain boundaries. It was considered that the significant reduction in grain size is attributed to the presence of CNTs hindering the dislocation absorption and annihilation at grain boundaries.     

Coupling effect of primary voids and secondary voids on the ductile fracture of heat-treatable aluminum alloys

Ductile fracture of commercial aluminum alloys is controlled not only by the primary voids but also by the secondary voids, which are respectively nucleated at cracked constituents and at decohered dispersoid. In this paper, experiment and modeling were carried out to study the combined effect of the two populations of voids on the ductile fracture in two kinds of heat-treatable aluminum alloys, i.e., Al-Cu-Mg alloys and Al-Mg-Si alloys. Different heat treatments were applied to the alloys to achieve various combinations of the two voids, which were subsequently related to the mechanical properties. A multiscale fracture model was proposed to describe quantitatively the relationships between parameters of the two voids and the ductility and fracture toughness of heat-treatable aluminum alloys. It is revealed experimentally and theoretically that the presence of secondary voids will reduce the ductile properties especially when the intervoid spacing is less than about 0.5 μm. All calculations are in good agreement with experimental results.     

Influence of Y on microstructures and mechanical properties of high strength steel weld metal

A comprehensive investigation is conducted into the effect of yttrium oxide on microstructures of weld metal deposits and mechanical properties of high strength steel electrode measured in the Ni–Cr–Mo–V alloy system. The results demonstrate a gradual decrease of the content of proeutectoid ferrites and a gradual increase of acicular ferrites, as the content of yttrium oxide increases from 0% to 0.02%. However, as the content of yttrium oxide surpasses 0.02%, the content of acicular ferrites reduces significantly. Meanwhile, the toughness under low-temperature impact increases and then decreases, as the content of yttrium varies from 0% to 0.03%, reaching the maximum of 102J at the field of 0.02%. However, the strength fails to change significantly. The results also indicate that the cold cracking sensitivity is lower when the content of yttrium oxide is 0.02%, but the values would increase as the content of yttrium oxide fluctuates.     

低碳微合金直接淬火钢的组织与力学性能

为了提高低碳直接淬火钢的强韧性能,对一种低碳Nb-V微合金钢进行了轧后直接淬火(DQ)和再加热淬火(RQ)热处理实验,分析了低碳直接淬火钢的的强韧化机理.采用光学显微镜、透射电子显微镜、硬度计、拉伸试验机以及冲击试验机研究了轧后热处理工艺对低碳Nb-V微合金钢组织和力学性能的影响.结果表明,DQ工艺钢马氏体板条间距细小,含有较多的位错亚结构,因此具有较高的强度和韧性.DQ工艺钢马氏体中的大量位错,促进了碳化物弥散析出,产生了显著的二次硬化效果.由于基体中固溶的Nb、V等元素推迟淬火马氏体在回火过程中的各种转变,以及回火时析出的细小弥散碳化物抑制马氏体铁素体回复、再结晶过程,DQ工艺钢表现出较高的回火稳定性.