Dispersoid precipitation and process modelling in zirconium containing commercial aluminium alloys

Small dispersoid particles inhibit recrystallisation and are thus critical in controlling the grain structure of many high strength commercial aluminium alloys. A general, physical model has been developed for the precipitation of Al₃Zr dispersoids in aluminium alloys. The predictions of the model have been compared with results of an experimental investigation of Al₃Zr precipitation in 7050. The model has been shown to faithfully reproduce the distribution of dispersoids observed in this alloy, correctly predicting dispersoid free zones observed in interdendritic regions and at grain boundaries. Furthermore, the predicted precipitation kinetics agree well with experimental observation. The model has been used to study the effects of homogenisation conditions and alloy composition on dispersoid formation and has been shown to be a powerful tool for optimising the dispersoid distribution in 7xxx series aluminium alloys.     

奥氏体化条件对M2高速钢组织和性能的影响

研究了不同规格的M2高速钢经不同工艺奥氏体化处理后的组织和性能。结果表明,较低温度（1150℃）奥氏体化时,原材料规格（即碳化物不均匀度）和奥氏体化时间对淬火晶粒度和抗弯强度的影响不大;1210℃奥氏体化时间对淬火晶粒度和抗弯强度影响很大;1210℃长时间奥氏体化对淬火晶粒度和抗弯强度的影响大于原材料规格的影响。     

Strength and tension/compression asymmetry in nanostructured and ultrafine-grain metals

The recent literature is reviewed with respect to the strength-limiting deformation mechanisms in nanocrystalline and ultrafine-grain metals. Based on these results, a deformation mechanism map is proposed for FCC metals with ultrafine-grain sizes. In the absence of flaw-controlled brittle fracture, it is concluded that the strength-limiting mechanism in metals with grain sizes between approximately 10 and 500–1000 nm is dislocation emission from grain boundary sources. A simple model for the strength in this regime of grain sizes is developed from classical dislocation theory, based on the bow-out of a dislocation from a grain boundary dislocation source. The model predicts not only the strength as a function of grain size, but also the observed tension/compression asymmetry of the yield strength. The tension/compression asymmetry arises from the pressure dependence of the dislocation self-energy during bow-out. The pressure dependence is a function of material and grain size, consistent with experimental observations. Finally, the model provides a physical basis for a pressure-dependent yield criterion.     

Effects of homogenisation conditions on semisolid microstructures of Al–Mg–Si–Mn alloys produced by D-SSF process

Abstract

The effects of different heating rates to a homogenisation temperature on the semisolid microstructure of Al–Mg–Si–Mn alloys are investigated. It is found that the size, morphology and distribution of the α-Al₁₂Mn₃Si₂ intermetallic compound (Mn containing dispersoid) depend on the heating rate in the homogenisation process. Fine spherical and homogeneously distributed Mn containing dispersoid particles are found in the slow heated samples (0˙7°C min^?1), while inhomogeneously distributed coarser particles with a rod-like shape are found in the rapid heated samples (110°C min^?1). The homogenised sample is deformed by 60% cold rolling. It is found that the recrystallised and semisolid grain sizes of the rapid heated sample are smaller than those of the slow heated sample in all conditions. Compared with the M4 alloy (0˙4 mass-%Mn), the M7 alloy (0˙72 mass-%Mn) has much finer semisolid grain size and smaller values of the shape factor close to 1. The Mn containing dispersoid greatly affects the semisolid grain size of the alloys. The results in this work show that the rapid heating in the homogenisation process is useful to produce high quality semisolid products of the Al–Mg–Si–Mn alloys.     

Application of a modified slip-distance theory to the indentation of single-crystal and polycrystalline copper to model the interactions between indentation size and structure size effects

Plasticity size effects offer both measurement challenges and opportunities for material engineering. We have used nano-indentation to study the relationship between different size effects. Hardness varies significantly with indent size in single crystals, and also in polycrystals, whenever indent sizes and structure sizes are within an order of magnitude of each other. We exploit the geometric self-similarity of a Berkovich indenter and apply slip distance theory to indents of different sizes at a constant indentation strain. We show that indent size, grain size and pinning defects combine in a single, length-scale-dependent deformation mechanism, to determine the yield strength (hardness) of a material. This provides an excellent foundation for: improved grain size determination by indentation, design rules for combining different methods of yield stress enhancement and using indentation to probe local stress–strain properties of a material, or for mapping residual stress.     

Action of chills on soundness and ultimate tensile strength (UTS) of aluminum–quartz particulate composite

The present investigation aims at producing cast aluminum alloy–quartz particulate composites in moulds containing metallic and non-metallic chills by dispersing quartz particles in molten aluminum alloy above the liquidus temperature, the size of the particles dispersed being between 30 μm and 100 μm. The dispersoid being added ranges from 3 to 9 wt.% in steps of 3%. The resulting composites cast using chills were tested for their strength and soundness. Results of the investigation indicate that (1) the strength of the composite developed is highly dependent on the location of the casting from where the test specimens are taken and also on the dispersoid content of the composite. (2) Chill thickness and chill material however does significantly affect the strength and soundness of the composite. (3) Soundness of the composite developed is highly dependent on the chilling rate as well as the dispersoid content. An increase in the rate of chilling and increase in the dispersoid content of the material both result in an increase in the UTS (ultimate tensile strength) of the material. The temperature gradient developed during solidification and VHC (volumetric heat capacity) of the chill used are the important parameters controlling the soundness of the composite. (4) Maximum interface temperature attained by the chill decreases with the increase in their VHC (volumetric heat capacity) and the total heat absorbed by the chill increases with increase in VHC. (5) Thermal properties of the end chills are used to determine the magnitude of the temperature gradients developed along the length of the casting solidifying under the influence of chills.     

Estimate of the Hall-Petch and Orowan effects in the nanocrystalline Cu with Al2O3 dispersoid

Nanocrystalline (nc) Cu materials with Al₂O₃ dispersoid (Cu-4 vol. % Al₂O₃) were successfully produced by a simple cryo-milling at 210 K with a mixture of Cu₂O, Al, and Cu ingredient powders, followed by hot pressing at 1123 K. The results of microstructural analysis showed that the hot pressed material was comprised of a mixture of Cu matrix (25.1 nm) with a homogeneous size distribution of the Al₂O₃ dispersoid (4 nm in radius). Grain size of Cu was measured by XRD (Scherrer's formula); dispersoid size of Al₂O₃ was confirmed by STEM-EDS (Scanning Transmission Electron Microscopy-Energy Dispersive Spectroscopy) and TEM (Transmission Electron Microscopy). Result of micro hardness tests indicated that the hot pressed materials have a significantly high hardness (265 ± 8, VHN; 2.6 GPa, SI units) at room temperature. Two strengthening mechanisms are considered for high hardness: strengthening by grain refinement and dispersion hardening. An estimate of the individual strengthening effect to total strength of the material at room temperature, based on the Hall-Petch and Orowan equations, is presented.     

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.     

Grain size dependent alumina fracture mechanics stress intensity

Various grain size dependent hardness test results obtained on alumina materials are brought together on a Hall–Petch (H–P) inverse square root of grain diameter basis to illustrate the normal expectation of greater strengthening for finer grained material, not unrelated to the observation that structural ceramic materials are employed often in a very much finer grain size condition than counterpart structural metals and alloys. Here, the H–P dependence is shown to carry over to describing recent, mutually confirming, hardness-based measurements of the fracture mechanics stress intensity of alumina materials at small grain sizes. The somewhat surprising dependence relates to the frequent observation that an increase in plastic flow strength of a structural material is associated with a decrease in its fracture toughness. For the case of strengthening by grain size refinement, however, the fracture stress of a material is raised more than the plastic flow stress and so greater local plastic work is required for fracturing.     

Superplasticity of fine-grained 7475 Al alloy and a proposed new deformation mechanism

A study has been made to investigate the superplastic deformation mechanisms of 7475 Al alloy in relation to the variation of grain size ranging between 5.5 μm and 13 μm. The strain-rate sensitivity (m) was increased with decreasing grain size in the superplastic deformation regime. Microstructural investigation after tension tests revealed that the dispersoid free zones were produced mostly at the grain boundaries normal to the tensile direction. A new model for describing the deformation behavior of the 7475 Al alloy has been proposed based on the assumption that the grain boundary sliding was accommodated by both diffusional flow and slip. This new model well predicts many aspects of experimental results.     

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.     

黄铜箔拉伸屈服强度的尺寸效应

为了研究金属箔的塑性变形性能与尺寸的相关性,在常温下对不同厚度和晶粒尺寸的黄铜箔试样进行了单向拉伸实验.结果表明:随着厚度或晶粒尺寸的减小,箔的屈服强度都会升高,晶粒尺寸对屈服强度的影响满足Hall-Petch细晶强化关系,厚度减小使屈服强度升高也可以主要归结于晶粒尺寸的减小.此外,当箔的厚度小于100 靘时,厚度/晶粒尺寸比不能表征屈服强度的尺寸效应.     

Mechanical performance and oxidation resistance of an ODS γ-TiAl alloy processed by spark plasma sintering and laser additive manufacturing

In this work, the influence of Y₂O₃ additions on the mechanical properties and oxidation resistance of a Ti-45Al-3Nb (at.%) alloy have been studied. In particular, the mechanical properties from 293 K to 1073 K and oxidation resistance at 1073 K of spark plasma sintered and direct metal deposited material have been examined. At room temperature, higher yield stress (+34%) and ultimate tensile strength (+14%) at reduced ductility (−17%) is observed for the oxide dispersion strengthened variant compared to its non-strengthened counterpart. The strengthened variant shows superior strength retention up to 1073 K. Strengthened direct metal deposited material shows similar deformation characteristics as sintered material but suffers from premature fracture due to residual porosity. The addition of Y₂O₃ increases the oxidation resistance of both sintered and direct metal deposited material. Parabolic growth constants are decreased by −49% and −75% in sintered and direct metal deposited material, respectively. In sintered material the dispersoid size shows only slight changes from 29 nm to 26 nm at 923 K after 987 h and to 32 nm at 1073 K after 924 h demonstrating the high stability of the added particles. TEM analysis reveals abundant grain boundary pinning by the particles contributing to microstructural stability. The results show the potential of oxide dispersion strengthening in titanium aluminides for conventional sintering as well as for additive manufacturing processing routes.     

晶粒尺寸对42CrMoVNb钢超高周疲劳性能的影响

研究了不同热处理制度下得到的3种具有不同晶粒尺寸的42CrMoVNb高强度钢的超高周疲劳性能. 结果表明, 超高周疲劳强度和疲劳强度比并不随晶粒尺寸的减小而单调提高, 中等晶粒尺寸的试样具有最高的疲劳强度和疲劳强度比. SEM断口观察表明, 绝大部分试样的疲劳裂纹起源于夹杂物. 随着疲劳断口裂纹源夹杂物处应力强度因子幅ΔK_inc的减小, 疲劳寿命N_f增加; 而在夹杂物周围的粗糙粒状区域(GBF)的应力强度因子幅ΔK_GBF并不随N_f变化而变化, 基本为一常数, 且粗晶粒试样的ΔK_GBF高于细晶粒试样. 这表明, 细化晶粒对高强度钢的超高周疲劳性能有着复杂的影响,存在一个合理的细化晶粒范围.     

Effect of Grain Size Variations on the Long-Time Stability of Alloy 718

Uniform, fine grain size is generally preferred in forgings of Alloy 718 for optimum properties. However, due to limitations imposed by forging equipment, forging size and forging configuration, it is often impossible to produce this desired grain size; instead, some grain duplexing or grain coarsening occurs in certain areas throughout the forging. Stress-rupture tests were conducted before exposure, and after exposing specimens at 1250°F, 50 ksi or 1300°F, 50 ksi for 500 hrs. on material having (1) uniform fine grain size (ASTM 7 or finer), (2) duplex grain size, and (3) uniform coarse grain size (ASTM 5 or coarser). These grain sizes were obtained in forgings with a simple pansake configuration using varying forging techniques. The results of the rupture tests conducted before exposure showed an excellent correlation with grain size and Ni3Cb plate size. After exposure, rupture ductility increased and strength decreased due to overaging. In duplexed samples the controlling factor appears to be the amount and size of the coarse grain. The optimum grain size for the best balance of rupture properties appears to be ASTM 6–7.     

Notch toughness in hot-rolled low carbon steel wire rod

Charpy V-notch toughness has been investigated in four hot-rolled, low carbon steels with different grain sizes and carbon contents between 0.019 and 0.057%. The raw material was wire rod designed for drawing and possible subsequent cold heading operations and manufactured from continuous cast billets. In this study, the influence of microstructure, mechanical properties, and alloying elements on the ductile-brittle transition behavior has been assessed. A particular emphasis has been given to the influence of boron with contents up to 0.0097%. As a result, transition temperatures between −29 and +50°C explicated by the material properties have been obtained. The examination also shows that the transition temperature raises with circa 0.5°C for each added ppm boron most likely as a consequence of an enlargement of the ferrite grain size and the reduction of yield and tensile strength. The highest upper shelf energy and lowest transition temperature can be observed in a steel without boron additions and with maximum contents of carbon, silicon, and manganese.     

Role of Mn-dispersoid in the fracture toughness enhancement of Al-Zn-Mg-(Mn) alloys

The effects of Mn dispersoid and fabrication methods on the fracture toughness in peak aged Al-Zn-Mg-(Mn) alloys have been studied. Sphere- or rod-shaped Mn dispersoids of the size in range from 0.05 μm to 0.5 μrn are formed by the addition of Mn in Al-Zn-Mg alloy. The extruded alloys containing Mn have higher fracture toughness with higher strength and show transgranular ductile fracture surface. as compared with those of Al-Zn-Mg alloy. These phenomena are found to be obtained from the improved load bearing capacity and the effective accommodation of the applied stress due to the dispersion hardening effect and homogeneous deformation by the Mn-dispersoids. Comparing the mechanical properties between the extruded and the rolled alloys containing 0.8 wt% Mn. the extruded alloy is shown to have higher strength and better fracture toughness than those of the rolled one. This result can be explained by the dispersion of stress concentration and the improved homogeneous deformation attributed to the fine grain structure and the existence of deformation texture for the extruded alloy.     

Tempered martensite embrittlement in a 32NiCrMoV125 steel

This study focused on tempered martensite embrittlement in a 32NiCrMoV125 steel through examination of the effects of austenite grain size and tempering temperature on the mechanical properties and fracture morphology of this material. Two different austenite grain sizes were obtained by austenitizing at 870 and 950 °C. After quenching, the specimens were tempered in the temperature range of 200–650 °C. The results obtained in this research indicate that by increasing the tempering temperature, the strength and hardness decrease, but ductility increases. However, impact testing indicated that tempered martensite embrittlement occurred when samples were tempered in the range of 250–400 °C. Fractography revealed intergranular and quasi-cleavage fracture. In summary, increasing the austenite grain size decreased strength, but increased impact toughness, except for samples tempered between 200 and 350 °C.     

微镦粗非均匀变形的显微硬度分析

为研究微小尺度材料流动特点,采用相同几何尺寸、不同晶粒尺寸的纯铜圆柱体,在不同的摩擦条件下进行镦粗实验,并通过测量端面显微硬度研究材料流动的情况。结果显示,小变形条件下,小晶粒尺寸和较大摩擦力有利于材料均匀流动;大变形条件下,大晶粒尺寸和较小摩擦力有利于材料的均匀流动。     

Effects of grain size and grain boundaries on defect production in nanocrystalline 3C–SiC

Cascade simulations in single crystal and nanocrystalline SiC have been conducted in order to determine the role of grain boundaries and grain size on defect production during primary radiation damage. Cascades are performed with 4 and 10 keV silicon as the primary knock-on atom (PKA). Total defect production is found to increase with decreasing grain size, and this effect is shown to be due to increased production in grain boundaries and changing grain boundary volume fraction. In order to consider in-grain defect production, a new mapping methodology is developed to properly normalize in-grain defect production rates for nanocrystalline materials. It is shown that the presence of grain boundaries does not affect the total normalized in-grain defect production significantly (the changes are lower than ～20%) for the PKA energies considered. Defect production in the single grain containing the PKA is also studied and found to increase for smaller grain sizes. In particular, for smaller grain sizes the defect production decreases with increasing distance from the grain boundary while for larger grain sizes the presence of the grain boundaries has negligible effect on defect production. The results suggest that experimentally observed changes in radiation resistance of nanocrystalline materials may be due to long-term damage evolution rather than changes in defect production rates from primary damage.