Effect of Microstructure on the Creep Properties of Ti–6Al–4V Alloys: An Analysis

The effects of microstructure types and microstructural parameters on creep properties were investigated systematically through an analysis of microstructure and creep properties of Ti–6Al–4V alloys based on the available literature data. The results indicated that the creep properties of the Ti–6Al–4V alloy are strongly dependent on microstructure type. Creep resistance of Ti–6Al–4V alloys is better in lamellar microstructure followed by bimodal and equiaxed microstructure respectively. Also, microstructural parameters such as the size of both prior beta grain and alpha colony and thickness of alpha lamellae in the lamellar microstructure, the volume fraction of primary alpha phase in bimodal microstructure and size of alpha phase in equiaxed microstructure can influence the creep properties.     

Influence of Microstructure on High Temperature Solid Particle Erosion Behaviour of Ti–6Al–4V Alloy

The present article describes the influence of microstructure i.e. lamellar, bimodal and equiaxed on solid particle erosion behaviour of Ti–6AL–4V alloy at service exposed temperature i.e. 400 °C. Erosion tests have been carried out using an air jet type test rig and Taguchi’s experimental design. From erosion test, it has been observed that impact velocity is the most significant controlling factor influencing the erosion of Ti–6Al–4V alloy followed by impingement angle, size of erodent and microstructural variation.The results indicated that erosion rate of Ti–6Al–4V alloy was less in bimodal microstructure followed by equiaxed and lamellar microstructure.     

Microstructural and Tribological Characteristics of Al–10Cu–Fe alloy Produced by Vertical Centrifugal Casting process

In this study, an attempt has been made to produce Al–10Cu–Fe alloy by vertical centrifugal casting at speeds ranging from 800 to 2850 rpm. The microstructural features, mechanical and wear properties have been investigated. The microstructure of Al–10Cu–Fe alloy consists of equiaxed grain morphology of the primary α-phase with eutectic phases in the interdendritic regions. It has been observed that there is a variation in the grain size from the inner surface of the casting to its outer surface. The speed also has a strong influence on the grain size and subsequent mechanical properties of the alloy. The wear properties of the alloy have been evaluated at a constant sliding velocity of 1 m/s for a range of applied load and sliding distance. The variations in the wear behavior are attributed to the size and solidification morphology of the castings.     

TA12A钛合金热处理过程中等轴和片层α相演变行为研究

对近α 型TA12A钛合金进行热处理实验,利用扫描电子显微镜(SEM)和电子背散射衍射(EBSD)技术对热处理后的微观组织进行观察,研究了两相区固溶温度和冷却速率对微观组织的影响.研究表明:TA12A钛合金在980和1000℃保温后冷却时,β相向α相转变,一方面可以使得等轴α相长大,另一方面也可析出片层α相.等轴α相长...     

On the mechanism of grain formation during spray atomization and deposition

The mechanisms governing the morphological changes in the microstructure of spray atomized and deposited Ni₃Al were studied, with particular emphasis on the formation of a spheroidal grain morphology. Accordingly, the various microstructural features present in the spray deposited material were rationalized on the basis of thermal energy considerations. The formation of spheroidal grains was proposed to evolve from: (a) the homogenization of dendrites that did not deform extensively during deposition; and (b) the growth and coalescence of deformed or fractured dendrite fragments. Support for this suggestion was provided by experimental results and numerical analyses which show that the microstructure of Ni₃Al is exposed to a high temperature anneal during deposition. Moreover, the results show that during high temperature annealing, the deformed or fractured dendrite fragments that were initially present in the spray deposited materials grew and coalesced leading to the development of a spheroidal grain morphology. On the basis of a coarsening mechanism, the relative annealing time under a particular cooling rate may be calculated and converted into a spheroidal grain size, d_sph, from the following equation, d_sph = 15.2· Ṫ^−0.35. The experimental results were observed to concur with this relationship.     

Electron Beam Melting of High Niobium Containing TiAl Alloy: Feasibility Investigation

Third generation γ‐TiAl alloys with a high niobium content, Ti–(47–48)Al–2Cr–8Nb, were processed by electron beam melting (EBM). This near‐net‐shape additive manufacturing process produces complex parts according to a CAD design. The starting powder is deposited layer by layer on the building table and selectively melted to progressively form the massive part. The EBM parameters such as layer thickness, melting temperature, scanning speed, or building strategy were set up to minimize porosity. The chemical composition of the built material is similar to the composition of the base powder despite a slight evaporation of aluminum and reveals a neglectable oxygen pick‐up. The very fine equiaxed microstructure resulting after EBM can be then set up by heat treatment (HT). According to the HT temperature in particular, an equiaxed microstructure, a duplex microstructure with different lamellar ratio and a fully lamellar microstructure is obtained. Not only test bars have been produced but also complex parts such as demo low pressure turbine blades.     

喷射成形硅钢板坯的工艺研究

利用自行研制的大型喷射成形中装置, 对Fe-4.5wt%硅钢板坯的喷射成形工艺进行了研究。还研究了雾化压力和沉积距离对其显微组织的影响。结果表明,在本试验条件下得出的最佳喷射成形工艺,可形成厚度均匀、晶粒细小并致密度良好的板坯,使随后的轧制成为可能。     

Microstructural features and heat flow analysis of atomized and spray-formed Al-Fe-V-Si alloy

Microstructural features of rapidly solidified powders and preforms of Al₈₀Fe₁₀V₄Si₆ alloy produced by spray forming process have been studied. The atomization and spray deposition were carried out using a confined gas atomization process and the microstructural features were characterized using scanning electron microscopy and transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. The microstructure of a wide size range of atomized powders invariably revealed cellular and dendritic morphology. The extent of dendritic region and the dendritic arm spacing were observed to increase with powder particle size. The TEM investigations indicated the presence of ultrafine second-phase particles in the intercellular or interdendritic regions. In contrast, the spray deposits of the alloy showed considerable variation in microstructure and size and dispersion of the second-phase particles at specific distances from the deposit-substrate interface and the exterior regions of the deposit. Nevertheless, considerable homogeneity was observed in the microstructure toward the center of the spray deposit. The formation and distribution of a cubic phase α-Al(Fe,V)Si has been characterized in both atomized powders and spray deposits. A one-dimensional heat flow model has been used to analyze the evolution of microstructure during atomization and also during spray deposition processing of this alloy. The results indicate that thermal history of droplets in the spray on deposition surface and their solidification behavior considerably influence the micro-structural features of the spray deposits.     

Solidification of the Undercooled Al-Si Alloy Containing 1.0 PctRE

Al-80 pctSi-1.0 pctRE alloy was levitated and melted using the electromagnetic levitation facility in combination with a laser heating unit. The growth morphologies of primary silicon were observed using a high-speed video, and the microstructure was analyzed by the scanning electron microscopy. The morphologies of primary silicon at low, intermediate, and high undercooling are dendrites, fragmented bulks and granular grains, and equiaxed grains, respectively. In addition, the growth velocities of primary silicon were measured, which were consistent with the theoretical prediction. The microstructure refinements of primary silicon played a dominant role in its large microhardness, which increased with the increase of undercooling. Moreover, the hardening effect of dendritic structure was stronger than that of equiaxed grain.

     

Quantitative characterization of microstructures of liquid-phase-sintered two-phase materials

Many powder metallurgy materials have a common microstructure character—spherical or equiaxed particles embedded in a matrix. This article presents an analytical approach that establishes relationships between a broad range of three-dimensional (3-D) microstructure parameters and two-dimensional (2-D) measurements for these materials. Specifically, the grain coordination number, measured dihedral angle, connectivity, and contiguity are directly related to the solid volume fraction, interfacial energies (equilibrium dihedral angle), and grain size. The volume fraction, the equilibrium dihedral angle, and the grain sizes are treated as independent variables. Other microstructural parameters can be expressed as functions of these independent variables. Results show that when the mean grain size is smaller than a critical value, these microstructure parameters change rapidly as the grain grows during processing at high temperatures. When the mean grain size is larger than a critical value, these microstructure measurements approach corresponding stable values.     

Ti 465Al 25V 10Cr 03Ni合金高温变形动态再结晶行为

 采用热模拟实验研究了变形参数(应变量、变形温度、应变速率)对Ti 465Al 25V 10Cr 03Ni合金微观变形组织的影响。研究结果表明,在应变量逐渐增加的热压缩变形过程中,一定变形量后开始出现层片弯曲、折曲或破碎,随着应变量的增加,动态再结晶形成的等轴晶粒增多,残余层片团减少。变形温度对钛铝合金的变形组织有着显著影响,提高热变形温度,有利于合金中的动态再结晶以及变形组织的均匀化,降低残余层片团的体积分数。较低的应变速率能促进变形时的动态再结晶,有利于提高变形组织的均匀性。还对γ TiAl合金动态再结晶新晶粒形成过程进行了讨论。     

Laser Direct Metal Deposition Technology and Microstructure and Composition Segregation of Inconel 718 Superalloy

  Multilayer of laser direct metal deposition (DMD) was prepared by depositing a gas atomized pre alloyed powder with a composition close to Inconel 718 alloy on Inconel 718 high temperature alloy substrate. The effects of the DMD parameters on the build up rate and the structure of the deposited layer were studied. The laser DMD sample was further processed by a solution treatment. The microstructure and property of the laser DMD zone before and after heat treatment were investigated as well. The results show that the laser parameters of actual laser power of 650 W, scanning speed of 58 mm/s, beam diameter of 1 mm, powder feed rate of 645 g/min, with a corresponding specific energy of 90-130 J/mm2, can be recommended as optimum parameters for high build up rate of Inconel 718 alloy. Under the condition of optimized parameters, a directional solidification microstructure was obtained and the average distance between the columnar crystals was 5-10 μm. The microcomposition segregation was found between the columnar crystal trunk and columnar crystal. The elements of Nb, Mo, Ti concentrated in the columnar crystal trunk. After the heat treatment, the segregation was greatly minimized, and the segregation ratios were close to 1. The hardness of the laser deposited layer did not show obvious difference along the height of the layer either for the as deposited layer or for the heat treated layer. However, the microhardness of the laser DMD zone after heat treatment was obviously higher than that after the as deposited treatment. During the heat treatment process, some Nb and Mo rich phases precipitated and strengthened DMD layer.     

喷射成形M3型高速钢碳化物组织特征与加热过程演化

采用常规铸造和喷射成形工艺分别制备了M3型高速钢铸坯和沉积坯.利用扫描电子显微镜、X射线能谱和X射线衍射等分析方法对冷却速度对合金的显微组织的影响,加热温度对M3高速钢中M₂C共晶碳化物分解行为的影响,以及热加工变形后铸态和沉积态组织的变化进行了研究.结果表明:铸态合金含有粗大的一次枝晶和M₂C共晶碳化物,而喷射成形沉积坯主要为等轴晶且碳化物细小均匀;冷却速度的提高极大地抑制了碳化物的析出和晶粒长大;加热温度的提高有利于M₂C共晶碳化物分解,过高的温度使得分解后的M₆C长大,不利于合金性能的提高;沉积坯经恰当的预热处理和热变形可以获得理想的变形组织.      

The coupled zone of rapidly solidified AlSi alloys in laser treatment

Laser-melted tracks were produced on AlSi samples containing between 15.5 and 26 wt% Si with the resultant solidification rates being measured by taking a longitudinal section through the centre of the laser trace. The Al-rich boundary of the coupled zone, i.e.the growth rate-concentration limit at which the transition from fibrous AlSi eutectic to α-Al dendrites plus interdendritic eutectic takes place, has been experimentally determined for concentrations of Si varying from 15.5 to 20 wt%. Supposing that the growing structure, for a given growth rate, is the one having the higher growth temperature, good agreement is found with the more recent microstructural growth models when kinetic effects are taken into account. For concentrations of Si higher than 20 wt%, primary Si crystals imbedded in equiaxed eutectic grains are observed which replace columnar eutectic and dendritic growth.     

微观组织参数及工艺条件对430不锈钢凝固显微结构的影响

在多元合金CAFE模型的基础上,分析了微观组织参数（形核密度、高斯分解参数、Gibbs-Thomson系数等）与430不锈钢凝固过程中晶粒形貌的复杂关系,以及过热度与冷却强度等工艺参数对凝固组织的影响.研究发现,晶粒尺寸和柱状晶向等轴晶转变不仅与体最大形核过冷度有关,也受体形核密度的影响.高斯分解参数和Gibbs-Thomson系数增大时,一次枝晶间距减小,等轴晶范围增大;但当它们增加至一定范围后,其对显微结构的影响逐渐变得不明显.过热度或冷却强度增大时,等轴晶范围减小,但一次枝晶间距的变化不明显.      

Microstructures and hardness of ultrafine-grained Ni3Al

The microstructural evolution of the ultrafine-grained intermetallic compound Ni₃Al is studied as a function of annealing at different temperatures. The ultrafine microstructure is produced by a high plastic torsional straining. Transmission electron microscopy, X-ray diffraction and differential scanning calorimetry are used to characterize the microstructural evolution and microhardness is used to determine mechanical behaviour. The as-deformed microstructure exhibits an almost fully disordered crystalline structure with coherent domain size of about 18 nm, a strong torsional texture and high internal elastic strains. On annealing the as-deformed samples at different temperatures, the recrystallization of the material into a granular type structure containing non-equilibrium grain boundaries is first observed. This is followed by the transformation from non-equilibrium grain boundaries with simultaneous grain growth. This transformation is correlated with an increase of hardness. A new concept of non-equilibrium grain boundaries transparency is presented to interpret this singular behaviour. The results are compared to those obtained on an ultrafine-grained Al-1.5% Mg alloy produced by the same technique and which exhibits the same mechanical behaviour.     

Microstructure and Texture Evolution during Friction Stir Processing of Fully Lamellar Ti-6Al-4V

Friction stir processing (FSP) was used to locally refine a thin surface layer of the coarse, fully lamellar microstructure of investment-cast Ti-6Al-4V. Depending on the peak temperature reached in the stir zone during processing relative to the β transus, three distinct classes of microstructures were observed. Using accepted wrought product terminology, they are equiaxed, bimodal, and lamellar, except for the case of FSP, the length scale of each was smaller by at least an order of magnitude compared to typical wrought material. The evolution of an initially strain-free fully lamellar microstructure to each of these three refined conditions was characterized with scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction. The fundamental mechanisms underlying grain refinement during FSP, including both the morphological changes and the formation of high-angle grain boundaries, were discussed.     

Modeling of microstructural evolution with tracking of equiaxed grain movement for multicomponent Al-Si alloy

A new model for the simulation of microstructure evolution of multicomponent alloys with equiaxed dendritic and eutectic morphology has been developed based upon the mixture-theory model (continuum approach). The model can account for the effects of natural convection, solidification contraction, solidification kinetics, and grain movement on the solidification microstructure evolution. The novelty of this model is that it includes tracking of equiaxed dendritic and eutectic grains movement during solidification and, thus, eliminates the assumption of uniform grain size in a given volume element, which is standard in current advanced solidification models. This is achieved through the implementation of continuous nucleation laws and of a grain distribution function over the volume element, in addition to solid transport simulation through the energy equation. To track grain movement, rules of tracking grain movement are proposed. The model deals with nonequilibrium solidification and describes competitive growth of primary and eutectic phases. The proposed model was implemented to simulate the microstructural evolution of an Al-Si-Mg alloy (A356) during solidification. An equivalent pseudobinary approach was developed to calculate the solidification parameters required in modeling of this multicomponent alloy. Computational experiments with the new model have demonstrated that significant variations in the volumetric grain density exist throughout the casting because of natural convection. These differences can be traced with the proposed grain tracking technique but not with current solidification models.     

Effect of transformation substructure on the strength and toughness of Fe−Mn alloys

Phase transformations in Fe?Mn alloys containing up to 9 pct Mn were studied by optical and electron transmission microscopy. Either equiaxed ferrite, massive ferrite, or massive martensite can form on cooling from austenite. The particular type of transformation product formed was found to depend on the alloy content, austenite grain size, and cooling rate. The mechanical properties of all the transformation products were evaluated using tensile and impact testing and are discussed in terms of the observed microstructural features. Yield strength and impact transition temperature were found to be relatively insensitive to manganese content but were strongly influenced by the transformation substructure and grain size of the transformed phase. In martensite it has been shown that the structural unit analogous to grain size in ferrite is the martensite packet size, which in turn is controlled by the prior austenite grain size. The fracture surface of broken impact specimens and the fracture profile were examined by means of electron and optical microscopy techniques. These fractographic observations were correlated with impact test data and microstructural observations of the various transformation products.