Microstructural study of pre-yielding and pre-yield cracking in TiAl-based alloys

Samples of Ti44Al8Nb1B (at.%), Ti46Al8Nb (700 ppm oxygen) and of the alloy K5 (Ti–46Al–2Cr–3Nb–0.2W–0.15B–0.4C (800 ppm oxygen) which have been tested in tension at stresses below their macroscopic yield stresses, have been examined using transmission electron microscopy. In lamellar samples it has been shown that dislocation multiplication takes place at stresses from about 400 MPa, well below their 0.2% proof stress. In samples with a duplex microstructure no dislocation activity is observed until the applied stress exceeds the macroscopic yield stress. Deformation twinning is initially observed in lamellar samples at stresses just below the 0.2% proof stress. No acoustic emission events are observed corresponding to the twinning seen in the fully lamellar samples. These observations are discussed in terms of the different pre-yielding behaviour of lamellar and duplex samples and in terms of acoustic emission signals, pre-yield cracking and pre-yield twinning. It is concluded that pre-yield cracking is caused by slip in gamma grains in near fully lamellar samples and by slip in lamellae longer than about 70 μm in fully lamellar samples. It is further concluded that all pre-yield acoustic emission is caused by cracking.     

Alloy design of gamma titanium aluminides based on phase diagrams

Phase stability in the Ti---Al---X (X = Cr, Mo and W) system has been investigated at 1473 and 1573 K by the following methods: microstructure observation of quenched specimens, diffusion coupling experiments, thermal analysis of DTA and in situ observation of X-ray diffraction. The ternary phase diagrams of Ti---Al---X system are proposed; additions of Cr, Mo and W stabilize the β phase in the ternary phase diagrams. The tendency of ‘β stabilizer’ is in the following order: 6 at% Cr = 3 at% Mo = 1 at% W 10 vol% β phase. In these Ti-Al-X ternary systems, the + β + γ three-phase coexisting region is close to the Ti---Al binary line and sifts slightly toward a higher concentration of Al as temperature increases. Based on the ternary phase diagrams, (γ + β) TiAl having a super-plastic capability and partial lamellar (P-L) microstructure which shows relatively well balanced mechanical properties from room temperature to elevated temperatures have been rationalized.     

Effects of discontinuour coarsening of lamellae on creep strength of fully lamellar TiAl alloys

High temperature creep of a binary Ti-42mol%Al alloy with fully lamellar structure was studied to examine effects of lamellar spacing on creep strength. Strain hardening is more significant in a finer lamellar material, resulting in higher creep strength at high stresses. Discontinuous coarsening of lamellae takes place during creep, and is more substantial in the finer lamellar material at low stresses. Because of the microstructural degradation, the strengthening by fine lamellae diminishes at low stresses. Some specimens were annealed at high temperatures to finish the discontinuous coarsening prior to creep testing. In these specimens, the strengthening by fine lamellae becomes effective even at low stresses.     

Effect of capping layer on hillock formation in thin Al films

The effect of capping layers on hillock formation has been studied in pure aluminum films. For this purpose, different capping materials (Ti, Mo, SiO₂) of various thicknesses (from 200 Å to 1000 Å) were deposited on an Al/glass substrate. The density, diameter and height of hillocks were analyzed after annealing for 400 min at 280 °C. As the thickness of the capping layer increases, the hillock density, diameter and height decreases. The total volume of hillocks per unit area of the film decreases while both the thickness and the biaxial modulus of the capping layers increases. Using a spherical cap model of the bulge test, a simple equation which can predict the total volume of hillocks per unit area of the film is suggested. The comparison shows excellent agreement between the modeled and experimental profiles.     

γ-TiAl片层界面在裂纹形核中的双重作用

通过全片层γ－ＴｉＡｌ基合金ＳＥＭ原位拉伸实验以及对裂纹前方滑移面及解理面上的应用力进行有限元计算,研究了片层界面在形核中的作用,当原裂纹与片层平行时,裂纹尖端滑移秕的分切应力较小,滑移相对困难,片层面上的正应力比其它解理面上的正应力大,从而解理裂纹优先沿片层界面形核;当裂纹与片层界面垂直时,裂纹尖端很多滑移系上的分切应力较大,滑移相对容易,片层面上的正应力远比其它解理面上的正应力小,从而裂纹优等     

Microstructure and high-temperature strength in MoSi2/NbSi2 duplex silicides

Pseudo-binary (Mo_0.90Nb_0.10)Si₂ and (Mo_0.85Nb_0.15)Si₂ containing duplex C11_b and C40 phases were prepared in a slow solidification process using the floating zone method to obtain basic knowledge regarding control of their microstructure. In (Mo_0.90Nb_0.10)Si₂ a thick band-like C40 phase was formed along grain boundaries in the C11_b matrix, while peculiar fine lamellar colonies composed of the C11_b and C40 phases appeared in (Mo_0.85Nb_0.15)Si₂ accompanied by constituent large C11_b and C40 grains. The orientation relationship between the two phases at the lamellar boundary was determined to be (0001)_C40(110)_C11b and [11 0]_C40[1 1]_C11b. Although the atomic stacking sequence changed from three-fold periodical layers on (0001) planes in the C40 phase to two-fold layers on (110) planes in the C11_b phase at the lamellar boundary, the lattice mismatch at the interface was very small (within 3%), resulting in good thermal stability of this microstructure and high strength at high temperatures. The phase transformation must proceed by the motion of all four 1/61 2-type partial dislocations on two layers of every three (0001) layers in the C40 phase to avoid large lattice distortion.     

Tension-Compression Yield Asymmetry Influenced by the Variable Deformation Modes in Gradient Structure Mg Alloys

Surface mechanical attrition treatment(SMAT) was carried out on hot-rolled AZ31 Mg samples along two orthogonal directions;as a result,two types of gradient structures with different grain sizes and texture components in different layers were produced.The tension-compression yield asymmetry(YA) was studied using samples with different thicknesses,in order to elucidate the effect of combinations of variable deformation modes operating in different layers of the two oriented SMAT samples.The 0° oriented SMAT sample containing layers with strong basal texture displayed significant YA,because of either dislocation slip or extension twinning domination during tension or compression.By contrast,the 90° oriented SMAT sample containing layers with coexisting orthogonal texture components had an obviously weakened YA,which was attributed to the multi-deformation modes cooperating during tension or compression,i.e.,extension twinning or detwinning in conjunction with dislocation slips,leading to close yield stresses compared between tension and compression.     

Pre-yielding and pre-yield cracking in TiAl-based alloys

Samples of Ti44Al8Nb1B (at.%) and Ti46Al8Nb (700 wt ppm oxygen) and of the alloy K5 (Ti-46.5Al-2Cr-3Nb-0.2W-0.15B-0.4C (800 wt ppm oxygen) have been heat treated to produce duplex, near fully lamellar and fully lamellar microstructures. These samples have been tested in tension, using detectors to detect acoustic events during pre-yielding. Many acoustic emission events and corresponding cracking, occur well below the 0.2% proof stress in the near fully lamellar samples of Ti44Al8Nb1B, in fully lamellar Ti46Al8Nb and fully lamellar K5. In fully lamellar Ti44Al8Nb1B alloy some samples show no acoustic events before yield and others show only single acoustic emission events. Duplex samples generate no signals until the applied stress exceeds the macroscopic yield point. The stress–strain curves from Ti448Nb1B, Ti46Al8Nb and from K5 show that in near fully lamellar and fully lamellar samples significant pre-yielding occurs, but none is obvious in the duplex samples. These observations are discussed in terms of the factors contributing to pre-yield plasticity and to pre-yield cracking in these alloys and with reference to earlier acoustic emission work where the reported behaviour is very different. It is concluded that yield, within sufficiently long lamellae or within gamma grains, can give rise to pre-yield cracking and thus to acoustic emission in lamellar samples.     

Solid-state hot pressing of elemental aluminum and titanium powders to form TiAl (γ + α2) intermetallic microstructure

The elemental powder metallurgy (EPM) process is used to prepare TiAl-base intermetallics. An EPM process conducted by two-stage solid-state hot pressing was employed to prepare TiAl-base intermetallics and to investigate the resulting microstructural changes. The results showed that the TiAl₃ phase forms in the first stage. During the temperature increase to the second sintering stage, lamellar phases start to precipitate in the TiAl₃ matrix. Further, the TiAl₃ phase transforms to TiAl, and Ti₃Al layers develop in the remaining titanium particles. Meanwhile, the lamellar phases grow into ring-type structures between the TiAl matrix and the Ti₃ Al layers. After the second stage, the remaining titanium particles are fully reacted, and a microstructure of Ti₃Al phases enclosed by fine-grained lamellar rings in the TiAl matrix is developed.     

The significance of acoustic emission during stressing of TiAl-based alloys. Part I: Detection of cracking during loading up in tension

Samples of fully lamellar Ti44Al8Nb1B and Ti44Al4Nb4Hf0.2Si have been tested in tension at room temperature while they were interfaced to an acoustic emission system. The generation of acoustic signals, at stresses well below the 0.2% yield stress and at higher stresses, has been shown to be due to the formation of cracks within individual lamellar colonies. Data obtained in the micro-strain region in loading–unloading measurements confirmed that there is significant plastic flow at stresses 200 MPa below the 0.2% proof stress of 625 MPa, and it is inferred that this flow gives rise to the cracking. The mechanism whereby such cracking occurs and the possible influence of these cracks on subsequent fatigue behaviour are discussed.     

全层状结构的γ—TiAl中裂纹扩展的TEM原位观察

本文采用ＴＥＭ原位拉伸技术研究了处于不同取向的全层状结构的Ｔｉ－４９％Ａｌ（原子分数）合金（ＰＳＴ晶体）的断裂机制，结果表明：ＰＳＴ晶体裂纹萌发及扩展机制强烈依赖于ＰＳＴ晶体的晶体学取向，裂纹尖端位错发射能力的不同和界面的约束是造成不同取向ＰＳＴ晶体具有不为断裂机制的根本原因。     

一种新型化学溶液沉积法制备涂层导体用RE2O3（RE=Y, Sm, Eu, Dy, Yb）缓冲层

用一种新的化学溶液沉积方法在双轴织构NiW (200)合金基底上制备了涂层导体用稀土氧化物RE2O3(RE=Y, Sm, Eu, Dy, Yb)缓冲层.分别利用X射线衍射,扫描电子显微镜,原子力显微镜对制得的RE2O3缓冲层的相结构、织构、表面形貌和平整度进行了检测.结果表明,RE2O3缓冲层具有较好的双轴织构,表面平整无裂纹.     

Yield locus evolution and constitutive parameter identification using plane strain tension and tensile tests

An inverse calibration strategy to determine the constitutive parameters of phenomenological advanced yield criteria in a convenient and economical manner is presented. The studies on the shape of the yield loci for various steel types revealed that their work contours exhibit almost no evolution in the vicinity of the equibiaxial tension after roughly 4% equivalent strain. In other words, the ratio between balanced biaxial and uniaxial stresses of the yield locus reaches a saturation value after undergoing some deformation. Accordingly, the balanced biaxial stresses can be associated with the tensile flow stresses in the rolling direction by means of a constant factor, which can be generalized for different steel families. Based on this, an alternative inverse-analysis strategy using the tensile and plane strain tension tests will be proposed and validated within this work. Cup drawing tests have been applied to assess the accuracy of the optimized yield loci for different strain paths.     

Effect of lamellar plates on creep resistance in near gamma TiAl alloys

Effect of interlamellar spacings on creep rate in Ti-48 at% Al alloy with fully transformed lamellar structure (FL) has been investigated at 1123 K/98 MPa. In addition, the correlation between creep rate and lamellar orientation to stress axis was elucidated by conducting creep tests at 1148 K/68.6 MPa for three single crystal (SC) specimens of Ti-48 at% Al with different lamellar orientations to stress axis. The difference in creep rate among FL specimens having different interlamellar spacings could not be defined in the early stage of transient creep. The onset of accelerating creep was slightly retarded by decreasing the inter-lamellar spacing. While the creep rate of the SC specimen whose lamellar orientation to stress axis 30–63 ° gradually decreases with increasing strain, and is larger than that of the FL specimen, the creep rate of the SC specimen with lamellar orientation nearly parallel to stress axis drastically decreases within small strain, and is 1/50 smaller than that of the FL specimen. This strong lamellar orientation dependence of creep rate is interpreted by the correlation between dislocation slip system and the lamellar plate.     

Feasibility of laser surface treatment of pearlitic and bainitic ductile irons for hot rolls

The effects of laser surface treatment on the microstructure, crackability and stresses generated on laser hardened layers produced in several ductile cast iron materials were investigated. Two kinds of alloys having pearlitic (SGP) and acicular (SGA) matrix microstructures were selected. Hardened layers with thicknesses ranging from 1.5 to 2.5 mm were obtained by means of laser remelting (LSRm) or laser hardening (LSH). Thermal stresses generated upon laser processing have been estimated by a simple thermal model. For energy densities delivered onto the material at above 40 J/mm², extensive cracking was developed in SGA and SGP irons due to the contribution of thermal stresses. By lowering the energy density, crack formation was avoided in SGP irons only. At low energy densities, crack formation is controlled by the generation of transformational stresses due to excessive austenite retention. An increase of the surface temperature or the alloying content gave rise to an increase of the retained austenite and the formation of lower bainite at the remelted zone and the heat affected zones, respectively. K_C fracture toughness of Fe₃C carbides embedded in pearlitic and acicular matrixes was measured by means of the nano-indentation technique. Fracture toughness of cementite in SGP irons was slightly higher than in SGA irons, which can help to reduce the crackability of LSH layers.     

Analysis of low transformation temperature welding (LTTW) consumables – distortion control and evolution of stresses

Low transformation temperature welding (LTTW) consumables have been reported to reduce the tensile residual stresses in weldments. Martensitic transformation induces compressive residual stresses and improves the fatigue resistance of welded joints. Several of these LTTW consumables have been developed at the Colorado School of Mines. This research work presents the comparisons of the experimentally and Sysweld calculated measurements for distortions and residual stresses for different plate thicknesses. In addition, residual stress evolution with time graphs were plotted to determine the amount of martensite required to promote compressive residual stresses and to calculate the time required to induce compressive residual stresses. The main aspect of this research is to analyse the behaviour of LTTW consumables in terms of distortion and residual stresses on various plate thicknesses.     

TiN/Si3N4纳米多层膜硬度对Si3N4层厚敏感性的研究

通过反应磁控溅射制备了一系列不同Si3N4层厚的TiN/Si3N4纳米多层膜,利用X射线衍射仪、高分辨透射电子显微镜、扫描电子显微镜和微力学探针表征了多层膜的微结构和硬度,研究了其硬度随Si3N4层厚微小改变而显著变化的原因.结果表明,在TiN调制层晶体结构的模板作用下,溅射态以非晶存在的Si3N4层在其厚度小于0.7 nm时被强制晶化为NaCl结构的赝晶体,多层膜形成共格外延生长的{111}择优取向超晶格柱状晶,并相应产生硬度显著升高的超硬效应,最高硬度达到38.5GPa.Si3N4随自身层厚进一步的微小增加便转变为非晶态,多层膜的共格生长结构因而受到破坏,其硬度也随之降低.     

Mechanical behavior of nanoscale Cu/PdSi multilayers

Berkovich nanoindentation and uniaxial microcompression tests have been performed on sputter-deposited crystalline Cu/amorphous Pd_0.77Si_0.23 multilayered films with individual layer thicknesses ranging from 10 to 120 nm. Elastic moduli, strengths and deformation morphologies have been compared for all samples to identify trends with layer thicknesses and volume fractions. The multilayer films have strengths on the order of 2 GPa, from which Cu layer strengths on the order of 2 GPa can be inferred. The high strength is attributed to extraordinarily high strain hardening in the polycrystalline Cu layers through the inhibition of dislocation annihilation or transmission at the crystalline/amorphous interfaces. Cross-sectional microscopy shows uniform deformation within the layers, the absence of delamination at the interfaces, and folding and rotation of layers to form interlayer shear bands. Shear bands form where shear stresses are present parallel to the interfaces and involve tensile plastic strains as large as 85% without rupture of the layers. The homogeneous deformation and high strains to failure are attributed to load sharing between the amorphous and polycrystalline layers and the inhibition of strain localization within the layers.     

Characterization and comparison of materials produced by Electron Beam Melting (EBM) of two different Ti–6Al–4V powder fractions

Electron Beam Melting (EBM) has been recognized as a revolutionary technique to produce mass-customized parts to near-net-shape from various metallic materials. The technique produces parts with unique geometries from a powder stock material and uses an electron beam to melt the powder layer-by-layer to fully solid structures. In this study we have investigated the use of two different Ti–6Al–4V powders of different size fractions in the EBM process; a larger 45–100 μm powder, and a smaller 25–45 μm powder. We have also investigated the effects of two build layer thicknesses, 70 μm and 50 μm, respectively. We hypothesize that the smaller powder has the potential to improve surface resolution of parts produced in the EBM process. The EBM as-built parts were investigated regarding surface and bulk chemistry, surface oxide thickness, macro- and microstructure, surface appearance and mechanical properties. We conclude from the results that both powders and both build layer thicknesses are feasible to use in the EBM process. The investigated material properties were not significantly affected by powder size or layer thickness within the studied range of process parameters. However, the surface appearance was found to be different for the samples made with the different powder sizes.     

The effect of depth-dependent residual stresses on the propagation of surface acoustic waves in thin Ag films on Si

We give detailed calculations of the acoustoelastic effect (AE) in highly textured thin silver films on silicon substrates. The AE relates the phase velocity of the surface waves to the stresses within the films. Therefore, we have investigated several stress states that were produced by annealing and measured by X-ray grazing incidence diffraction. The selected temperatures range from 25 to 700 °C. The stress profiles are modelled by a stratification of the films into different layers. Subsequently, by application of the stiffness-matrix method (SMM) for acoustic wave propagation, the corresponding AE is calculated. No measurements of the AE itself will be reported here. Our calculations show that the AE is fairly weak and becomes maximal when the wavelength is on the order of some few film thicknesses.