Analysis of local crystallographic orientation in an annealed Ti60 billet

A heterogeneous microstructure in terms of local orientation distribution is often found in near-α titanium alloys. The presence of large regions with similar crystallographic orientation, called ‘macrozones',could drastically decrease the fatigue performance of titanium alloys. The present work reports on the crystallographic orientation of a near-α titanium alloy, Ti60, billet after annealing in an α+β phase field.The texture was found to be weak, and no obvious macrozone was found in our measured zone where the variant selection is suppressed. Meanwhile, in-depth electron backscattered diffraction(EBSD) analysis was applied to evaluate the final microstructure, and the mechanisms by which they formed were analyzed.     

Elastic and plastic stress analysis of an interface crack between two dissimilar layers

In the present paper, the mixed-mode Dugdale model is applied to investigate the plastic zone size and the crack tip opening displacement of an interface crack between two dissimilar layers. In the analysis, both normal and shear stresses are assumed to exist in the plastic zones and satisfy the Von Mises yield criterion. The plastic zone sizes can be determined on condition that the stress intensity factors caused by the stresses in the plastic zones and applied loading are zero. Then, the crack tip opening displacement can be obtained by dislocation theories. In numerical examples, the plane stress condition is considered. The plastic zone size and the crack tip opening displacement of an interface crack between two dissimilar layers under a uniform load are examined. The effects of Dundurs’ parameters and the thickness of materials on the plastic zone size and the crack tip opening displacement are investigated in detail. Numerical results show that in the case of small thickness, the values of the normalized plastic zone size and the normalized crack tip opening displacement decrease with increasing Dundurs’ parameters, α and β, while, in the case of infinite thickness, the value of the normalized plastic zone size is independent of α, and the value is symmetric about the axis on which β = 0.     

Effect of 2–6 at.% Mo addition on microstructural evolution of Ti-44Al alloy

In order to understand the effect of Mo alloying on the microstructural evolution of TiAl alloy, the as-cast microstructure, heat treated microstructure characteristic, and hot compression microstructure evolution of Ti-44 A1 alloy have been studied in this work. The as-cast microstructure morphology changes from(γ+α_2)lamellar colony and β/β_0+γ mixture structure to β/β_0 phase matrix widmannstatten structure,when Mo content increases from 2 at.% to 6 at.%. Affected by the relationship between β phase and αphase, the angles between the lamellar orientation and the block β/β_0 phase are roughly at 0°, 45° and90°. Comparing with heat treatment microstructure, the hot compression microstructure contains lessβ/β_0 phase, however, the β/β_0 phase containing 2 Mo alloy and 3 Mo alloy hot compressed at 1275 ℃ has the inverse tendency. In addition,(α_2 +γ) colony is decomposed by the discontinuous transformation.     

Microstructure and mechanical properties of TA15 titanium alloy under multi-step local loading forming

In this paper six different local loading processes were proposed to study the effects of local loading conditions (temperature, deformation degree, loading pass, heats, cooling modes and heat treatment) on the microstructure and mechanical properties of TA15 titanium alloy workpieces including room and high temperature tensile properties, impact property, fracture toughness and high temperature duration property.It is found that it is better to finish the local loading forming in one heating time, if multi-fire forging needed the optimal forging technique as follows: adopting conventional forging (950 °C) at first and then following near-beta forging to control the proportion of the equiaxed primary α phase and the transformed β phase, allocating deformation degree of each loading pass rationally and using WQ cooling mode. Thus the workpiece with good compositive mechanical properties can be obtained.     

Effects of welding speed on the microstructure and hardness in friction stir welding joints of 6005A-T6 aluminum alloy

The 6005A-T6 aluminum alloy was friction stir welded at different welding speeds. The peak temperature, microstructure and mechanical properties were examined for these joints. A special attention was devoted to the relationship between the precipitates evolution within different zones and the local hardness. In the nugget zone (NZ) experiencing the highest peak temperature, the β″ precipitates dissolved into α-Al matrix during welding, and the hardness of NZ depended on the level of natural aging (NA) at different welding speeds. The thermo-mechanically affected zone (TMAZ) is characterized by elongated grains with a high density of dislocations. The welding speed had not a significant effect on hardness in this zone. The heat-affected zone (HAZ) contains the transformation of β″–β′, the precipitation of Q′ and the coarsening of precipitates. The HAZ close to the joint center line exhibited the minimum hardness due to the coarsening of β′ and Q′ precipitates while the HAZ far from it having a high hardness level was mainly related to coherent β″ precipitates. The HAZ hardness and joint strength have an increased tendency with increasing the welding speed. It can be explained by increasing the density of Q′ or β″ precipitates.     

Microplasticity in HCF of Ti–6Al–4V

Previous research has shown that Ti–6Al–4V exhibits pronounced stress ratio effects under high cycle fatigue (HCF) loading. At high stress ratios (R>0.7), a transition of failure mode occurs from traditional surface fatigue crack initiation and growth to bulk-dominated damage initiation and coalescence of multiple microcracks consistent with a ductile tensile test. At these high stress ratios, ratchetting was shown to occur (Int. J. Fatigue 21 (1999) 679; Mech. Time-Dependent Mater. 2 (1999) 195), leading to progressive strain accumulation until final failure. This study explores the microstructural origins of this stress ratio transition in HCF using computational micromechanics. The material being studied is a two-phase Ti–6Al–4V plate forging, consisting of a duplex microstructure with a hexagonal close-packed (hcp) α-phase and lamellar grains with layers of body-centered cubic (bcc) β-phase and secondary hcp α-phase. Crystallographic slip is the dominant mode of plastic deformation in this material. A 2-D crystal plasticity model that incorporates nonlinear kinematic and isotropic hardening at the slip system level is implemented into the finite element method to simulate the cyclic plasticity behavior. The finite element model is used to qualitatively understand the distribution of microplasticity in this alloy under various loading conditions. For typical HCF stress amplitudes, it is shown that microstructure scale ratchetting becomes dominant at R=0.8, but is insignificant at R=0.1 and 0.5. Reversed cyclic microplasticity is insignificant at all three stress ratios. The effects of phase morphology and orientation distribution are shown to affect the microscale plastic strain distribution in terms of the location and magnitudes of the plastic shear bands that form within clusters or chains of primary α grains. The results of the finite element modeling are also considered in light of previous experimental results.     

航空紧固件用Ti-5553合金的组织和性能

采用扫描电镜和透射电子显微镜研究不同热处理制度对Ti-5553高强钛合金显微组织与力学性能的影响。结果表明:在(α+β)两相区进行固溶处理时,随着固溶温度的升高,Ti-5553合金组织中的初生α相含量逐渐减少,β相的尺寸和体积分数均增加,合金强度逐渐降低。时效后β基体发生转变,晶界和晶内析出大量次生α相。次生α相的尺寸对力学性能产生重要影响,随着时效温度的升高,次生α相逐渐粗化,导致抗拉强度逐渐下降。1240MPa级航空紧固件用Ti-5553的固溶温度应选择Tβ以下,使组织中留有足够的β相,从而时效时在β相中有大量次生α相析出,获得需要的高强度。同时,保留一定含量的初生α相,以便获得良好的塑韧性。经810~820℃,1.5h,水淬+510℃,10h,空冷热处理后,合金可以获得较好的综合性能,抗拉强度达1500MPa,伸长率达14.8%,断面收缩率为38.6%。固溶和时效态的拉伸断口均存在大量韧窝,材料具有良好的塑韧性。     

Distinct dendritic α phase emerging on the surface of primary α phase in a compressed near-α titanium alloy

The characteristic of the surface morphology of primary α phase was studied in a deformed near-α titanium alloy. Dendritic α phase emerged on the surfaces of primary α phase when the alloy was air-cooled in α + β phase field after deformation. The dendritic α grain has the same orientation with its original primary α grain. The formation of the dendritic α phase could be explained by interface instability in epitaxial growth process of the primary α phase. The dislocations induced by deformation could facilitate the formation of dendritic α phase leading to the dendritic α phase and more obvious with the increase of strain. The growth of dendritic α phase was finally limited by the nucleation of second α phase with cooling.     

双相γ+α2钛合金片层承载力的数学模型及预测

为了研究双相γ+α2钛合金晶体中片层组织的承载特性,建立了相应晶体不同取向力学性能的数学模型,并计算了γ相及γ+α2片层组织不同取向的承载力.通过对模型的数值求解,得到γ相和γ+α2片层组织的承载力随不同加载方向的变化规律.计算结果表明:随着α2相体积分数的增加,γ+α2片层组织的最大承载力方向逐渐趋向平行于α2相的(0001)面,临界α2相等价体积分数为λe=30%.这从理论上解释了γ+α2片层组织的最大承载力方向为平行于α2相的(0001)面这一传统观点.这一结论成立的条件为α2相等价体积分数λe>30%.     

Dynamic mechanical behavior of a Zr-based bulk metallic glass composite

Dynamic mechanical analysis (DMA) was used to study the viscoelastic properties of a Zr_56.2Ti_13.8Nb_5.0Cu_6.9Ni_5.6Be_12.5 (LM2) bulk metallic glass (BMG) matrix composite. The temperature spectrum and the frequency spectrum were tested by three point bending. We revealed that the relaxation process of the LM2 composite is different from the single amorphous alloys. The unique superimposition of glass transition (α-relaxation) and β-phase transformation during DMA results in double peaks on the loss modulus evolution and a plateau on the storage modulus evolution. The numerical analysis of the isothermal spectrum suggests that the dynamic relaxation behavior of the LM2 composite cannot be described by a single classical relaxation model. A coupling model is instead proposed to well quantify the coupling effect of the α-relaxation and β-phase transformation in the LM2 BMG composite. Our findings provide an insight into understanding the more complex structural relaxation of BMG composite than monolithic BMGs.     

Microstructure transformations of laser-surface-melted near-alpha titanium alloy

Microstructural transformations of near-alpha titanium alloy under laser-surface -melted and post-heat treated conditions were systematically investigated. The results show that, after laser surface melting treatment, a melted zone and a heat-affected zone are formed in the titanium alloy. The melted zone consists of columnar grains, which are characterized by fine a″ phase, distributed dispersively in acicular martensite phase α′. Martensite transformation occurs at a lower temperature in post-heating process. The α phase in the laser-surface-melted zone is further refined during the recrystallization, and a fine dispersive α + β microstructure is formed when experienced post-heat treatment at an elevated temperature.     

Investigation on microstructure and mechanical properties of Ti14 alloy after semisolid deformation

Abstract

This study details the development of microstructure of Ti14 alloy as a function of the forging temperature and forging ratio in semisolid state and influence of resulting microstructure on the mechanical properties. The results reveal that dynamic recrystallisation occurred during semisolid forging, and the grain refinement was attained. Grain size increased in the forging temperature and decreased in the forging ratio. High ultimate tensile strengths and low elongation have been achieved after semisolid forging. The strength decreased with increasing forging temperature, while the ductility increased with increasing forging ratio. The relative contributions of tensile properties were attributed to the varieties of grain size obtained by thixoforging.     

On mechanism of flow softening in Ti–5Al–5Mo–5V–3Cr

Abstract

The flow behaviour of Ti–5Al–5Mo–5V–3Cr with an initial microstructure containing acicular α platelets has been characterised during isothermal subtransus forging. Flow softening was observed, following yielding and limited hardening, for all investigated temperatures and strain rates, before a steady state flow regime being reached at a strain of ～0·5. The acicular α plates were found to have been fragmented by the forging process, which is concurrent with previous findings. The flow behaviour of the fully retained β phase below the β transus temperature has been established and found to be similar to that of the steady state flow of platelet α in a β matrix. Forging the acicular α microstructure to low strains resulted in higher dislocation concentrations in the β matrix than could be observed in the α precipitates, supporting the hypothesis of hardening through dislocation impedance. Evidence of fragmentation via a pinch-off mechanism was found where slip was observed to have been transmitted across an α plate. Thus following yield the flow behaviour is dominated by the pile-up of dislocations at the α/β interface before the transmission of slip leads to plate fragmentation resulting in flow softening to a steady state regime governed entirely by the β matrix.     

The cubic-to-β martensitic transformation in ZrO2-Sc2O3

The nature of the cubic-to-β (c-β) transformation and the microstructure of theβ-phase were examined in ZrO₂-10.5 and 12.5 mol % Sc₂O₃ alloys. The c -β transformation is induced during cooling from the high-temperature cubic phase region by martensitic transformation. The microstructure of theβ-phase usually has a herring-bone appearance, which is made of a unique array of four orientation variants. Two types of interfaces are formed in the structure; long straight interfaces and short interfaces. The former and the latter interfaces are nearly parallel to the {011} _β and {012} _β type planes, respectively, which correspond to the {001}_c and {011}_c planes in the parent cubic phase. The {011} _β and {012} _β type interfaces are fully coherent, low-energy interfaces. The herring-bone structure is likely to be a favourable one for minimizing the strain energy and interfacial energy associated with the c -β transformation, which is a characteristic microstructure developed by martensitic transformation.     

Formation and crystallization of Al-Fe-Si amorphous alloys

Rapid solidification experiments of Al-based Al-Fe-Si ternary alloys have been performed both by the gun and single roller methods. Glass forming tendency is found to be largest near theβ-phase (Al₉Fe₂Si₂) composition, where the amorphous state is obtained both by the gun and single roller methods. Crystallization behaviour of amorphous AlFe_13.0Si_17.4 alloy, nearβ-phase composition, followed the course of (i) the appearance of fineα-Al particles and (ii) the transformation of the matrix intoβ-phase. The ratio of crystallization temperature and the melting point,T _x/T _m, for AlFe_13.0Si₂17.4 amorphous alloy is smaller than 0.5 indicating the difficulty of glass formation of these alloys.     

Modeling and analyzing the effects of heat treatment on the characteristics of magnesium alloy joint welded by the tungsten-arc inert gas welding

The objective of this paper is to present the mathematical models for modeling and analysis of the effects of heat treatment on the characteristics of magnesium alloy joint welded by the tungsten-arc inert gas (TIG) welding. The process of heat treatment adopts the tempering process with varying processing parameters, including tempering temperature and tempering time. The microstructure and mechanical properties of the welded joint are considered in the characteristic evaluation and explored by experiment. An experimental plan of the face-centered central composite design (CCD) based on the response surface methodology (RSM) has been employed to carry out the experimental study. The results of analysis of variance (ANOVA) and comparisons of experimental data show that the mathematical models of the value of the maximum tensile strength and elongation are fairly well fitted with the experimental values with a 95% confidence interval. In the tempering process, the microstructure of welded joint in the weld bead displays two main microstructures of hcp-α-phase Mg and bcc-β-phase Mg₁₇Al₁₂. Results show that the average size and proportion of α-phase Mg grains decreases with the increase of the tempering time and temperature. But, the increase of the tempering time and temperature promote increasing the average size and proportion of β-phase Mg₁₇Al₁₂ grains. An increase in both the high value of tempering temperature and tempering time leads to an increase of the maximum tensile strength. The values of the elongation increase with increasing in both the value of tempering temperature and tempering time.     

TC4钛合金线性摩擦焊接头的组织与硬度分析

采用线性摩擦焊焊接TC4钛合金,对焊态下接头的显微组织及硬度进行了分析与测试。结果表明：焊接接头可分为母材区、热机影响区和焊核区。热机影响区组织由母材至焊核区依次为等轴α相和层片状（α＋β）相沿受力方向被拉长组织、纤维状组织中伴有等轴状α和层片状（α＋β）再结晶晶粒、针状α’和少量的α再结晶组织。焊核区组织为针状α’,而且纵向由中心至边缘组织逐渐粗大。垂直于焊缝方向由母材过渡到焊缝中心硬度逐渐由360HV增大到390HV左右,焊核区纵向硬度由中心向边缘逐渐减小到330HV左右。     

热处理对AZ91D及AZ71镁合金组织及力学性能的影响

研究了热处理工艺对AZ91D及AZ71镁合金显微组织及力学性能的影响。结果表明,在AZ91D和AZ71镁合金中,温度升高,β相含量都是先增加后减小,400℃时合金中的β相几乎消失,完全由α基体相组成;硬度都是先减小后增加,235℃时硬度最大;耐冲击性都是先减小后增加,235℃时耐冲击性最低。     

Superplasticity and microstructure of TC21 titanium alloy during thermomechanical treatment

Abstract

As rolled TC21 titanium alloy was subjected to isothermal constant strain rate tensile tests using an electronic tensile testing machine. After tensile deformation, the alloys were subjected to double annealing. Superplastic behaviour and microstructure evolution were systematically investigated. Experimental results show that as rolled TC21 alloy exhibits good superplasticity at temperatures ranging from 870 to 930°C and strain rates ranging from 3×10^?4 to 3×10^?2 s^?1. A maximum elongation of 373·3% was obtained at 910°C and 3×10^?4 s^?1. In addition, the alloy microstructure comprises α and β phases during plastic deformation. The primary α-grains aggregate and merge to form new crystal grains with irregular grain boundaries because of dynamic recrystallisation. Furthermore, the primary α phase content gradually decreases with increasing temperature. The resulting microstructure after deformation and double annealing is a duplex microstructure comprising a primary equiaxed α phase and a β-transformed lamellar structure. The acicular α phase transformed from the β phase is mutually interlaced as a basketweave structure after deformation at 930°C and double annealing.     

Microstructure evolution of Ti–30Nb–(4Sn) alloys during classical and step-quench aging heat treatments

Recent studies involving step-quench (SQ) heat treatments have shown that, unlike classical quench-and-aging (QA) heat treatments, SQ route can favour precipitation mechanisms that are strongly dependent on decomposition of the parent β-phase. In this work, Ti–30Nb and Ti–30Nb–4Sn (wt-%) alloys were subjected to cold-working, recrystallisation and then to either classical or SQ heat treatments at 400°C. Among QA samples, ω-phase is formed in both alloys during the heating ramp. By the SQ route, we found extensive isothermal ω-phase precipitation in Ti–30Nb during aging. On the other hand, a complete suppression of ω-phase in Ti–30Nb–4Sn was observed, thus, α-phase precipitation could take place improving hardness while still preserving a low elastic modulus.