Influence of Two-Step Heat Treatments on Microstructure and Mechanical Properties of a β-Solidifying Titanium Aluminide Alloy Fabricated via Electron Beam Powder Bed Fusion

Additive manufacturing technologies, particularly electron beam powder bed fusion (PBF-EB/M), are becoming increasingly important for the processing of intermetallic titanium aluminides. This study presents the effects of hot isostatic pressing (HIP) and subsequent two-step heat treatments on the microstructure and mechanical properties of the TNM-B1 alloy (Ti–43.5Al–4Nb–1Mo–0.1B) fabricated via PBF-EB/M. Adequate solution heat treatment temperatures allow the adjustment of fully lamellar (FL) and nearly lamellar (NL-β) microstructures. The specimens are characterized by optical microscopy and scanning electron microscopy (SEM), X-ray computed tomography (CT), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD). The mechanical properties at ambient temperatures are evaluated via tensile testing and subsequent fractography. While lack-of-fusion defects are the main causes of failure in the as-built condition, the mechanical properties in the heat-treated conditions are predominantly controlled by the microstructure. The highest ultimate tensile strength is achieved after HIP due to the elimination of lack-of-fusion defects. The results reveal challenges originating from the PBF-EB/M process, for example, local variations in chemical composition due to aluminum evaporation, which in turn affect the microstructures after heat treatment. For designing suitable heat treatment strategies, particular attention should therefore be paid to the microstructural characteristics associated with additive manufacturing.     

Influence of Electron Beam Powder Bed Fusion Process Parameters at Constant Volumetric Energy Density on Surface Topography and Microstructural Homogeneity of a Titanium Aluminide Alloy

In powder bed fusion additive manufacturing, the volumetric energy density E _V is a commonly used parameter to quantify process energy input. However, recent results question the suitability of E _V as a design parameter, as varying the contributing parameters may yield different part properties. Herein, beam current, scan velocity, and line offset in electron beam powder bed fusion (PBF-EB) of the titanium aluminide alloy TNM–B1 are systematically varied while maintaining an overall constant E _V. The samples are evaluated regarding surface morphology, relative density, microstructure, hardness, and aluminum loss due to evaporation. Moreover, the specimens are subjected to two different heat treatments to obtain fully lamellar (FL) and nearly lamellar (NLγ) microstructures, respectively. With a combination of low beam currents, low-to-intermediate scan velocities, and low line offsets, parts with even surfaces, relative densities above 99.9%, and homogeneous microstructures are achieved. On the other hand, especially high beam currents promote the formation of surface bulges and pronounced aluminum evaporation, resulting in inhomogeneous banded microstructures after heat treatment. The results demonstrate the importance of considering the individual parameters instead of E _V in process optimization for PBF-EB.     

Using Selective Electron Beam Melting to Enhance the High-Temperature Strength and Creep Resistance of NiAl–28Cr–6Mo In Situ Composites

By increasing the density of interfaces in NiAl–CrMo in situ composites, the mechanical properties can be significantly improved compared to conventionally cast material. The refined microstructure is achieved by manufacturing through electron beam powder bed fusion (PBF-EB). By varying the process parameters, an equiaxed or columnar cell morphology can be obtained, exhibiting a plate-like or an interconnected network of the (Cr,Mo) reinforcement phase which is embedded in a NiAl matrix. The microstructure of the different cell morphologies is investigated in detail using scanning electron microscope, transmission electron microscopy, and atom probe tomography. For both morphologies, the mechanical properties at elevated temperatures are analyzed by compression and creep experiments parallel and perpendicular to the building direction. In comparison to cast NiAl and NiAl–(Cr, Mo), the yield strength of the PBF-EB fabricated specimens is significantly improved at temperatures up to 1,027 °C. While the columnar morphology exhibits the best improved mechanical properties at high temperatures, the equiaxial morphology shows nearly ideal isotropic mechanical behavior, which is a substantial advantage over directionally solidified material.     

Effect of microstructure on the fatigue properties of Ti–6Al–4V titanium alloys

Through an analysis on microstructure and high cycle fatigue (HCF) properties of Ti–6Al–4V alloys which were selected from literature, the effects of microstructure types and microstructure parameters on HCF properties were investigated systematically. The results show that the HCF properties are strongly determined by microstructure types for Ti–6Al–4V. Generally the HCF strengths of different microstructures decrease in the order of bimodal, lamellar and equiaxed microstructure. Additionally, microstructure parameters such as the primary α (α_p) content and the α_p grain size in bimodal microstructures, the α lamellar width in lamellar microstructure and the α grain size in equiaxed microstructures, can influence the HCF properties.     

铝合金表面冷喷涂铜涂层研究

目的 进一步提高铝合金构件的散热能力。方法 采用冷喷涂技术在铝合金试件表面上制备了纯铜涂层。借助光学显微镜、扫描电子显微镜、拉伸试验、三点弯曲试验和激光闪射法等试验,研究铝合金表面铜涂层微观组织和性能,并探究热处理铜涂层组织形貌、抗弯强度以及导热系数的影响。结果 铜涂层与铝合金基体形成了良好的结合,纯铜涂层导热系数能达到纯铜块材50%的水平。结论 经过热处理,铜涂层内颗粒发生静态再结晶,部分晶粒细化。涂层的抗弯曲强度下降,导热系数上升。     

Phase diagram calculation and experimental research on Fe–12Cr–B–Al–alloys

The vertical sections of Fe–12%Cr–B–xAl–C system with different aluminum contents have been calculated by use of Thermo‐Calc software and the influence of aluminum content on the phase regions and the parameters of eutectic point have been analyzed. Fe–12.0%Cr–1.0%B–2.0%Al–0.3%C and Fe–12.0%Cr–1.0%B–4.0%Al–0.3%C alloy were chosen to be studied by experiment. The phase transition temperatures were measured by differential scanning calorimetry and the microstructure and the phase type was detected by scanning electrone microscope‐energy dispersive X‐ray spectroscopy and X‐ray diffraction. The results indicate that calculated phase diagrams agree well with the experimental results and further prove the thermodynamics database of Thermo‐Calc software is reliable and it can be used to help design the alloy composition and heat treatment process.     

Influence of sharp microstructural gradients on the fatigue crack growth resistance of α+β and near-α titanium alloys

The present study focuses on the effect of microstructural gradients on the fatigue crack growth resistance of Ti‐6Al‐4V and Ti‐6242 titanium alloys. Sharp microstructural gradients from fine‐grained bimodal to coarse‐grained lamellar microstructures were obtained by heat treating only a portion of fine‐grained plates in the β single‐phase field using a high‐frequency induction coil. For fatigue crack growth from a bimodal into a lamellar microstructure, it was found that the initial crack extension past the microstructural transition within the lamellar microstructure shows the same crack growth resistance as the reference bimodal microstructure. Similarly, for fatigue crack growth from a lamellar into a bimodal microstructure, the initial crack extension past the microstructural transition within the bimodal microstructure shows same crack growth resistance as the reference lamellar microstructure. Based on detailed crack front profile investigations using optical light and scanning electron microscopy as well as heat tinting procedures, these findings can be mainly attributed to the effect of the crack front geometry.     

TiAl基合金的组织演变

针对TiAl基合金是显微组织控制,综述了TiAl基合金中几种常见的组织演变,着重论述了变形TiAl基合金在热处理过程中的晶粒长大及动力学分析,TiAl基合金在冷却时层状组织的形成和全层状TiAl基合金在高温时的非连续粗化这3种组织演变的研究现状和面临的问题。     

The relationship between fracture toughness and microstructure of fully lamellar TiAl alloy

Fully lamellar (FL) Ti–46.5Al–2Cr–1.5Nb–1V (at%) alloy is used to study the relationship between microstructure and fracture toughness. A heat treatment process is adopted to control the microstructural parameters of the studied alloy. Fracture toughness experiments and scanning electron microscope (SEM) in-situ straining experiments are carried out to determine the influence of lamellar spacing and grain size on the fracture toughness of FL TiAl alloys. It is found that ligament length depends on the lamellar spacing, and fracture toughness varies non-monotonously with the increase of grain size. The results are ascribed to the competition between the microcrack nucleation and microcrack propagation. Finally a semi-empirical relationship between the fracture toughness and microstructure parameters was established.     

Microstructures and mechanical properties of titanium alloy connecting rod made by powder forging process

A new powder metallurgy (P/M) titanium alloy connecting rod with the composition of Ti–1.5Fe–2.25Mo (wt.%), formed by a powder forging process, was successfully developed with the help of a pre-processing simulation using a commercial finite element software (DEFORM-3D). The microstructures and the mechanical properties of the deformed material were evaluated. The results indicate that the microstructures of the crank pin end, fork part and piston pin end are lamellar α + β structures and the microstructure of shank is of through-transus bi-modal phase. The tensile strength and elongation are much higher than those of the most widely used PF-11C50/60 steels, and can well meet the requirements of the connecting-rod industry.     

Investigation on the Microstructure Homogenization in a TiAl Based Alloy Prepared by Elemental Powder Metallurgy

1.IntroductionTiAlbasedalloyhasdrawnintensiveattentioninhightemperaturestructuralmaterialarea,be-causeofitslowdensity,excellellthightemperaturepropertiesf1~2].However,itspoorworkabilityisanobstacleforitsapplication.Inordertoovercomethisdifficulty,extensiveworkhasbeenconductedtoim-proveitsworkabi1ityandnearnetshaping.Elementalpowdermetallurgy(EPM)hasbeenusedforfabricatingTiAl-basedalloys,becauseofitstwoapparentadvantages'lowcostandconvenientad-ditionofalloyelemellts[3].Thistechniqueincludesr…     

Fatigue Behavior of Ti-6Al-4V

Low-cycle-fatigue texts in vacuum and air were performed. Under cyclic loading the Ti-6Al-4V showed both cyclic hardening and cyclic softening depending on heat treatment, stress amplitude, and microstructure. Plastic deformation of the β-phase in the unaged condition due to stress induced martensitic transformation caused cyclic hardening. Cyclic softening was observed if the α-phase hardened by coherent Ti₃Al particles was plastically deformed. Equiaxed microstructures exhibited a stronger cyclic softening than lamellar structures. This behavior could be explained by the pronounced texture of the equiaxed microstructures, whereas the lamellar structures were texture-free. The fatigue life was influenced by the cyclic softening process mainly in the low-cycle-fatigue regime. The fatigue life at normalized stress amplitude (σ_a/σ_y) was shorter for microstructures with strong cyclic softening as compared to microstructures with lower cyclic softening.     

A STUDY ON THE CHANGE OF FATIGUE FRACTURE MODE IN TWO TITANIUM ALLOYS

The appearance of the fatigue fracture surface and crack growth curve have been examined for a Ti–2.5Cu alloy with different microstructures (two equiaxed and two lamellar microstructures), and for TIMETAL 1100 with a lamellar microstructure. With increasing Δ K , a slope change in the crack growth curve correlates with a transition in the fracture surface appearance (induced by a fracture mode transition); this being found in each microstructure. The microstructure size that controls the fatigue fracture is found to be the grain size for equiaxed microstructures and the lamella width for lamellar microstructures. The transitional behaviour can be interpreted in terms of a monotonic plastic zone size model in microstructures having a coarse microstructure size and in terms of a cyclic plastic zone size model for microstructures having a fine microstructure size.     

Creep behaviour of cast TiAl based intermetallics

Abstract

Constant load tensile creep tests were carried out on the cast TiAl based intermetallics Ti–47 Al–2Mn, Ti–47 Al–2Zr, and Ti–48Al (at.-%), prepared by plasma arc melting. Two microstructural conditions dependent on heat treatment were evaluated as follows: a fully lamellar (FL) scheme consisting of a fully transformed coarse lamellar structure with α₂ laths plus γ laths within the grain interiors; and a duplex scheme consisting of fine equiaxed grains of γ with α₂/γ lamellae. The steady state creep behaviour of both microstructural conditions, for each composition, was studied under stresses of 70–300 MN m^?2 in the temperature range 700–900°C. The microstructure was found to have a pronounced influence on the creep resistance. The FL microstructure exhibited superior creep resistance to the duplex microstructure. At temperatures and stress levels at which direct comparisons can be made, the steady state creep rates of the FL structures are an order of magnitude lower than those of the duplex structure. The apparent creep activation energies and stress exponents were measured for both microstructural conditions for each composition. The temperature and stress dependence of the steady state creep rate of both microstructures can be described by the power law creep equation, suggesting dislocation motion as the operative deformation mechanism.

MST/1962     

Effects of Al–5Ti–1B on the structure and hardness of a super high strength aluminum alloy produced by strain-induced melt activation process

In this study the effect of Al–5Ti–1B grain refiner on the structural characteristics and hardness of Al–12Zn–3Mg–2.5Cu aluminum alloy has been investigated. The alloy was produced by modified strain-induced melt activation (SIMA) process. Reheating condition to obtain a fine globular microstructure was optimized. The specimens subjected to deformation ratio of 40% (at 300 °C) and various heat treatment times (5–40 min) and temperature (550–620 °C) regimes were characterized in this study. Microstructural study was carried out on the alloy by the use of optical and scanning electron microscopy (SEM) in both unrefined and Ti-refined conditions. The results showed that for the desired microstructures of the alloy during SIMA process, the optimum temperature and time are 575 °C and 20 min respectively. The hardness test results of the alloy also revealed that T6 heat treatment is more effective in hardness enhancement of all specimens in comparison with SIMA processing.     

Microstructure and Mechanical Properties of TiAl-based Alloy

Two phases gamma titanium aluminide alloy,Ti-46.5Al-2.5V-1Cr.was investigated to characterizemicrostructures and to define the microstructure/mechanical property relationship.Many kinds ofmicrostructure of gamma and α_2 phases were obtained by heat treatments in the α+γ,α_2+γ and αfields.The effects of microstructure on tensile properties,fracture toughness and J-R resistancecurve at room temperature,were systematically studied.The experimental results showed that themicrostructure had a strong effect on mechanical properties,The duplex microstructure produced byheat treatment at 1250℃×4 h with controlled cooling resulted in the highest ductility of 4.8% tensileelongation,low fracture toughness and crack growth resistance.The fully lamellar microstructureproduced by heat treatment in the α field having large grain sizes resulted in the highest fracturetoughness but the lowest ductility.     

Evaluation of properties and microstructure as a function of tempering time at intercritical temperatures in HY-80 steel castings

Prior work on a failed HY-80 Bridge Access Trunk (BAT) casting indicated the cause of failure to be improper processing techniques. As-received HY-80 casting material showed a non-homogenous microstructure with two distinct microconstituents present: undertempered martensite and a layered martensite–ferrite structure. Heat treatment temperatures from in the intercritical range (721–799 °C) produced microstructures that differ from the desired uniform microstructure of tempered martensite and were similar to those found in the failed casting. In order to further examine the relationship between processing and microstructure, it was decided to vary the time for which the steel was held in the intercritical temperature range. This additional work was warranted by the medium intercritical heat treatment results in the previous study [Holthaus JE, Koul MG, Moran AL. Property and microstructure evaluation as a function of processing parameters: large hy-80 steel casting for a US navy submarine. Eng Fail Anal 2006;13(1):1397–409] and its similarity to the failed casting microstructures. An important finding of this study is that, contrary to normal behavior during tempering, HY-80 steel tempered in the intercritical range demonstrates a severe loss of toughness; which can be exaggerated for longer hold times and higher temperatures. To confirm the hypothesis that the presence of brittle martensite formed by improper heat treatment was the cause of failure, SEM section fractography was employed to directly examine the microstructure underlying the fracture surface and to identify a correlation between microstructure and fracture mode.     

Targeted heat treatment of additively manufactured Ti-6Al-4V for controlled formation of Bi-lamellar microstructures

Laser powder bed fusion (L-PBF) was utilized to produce specimens in Ti-6Al-4V,which were subjected to a bi-lamellar heat treatment,which produces microstructures consisting of primary α-lamellae and a fine secondary α-phase inside the inter-lamellar β-regions.The bi-lamellar microstructure was obtained as (i)a direct bi-lamellar heat treatment from the asbuilt condition or (ii) a bi-lamellar heat treatment preceded by a β-homogenization.For the bi-lamellar treatment with β-homogenization,cooling rates in the range 1-500 K/min were applied after homogenization in β-region followed by inter-critical annealing in the α + β region at various temperatures in the range 850-950 ℃.The microstructures were characterized using various microscopical techniques.Mechanical testing with Vickers hardness indentation and tensile testing was performed.The bi-lamellar microstructure was harder when compared to a soft fully lamellar microstructure,because of the presence of fine α-platelets inside the β-lamellae.Final low temperature ageing provided an additional hardness increase by precipitation hardening of the primary α-regions.The age hardened bi-lamellar microstructure shows a similar hardness as the very fine,as-built martensitic microstructure.The bi-lamellar microstructure has more favorable mechanical properties than the as-built condition,which has high strength,but poor ductility.After the bi-lamellar heat treatment,the elongation was improved by more than 250 ％.Due to the very high strength of the as-built condition,loss of tensile strength is unavoidable,resulting in a reduction of tensile strength of～18 ％.     

Microstructural evolution during accumulative roll-bonding of commercial purity aluminum

The microstructure in commercial purity aluminum deformed from medium to high strain (_vM=1.6–6.4) by accumulative roll-bonding (ARB) at 473 K was quantitatively examined by transmission electron microscopy. It was found that a sub-micrometer lamellar structure characterizes the microstructure at high strains (_vM>1.6), and that the lamellar boundary spacing decreases and the misorientation across the lamellar boundaries increases with increasing rolling strain. This characteristic evolution has also been observed during conventional cold-rolling of commercial purity aluminum. However, a comparison between the two processes shows a significant difference in the evolution of the microstructural parameters. These differences are discussed based on the different processing conditions characterizing ARB and conventional rolling, respectively.     

Influence of high-pressure torsion and hot rolling on the microstructure and mechanical properties of aluminum–fullerene composites

In this study, we investigate the impacts of working process, high-pressure torsion (HPT) and hot rolling (HR) on the microstructure and mechanical performance of aluminum-based nanocomposites containing fullerenes. HPT caused severe plastic deformations that generate numerous dislocations and lattice strains, and this stimulated the formation of aluminum carbides (Al₄C₃) and reduced the hardness during heat treatment. In contrast, the HRed specimens experienced dynamic recovery, and their initial dislocation densities and lattice strains were lower than those of the HPTed specimens. Thus, the HRed composites formed supersaturated aluminum phases as well as aluminum carbides during the heat treatment. The supersaturated phases provided high-density dislocations and severe lattice strains, resulting in an increase in the hardness during the heat treatment. This comparison suggests that the mechanical properties of aluminum–fullerene composites can be controlled by working processes in practical situations.