Microstructures and mechanical properties of Ti–Al–V–Nb alloys with cluster formula manufactured by laser additive manufacturing

Ti–Al–V–Nb alloys with the cluster formula, 12[Al–Ti₁₂](AlTi₂)+5[Al–Ti₁₄](V,Nb)₂Ti, were designed by replacing V with Nb based on the Ti–6Al–4V alloy. Single-track cladding layers and bulk samples of the alloys with Nb contents ranging from 0 to 6.96 wt.% were prepared by laser additive manufacturing to examine their formability, microstructure, and mechanical properties. For single-track cladding layers, the addition of Nb increased the surface roughness slightly and decreased the molten pool height to improve its spreadability. The alloy, Ti–5.96Al–1.94V– 3.54Nb (wt.%), exhibited better geometrical accuracy than the other alloys because its molten pool height was consistent with the spread layer thickness of the powder. The microstructures of the bulk samples contained similar columnar β-phase grains, regardless of Nb content. These grains grew epitaxially from the Ti substrate along the deposition direction, with basket-weave α-phase laths within the columnar grains. The α-phase size increased with increasing Nb contents, but its uniformity decreased. Along the deposition direction, the Vickers hardness increased from the substrate to the surface. The Ti–5.96Al–1.94V–3.54Nb alloy exhibited the highest Vickers hardness regardless of deposition position because of the optimal matching relationship between the α-phase size and its content among the designed alloys.     

Microstructures and mechanical properties of friction stir welded joints of Zr–Ti alloy

Zirconium–titanium alloy joints were successfully produced by friction stir welding. Unlike the (α+β) dual phase microstructure in base metal, only the β phase existed in the region in which temperature exceeded the β transient point for the as welded joint. Accordingly, the hardness in these regions exhibited integral decrement and uniform distribution features. The thermal simulation further showed that hardness variation was mainly determined by phase composition. Microstructure development in the nugget zone was mainly governed by continuous dynamic recrystallisation. Satisfactory ultimate tensile strength and elongation equal to the base metal were achieved in the as welded joint. Tensile fracture occurred at the heat affected zone near the retreating side of the joint. The fracture surface of the joint exhibited a mixing feature with quasi-cleavage facets and small dimples.     

Tensile properties and failure analysis of Ti–6Al–4V joints by electron beam welding

The microstructural and mechanical characterization of electron beam welded joints of forged Ti–6Al–4V were investigated. Microhardness tests indicate that the hardness of the fusion zone(FZ) is higher than that of the heat-affected zone(HAZ) and base metal. The tensile results show that the mechanical properties of the welded joints are comparable with those of the base metal in terms of static strength and are in accordance with the relationship between microstructure and mechanical properties of welded joints. The ultimate tensile strength of the weld is equal to that of the hourglass joint, which indicates that the mechanical properties of the longitudinal FZ and those of the transverse FZ are the same. Macromechanical behavior and macrofracture and microfracture of the base material,joint, and weld specimens are observed. A comparison among the three types of specimen fracture phenomena reveals the following distinctive differences:(1) the fracture mode,(2) the micrograph of the dimple pattern at the central region, and(3) the size of the dimple at the central region and the transition region.     

Microstructure evolution and mechanical properties of Cu–Al joints by ultrasonic welding

Dissimilar joints of copper to aluminium were produced by high power ultrasonic welding (USW). The interfacial reaction between copper and 6061 aluminium alloy as a function of welding time was studied. The intermetallic compound (IMC) layer is mainly composed of CuAl₂ and Cu₉Al₄. The thickness of the IMC layer increases with the welding time. For a relatively long welding time (0·7 s) in USW, the dendritic solidification microstructure was observed in local regions, owing to the occurrence of the eutectic reaction, α-Al+θ→L, in the welding process. The lap shear load (or strength) of the joints first increases and then decreases with increasing welding time, and the failure of the joints occurred dominantly at the interface. This is mainly attributed to the development of IMC layer at the interface.     

Microstructure and mechanical properties of dissimilar welded Mg–Al joints by ultrasonic spot welding technique

Abstract

Dissimilar spot welds of magnesium–aluminium alloy were produced via a solid state welding process, i.e. ultrasonic spot welding, and a sound joint was obtained under most of the welding conditions. It was observed that a layer of intermetallic compound (IMC) consisting of Al₁₂M₁₇ formed at the weld centre where the hardness became higher. The lap shear strength and failure energy of the welds first increased and then decreased with increasing welding energy, with the maximum lap shear strength and failure energy occurring at ～1250 J. This was a consequence of the competition between the increasing diffusion bonding arising from higher temperatures and the deterioration effect of the intermetallic layer of increasing thicknesses. Failure predominantly occurred in between the aluminium alloy and the intermetallic layer, which normally stayed at the magnesium side or from the cracks of the IMCs in the reaction layer.     

Electron beam melting of Ti–48Al–2Cr–2Nb alloy: Microstructure and mechanical properties investigation

Gas atomized Ti–48Al–2Cr–2Nb powders have been used as precursor material in order to evaluate additive manufacturing for the production of near-net-shape γ-TiAl specimens to be employed in the field of aero-engines. In particular electron beam melting (EBM) is used to realize a selective densification of metal powder by melting it in a layerwise manner following a CAD design. The microstructure, the residual porosity and the chemical composition of the samples have been investigated both immediately after EBM and after heat treatments. High homogeneity of the samples, very low pickup of impurities (oxygen and nitrogen) with respect to the starting powders have been observed and due to an extremely low level of internal defects, intrinsic to EBM process, the tensile properties of the EBM γ-TiAl appear very consistent with a small scatter.     

Microstructure and mechanical behaviour of Nb–Al–V alloys with 10–25 at.% Al and 20–40 at.% V—II: mechanical behaviour and deformation mechanisms

The mechanical behaviour of three Nb–Al–V alloys with nominal compositions Nb–10Al–20V, Nb–15Al–20V and Nb–25Al–40V (in at.%) have been investigated. Both conventional constant strain rate deformation and compressive creep tests have been performed and the deformation microstructures have been examined by transmission electron microscopy (TEM). At room temperature all three alloys deform by planar slip, with dislocation/particle interactions giving significant strengthening for the two phase alloys. Deformation at higher temperatures occurs by a combination of dislocation glide and climb processes, giving more homogeneous microstructures. All of the dislocations in the B2 phase of these alloys are uncoupled superpartial dislocations with b=1/2<111>. The influence of dislocation/domain boundary interactions on the formation of slip bands and uncoupled superpartials is discussed.     

Effect of Nb on the high temperature oxidation of Ti–(0–50 at.%)Al

The oxidation behavior of Ti–Nb, Ti₃Al–Nb and TiAl–Nb (Nb: 0–30 at.%) has been investigated at 1173 K in air. When Nb is in solid solution with TiO₂, the addition of Nb can improve the oxidation resistance of the alloys by impeding mass transfer in TiO₂. However, Nb decreases the oxidation resistance when the amount of Nb is too high and forms TiNb₂O₇ or AlNbO₄ phases in the scale.     

Influence of beam oscillation patterns on the structure and mechanical properties of Ti–6Al–4V electron beam weldments

Abstract

The present study deals with the effect of beam oscillation patterns on the structure and mechanical properties of Ti–6Al–4V electron beam weldments. Electron beam welding of Ti–6Al–4V sheets was done with beam oscillation in various patterns (sinusoidal, square, triangular and elliptical). Welding without beam oscillation was also done for comparison. Room temperature hardness and tensile properties at different temperatures (room temperature, 150, 300 and 450°C) of the weldments in both as welded and post-welding heat treated conditions were observed and their correlations with the microstructure studied. The beam oscillated weldments showed higher ductility and lower strength (hardness) compared with those without being beam oscillated. This was attributed to wider diffusional α plates in the beam oscillated welds owing to slower cooling rates. Elliptical oscillation pattern produced weldments possessing the highest ductility and lowest strength values compared with other oscillation patterns. The oscillating patterns of sinusoidal, square or triangular forms resulted in weldments with almost the same structure and properties.     

Microstructure and mechanical behaviour of Nb–Al–V alloys with 10–25 at.% Al and 20–40 at.% V—I: microstructural observations

The microstructures of three Nb–Al–V alloys with nominal compositions Nb–10Al–20V, Nb–15Al–20V and Nb–25Al–40V (in at.%) have been investigated. It is shown that the alloys each exhibit an A2 or B2 matrix and often contain A15 and/or σ phase precipitates depending on thermal history. Both the A15 and σ phase precipitates exhibit two different well-defined orientation relationships and for the former these correspond to minimisation of elastic strain energy. ALCHEMI data from the B2 phase indicate that this is more stable for higher Al concentrations, and this is consistent with measurements of A2/B2 order-disorder transformation temperatures. In the alloy Nb–15Al–20V, the precipitation of the A15 phase in a supersaturated B2 matrix is preceded by the separation of the B2 phase into Al-rich domains in an Al-lean matrix.     

Microstructures and mechanical properties of Fe–Al–Ta alloys with strengthening Laves phase

The addition of Ta to Fe–Al alloys results in the formation of a stable Ta(Fe,Al)2 Laves phase with hexagonal C14 structure in the Fe–Al phase at temperatures of 800, 1000 and 1150 °C. It was found that the solubility of Ta in Fe–Al is generally low and the solubility of Ta varies with Al content. Respective isothermal sections of the Fe–Al–Ta system have been established. Particular attention has been given to precipitation in the Fe₃Al phase with a small addition of Ta. At intermediate temperatures, 600–750 °C, an additional Heusler-type phase with L2₁-structure precipitates, which transforms at longer times and high temperatures to the stable C14 Laves phase. The yield stress in compression and the creep behaviour of the Fe–Al–Ta alloys with various microstructures were studied. Due to the presence of the L2₁-Heusler phase, the yield stress and the creep resistance at temperatures below 700 °C was increased considerably.     

Microstructure and mechanical properties of butt joints of QCr0.8/1Cr21Ni5Ti equal-thickness dissimilar materials by electron beam welding

0　IntroductionThedissimilarmaterialsjointisusuallyemployedinindustrialproductiontomakefulluseofthematerialsperformanceadvantageorspecialstructuralpurpose.Thedissimilarmaterialsjoiningtechnologybetweencopperalloyandstainlesssteelhasattractedagreatdealofattention,botheffectiveassemblageputweldedcomponent’scoolingandhighstrengthintoeffect.However,becauseofbothmaterialschemicalandthermophysicalperformancedifferenceandcomplexityofweldingmetallurgy,thedefectsuchaspore,crackandbrittlephaseandarathe…     

Microstructure and tensile properties of orthorhombic Ti–Al–Nb–Ta alloys

Microstructure and tensile properties of orthorhombic Ti–Al–Nb–Ta alloys have been studied. In order to optimize ductility and strength of the orthorhombic alloys with the nominal compositions of Ti–22Al–23Nb–3Ta and Ti–22Al–20Nb–7Ta, various thermomechanical processing steps were implemented as part of the processing route. With a special heat treatment before rolling to obtain a fine and homogeneous rolled microstructure, the rolled microstructure resulted in a good combination of high tensile yield strength and good ductility of the alloys through available solution and age treatments. The duplex microstructure with equiaxed α₂/O particles and fine O phase laths in a B2 matrix, deforming in α₂+B2+O phase field and treating in O+B2 phase field, possesses the highest tensile properties. The R.T. yield strength and ductility of the Ti–22Al–20Nb–7Ta alloy are 1200 MPa, and 9.8% respectively. The yield strength and ductility values of 970 MPa and 14% were also maintained at elevated temperature (650°C).     

Microstructure and mechanical properties of Fe–Si–Ti–(Cu,Al) heterostructured ultrafine composites

The influence of partial substitution of Fe by Cu or Al in Fe_75?xSi₁₅Ti₁₀(Cu, Al)_x (x = 0 and 4) ultrafine composites on the microstructure and mechanical properties has been investigated. The Fe₇₁Si₁₅Ti₁₀Cu₄ ultrafine composite exhibits a favorable microstructural evolution and improved mechanical properties, i.e., large plastic strain of ～5% and pronounced work hardening characteristics. The mechanical properties of the ultrafine eutectic composite are strongly linked to the length scale heterogeneity and the distribution of the constituent phases.     

The phase evolution,mechanical behavior,and microstructural instability of a fully-lamellar Ti–46Al(at.%) alloy

A Ti–46Al(at.%) polysynthetically twinned alloy, which contained a characteristic lamellar spacing of λ=1.3 μm, was converted to a polycrystalline fully-lamellar microstructure with λ=19.9 nm using an α-phase solution treatment followed by α₂+γ aging. Tensile experiments, performed on microsamples extracted from within single grains of the polycrystalline material, illustrated that ultrafine lamellae led to exceptional tensile strength. However, the ultrafine lamellae were not stable at elevated temperatures and the ultrafine material was found to have relatively poor creep resistance.     

Hot corrosion of Ti–46Al–8Ta (at.%) intermetallic alloy

Hot corrosion behaviour of a fully lamellar Ti–46Al–8Ta (at.%) alloy was studied in air under thermal cycling conditions (20-h cycles) at 700 and 800 °C. The samples were purposely contaminated with salt deposits consisting of NaCl or Na₂SO₄ or a mixture of these. The progress of degradation was followed by mass change measurements and visual inspection. Post-exposure examination involved scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The composition of salt deposits clearly influenced the rate and type of corrosion. Sodium chloride appeared especially harmful because of the formation of volatile chloride species.     

Creep- and coarsening properties of Al–0.06 at.% Sc–0.06 at.% Ti at 300–450 °C

Upon aging at 300–450 °C, nanosize, coherent Al₃(Sc_1−xTi_x) precipitates are formed in pure aluminum micro-alloyed with 0.06 at.% Sc and 0.06 at.% Ti. The outstanding coarsening resistance of these precipitates at these elevated temperatures (61–77% of the melting temperature of aluminum) is explained by the significantly smaller diffusivity of Ti in Al when compared to that of Sc in Al. Furthermore, this coarse-grained alloy exhibits good compressive creep resistance for a castable, heat-treatable aluminum alloy: the creep threshold stress varies from 17 MPa at 300 °C to 7 MPa at 425 °C, as expected if the climb bypass by dislocations of the mismatching precipitates is hindered by their elastic stress fields.     

Microstructure,mechanical properties and creep resistance of Mg–(8%–12%)Zn–(2%–6%)Al alloys

The microstructural characteristics, mechanical properties and creep resistance of Mg–(8%–12%)Zn–(2%–6%)Al alloys were investigated to get a better overall understanding of these series alloys. The results indicate that the microstructure of the alloys ZA82, ZA102 and ZA122 with the mass ratio of Zn to Al of 4–6 is mainly composed of α-Mg matrix and two different morphologies of precipitates (block τ-Mg₃₂(Al, Zn)₄₉ and dense lamellar ε-Mg₅₁Zn₂₀), the alloys ZA84, ZA104 and ZA124 with the mass ratio of 2–3 contain α-Mg matrix and only block τ phases, and the alloys ZA86, ZA106 and ZA126 with the mass ratio of 1–2 consist of α-Mg matrix, block τ precipitates, lamellar ?-Al₂Mg₅Zn₂ eutectics and flocculent β-Mg₁₇Al₁₂ compounds. The alloys studied with the mass ratio of Zn to Al of 2–3 exhibit high creep resistance, and the alloy ZA124 with the continuous network of τ precipitating along grain boundaries shows the highest creep resistance.     

Effect of interlayer on microstructure and mechanical properties of Al–Ti ultrasonic welds

Joints of Al6061 and Ti6Al4?V alloys with pure Al-particle interlayers were conducted using ultrasonic spot welding. The microstructure, hardness, lap shear strength and fracture energy were measured for different welding energies. With increasing welding energy delivered through the sonotrode, the lap shear strength of the joints increased, reaching about 106?MPa at a welding energy of 1100?J, at which failure occurred in the pull-out mode. In the weld region, the hardness of Al6061 alloy increased with increasing weld energy, whereas the hardness of Ti6Al4?V did not change discernibly. No brittle intermetallic compounds were observed in the joints. Moreover, two simple mechanisms were described for the formation of ultrasonic spot-welded Al–Ti joints with and without the pure Al interlayer.     

Microstructures and hydrogen embrittlement of Ti–49Al alloy

Microstructures and hydrogen embrittlement of Ti–49 at% Al were investigated. Results showed that there were three kinds of microstructures formed by heat treatment at 1423, 1573 and 1703 K. The specimens which were heat-treated at 1573 K, showed better elongation than the others on tensile tests at room temperature, and this kind of specimens were used to investigate the effect of hydrogen on tensile properties of this alloy. After heat-treatment in hydrogen gas at 1073 K for 3 h, the specimens were divided into three groups. In the first group, they were tensile-tested at room temperature in vacuum; in the second group, they were heat-treated at 823 K for 1.5 h in argon gas followed by tensile-testing at room temperature in vaccum; and in the third group, they were tensile-tested at 473 or 573 K in vacuum. Results showed that for the specimens precharged with hydrogen, the elongation was decreased significantly at room temperature, and that the decreased elongation was recovered by removing hydrogen at 823 K in Ar gas, although it was not recovered to that of the specimens without hydrogen. This means that hydrogen decreases the room temperature elongation of this alloy. For the precharged specimens the elongation was also decreased at 473 and 573 K in vacuum. This may indicate that hydrides in the precharged specimens affect the tensile properties in vacuum.