β Phase decomposition in Ti–5Al–5Mo–5V–3Cr

The decomposition of the β phase during rapid cooling of the near β titanium alloy Ti–5Al–5Mo–5V–3Cr has been studied using in situ X-ray synchrotron diffraction combined with ex situ conventional laboratory X-ray diffraction and transmission electron microscopy (TEM). Evidence is found supporting the suggestion by De Fontaine et al. (Acta Mater. 1971;19) that embryonic ω structures form by the correlation of linear (1 1 1)β defects at high temperatures. Further cooling causes increased correlation of these defects and the formation of athermal ω structures within the β matrix at temperatures ～500 °C. Post-quench aging at 570 °C resulted in the nucleation of α laths after ～90 s at temperature, with the laths all initially belonging to a single variant type. Aging for 30 min produced an even distribution of α precipitates with a lath morphology ～1.5 μm × 0.2 μm in size composed of both the expected Burgers variants. Mechanical property data suggests that the ω structures alone have no real effect; however, hardness increases were observed as the α phase developed. The utilization of thermal regimes similar to those presented in this paper could offer a method to engineer the α phase in near β titanium alloys and hence control mechanical properties.     

Grain refining action of Al–5Ti–C and Al–TiC master alloys with Al–5Ti master alloy addition for commercial purity aluminium

Al–Ti–C master alloys have a great potential as efficient grain refiners for aluminium and its alloys. In the present work, the Al–5Ti–C, Al–TiC and Al–5Ti master alloys have been successfully prepared by a method of liquid solidification reactions. While the Al–5Ti–C master alloy consists of some strip- or needle-like TiAl₃, and in addition to TiC particles in the Al matrix, the Al–TiC master alloy revealed the presence of only TiC particles, and the Al–5Ti master alloy consists of only some blocky TiAl₃ particles. A united refinement technology by Al–5Ti–C+Al–5Ti and Al–TiC+Al–5Ti master alloys was put forward in this paper. The blocky TiAl₃ particles in Al–5Ti master alloy can not only improve the grain refinement efficiency of Al–5Ti–C and Al–TiC master alloys but also reduce the consumption because the blocky TiAl₃ particles improve the grain refinement efficiency of TiC particles in Al–5Ti–C and Al–TiC master alloys.     

Refining performance of Al–3Ti–0.2C–5Sr on A356 alloy and electron microanalysis of ternary Al–Ti–Sr phases

The effect of Al-3Ti-0.2C-5Sr(wt%) grain refiner on the refining performance and modification of A356 alloy was investigated using optical microscope(OM).The morphology and crystal structure of ternary Al-Ti-Sr phases in Al-3Ti-0.2C-5Sr refiner were analyzed by scanning electron microscopy(SEM) and transmission electron microscopy(TEM).The results show that the ternary Al-TiSr phases in Al-3Ti-0.2C-5Sr refiner can promote the grain refining efficiency of A356 alloy.The ternary Al-Ti-Sr phases co-exist in two morphologies,i.e.,blocky-like phase and surround-like phase,besides,which both have the same chemical composition of Al_(34)Ti_3Sr.The crystal structure of Al_(34)Ti_3Sr is face-centered cubic,and the lattice parameter is determined to be about 1.52 nm.     

Deformation and creep modeling in polycrystalline Ti–6Al alloys

This paper develops an experimentally validated computational model for titanium alloys accounting for plastic anisotropy and time-dependent plasticity for analyzing creep and dwell phenomena. A time-dependent crystal plasticity formulation is developed for hcp crystalline structure, with the inclusion of microstructural crystallographic orientation distribution. A multi-variable optimization method is developed to calibrate crystal plasticity parameters from experimental results of single crystals of α-Ti–6Al. Statistically equivalent orientation distributions of orientation imaging microscopy data are used in constructing the polycrystalline aggregate model. The model is used to study global and local response of the polycrystalline model for constant strain rate, creep, dwell and cyclic tests. Effects of stress localization and load shedding with orientation mismatch are also studied for potential crack initiation.     

Microstructure and mechanical properties of spot welded Al–5·5Mg–0·3Cu alloy

Abstract

The influences of spot welding on the microstructure and mechanical properties of an Al–5·5Mg–O·3Cu alloy have been investigated. Results showed that dendrites were formed with porosity and cracks in the nugget. Grain boundary melting occurred in the heat affected zone and wide grain boundaries appeared. The alloy exhibited low hardness in the nugget centre. Tensile cracks propagated at the edge of the nugget and mixed rupture with dimples and intergranular fracture occurred. Fatigue fracture initiated at the edge of the nugget and propagated perpendicularly to the tensile axis. Transgranular fracture with striations was also observed.     

Cytotoxicity of Ti–6Al–4V–5Cu Alloy to MC3T3-E1 Cells

The aim of this work was to study the cytotoxicity of Ti–6 Al–4 V–5 Cu(TC4–Cu) alloy for dental applications and evaluate the cell viability of diff erent fabricated TC4–Cu alloys in contact with MC3 T3-E1 cells in vitro. Ti–6 Al–4 V(TC4) alloy was used as a negative control to evaluate the cytotoxicity level of TC4–Cu alloy so as to provide basic support for the dental clinical application. Control group TC4 and experimental group TC4–Cu with diff erent fabrications were incubated in the cell culture medium. The absorbance value of mouse osteoblast MC3 T3-E1 cells was detected by CCK-8 assay after 1, 3 and 7 days, respectively. The relative proliferation rate of cells was calculated, and then the toxicity level was valued for each group. Cell morphology on the surface was also studied by observing the cytoskeleton through F-actin filament staining. The experimental results showed that the absorbance values for the experimental and the negative control groups were not same at diff erent time points. Compared with diff erent fabricated TC4–Cu alloys, the annealing-TC4–Cu alloy showed much better biocompatibility. The mouse osteoblast MC3 T3-E1 cells cultured with annealing-TC4–Cu, rolling-TC4–Cu and solution + aging-TC4–Cu have no toxic eff ects, and these alloys could promote the proliferation of mouse osteoblasts.     

Strength and fracture of two hot-rolled Ti–Al–Si–Nb dual phase alloys based on Ti3Al

Ti–Al–Si–Nb dual phase alloys are mainly composed of α₂-Ti₃Al matrix and Ti₅Si₃ silicide phases. In this paper, two alloys (402 and 405) whose Si contents are 2 and 5 at% respectively were arc melted and hot-rolled into sheets with different amounts of deformation. The silicide phase (Ti,Nb)₅(Si,Al)₃ was broken up into small pieces and redistributed in the α₂ matrix during the hot-rolling. Improved strength and ductility of the two alloys were observed after hot-rolling, which can be attributed to both the finely distributed reinforcement silicide phase and refinement of the matrix grain size. The mechanical properties of the two alloys are dependent on their volume fractions of the silicide phase: the strength of alloy 405 is higher than that of alloy 402, while alloy 402 is more ductile than alloy 405. The brittle–ductile transition temperature of the two dual phase alloys is between 600 and 800°C. The surface slip on the dual phase alloys was also observed. Obvious separation between the (Ti,Nb)₅(Si,Al)₃ particles and the α₂ matrix is found on the fracture surfaces obtained at high temperature, showing dimple-like morphology.     

Interlayer growth at interfaces of Ti/Al–1%Mn,Ti/Al–4·6%Mg and Ti/pure Al friction weld joints by post-weld heat treatment

Abstract

The interlayer growth at interfaces of Ti/Al–1%Mn and Ti/Al–4·6%Mg weld joints was studied by postweld heat treatment. The heating temperatures ranged from 676 to 873 K (400–600°C) and maximum heating time was 360 ks (100 h). The basic mechanism of interlayer growth for pure Ti/pure Al friction weld joint was also estimated. The interlayer growth rate of Ti/Al–4·6%Mg joint was much faster than for the Ti/ Al–1%Mn joint. The interlayer mainly consisted of (Al,Si)₃Ti for the Ti/Al–1%Mn joint, and Al₁₈Mg₃Ti₂ for the Ti/Al–4·6%Mg joint. While the interlayer grew from Al alloy substrate to the Ti side for the Ti/Al–1%Mn joint, it grew from the Ti substrate to the Al alloy side for the Ti/Al–4·6%Mg joint. The interlayer growth stopped for several hours on heating for 36 ks (10 h). Neither linear nor parabolic time-dependence relations could be exactly fit to the interlayer growth rate for both joints. The interlayer growth of Ti/Al–1%Mn was proportional to heating time raised to approximately 0·85. The crystal direction of Al₃Ti interlayer growth of the Ti/Al joint was close to 〈001〉 and 〈111〉 directions obtained by OIM method. Nucleation and nuclei growth were observed at the interface of the Ti/Al joint. The nucleation and the nuclei growth are the reason for the phenomena (time dependence) described above.     

High-Temperature Oxidation Kinetics and Behavior of Ti–6Al–4V Alloy

Owing to the high-temperature reactivity of titanium, the oxidation and alloying of titanium during hot working processes is an important variable. The oxidation behavior of Ti–6Al–4V alloy in air was investigated at various temperatures between 850 and 1100 °C for different times. The oxidation kinetics were determined by isothermal oxidation weight gain experiments. The results showed that the oxidation kinetics approximately obeyed a parabolic law. The activation energy of oxidation was estimated to be 199 and 281 kJ mol^?1 when temperature was above and below the beta transformation temperature (T _β), respectively. A model to predict oxidation extent was established based on experimental observations. The oxide scales mainly consisted of TiO₂ with a small amount of Al₂O₃ and TiVO₄. The alpha case was defined as solid solution formed because of oxygen diffusion into the substrate. The difference in the morphology and the formation mechanism of the alpha case at different temperature ranges was mainly owing to the participation of the grain boundary and grain orientation of the nucleation site.     

3D modelling of Ti–6Al–4V linear friction welds

Linear friction welding (LFW) is a solid-state joining process that significantly reduces manufacturing costs when fabricating Ti–6Al–4V aircraft components. This article describes the development of a novel 3D LFW process model for joining Ti–6Al–4V. Displacement histories were taken from experiments and used as modelling inputs; herein is the novelty of the approach, which resulted in decreased computational time and memory storage requirements. In general, the models captured the experimental weld phenomena and showed that the thermo-mechanically affected zone and interface temperature are reduced when the workpieces are oscillated along the shorter of the two interface contact dimensions. Moreover, the models showed that unbonded regions occur at the corners of the weld interface, which are eliminated by increasing the burn-off.     

Tensile properties of gas tungsten arc weldments in commercially pure titanium,Ti–6Al–4V and Ti–15V–3Al–3Sn–3Cr alloys at different strain rates

Abstract

The influence of microstructure and strain rate on the mechanical behaviour of three titanium alloys having applications in aerospace, namely, commercially pure titanium (α phase), Ti–6Al–4V (α + β phases) and Ti– 15V–3Cr–3Sn–3Al (β phase) is investigated for both the parent metals and their gas tungsten arc weldments. The results indicate that the tensile strengths of the three as received titanium alloys and their weldments increase with increasing strain rate. However, their elongations decrease with increasing strain rate. The as received Ti–6Al–4V alloy and its weldment, with a mixed α and β phase microstructure, have the maximum strength and microhardness. Commercial purity titanium metal and its weldment exhibit the minimum strength and microhardness. The tough Ti–15V–3Cr–3Sn–3Al alloy and its weldment, having a fully β phase microstructure, appear to have optimum strength and microhardness. The tensile properties of all three titanium alloy weldments are inferior to those of the as received metals.     

Dynamic response and plastic deformation behavior of Ti–5Al–2.5Sn ELI and Ti–8Al–1Mo–1V alloys under high-strain rate

Split Hopkinson pressure bar test system was used to investigate the plastic deformation behavior and dynamic response character of a-type Ti–5Al–2.5Sn ELI and near a-type Ti–8Al–1Mo–1V titanium alloy when subjected to dynamic loading. In the present work, stress–strain curves at strain rate from 1.5 9 103to 5.0 9 103s-1were analyzed, and optical microscope(OM) was used to reveal adiabatic shearing behavior of recovered samples. Results show that both the two alloys manifest significant strain hardening effects. Critical damage strain rate of the two alloys is about 4.3 9 103s-1, under which the impact absorbs energy of Ti–5Al–2.5Sn ELI and Ti–8Al–1Mo–1V are 560 and 470 MJ m-3, respectively. Both of them fracture along the maximum shearing strength orientation, an angle of 45° to the compression axis. No adiabatic shear band(ASB) is found in Ti–5Al–2.5Sn ELI alloy, whereas several ASBs with different widths exist without regular direction in Ti–8Al–1Mo–1V alloy.     

Characterization of α_2 Precipitates in Ti–6Al and Ti–8Al Binary Alloys: A Comparative Investigation

Transmission electron microscopy (TEM) and atom probe tomography (APT) techniques were used to investigate the nanoscale orderedα_2 (Ti _(3 )Al) precipitates in Ti–Al binary alloys.Ti–6Al and Ti–8Al binary alloys were solution treated and aged to obtain Widmanstatten microstructure and promoteα_(2 )precipitates.The TEM results displayed strong short-range ordering ofα_(2 )precipitates in Ti–8Al alloy,while no evidence of the superlattice reflections ofα_(2 )in Ti–6Al alloy.The results acquired from APT showed theα_(2 )clusters and atoms distribution at the interface between the matrix andα_(2 )precipitates.The size and morphology ofα_(2 )particles in Ti–8Al alloy,respectively,obtained by TEM and APT are closely consistent.Meanwhile,the APT results displayed tiny size clusters in Ti–6Al alloy,which supposed to give evidence of the initial ordering process ofα_(2 )precipitates in the absence of correlative results from TEM.     

Resistance-curve and fracture behavior of Ti–Al3Ti metallic–intermetallic laminate (MIL) composites

The R-curve and fracture toughness behavior of single-edge notch beams of Ti–Al₃Ti metallic–intermetallic laminate (MIL) composites has been investigated. Composites with 14, 20, and 35% volume fraction Ti, with a corresponding intermetallic layer thickness of ~540, ~440, and ~300 microns, respectively, were tested in crack arrester and crack divider orientations. In the arrester orientation, the R-curve could not be determined for the two highest Ti volume fraction compositions as the main crack could not be grown through the test samples. In the divider orientation, R-curves were determined for all three Ti volume fractions tested. The laminate composites were found to exhibit more than an order of magnitude improvement in fracture toughness over monolithic Al₃Ti. Crack bridging and crack deflection by the Ti layers were primarily responsible for the large-scale bridging conditions leading to the R-curve behavior and enhanced fracture toughness. Estimates of steady-state toughness under small-scale bridging conditions were in close agreement with experimental results.     

Fatigue in Ti–6Al–4V–B alloys

The addition of small amounts of B to Ti–6Al–4V alloy reduces the as-cast grain size by an order of magnitude and introduces TiB phase into the microstructure. The effects of these microstructural modifications on both the high cycle fatigue and cyclic stress–strain response were investigated. Experimental results show that B addition markedly enhances the fatigue strength of the alloy; however, the influence of prior-β grain size was found to be only marginal. The presence of TiB particles in the matrix appears to be beneficial with the addition of 0.55 wt.% B to Ti–6Al–4V enhancing the fatigue strength by more than 50%. Strain-controlled fatigue experiments reveal softening in the cyclic stress–strain response, which increases with the B content in the alloy. Transmission electron microscopy of the fatigued specimens indicates that generation of dislocations during cyclic loading and creation of twins due to strain incompatibility between the matrix and the TiB phase are possible reasons for the observed softening.     

ω-Assisted nucleation and growth of α precipitates in the Ti–5Al–5Mo–5V–3Cr–0.5Fe β titanium alloy

This paper discusses the structural and compositional changes at the nanometer scale associated with the nucleation and growth of α precipitates in the β titanium alloy Ti-5553 (Ti–5Al–5Mo–5 V–3Cr–0.5Fe) with ω precipitates acting as heterogeneous nucleation sites. The microstructural evolution in this alloy, during β-solutionizing, quenching and aging type heat-treatments, has been investigated by combining results from scanning electron microscopy, orientation imaging microscopy, transmission electron microscopy, high-resolution TEM and three-dimensional atom probe (3DAP) tomography. Athermal ω precipitates form in this alloy on quenching from above the β transus temperature. On isothermal annealing at low temperatures, these ω precipitates coarsen to form chemically ordered ω precipitates, accompanied by the nucleation of the stable α phase. Annealing at higher temperatures leads to dissolution of ω and further growth of α precipitates accompanied by clustering of different α variants in self-accommodating morphologies. 3DAP results indicate that annealing at lower temperatures (～350 °C) leads to initial nucleation of α precipitates with a non-equilibrium composition, nearly identical to that of the β matrix. Subsequent aging at higher temperatures (～600 °C) leads to more pronounced partitioning of alloying elements between the two phases. These results indicate that the structural body-centered cubic to hexagonal close-packed transformation and the compositional partitioning of alloying elements occur in sequential steps, resulting in a mixed-mode displacive-diffusional transformation, similar to the bainite transformation in steels.     

On the ω phase formation in Cr–Al and Ti–Al–Cr alloys

The interaction between Al and the transition metals Ti and Cr on the stability of the ω phase in metastable β-based structures was studied. Alloys were quenched from the melt to retain at room temperature a metastable β phase (B2 structure), which is stable at high temperatures. The structural study of the ω phase was carried out by correlating the deviation of ω structure from the ideal ω phase to the compositions of the parent β phase. Deviation of ω structures from the ideal one was related to the electron concentration of the parent β phase. A diffuse ω structure is reported in the Cr₂Al phase (C11_b structure) for the first time. The results are consistent with our previous suggestions that Al stabilises the ω phase in transition metals by lowering the spatial conduction electron concentration in the parent β phase and by enhancing p–d hybridisation of valence electrons. In the ternary Ti–Al–Cr alloys, prolonged annealing of the Ti–30Al–10Cr and Ti–20Al–10Cr alloys at 450°C led to the formation of two types of ordered crystalline ω structure.