Although massive transformation has been increasingly observed in titanium alloys, the decomposition mechanism of the massive transformation product (αm) is still unclear. Here, the β→αm→α+β ...continuous phase transformation in Ti-6Al-4V alloy during cooling was studied. We found that massive transformation occurs at a moderate cooling rate below 291.5 °C/s, forming irregular featureless αm. The thermodynamically unstable αm is supersaturated with β stabilizers, and it tends to decompose into ultrafine α+β structure during the continuous cooling at ever-lower cooling rates and temperatures. In the initial stage of αm decomposition, misfit dislocations appear as a predecessor and split αm into strips. Accompanied by the elemental redistribution, misfit dislocations are accommodated and replaced by β phase. Both the formation and decomposition of αm follow the typical Burgers orientation relationship.
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The most popular additive manufacturing (AM) technologies to produce titanium alloy parts are electron beam melting (EBM), selective laser melting (SLM) and directed energy deposition (DED). This ...investigation explores mainly these three techniques and compares these three methods comprehensively in terms of microstructure, tensile properties, porosity, surface roughness and residual stress based on the information available in the literature. It was found that the microstructure is affected by the highest temperature generated and the cooling rate which can be tailored by the input variables of the AM processes. The parts produced from EBM have strength comparable to that of conventionally fabricated counterparts. SLM and DED yield superior strength, which can be up to 25% higher than traditionally manufactured products. Due to the presence of larger tensile residual stress, surface roughness and porosity, AM fabricated parts have lower fatigue life compared to those of from traditional methods. EBM parts have slightly lower fracture toughness (i.e., lower fatigue life) than conventionally produced parts while SLM and DED have significantly lower fracture toughness. Annealing, hot isostatic pressing, stress relief and additional machining processes improve the characteristics of parts produced from AM. Ti–6Al–4V alloy parts fabricated via AM may have limited applications despite the high demands in aerospace or biomedical engineering. Since rapid product development using 3D printers leads to significant cost reductions more recently, it is expected that more opportunities may soon be available for the AM of titanium alloys with newer AM processes such as cold spray additive manufacturing (CSAM) and additive friction stir deposition (AFSD).
This study focuses on the role of pyramidal 〈c + a〉 dislocations in the grain refinement mechanism in the Ti-6Al-4V alloy with an initial {112¯0}<101¯0>rolling texture. A large number of pyramidal ...〈c + a〉 dislocations were activated in the sample subjected to the severe shot peening process. Two important roles of pyramidal 〈c + a〉 dislocations were discovered. First, pyramidal 〈c + a〉 slip coordinates the large c-axis strain, thereby achieving generalized plastic flow, especially in nanograins. Second, the unique low-angle grain boundaries (LAGBs) with basal-pyramidal dislocation locks (prismatic 〈c〉 and prismatic 〈c + a〉 dislocations) were produced for the first time by pyramidal 〈c + a〉 interacting with basal 〈a〉 dislocations. This unique low-energy boundary greatly enhances the stability of the strain-induced grain boundary and dislocation density (~6.6 × 1015m − 2 in nanograins). The grain refinement process contains three types of subdivision modes: (I) dislocation walls with pyramidal 〈c + a〉 dislocations in coarse grains; (II) basal 〈a〉 intersecting with prismatic 〈a〉 dislocations in coarse grains; and (III) basal 〈a〉 intersecting with pyramidal 〈c + a〉 dislocations in coarse grains, ultrafine-grains and nanograins. The occurrence of slip modes depends on the initial texture and texture evolution during dynamic recrystallization. Besides, Hall-Petch breakdown at the nanoscale was found and is attributed to the decreasing critical resolved shear stress of pyramidal 〈c + a〉 slip at the nanoscale. This study provides a new approach for the design of stable nanostructured hexagonal close-packed metals by the unique LAGBs with basal-pyramidal dislocation locks.
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A high-temperature antioxidant layer on Ti-6Al-4V alloy was prepared by a two-step method of REO-boriding followed by aluminizing (REOs are rare-earth oxides, which are Nd2O3, Gd2O3, and Tm2O3). The ...microstructure and oxidation behavior of the layer were investigated at temperatures of 700 °C, 800 °C, and 900 °C in air. The findings unveiled that the layer significantly enhances the antioxidant capacity of Ti-6Al-4V alloy. After cyclic oxidation at 900 °C for 100 h, the average weight gain of the layer is only one fortieth of that of the matrix. TiAl3 in the layer can react with O2 to form a dense continuous Al2O3 layer, which effectively inhibits the diffusion of O2 to the matrix. During the cyclic oxidation, the thicker TiAl2, TiAl, and Ti3Al diffusion layers are formed sequentially between TiAl3 layer and matrix, and TiAl2 layer can also provide the good oxidation resistance.
•REO-borided and aluminized coatings were prepared by a simple two-step process.•The coatings have excellent high-temperature oxidation resistance.•Oxygen reacts with Al3Ti to form a dense continuous Al2O3 layer to resist oxidation.
•DSOLSP processing is proposed for the small-sized blisk.•DSOLSP causes a two-way bending deformation of the blade.•The gradient microstructure and full-thickness CRS are induced by DSOLSP.•The ...vibration fatigue strength of the DSOLSPed blades increased by 25.9%.•The anti-fatigue mechanism of DSOLSP is elucidated.
Double-sided symmetric oblique laser shock peening (DSOLSP) was adopted to experiment on a small-sized blisk with restricted space. The influences of different laser energies on the shape deviation, surface roughness, microhardness, and residual stress of the blades were studied. The optimum process parameters were selected based on the shape deviation results of the blades and the microstructural evolution of the surface layers was investigated. The strengthening effect of DSOLSP treatment on notched blades was evaluated by vibration fatigue tests. It was found that DSOLSP can effectively cover the fatigue vulnerable regions without interference. Two-way bending deformation was formed after DSOLSP, and the shape deviation of the blades could be controlled within ±0.02 mm. With the increase of laser energy, the surface roughness and the depth of work-hardened layer increased. The full-thickness compressive residual stress (CRS) field was induced. A gradient microstructure was generated on the surface of the blade, and the size of average nanograins on the topmost surface is approximately 36.5 nm. The fatigue strength of the DSOLSP treated blades with notches was increased by 25.9 %. The enhancement can be attributed to the synergistic strengthening of CRS and gradient microstructure.
•An analytical framework for defect-based fatigue life prediction of AM metals.•Accounting for different failure modes and crack growth mechanisms in modeling.•Use of Extreme Value Statistics for ...approximation of defect size.•Sensitivity analysis for the effects of defect size and shape and FCG properties on predictions.
Metal additive manufacturing (AM) while offering advantages such as generating parts with intricate geometries, introduces challenges such as intrinsic defects. Since fatigue cracks often start at defects, developing analytical methods to predict fatigue failure is necessary for critical AM applications involving cyclic loadings. In this work a computational framework is presented where a generalized Paris equation and the Hartman-Schijve variant of NASGRO equation are used to predict the fatigue life of L-PBF Ti-6Al-4V and 17-4 PH stainless steel specimens. A variety of conditions resulting in different failure modes and defect sized were considered, including annealed and HIPed treatments as well as as-built and machined surface conditions. In contrast to the commonly assumed mode I crack growth in the literature, in this work the appropriate crack growth mode based on the observed failure mechanism is used for fatigue life predictions. By using Extreme Value Statistics (EVS) and equivalent defect size based on Murakami’s method for approximation of the initial defect size, and appropriate crack growth equation and properties, good correlations between experimental results and prediction curves are demonstrated.
This paper presents the results of a comparative study on the mechanical and tribological properties of Ti-6Al-4V alloy by local heat treatment (LHT) with (w/) and without (w/o) ultrasonic ...nanocrystal surface modification (UNSM) technique at high temperatures of 400, 600 and 800°C. The objective of this study is to further improve the mechanical and tribological properties of Ti-6Al-4V alloy by the combination of LHT+UNSM treatment. The hardness measurement results revealed that the hardness of Ti-6Al-4V alloy after LHT was increased with increasing the temperature, but it was further increased by the combination of LHT+UNSM treatment. It was also found that the wear resistance was increased in the same temperature order, where the highest temperature of 800°C exhibited the highest wear resistance. However, even though the hardness was increased after LHT, but no significant difference in wear resistance was found between the untreated and heat treated (400°C) Ti-6Al-4V alloy. The wear mechanisms of the samples treated at various temperatures were also discussed based on the obtained SEM images of the wear tracks. The development of the combination of LHT+UNSM treatment was successfully demonstrated the possibility of being applied to Ti-6Al-4V alloy to achieve a significant high hardness.
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•A local high-temperature UNSM treatment was employed for the first time.•The hardness was improved with increasing the temperature.•The friction and wear were improved by local high-temperature UNSM treatment.•The combination of local heat treatment and UNSM was found to be beneficial.
The brazing joint of the Ti-6Al-4V alloy was produced with a designed brazing filler alloy and the optimized brazing temperature which is lower than the β-phase transformation of the matrix. The ...strength and the ductility of brazing joined Ti-6Al-4V samples were evaluated by conventional tensile tests with a DIC 2D–strain field measurement. The Widmanstätten microstructure with no voids or cracks or intermetallic compounds was found throughout the joint with a width of β-lamellar as ~1μm. Due to the fine acicular α-Widmanstätten and β-lamellar, and the uniformly diffused filler elements throughout the entire joint, the strength of the joint was as much as the matrix. In addition, the hardness test results agreed well with the tensile strength tests. All fractures occurred in the matrix rather than the brazing joints. Furthermore, the maximum local tensile strain was measured as 20% in the matrix, while under the same stress, the brazing joint only reached 6.3% tensile plastic strain. Thus, the mechanical properties of the joint with the associated microstructure demonstrated that a successful brazing filler alloy has been developed for the Ti-6Al-4V alloy.
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•Fine and interlocking acicular Widmanstätten microstructure is formed across the entire brazing joint section.•Preferential grain orientations and the solid solution strengthening result in the ultimate strength of the brazed samples.•The localized strain distribution at the braze joint was measured as ~6.3%. All the fractures occurred in the matrix with local tensile strain up to 20%.
The formation of the coarse columnar crystal structure of Ti-6Al-4V alloy in the process of additive manufacturing greatly reduces the mechanical performance of the additive manufactured parts, which ...hinders the applications of additive manufacturing techniques in the engineering fields. In order to refine the microstructure of the materials using the high intensity ultrasonic via the acoustic cavitation and acoustic flow effect in the process of metal solidification, an ultrasonic vibration technique was developed to a synchronous couple in the process of Laser and Wire Additive Manufacturing (LWAM) in this work. It is found that the introduction of high-intensity ultrasound effectively interrupts the epitaxial growth tendency of prior-β crystal and weakens the texture strength of prior-β crystal. The microstructure of Ti-6Al-4V alloy converts to fine columnar crystals from typical coarse columnar crystals. The simulation results confirm that the acoustic cavitation effect applied to the molten pool created by the high-intensity ultrasound is the key factor that affects the crystal characteristics.
It is the purpose of this study to investigate the effects of heat treatment on the microstructure, corrosion, and biological behavior of Ti-6Al-4V alloy manufactured using electron beam melting ...(EBM). The EBM parts were exposed to different heat treatments at 1000 °C and various cooling rates (HT-1, HT-2, and HT-3) to obtain a combination of α and β phases. Electrochemical analyses indicate that the non-heat treated alloy (Ref) exhibits superior corrosion resistance during early immersion, while all samples display similar corrosion performance after one month of immersion. The higher corrosion performance was associated with the content of β phases in the manufactured alloy. The Ref, HT-1, HT-2, and HT-3 EBMed alloys had corrosion current densities of 0.46, 0.66, 0.67, and 0.92 µA cm−2, respectively, after one month of immersion. Cell adhesion and durability were shown to be excellent on HT-3 samples. In conclusion, a suitable heat treatment can improve the biological performance of the EBMed alloy, despite resulting in lower corrosion performance in the early stages of corrosion.
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