Sintering is a critical step to obtain the desired mechanical properties for titanium alloys. However, the residual porosity and the coarse microstructure caused by traditional pressureless sintering adversely affect the me-chanical properties. Herein, we proposed a novel two-step pressureless sintering route to achieve sintering densification and grain refinement. The high density of 99.0% and the refined parent beta grains with a size of 90.57 mu m for Ti-6Al-4V alloy are acquired ultimately, which is among the best pressureless sintering practice reported in the literatures. The selection of two-sintering temperature (< 1250 C) is confirmed to be the key to improving density and reducing grain size. Benefitting from grain refinement, the increased number of grain boundaries provides more channels for vacancy diffusion, and the formation of small alpha colonies with multiple orientations makes the Widmansta center dot tten microstructure more uniform. During plastic deformation, the abundant dislocation cross slips and nanoscale twins of face-centered cubic phase within these colonies relieve the stress accumulation of phase interfaces, thereby delaying the formation of cracks and promoting the large plastic flow with the high-stress level. Compared with standard sintering, two-step sintered samples exhibit better me-chanical properties (ultimate tensile strength of 968 MPa, yield strength of 871 MPa and elongation of 16.1%), achieving the strength-ductility synergy. This work casts light on developing high-performance and low-cost titanium and its alloys.

To solve the low ductility of the uniformly particle-reinforced composites, novel (TiC + Ti5Si3)/Ti laminated composites are fabricated by electrophoretic deposition (EPD) silicon (Si) powders + graphene oxide (GOs) and spark plasma sintering (SPS) technology. The microstructure evolution, mechanical properties, strengthening and fracture mechanisms of laminated composites were systematically studied. Results show that the GOs and Si powders are evenly attached to Ti foils after EPD, and the (TiC + Ti5Si3) reinforcements are in situ formed at the interface after SPS. TiC phases are distributed along the original interface, and most of the Ti5Si3 reinforcements are located in the Ti foil matrix. The 50 mu m - 120 s sample possesses obvious shell nacre containing particular "brick-and-mortar" architecture structure. And the 100 mu m - 120 s sample displays excellent room temperature tensile properties, with the ultimate tensile strength (UTS) of 759 MPa, yield strength (YS) of 688 MPa and elongation (EL) of 24.3 %. The change of microstructure leads to the increase of strength, which is caused by the solid-solution strengthening, grain boundary strengthening and reinforcements strengthening. At this time, the design of laminated structure can change the crack propagation direction, extend the crack propagation path and thereby increases the energy required for crack propagation, obstructing the fracture of composites. This work provides an effective method to prepare Ti matrix composites with synergy of strength and ductility.

Rapid fabrication of the Ti-6Al-4 V alloy with the target hardness via laser powder bed fusion (LPBF) is essential to meet the requirements of specific applications. Herein, the explainable machine learning (xML) models were employed to predict the hardness of LPBF-ed parts and to provide insights into the mechanisms linking process-microstructure-hardness. Results indicate that gradient boosting decision tree (GBDT) model outperforms others on test set, achieving an R square (R-2) of 0.963, a mean absolute error (MAE) of 2.059 HV, and a root mean squared error (RMSE) of 3.844 HV. The contributions of process parameters were evaluated using SHapley Additive exPlanations (SHAP) values. The results demonstrate that the scanning speed (v) has the most significant influence on hardness, with the mean |SHAP value| accounting for approximately 51.2 % of the overall value, and shows an overall positive correlation with hardness. Experimental validation is consistent with the prediction results that S6 fabricated under the high v displayed a high hardness of 404.58 HV. The high v results in a large number of dislocations in as-built S6, impeding dislocation slip during loading and thus increasing resistance to deformation. The {10-11}<-1012> compression twins, nano-stacking faults and numerous fine alpha'-colonies provide extra grain boundary strengthening. Furthermore, the substantial volume fractions of interfaces between randomly oriented alpha' martensite act as a strong barrier to dislocation slip transmission, thereby offering effective strengthening. This work offers guidance for predicting properties and exploring the underlying mechanisms of process parameters on material properties.

Discontinuously reinforced titanium (Ti) matrix composites (DRTMCs) strengthened by in-situ TiCx particles are synthesized via pyrolysis of polyzirconocarbosilane (PZCS) and pressureless sintering. The Ti matrix reacts with the pyrolysis product of PZCS to form TiCx particles, which effectively refine the alpha-Ti grains and create a clean, well-bonded semi-coherent interface. A notable orientation relationship is observed between the TiCx and alpha-Ti phases: (111)TiCx||(1011)Ti and [110]TiCx||[1210]Ti. The Ti-2PZCS composite demonstrates a significant reduction in average grain size, from 101.5 mu m in pure Ti to 39.49 mu m, coupled with superior comprehensive mechanical properties: an ultimate tensile strength of 710 MPa, a yield strength of 588 MPa, and an elongation of 9.5 %. The enhanced strength of Ti/PZCS composites is mainly due to grain refinement, solid solution strengthening, and load transfer mechanisms. This study introduces a novel approach for fabricating highperformance Ti composites, highlighting the potential for advanced material applications.

Machine learning methods can accurately predict the density of as-built parts by laser power bed fusion (LPBF), providing a reference for optimizing process parameters. However, obtaining massive training data via experiments is time-consuming and high-cost. Herein, a novel data augmentation method based on the Wasserstein generative adversarial network and regularization strategy (LC-WGAN) was developed to generate new training data for density prediction modelling. Four machine learning (ML) algorithms, support vector regression (SVR), multilayer perceptron (MLP), random forest (RF) and gradient boosting decision tree (GBDT), were adopted to construct the density prediction models. Results show that the proposed LC-WGAN effectively enhanced the prediction performance of all four models. R2 of MLP dramatically increased by 73.06 %, while that of RF and GBDT was marginally improved by 10.07 % and 7.48 %, respectively. RF trained by augmented dataset has the highest prediction performance on test set with MAE, RMSE and R2 of 0.7589, 0.9584 and 0.9822, respectively, which is suitable for density prediction. Additionally, SHAP analysis reveals that energy density is the most important parameter with an overall positive impact on density. The proposed method can provide valuable insights for the limited sample modelling in other fields.

In this study, concurrent enhancements in both strength and ductility of the Al-2Li-2Cu-0.5Mg-0.2Zr cast alloy (hereafter referred to as Al-Li) were achieved through an optimized forming process comprising ultrasonic treatment followed by squeeze casting, coupled with the incorporation of Sc. Initially, the variations in the microstructure and mechanical properties of the Sc-free Al-Li cast alloy (i.e., alloy A) during various forming processes were investigated. The results revealed that the grain size in the UT+SC (ultrasonic treatment + squeeze casting) alloy was reduced by 76.3% and 57.7%, respectively, compared to those of the GC (gravity casting) or SC alloys. Additionally, significant improvements were observed in its compositional segregation and porosity reduction. After UT+SC, the ultimate tensile strength (UTS), yield strength (YS), and elongation reached 235 MPa, 135 MPa, and 15%, respectively, which were 113.6%, 28.6%, and 1150% higher than those of the GC alloy. Subsequently, the Al-Li cast alloy containing 0.2 wt.% Sc (referred to as alloy B) exhibited even finer grains under the UT+SC process, resulting in simultaneous enhancements in its UTS, YS, and elongation. Interestingly, the product of ultimate tensile strength and elongation (i.e., UTS × EL) for both alloys reached 36 GPa•% and 42 GPa•%, respectively, which is much higher than that of other Al-Li cast alloys reported in the available literature.

Download This Paper Open PDF in Browser Add Paper to My Library Share: Permalink Using these links will ensure access to this page indefinitely Copy URL Copy DOI

Excessive interstitial oxygen (O) contamination, usually causing a dramatic loss of ductility, remains a longstanding challenge for titanium (Ti) and its alloys. Here, we propose to solve this critical problem by adding a minor CaC2 oxygen-scavenger via a simple powder metallurgy (PM) pressureless sintering approach. It is found that the surface oxide layer of hydride-dehydride (HDH) Ti powder begins to dissolve into the Ti matrix between 700 °C and 800 °C during the PM sintering process. The incorporation of CaC2 can react with the surface oxide layer (617-676 °C) prior to its active dissolution and form micrometer-sized TiC and nano-sized CaTiO3 particles, significantly refining the α-Ti grains and creating clean and well-bonded interface with Ti matrix. Therefore, the unique oxygen-scavenging effect by CaC2 produces high strength and superior ductility for Ti alloy. Even with an initially high oxygen content (∼4000 ppm), the Ti-0.4 wt.%CaC2 sample still exhibits a high ultimate tensile strength of 621±25 MPa and superior elongation of 29.3±2.6%, respectively. These values correspond to an increase of 17.6% and 301.4% compared to the as-sintered commercially pure titanium (CP-Ti) properties and far exceed the ASTM standard B381 for Grade 4 wrought Ti alloy (550 MPa and 15%) with the same O content. This work offers a novel method to develop high-strength and superior-ductility Ti materials from much more affordable Ti powder.

The challenges of micro-pores resistant to closure and significant ductility loss during the pressureless sintering of titanium (Ti) alloy powder mixtures have been overcome through an optimized design that capitalizes on the role of hydrogen (H). Explore novel mechanisms of hydrogen effects on phase transformation and densification during sintering, and innovatively fabricated unique microstructure and mechanical properties. The study employs thermodynamic calculations and analyses to understand the effect of dehydrogenation on the element diffusion mechanism. It optimizes sintering parameters to develop a bimodal microstructure featuring equiaxed alpha-Ti around the primary grain boundary alpha. More cracks in brittle powder compact enhanced micropore closure, leading to a high density with uniform element diffusion. The phase transition of H in the early stages creates lattice and surface defects, activating rapid diffusion channels at lower temperatures and reducing the apparent activation energy substantially compared to pure Ti. The optimized bimodal structure exhibits superior mechanical properties, maintaining a strength of 969 MPa while achieving a ductility of 16.8%. This research provides new insights into H applications in Ti powder metallurgy, achieving the expansion of titanium alloys into cost-effective domains.

In this work, a novel multi-scale microstructure of Ti6Al4V duplex alloy with nano-sized beta particles was designed based on powder metallurgy. The common lamellar structure was carefully adjusted to reveal the complex effects of microstructure on mechanical properties. The ideal microstructure transforms into a composition dominated by sparse equiaxed primary alpha, lots of beta-nanoprecipitates, and traces of secondary acicular alpha ' by heat treatment. The uniform nucleation of nano-sized beta particles, driven by intragranular element concentration difference and local distortions, is a pivotal reason for high yield strength by dispersion strengthening. More phase boundaries also act as sustainable sources for geometrically necessary dislocations. An uncoordinated phase interface relationship from variant selection improves strain-hardening ability. The intragranular misorientation causes the value inhomogeneity of the Schmid factor in primary alpha, resulting in an auxiliary slip effect that benefits ductility. Therefore, the fabricated Ti6Al4V alloy displays an ultrahigh ultimate tensile strength of 1329 MPa, yield strength of 1223 MPa, and reasonably large elongation of 8.5 %, respectively. This work underscores the intricate interplay of microstructure evolution and deformation mechanisms in powder metallurgy duplex titanium alloys, offering valuable insights into enhancing the mechanical properties.

The in-situ synthesized particle reinforced TC4 matrix composites were prepared by powder metallurgy pressureless sintering using polycarbosilane （PCS） as precursor. The thermal compression simulation experiments were conducted on TC4-1PCS （mass fraction of PCS is 1%） composites at 850-1100 ℃ and 0.001-1 s-1 to analyze the stress-strain curves of the composites under different parameters using the Gleeble-3500 thermal simulation testing machine. The effects of deformation parameters on the reinforced phase particles， matrix structure and densification were analyzed by OM， SEM and EBSD methods. The results indicate that the TiC reinforced phase particles with the size of 5-10 μm and large amount of residual pores are observed in the TC4-1PCS composites before hot deformation. The β transition temperature（Tβ） of TC4-1PCS matrix is 1000-1050 ℃. When deformed above Tβ， matrix of composite consists of lamellar quenched martensite， while the matrix turns into duplex microstructure， when deformed below Tβ. The deformation temperature determines the relative density and microstructure types of the composites， while the strain rate affects the phase size in the matrix and residual porosity. The densification of TC4-1PCS composites can be promoted by the increase of deformation temperature and the decrease of strain rate， while the increase of strain rate has obvious effect on the microstructure refinement. The microstructure refinement and densification of TC4-1PCS composites can be achieved by the deformation at 1050 ℃ and 0.1 s-1.

The application of the laser directed energy deposition (LDED) titanium (Ti) alloys has been severely impeded by their poor low cycle fatigue (LCF) properties in aerospace industry. Herein, we propose to improve the LCF properties of LDED-ed Ti6Al4V alloys via microstructure adjustment. A solid solution aging 850AA treatment is made to regulate the microstructure into a multi-scale composition dominated by discontinuous alpha GB and coarsened alpha P+(alpha S+beta) phases. The LCF properties of LDED-ed Ti6Al4V with a multi-scale microstructure even outperform the wrought Ti6Al4V at high strain amplitudes. Furthermore, this multi-scale microstructure has special local plastic deformation behavior and dislocation activities under cyclic stress loading, which strongly affects the internal stress evolution and crack propagation. The fatigue softening behavior is attributed to the combined effect of back stress and friction stress. Fatigue cracks are deflected by the alpha/beta boundary when the angle (phi) between the long axis of alpha lamellae and the crack direction is less than 35 degrees. The transgranular fracture occurs when phi is close to 90 degrees (70 degrees-90 degrees). Meanwhile, the fatigue crack usually propagates along the basal or prismatic planes with high Schmid factors. These findings provide novel insights into the microstructure design of LDED-ed Ti alloys with high fatigue damage tolerance.

Novel in-situ polycarbosilane (PCS)-derived TiC particles reinforced Ti6Al4V composites with ultrahigh strength and good ductility were fabricated by powder extrusion. The grain microstructure, dynamic recrystallization (DRX), texture orientation, mechanical properties and strengthening mechanisms were systematically investigated. The pyrolysis of PCS and in-situ reaction with Ti6Al4V matrix promote the formation of TiC particles and Si atoms solid-solution. TiC particles provide many heterogeneous nucleation sites for DRX and restrict grain growth by pinning effect, forming many fine equiaxed α grains. The average grain size of α-Ti is refined from 50.17 μm in as-sintered Ti6Al4V alloy to 3.17 μm in as-extruded Ti6Al4V-3PCS composite. The basal texture with {0001}<11 2‾ 0> orientation is dominant in the as-extruded microstructures, originating from the deformed α grains during β single-phase extrusion. The incorporation of PCS activates the occurrence of DRX and weakens the {0001}<11 2‾ 0> basal texture. Benefiting from the in situ formed TiC particles, solid solution Si atoms and low interstitial oxygen (O) contamination, the as-extruded Ti6Al4V-3PCS composite achieves the best combination of room-temperature strength and ductility, with 1307 ± 18 MPa in ultimate tensile strength (UTS), 1178 ± 25 MPa in yield strength (YS) and 10.0 ± 0.8% in elongation (EL), much higher than those of wrought Ti6Al4V alloy (ASTM B381). This work offers a feasible route for fabricating novel ultrastrong and ductile Ti6Al4V composites.

In this study, a new squeeze-cast Al-5Cu-xLi-0.5Mn-0.3Mg-0.15Ti (x = 0.3, 0.6, 0.9, 1.2 wt%) alloy (Al-5Cu-xLi alloy for short) was prepared with ultrasonic treatment (UT), and the effects of UT and Li content on the microstructure evolution and mechanical properties of Al-5Cu-xLi alloy were investigated. Unlike other aluminum alloys, UT applied to aluminum-lithium alloys can bring about significant degassing effects in addition to grain refinement and homogenization, which is beneficial for the high strength and toughness of Al-5Cu-xLi alloy. With the increase of Li content from 0.3 wt% to 1.2 wt%, the average grain size remains unchanged at first, and then increases significantly. Interestingly, the average size of & alpha;-Al grains is 30% smaller than that of Al-5Cu-0Li alloy (without Li element) after adding only 0.3 wt% of Li element, while the ultimate tensile strength (UTS), yield strength (YS) and elongation are increased by 11.3%, 15.6% and 26.9%, respectively. The optimal comprehensive mechanical properties are obtained in Al-5Cu-0.6Li alloy, which exhibits 300 MPa in UTS, 190 MPa in YS and 16.7% in elongation. However, by further increasing the Li content, the strength of the Al-5Cu-xLi alloy remains almost unchanged, while its elongation decreases significantly (but still more than 13.5%), due to grain coarsening and slight compositional segregation.& COPY; 2023 Published by Elsevier B.V.

In this work, we fabricated a high-performance Ti-6Al-4V alloy from TiH2 and 60Al40V powder mixtures by powder metallurgy (PM) pressureless sintering and hot extrusion. The correlation among sintering temperature, hot extrusion microstructure, and mechanical properties was systematically investigated. The dehydrogenation of the mixed powder during sintering promotes the closure of pores and obtains a fine, fully dense microstructure. The billets sintered at 1200 degrees C possess a denser lamellar microstructure consisting of small (alpha + beta) colonies, which provide more nucleation points during the later deformation. We made a detailed investigation into the deformation behavior and dislocation motion of the special lamellar structure in the fabricated Ti-6Al-4V via EBSD and TEM. The uniform alpha/beta lamellar interface generated by continuous dynamic recrystallization is a good location for geometrically necessary dislocation generation and storage. The synergistic improvement of strength and ductility after extrusion is attributed to the homogeneous and fine recrystallized microstructure and the smooth appearance of deformation twins. The ultimate tensile strength of extruded rod is up to 1213 MPa and the elongation increases to 14 %. This work proposes a simplified method to achieve low-cost PM Ti alloys with excellent performance.(c) 2023 Elsevier B.V. All rights reserved.

Sintering titanium (Ti) materials to high densities while maintaining fine grain sizes is a great challenge, which severely restrains their engineering application. Herein, we fabricated a high-density and fine-grain Ti material by a small addition of CaB6 via powder metallurgy (PM) pressureless sintering route. The effect of CaB6 addition on the grain growth kinetics and sintering densification mechanism were systematically investigated. The CaB6 oxygen-scavenger breaks the surface oxide layer and promotes the mutual diffusion of Ti atoms during sintering, resulting in the in-situ formation of nano-sized CaTiO3 particles and increase the sintered density. Moreover, the micro-sized TiB whiskers with the length from 10 gm to 30 gm and width from 1 gm to 4 gm are also formed in the Ti matrix. The grain microstructure of Ti/CaB6 composite displays a random orientation feature with the fine and uniform equiaxed crystallites. The presence of in situ TiB and CaTiO3 second-phase particles makes a greater grain growth activation energy Q for Ti/CaB6 composites, from the 373 kJ/mol of pure Ti to the 522 kJ/mol of Ti-1.0CaB6 composite, creating a smaller grain size. The grain refining effect by CaB6 addition is more effective at high temperature. As a result, the grain size of Ti-1.0CaB6 composite sintered at 1300 celcius/2 h is only 74 gm, much smaller than the 163 gm of pure Ti. This study offers a feasible way to produce the high-density and fine-grain Ti materials. (c) 2023 Elsevier B.V. All rights reserved.

Powder hot isostatic pressing (HIP) is an effective method to achieve near-net-shape manufacturing of high-quality complex thin-walled titanium alloy parts, and it has received extensive attention in recent years. However, there are few reports about the microstructure characteristics on the strengthening and toughening mechanisms of powder hot isostatic pressed (HIPed) titanium alloys. Therefore, TA15 powder was prepared into alloy by HIP approach, which was used to explore the microstructure characteristics at different HIP temperatures and the corresponding tensile properties and fracture toughness. Results show that the fabricated alloy has a “basket-like structure” when the HIP temperature is below 950°C, consisting of lath clusters and surrounding small equiaxed grains belts. When the HIP temperature is higher than 950°C, the microstructure gradually transforms into the Widmanstatten structure, accompanied by a significant increase in grain size. The tensile strength and elongation are reduced from 948 MPa and 17.3% for the 910°C specimen to 861 MPa and 10% for the 970°C specimen. The corresponding tensile fracture mode changes from transcrystalline plastic fracture to mixed fracture including intercrystalline cleavage. The fracture toughness of the specimens increases from 82.64 MPa·m 1/2 for the 910°C specimen to 140.18 MPa·m 1/2 for the 970°C specimen. Specimens below 950°C tend to form holes due to the prior particle boundaries (PPBs), which is not conducive to toughening. Specimens above 950°C have high fracture toughness due to the crack deflection, crack branching, and shear plastic deformation of the Widmanstatten structure. This study provides a valid reference for the development of powder HIPed titanium alloy.

Aiming at solving the interstitial oxygen contamination for titanium (Ti) parts fabricated by metal injection molding (MIM) technology, we innovatively introduced LaB6 oxygen scavenger to eliminate its adverse effect and produce a high-performance Ti material using inexpensive hydride-dehydride (HDH) Ti powders. Rheological behavior of feedstocks, decomposition behavior of binders, as-sintered microstructure and mechanical properties were systematically studied. The viscosity of feedstock F2 containing LaB6 powder meets the requirement of injection molding operation. The as-sintered 0.5 wt% LaB6/Ti composite has homogenous and fine microstructure with the average grain size of 65.6 mu m. The in-situ TiB and oxygen-containing (La2O3 and LaCl-O) reinforcements are formed by the reaction of LaB6 and Ti matrix, which are uniformly distributed in the composite. The generated TiC particle and Ti matrix exist an orientation relation of (111)TiC||(0110)Ti and [101]TiC||[0001]Ti. The LaB6/Ti composite possesses excellent mechanical properties, with the ultimate tensile strength of 632 MPa, yield strength of 548 MPa and elongation of 15 %. Attributed to the oxygen scavenger of LaB6, the elongation of the composite is greatly increased by 163 % compared with pure Ti. This work provides a valuable guidance for the development of low-cost and high-performance Ti parts by MIM technology.

In this paper, aging precipitates and their high effects on mechanical properties of squeeze-cast Al–5Cu-0.6Li-0.5Mn-0.3Mg-0.15Ti alloy (a novel Al–Cu–Li–Mn alloy) were investigated to reveal strengthening & toughening mechanism. After T6 heat treatment (i.e., solution treatment at 530 °C for 10 h + aging at 180 °C for 8 h), its ultimate tensile strength (UTS), yield strength (YS) and elongation (El.) are 465 MPa, 310 MPa and 16.5%, respectively. Compared with the as-cast Al–Cu–Li–Mn alloy, the UTS and YS are increased by 55% and 63.2%, respectively, with almost no loss in ductility. Interestingly, the product of UTS and El. (i.e., UTS•El.) of the T6-treated Al–Cu–Li–Mn alloy reaches 7.67 GPa%, which is 50.4% higher than that of the as-cast Al–Cu–Li–Mn alloy and better than that of most third-generation or fourth-generation Al–Li and Li-free 2xxx alloys prepared by casting or plastic deformation followed by heat treatment. The uniformly dispersed submicron-sized T (AlxMnyCuz), nano-sized T1 (Al2CuLi) and much smaller θ' (Al2Cu) phases precipitated in the T6-treated Al–Cu–Li–Mn alloy are coherent or semi-coherent with aluminum matrix, which not only enhances strength but also delays strain localization and fracture. In addition, the actual average thickness of T1 and θ′ precipitates are about 2 nm and 3.5 nm, which are much larger than their critical values for the conversion of their strengthening mechanisms. Therefore, the main strengthening mechanism of the squeeze-cast Al–Cu–Li–Mn alloy is the Orowan bypassing mechanism rather than the shearing mechanism.

Metal injection molding (MIM) is a very effective way to prepare brittle TiAl alloy parts. In this study, the rheological properties of feedstocks, as-sintered microstructure, interstitial impurities, and mechanical properties by using inexpensive non-spherical powders were systematically studied. By optimization of raw materials and strict process control, semi-alloyed Ti-48Al powder was prepared by self-propagation synthesis. The powder is composed of TiAl, Ti3Al, and TiAl3 with an O content of 0.34 wt