Solidification Of Alloys Research Articles

Formation mechanism and mechanical behavior of gradient nanograin structure in directional solidified Ti3Al alloy: Atomic-scale study

The formation mechanism of Ti3Al alloy during a directional solidification process was systemically investigated by means of molecular dynamics (MD) simulations, and its mechanical behavior was explored by comparing with its nanograined (NG), coarse-grained (CG) and gradient nanograined (GNG) counterparts. It is found that the solidified front forms equiaxed crystals first, then they transform into columnar crystals, and the GNG structure is formed finally. Noticeably, the grains will grow preferentially in the direction parallel to the solidification direction. Besides, it is also found that the directional solidified alloy with the GNG structure has higher tensile strength and better ductility than its NG and CG counterparts. The GNG structure not only suppresses strain localization and grain growth in its small grain regions, but also promotes more cross dislocations in the large grain regions, resulting in a better mechanical performance.

Achieving high strength-ductility synergy via Guinier-Preston zone formation in a new Mg–Zn–Al–Ca–Mn–Ce sheet alloy

ZAXME11100 (Mg–1Zn–1Al-0.5Ca-0.4Mn-0.2Ce, wt.%) is a recently developed sheet alloy with excellent formability at room temperature. The yield strength of this alloy is significantly improved by a short 1-h aging treatment at 210 °C (T6), from 159 MPa in solution-treated state (T4) to 270 MPa in the T6 condition, with a slight reduction in tensile elongation (from 31 % in T4 to 26 % in T6). In this work, precipitation microstructure was characterized to explain the aging hardening and high strength-ductility synergy of the new alloy. A high density (8 × 1023/m3) of ordered monolayer Guinier-Preston (GP) zones enriched in Al, Ca and Zn uniformly form on the Mg basal planes after T6 treatment. Grain boundary segregation of Al, Ca and Zn is also observed. Deformation microstructure of T4 and T6 samples tested to various strain levels was examined. At small strain levels, basal and non-basal <a> dislocation slip dominates the deformation process, and cross slip of <a> dislocations between basal and non-basal planes is observed in T6 samples. The GP zones are gradually destroyed by plastic deformation. Based on those results, the strengthening contribution of ordered GP zones was modeled to be 116 MPa. Therefore, formation of GP zones is an effective way to achieve high strength-ductility synergy in wrought Mg alloys.

Study of the irradiation hardening of a Fe9Cr ferritic model alloy by nanoindentations

This paper presents a series of Berkovich indentation results obtained on unirradiated and 7.2 MeV proton irradiated specimens of a Fe9Cr model alloy with an equiaxed ferritic microstructure. Using a energy degrader wheel made of 24 aluminum foils of different thickness, a flat damage profile of about 50 μm resulted from the irradiation, allowing to perform indentations within a volume material that is homogeneously irradiated. The indentation size effect on measured hardness was analyzed with a model based on the disloction-slip distance theory built on the concept of combined spatial frequency of all obstacles to dislocation motion. The effects on hardness of the tip-specimen contact size (or penetration depth), dislocation density and mean distance between irradiation defects were quantified and discussed. The irradiation hardening was characterized by the increase of hardness determined at large penetration depths.

Effect of Al content in CoCrFeNiAlx HEA on mechanical properties and high temperature oxidation resistance of WC-10%CoCrFeNiAlx hard alloy

The CoCrFeNiAl high-entropy alloy, with varying Al compositions, was utilized as the bonding phase in this study to fabricate WC-10%CoCrFeNiAlx (where x represents the mole ratio of Al element, ranging from 0 to 1) hard alloys using the Spark Plasma Sintering method. The investigation focused on elucidating the influence mechanism of microstructure and properties of CoCrFeNiAlx high-entropy alloys on the mechanical properties of WC-10%CoCrFeNiAlx hard alloys. Isothermal oxidation testing was subsequently conducted to examine the impact of Al content on the high-temperature oxidation resistance of WC-10%CoCrFeNiAlx hard alloys. Finally, a comparison was made between the mechanical properties and high temperature oxidation resistance of WC-10%CoCrFeNiAlx hard alloys and conventional WC-6Co cemented carbide. The findings suggest that as the Al content increases, the CoCrFeNiAlx high-entropy alloy undergoes a transition from a single Face-Centered Cubic (FCC) phase to a combination of FCC and Body Centered Cubic (BCC) phases, ultimately transforming into a BCC phase. This evolution results in a gradual enhancement in hardness. The mechanical properties of WC-10CoCrFeNi hard alloy exhibit exceptional characteristics when the Al content is eliminated, with hardness, fracture toughness, and bending strength reaching 1702 MPa, 13.0 MPa·m1/2, and 1378 MPa respectively. As the Al content increases in WC-10%CoCrFeNiAlx hard alloys, there is a corresponding increase in hardness while fracture toughness and bending strength gradually decrease. Furthermore, an enhancement in high temperature oxidation resistance is observed. Finally, in comparison to the conventional WC-6Co hard alloy, the mechanical properties of WC-10CoCrFeNiAlx hard alloys exhibit similarity while demonstrating a significant enhancement in oxidation resistance.

Analysis of slag chemistry in WEEE smelting using experimental and modelling study of the “CuO0.5”-ZnO-FeO-FeO1.5-CaO-SiO2-AlO1.5 system in equilibrium with Cu metal

The variability of the composition of recycled copper-containing materials requires enhanced control and understanding of slag chemistry in the secondary pyrometallurgical processing, when compared to primary copper smelting. In the black copper route for recycling waste electronic and electrical equipment (WEEE), slags exhibit high concentrations of alumina and zinc oxide. The simplest system governing phase equilibria at the reducing smelting stage of the black copper process is “CuO0.5”-ZnO-FeO-FeO1.5-CaO-SiO2-AlO1.5. In the present study, slags within this system were equilibrated with liquid copper Cu and solid Fe metallic alloys, quenched and characterized by the Electron Probe X-ray Microanalysis (EPMA). The range of compositions was selected based on the information about the process available in literature. The study focused on the liquidus and proportion of solid spinel in the temperature range from 1100 to 1300 °C. Precise control of the proportion of solids can enhance the stability of refractory materials against corrosive slags while maintaining entrained metal droplets at reasonably low level. Additionally, experimental correlations between the solubility of copper in the oxide liquid and the partial pressure of oxygen were developed for the Zn-free slags in equilibrium with metallic copper at 1200 and 1300 °C. All experimental results were compared to thermodynamic predictions using recent models and FactSage® software. Uncertainties were identified to be used in further model improvement.

Precipitation behavior of supersaturated solid-solubility CuCrZr alloy by additive manufacturing

As a typical precipitation-strengthened alloy, the improvement of CuCrZr alloy performance can be achieved by promoting the precipitation of alloying elements. However, the low solubility of Cr limits further enhancement of the performance of conventionally processed CuCrZr alloys. In this study, a supersaturated solid solution CuCrZr alloy was prepared using laser powder bed fusion (LPBF), and the precipitation of solute atoms was promoted through heat treatment, resulting in an alloy with excellent comprehensive performance. The influence of different heat treatment processes on the precipitation behavior and performance of the alloy was investigated. Microstructural analysis revealed the presence of micrometer-scale Cr particles in the specimens after solution treatment (ST) and solution-aging treatment (SAT), while the direct-age-hardening (DAH) specimen exhibited dispersed nanoscale Cr precipitations. Notably, a Nishiyama-Wassermann (N-W) relationship was observed between the Cr precipitations and the Cu matrix, significantly enhancing the mechanical properties of the alloy. The DAH specimen exhibited an ultimate tensile strength (UTS) of 661.54 MPa, a hardness of 231.03 HV and an electrical conductivity of 65.05 % IACS, demonstrating the best match between mechanical properties and electrical conductivity. The outstanding properties can be credited to the uniformly distributed Cr nano-precipitates and slim lattice distortion. Additionally, the main strengthening mechanism is precipitation strengthening for CuCrZr alloy.

General investigations on the heat treatment and thermal fatigue behavior of an experimental hot work tool steel tailored for laser powder bed fusion

An experimental hot work tool steel with a leaner chemistry compared to the standard AISI-H13 grade, aimed at enhanced processability, was gas atomized and processed using laser powder bed fusion (L-PBF) to >99.8 % relative density. The lower carbon content (∼0.25 wt% vs. ∼0.4 wt% in H13) resulted in a softer as-built (AB), and quenched martensite (∼485 HV1 vs. 625 HV1 in H13), higher martensite-start temperature (∼360 °C vs. 280–320 °C), and about ∼18 % reduction in dilatation, ascribed to the volume expansive martensitic transformation. Charpy V-notch impact toughness in AB condition (>30 J) was higher than those reported for AB H13. These features account for the improved L-PBF processability. The tempering response in comparison with a wrought H13, was characterized by a larger precipitation of thermally stable Mo- and V-rich secondary hardening alloy carbides at the expense of the easy-to-coarsen Cr-rich ones, attributed to the higher Mo and lower Cr content in experimental alloy. The combination of lower C- and V-contents in experimental alloy, reduced the driving force for the precipitation of coarse vanadium rich carbides during austenitization, as confirmed by electron diffraction spectroscopy (EDS) and synchrotron X ray diffraction. This led to a stronger fine secondary hardening alloy carbide precipitation during tempering, and enhanced temper resistance. As elaborated by isochronal tempering, as well as tempering curves, reduction of the elements Si, and Cr shifted the secondary hardness peak to higher tempering temperatures. Consequently, tempering, and thermomechanical softening resistance was significantly enhanced in the new steel with a maximum hardness of ∼550 HV1 in over-tempered condition (i.e., slightly above secondary hardness peak). Thermal fatigue (TF) tests revealed a denser and finer TF crack network with a larger oxidation in experimental steel. Thereby, the TF crack penetration depth scaling with the local plastic yielding of the matrix was evidently shorter in the new steel compared with wrought H13, thanks to significantly improved hot hardness, thermal conductivity, and enhanced thermomechanical softening resistance. Finally, a proof of concept is demonstrated by processing a relatively large injection molding die (150 mm × 110 mm × 30 mm) with promising tensile and impact toughness properties after direct double tempering to 51 HRC, and 45 HRC hardness levels.

Effect of natural aging by multifunction cavitation on plane bending fatigue behaviour of heat-treatable Al-Si7Mg aluminum alloys and its fatigue strength estimation

The effect of natural aging by multifunction cavitation (MFC) on the fatigue behaviour of heat-treatable Al-Si7Mg aluminum alloys was examined. Surface observation and plane bending fatigue tests were conducted for the MFC-treated aluminum alloys at a stress ratio, R, of −1. The hardness of aluminum alloy without T6 treatment was significantly increased by MFC due to both the work hardening and natural aging. MFC treatment improved the fatigue lives and the fatigue strength at N = 107 cycles of aluminum alloys due to the generation of compressive residual stress and increasing the surface hardness; however, surface pits were large enough to easily nucleate an initial fatigue crack. In addition, the fatigue strength at N = 107 cycles of MFC-treated aluminum alloys can be estimated considering the residual stress, pit size and surface hardness.

Effect of Al and Nb addition on the precipitation hardening and deformation mechanism of CoCrNi medium entropy alloy

Single-phase medium-entropy alloys (MEAs) with face-centered cubic (fcc) structure exhibit high elongation and toughness but suffer from insufficient strength. In this work, two novel (CrCoNi)Al4 and (CrCoNi)Al4Nb2 MEAs were designed with the aim of introducing precipitation hardening effect and thus improve strength of the alloys. It was revealed that addition of Al alone, only slightly increased the strength, but the elongation decreased severely. Conversely, the combination of Al and Nb can significantly increase strength and maintain acceptable elongation. The Al addition promotes the formation of a disperse distribution of BCC/B2 composite precipitates, however, the large size and incoherent nature of these composite precipitates produce minor hardening response. Except for the BCC/B2 composite precipitates, a high density of the nanoscale γ′ phase with L12 structure and stoichiometry of (Ni, Co, Cr)3(Al, Nb) is produced in the Al4Nb2 MEA, which significantly increases the strength of the alloy. After the addition of Al and Nb, the deformation mechanism of the alloys is changed from nanotwins to stacking fault (SF) dominated plasticity. Such phenomenon mainly relates to the increasing critical twinning stress affected by the channel width of the matrix. This study provides new insight for the composition design of precipitation-hardening MEAs.

Simulation of gas porosity formation and interaction with dendrite and eutectic structures during solidification of Al-Si alloys

A two-dimensional (2-D) multi-phase cellular automaton (CA)-finite difference (FD)-lattice Boltzmann (LB) coupled model is developed to simulate the hydrogen pore formation during dendritic and eutectic solidification of hypoeutectic Al-Si alloys. In this model, the dendrite and eutectic structures are simulated using a CA technique. The solute diffusion is solved by the FD method. The nucleation, growth, and motion behavior of gas pore, and hydrogen transport are simulated by a LB Shen-Chen scheme. It is found that the pores can grow fully but compete in the primary dendrite growth stage. Conversely, pores newly nucleated in residual inter-dendrite liquid during the eutectic solidification stage grow restrictedly and independently. Pore shape severely deformed by the compression of the surrounding solid structures. Moreover, the effects of the initial Si composition, initial hydrogen concentration and cooling rate on the pore formation are qualitatively and quantitatively studied. It is found that the initial Si composition significantly impacts on the pore size and distribution, but little effect on the porosity percentage which decreases markedly with the decreasing initial hydrogen concentration and increasing cooling rate. Comparison reveals that the simulated microstructure morphological characteristics and the predicted porosity percentage agree well with the experimental observation reported in the literature.

Annealing-induced hardening in single-phase FCC, equiatomic CrCoNi and non-equiatomic CrCoNiMo medium-entropy alloys

A two-step processing route including cold working (primary hardening) followed by annealing-induced (secondary) hardening is proposed to strengthen single-phase face-centered cubic CrCoNi-based medium-entropy alloys (MEAs). Here, we investigate two compositions (in at.%): Cr33·3Co33·3Ni33.3 and Cr27·3Co33·3Ni33·3Mo6. The former has received considerable interest and is now regarded as a model MEA. The latter was designed to investigate how replacing 6 at.% Cr with Mo affects mechanical properties and annealing-induced hardening. It is worth mentioning that this latter composition is similar to that of the industrial MP35 N alloy (Cr22·9Co35·4Ni35·5Mo6.2), which is well-known to exhibit a strong annealing-induced hardening, but Cr27·3Co33·3Ni33·3Mo6 is outside its specification. In MP35 N, it has long been thought that Mo segregation at planar defects is responsible for annealing-induced hardening. In the present work, we show that the strength of both Cr33·3Co33·3Ni33.3 and Cr27·3Co33·3Ni33·3Mo6 can be increased by ∼25% after post-deformation annealing, thus demonstrating that Mo is not essential to cause annealing-induced hardening. Either another element, which could be Cr as revealed by atom probe tomography, is able to segregate at planar defects and induce annealing-induced hardening, or different mechanisms produce a similar strengthening effect. Based on these results, we discuss different models that can be used to quantify annealing-induced hardening in CrCoNi-based MEAs.

Experimental Study on Turbine Blade Material

This experimental study investigates the nitriding effects on microstructure of wrought 17-4 PH stainless steel. Nitriding was carried out at 560 °C for 2 hours, resulting in significant improvements in hardness. Nitriding improves the hardness on the surface of the 17-4 PH steel and remains constant in the core. As a result of nitriding the harness increased 3.5 times the hardness of the wrought alloy. Microstructural analysis of both untreated and nitrided samples using a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS) revealed the formation of a nitride layer. The results show a relationship between nitriding temperature and nitride layer thickness, with higher temperatures leading to thicker layers.

Ultra strong FCC structured Ni8Cr4Co4Fe6W2 high entropy alloys with high strength and ductility by laser powder bed fusion

The high entropy alloys (HEAs) are widely used in high temperature service environment (such as aviation engine blades) with bright application potential in additive manufacturing. Laser powder bed fusion (LPBF) is technology currently offers the best reproducibility and dimensional accuracy for part production. The unique atomic lattice structure of HEA makes its plastic deformation mechanism different from that of traditional dilution alloy, resulting in the strengthening effect of face-centered cubic high-entropy alloy different from that of traditional metal alloy materials. In this paper, Ni8Cr4Co4Fe6W2 was designed by means of composition control. Its spherical powders with uniform composition were successfully prepared by high temperature remelting spheroidization (PHTR) method, compared with gas atomization (GA) method. The strain rate sensitivity m values of GA-Selective Laser Melting and PHTR-Selective Laser Melting samples were 0.030 and 0.027, respectively. The hardness of Ni8Cr4Co4Fe6W2 high-entropy alloy showed a trend of continuous increase with the decrease of grain size, while the elastic modulus didn’t change much (PHTR-SLM is 193 Gpa and GA-SLM is 189 Gpa). The simulated value of the contact stress decreased by 23 % compared with the theoretical value. The contact radius increased with the increase of plastic deformation in addition to the reduction of contact pressure. The printing sample value of the elongation, yield strength (YS) and Ultimate Tensile Strength (UTS) on PHTR-SLM has increased by 39.6 %, 33.7 % and 25.9 %, respectively in additive manufacturing, compared to GA-SLM. The ability to produce refractory HEAs through PHTR method will make it possible to manufacture complex geometry shapes at a reasonable cost.

Achieving high strength and uniform ductility in high-entropy alloys via dynamic-precipitation accelerated transformation-induced plasticity

This study presents a novel approach to fabricating advanced transformation-induced plasticity (TRIP) high-entropy alloys (HEAs) that exhibit an exceptional combination of high strength and uniform ductility, far surpassing the mechanical performance of previously developed TRIP HEAs and non-TRIP multiphase HEAs. The ultrafine-grained Al5Cr20Fe35Co35Ni5 TRIP HEA sample exhibits a yield strength of gigapascal-level, ranging from 1100 to 1260 MPa, with large uniform elongations that range from 29 % to 39 %. Simultaneously, the alloy displays outstanding toughness, reaching 46,000±5390 MPa·%. The achievement of this significant breakthrough rests upon two key factors: (1) achieving an ultrafine-grained FCC matrix in a fully recrystallized microstructure with the aid of thermally induced B2 phase, and (2) dynamic precipitation of nanometer-sized B2 phase particles on high-density stacking faults and slip bands created within the interiors of ultrafine grains. The dynamic precipitation of the B2 phase, facilitated by the formation of an amorphous phase that serves as a precursor to the B2 crystalline phase, is a rare occurrence in HEAs at room temperature. This dynamic precipitation process greatly enhances TRIP-assisted strain hardening (from FCC to HCP phase) of the current alloy at ultrafine grains, enabling the overcoming of the inherent tensile ductility constraints typically associated with ultrafine-grained metals. The current development of TRIP HEAs with an outstanding combination of high strength and high toughness is believed to mark a remarkable milestone in the application field of TRIP HEAs.

Effect of equal-channel angular pressing on microstructure, aging kinetics and impact behavior in a 7075 aluminum alloy

Among the precipitation hardenable 7xxx series aluminum alloys, AA7075 alloys (one of the most important engineering alloys) due to their low density (lightweight), high toughness and strength, and enhanced fatigue behavior have been used over the years in aerospace, aircraft, automotive, construction and marine applications. In the present study, the influence of the artificial aging through conventional heat treatment (i.e., precipitation hardening) and the thermomechanical treatment (equal-channel angular pressing (ECAP) + post-ECAP aging) on the quasi-static tensile, impact toughness and precipitation behavior as well as on the microstructure of an AA7075 alloy is investigated systematically. The results indicate that the ECAP process as well as post-ECAP aging result in considerably increased strength and hardness of the AA7075 alloys because of the superimposed effects of grain boundary strengthening (Hall-Petch relation), strain hardening (by means of the increased dislocation density) and precipitation hardening (due to the fine precipitates formed in Al matrix). Multi-pass ECAP processing has been found to result in a considerable decrease of the impact toughness (from 15.3 J·cm−2 to 7.6 J·cm−2) associated with the failure mode (i.e., ductile-to-brittle transition) of the AA7075 alloys, whereas the artificial peak aging leads to a marked improvement (∼67 %) in the impact toughness in ECAPed conditions. Moreover, the ECAP process noticeably accelerates the precipitation kinetics of AA7075 alloys: The peak aging time in the ECAPed alloy is reduced significantly – from 30 h to 4 h. The results of the present study provide a deeper understanding of the combination of ECAP and heat treatment, and their effect on the fracture mode and toughness under impact loading, the aging kinetics and the microstructural evolution of an AA7075 alloy.

The Effect of Heat Treatment on the Mechanical Properties and Oxidation Resistance of AlSi10Mg Alloy

In this study, the microstructure, hardness, tensile strength, and dry wear properties of the cast AlSi10Mg alloy as well as the effects of processing parameters on the oxidation behavior of this alloy were investigated. In this context, AlSi10Mg (wt%) alloy was produced by gravity die casting method, and then, air cooling, quenching, and T6 treatment were applied. The microstructure of the alloy was investigated using an optical microscope, SEM, and EDS. Additionally, thermogravimetric analysis (TGA) was used to determine the effect of processing parameters on the oxidation behavior of the samples. The friction and wear properties of the alloy were investigated using a ball-on-disk wear test device. It was founded that the as-cast AlSi10Mg alloy's microstructure contained phases of α(Al), Si, and β-Mg2Si. It has been observed that the microstructure of the cast AlSi10Mg alloy gradually turns into a spherical form when quenching and air-cooling heat treatments are applied. Also, T6 heat treatment led to further spheroidization of Si particles, formation of β-Mg2Si precipitates, and elimination of the Chinese script morphology of the β-Mg2Si phase. It was observed that the yield strength of the AlSi10Mg alloy in the as-cast state decreased negligibly compared to the air-cooled state, while there was an increase of up to 70% in the aging-treated condition. The results show that the tensile strength of the as-cast AlSi10Mg alloy increased by 115% after aging. Additionally, it was found that the AlSi10Mg alloy's wear resistance increased with the aging process.

Enhanced strength-ductility synergy in a gradient pseudo-precipitates heterostructured Al-2.5%Mg alloy: Design, fabrication, and deformation mechanism

Heterostructures of alloyed composites, comprising heterogeneous domains with dramatically different constitutive properties, hold remarkable potential to expand the realm of material design systems and resolve the trade-off between strength and ductility. This study introduces an innovative materials design method for synthesizing gradient pseudo-precipitates heterostructure (GPHS) in non-heat-treatable Al-2.5%Mg alloys. Utilizing cost-effective mild steel as both the diffusion source and protective layer, this heterostructure is achieved through pin-less friction stir-assisted cyclic localized deformation process. Exogenous Fe atoms diffuse across the interface by friction stir-induced heat conduction, forming Fe-Al second-phase particles in the Al alloy matrix. A rapid inter-diffusion mechanism is activated in conjunction with dense dislocation walls, grain boundaries, and sub-structures, resulting in the formation of pseudo-precipitates. These pseudo-precipitates are ultimately dispersed in a gradient distribution throughout the entire thickness of the Al alloy matrix induced by localized incremental deformation. The GPHSed Al-2.5%Mg alloy exhibits an enhanced synergy of strength and ductility, with a uniform elongation increase from 11 % to 21.2 %, while maintaining the strength. Multiple strengthening and hardening mechanisms, such as solid solution strengthening, dislocation hardening, and second phase strengthening, work synergistically to promote mechanical performance. Notably, the hetero-deformation between hard pseudo-precipitates and soft Al alloy matrix induces additional strain hardening, leading to high ductility. This work provides a fresh perspective on the design and fabrication of high-performance alloys with advanced heterostructures, especially for non-heat-treatable alloys.

From clinic to lab: Advances in porous titanium-based orthopedic implant research

Solid titanium alloys are extensively employed in orthopedic applications due to their reliable mechanical strength, but the stress shielding phenomenon caused by its high elastic modulus and the disadvantage of lack of bone integration may lead to clinical complications. The design of porous structure is a feasible approach to reduce the elastic modulus and promote bone and vascular ingrowth. This paper reviewed the application and current challenges of solid titanium implants in orthopedics. To address the drawbacks and explore porous titanium implants that can effectively integrate with bone tissue while possessing sufficient mechanical strength, this review discussed the latest preclinical research from the perspectives of structural design, surface modification, and material composition. The porous structure parameters that affect bone ingrowth and mechanical strength in implants, including porosity, pore size, cellular structure, and gradient structure, were presented. Based on the current research progress, various surface modification techniques aimed at enhancing the performance of porous titanium implants were overviewed. Moreover, a range of titanium alloys including nickel-titanium alloy, β-titanium alloy, high-entropy alloy, and composite alloys are investigated for their prospective applications in orthopedics. Overall, this review provided valuable insights into the current state of research and highlighted the potential of porous titanium alloys in improving clinical outcomes in orthopedics.

Influence of annealing temperature on the mechanical properties and corrosion behavior of Ti-5.5Al-2.0Zr-1.5Sn-0.5Mo-1.5Nb alloy

In this work, the effect of annealing temperature of Ti552 titanium alloy is systematically investigated based on mechanical property tests, electrochemical tests and static immersion tests. The results show that after the annealing treatment, the microstructure morphology of the α-phase transforms from striated to equiaxed, and the intergranular β-phase gradually increases. As the annealing temperature increases, the grain size gradually increases and the volume fraction of α-phase gradually decreases. The tensile strength and Vickers hardness are the maximum and the elongation is the minimum in the hot-rolled condition. With increasing annealing temperature, the strength and Vickers hardness of Ti552 alloy gradually decrease, while the elongation gradually increases. In 3.5 wt% NaCl solution, Ti552 alloy has similar corrosion behavior at different annealing temperatures, although its corrosion resistance varies. In potentiodynamic polarization and EIS testing, the annealed Ti552 alloy shows a low corrosion current density (10-6 A/cm2) and a high polarization impedance (106 Ω·cm2), indicating the formation of a dense and stable oxide film. Under annealing at 800 °C, the corrosion resistance is at its highest and the corrosion rate is the lowest, according to electrochemical and static immersion studies. The comprehensive analysis suggests that the corrosion behavior of Ti552 alloy in NaCl solution may be a result of the combined effect of the volume fraction of the β-phase and the grain size of the α-phase.

Multi-physics simulation of non-equilibrium solidification in Ti-Nb alloy during selective laser melting

This paper underscores the critical role of temperature distribution in the molten pool on the resulting solidification microstructure during selective laser melting (SLM) process. A powder-resolved computational fluid dynamics (CFD) model of Ti-Nb alloy is established to investigate heat transfer and fluid flow in the molten pool. When comparing the bulk model to the powder bed model, discrepancies in temperature gradient and cooling rate can reach up to 26.29 % and 18.32 %, respectively. Utilizing the powder bed CFD model, temperature gradient and cooling rate data at the molten pool boundary are integrated into the finite interface dissipation phase-field model. The simulation results illuminate the non-equilibrium solidification microstructure in the SLMed Ti-Nb alloy. Results indicate that the rapid solidification fosters a pronounced solute trapping effect, mitigating solute microsegregation. The microstructure manifests as cellular and dendritic structures, with Nb concentration in cellular cores and depletion in intercellular region. Predicted primary dendrite arm spacing (PDAS) or cell size range between 0.4 to 0.6 μm, consistent with experimental observations. In particular, the study reveals a transition from dendritic to cellular and from cellular to planar structures, with an accompanying analysis of the underlying mechanisms.