Prior Particle Boundaries Research Articles

Effect of directional anisotropy on mechanical properties of 9Cr Ferritic/Martensitic ODS steels processed by mechanical alloying and powder forging

The present work investigates the feasibility of the powder forging route as a novel approach for the consolidation of oxide dispersion strengthened (ODS) steel powders. Mechanically alloyed powder of the composition Fe-9Cr-2 W-0.2Ti-0.12 C-0.35Y2O3 was filled in a mild steel can and loose sintered in a reducing atmosphere at 1200 °C after which it was forged in a channel die using a friction screw forging machine. The forged ODS steel was investigated in terms of residual porosity, microstructure, hardness and tensile properties. Density as well as hardness variation was determined along both the forging directions, i.e., longitudinal and transverse. In the transverse direction density, hardness and tensile strength was slightly higher as compared to that in the longitudinal direction. Powder forging led to the elimination of the prior particle boundaries (PPB) network in the final forged slab. Further, near isotropic mechanical properties and homogeneous chemical composition were obtained throughout the forged sample. The proposed powder forging route has significant potential as an alternative consolidation process to hot isostatic pressing (HIP) and hot extrusion to produce ODS steel for nuclear applications.

Microstructure evolution, strength-ductility synergy and fracture mechanisms of Cu-18Sn-0.3Ti alloy prepared using Direct Current assisted Hot Pressed sintering and conventional solution treatment

Based on micron atomized spherical Cu-18Sn-0.3Ti alloy powders, the Cu-18Sn-0.3Ti alloy with an elongation of 23.4%, an ultimate tensile strength of 507.2 MPa and a yield strength of 229.4 MPa was prepared using Direct Current assisted Hot Pressed sintering (DCHP) and conventional solution treatment. The microstructure evolution mechanism of alloy during DCHP, the strength-ductility synergy and fracture mechanisms of solution treated DCHP (ST-DCHP) samples were systematically investigated. The results show that, under the synergistic effect of Joule heat, electron wind force and external load of DCHP, dislocations were consecutively generated and consumed as the driving force of dynamic recrystallization, fine equiaxed grains and a large number of annealing twins were formed, the prior particle boundaries (PPBs) of alloy powders gradually disappeared and good sintering necks were formed, which played key roles in improving the strength of alloy. Simultaneously, the δ phase was segregated from grain boundaries to PPBs during DHCP. During the subsequent solution treatment, the phase transformation of δ phase occurred, the β, γ and ε phases which were conducive to the plastic deformation of alloy were generated inside δ phase, resulting in a change in the fracture mechanism inside powders and at sintering necks. The high fraction of high-angle grain boundaries (HAGBs) was considered as the main reason for inducing the formation of high-density deformation twins with different directions inside fine α grains, which greatly improved the ductility of alloy. The strengthening contributions of microstructure were also evaluated, and the calculated yield strength was well consistent with the experimental ones. The results provide the theoretical guidance for the preparation of Cu-Sn-Ti alloy with higher Sn content and lay a foundation for the preparation of Nb3Sn superconducting wires with higher critical current density.

On the microstructure and high cycle fatigue of near-net shape PM-HIPed Inconel 625

This paper investigated the microstructure and fatigue behavior of PM-HIPed Inconel 625. The microstructure was composed of γ phase and (Mo, Nb) carbonitrides located mostly on prior particle boundaries. Despite the presence of these carbonitrides, the samples showed good tensile properties with high elongation. Two different surface conditions, pickled and machined, were used for high cycle fatigue testing under a four-point bending test. The results indicated that pickled samples had 6% lower fatigue strength (at 106 cycles) with three times higher standard deviation compared to the machined ones. Fatigue failure mechanisms were found to be dependent on surface conditions and showed different failure modes due to non-metallic oxide inclusions and surface defects in samples with machined and pickled surfaces, respectively. The effect of type, size, and location of defects, multiplicity of crack initiations, as well as surface roughness were analyzed and discussed.

A novel ODS high-entropy composite with improved strength and ductility

Body-centered cubic (BCC) structured high-entropy alloys (HEAs) usually exhibit excellent strength, thermal stability, irradiation tolerance and corrosion resistance but low ductility. In this study, a novel oxide dispersion strengthened (ODS) AlCrFeNi high entropy composite (with BCC-structure reinforced by face-centered cubic (FCC) ODS-CoCrFeNiMn HEA reinforcement was designed and prepared by mechanical alloying and spark plasma sintering. The effects of sintering temperatures (950 °C, 1000 °C, 1050 °C and 1100 °C) and reinforcement amount (5%, 10%,15% and 20%) on microstructure and mechanical properties were investigated. The results showed that the optimal sintering temperature of the ODS AlCrFeNi composites is 1100 °C. The hardness decreases with the increase of the reinforcement content (from 5% to 20%). Compared to the reference alloy (RA) without the addition of reinforcement, the compressive strain of the composite with 15% reinforcement (B–15%F) increases significantly from 8% (RA) to 18%, while the compressive strength increases from 2823 MPa (RA) to 3078 MPa. The B–15%F is composed of matrix, reinforcement and intermediate transition regions. The thickness of transition regions increases with increasing sintering temperature. The matrix consists of fine BCC FeCr phase and B2 NiAl phase, due to spinodal decomposition, together with high-density nanoscale oxides Y4Zr3O12 and a small amount of Y–Al–O. Surprisingly, the crystal structure of reinforcement transforms from FCC into the BCC during SPS. Meanwhile, the mutual diffusion of Al elements in the matrix and Co and Mn elements in the reinforcement also results in similar spinodal decomposition in the reinforcement. The domain size of spinodal decomposition in the reinforcement is coarser than that in the matrix. The reinforcement is mainly distributed on the prior particle boundaries (PPBs), which can effectively reduce/suppress the formation and propagation of micro-cracks along the PPBs, and thus improve both the strength and ductility of the composite.

Effect of humid atmosphere on the microstructure and mechanical properties of a PM Ni-based superalloy: From powders to bulk alloys

Understanding the effects of powder storage condition is helpful to improve the mechanical properties of powder metallurgy (PM) and additive manufacturing (AM) superalloys. In this work, nickel-based FGH96 superalloy powders were stored under ambient air (relatively dry) and humid (25 °C/95%RH and 90 °C/95%RH) atmospheres and then consolidated by hot isostatic pressing (HIP). The powders and corresponding bulk alloys were characterized by XPS, OM, FESEM, EPMA, EBSD, and TEM. The results indicate that the humid atmosphere mainly affects the gas content, chemical state, and oxide/hydroxide layer thickness of powders, leading to highest values of oxygen content (~450 ppm), hydrogen content (~25 ppm), and NiO/Ni(OH)2 thickness (~13.9 nm) for 90 °C/95%RH powders, and further forms much severe prior particle boundaries (PPBs) defects in HIPed alloy. The detrimental PPBs are composed of large-sized γ', MC carbides-(Ti, Nb)C, Al2O3 and ZrO2 oxides. In comparison with ambient air powders compact, the strength of 90 °C/95%RH powders compact is unexpectedly increased, which is due to the internal grains pinned inside the original particle boundaries by higher severity degree of PPBs, and leads to the increase of grain boundary strength. Meanwhile, the ductility is significantly decreased, because the fracture mode transited from ductile transgranular fracture to brittle inter-particle debonding. The study reveals the relationship between humid atmosphere and properties of both powders and bulk alloys, and intends to obtain good combination of strength and ductility for PM Ni-based superalloys.

Evolution Behavior of Rapidly Solidified Microstructure of a Ti-48Al-3Nb-1.5Ta Alloy Powder during Hot Isostatic Pressing

In this study, Ti-48Al-3Nb-1.5Ta powders were manufactured from cast bars by the supreme-speed plasma rotating electrode process (SS-PREP) and used to prepare hot isostatically pressed (HIPed) material at 1050–1260 °C with 150 MPa for 4 h. The phase, microstructure and mechanical performance were analyzed by XRD, SEM, electrical universal material testing machine and other methods. The results revealed that the phase constitution changed from γ phase to α2 phase and then to γ phase with the material changing from as-cast to powders and then to as-HIPed. Compared with the as-cast material, the grain size and element segregation were significantly reduced for both powders and as-HIPed. When the hot isostatic pressing (HIP) temperature was low, the genetic characteristics of the powder microstructure were evident. With the HIP temperature increasing, the homogeneity of the composition and microstructure increased, and the prior particle boundaries (PPBs) gradually disappeared. The elastic moduli of powder and as-HIPed were superior to those of as-cast, which increased with the HIP temperature increasing. The hardness of as-HIPed was lower than that of the powder. The compressive strength, compressive strain, bending strength, and tensile strength of as-HIPed were higher than those of as-cast. With an increase in the HIP temperature, the compressive strength decreased gradually, and the compressive strain first decreased and then increased.

Microstructural Characterization and Prior Particle Boundary (PPB) of PM Nickel-Based Superalloys by Spark Plasma Sintering (SPS).

This research investigates the microstructure and defects of powder metallurgy (PM) nickel-based superalloys prepared by spark plasma sintering (SPS). The densification, microstructural evolution, and precipitate phase evolution processes of FGH96 superalloy after powder heat treatment (PHT) and sintering via SPS are specifically analyzed. Experimental results demonstrate that SPS technology, when applied to sinter at the sub-solidus temperature of the γ' phase, effectively mitigates the formation of a prior particle boundary (PPB). Based on experimental and computational findings, it has been determined that the presence of elemental segregation and Al2O3 oxides on the surface of pre-alloyed powders leads to the preferential precipitation of MC-type carbides and Al2O3 and ZrO2 oxides in the sintering necks during the hot consolidation process, resulting in the formation of PPB. This study contributes to the understanding of microstructural modifications achieved through SPS technology, providing crucial information for optimizing sintering conditions and reducing the widespread occurrence of PPB, ultimately enhancing the material performance of PM nickel-based superalloys.

Effect of processing route on microstructure and tensile properties of hot isostatic pressed 30CrMnSiNi2A steel: Comparison of prealloy and master alloy routes

The prior particle boundaries (PPBs) are non-negligible powder metallurgy defect, which seriously weaken the mechanical properties of hot isostatic pressed (HIPed) steels. In present study, the master alloy route is proposed to improve the PPBs in HIPed 30CrMnSiNi2A steel, and the Fe-4.40Cr-4.80Mn-4.60Si-6.80Ni-1.00Mo-0.40V-1.12C master alloy system with low solidus temperature and wide solid-liquid temperature interval is designed. The microstructure, tensile properties and fracture behavior of HIPed compacts prepared by prealloy and master alloy routes are systematically compared. The results show that the structure in prealloyed sample is granular bainite (GB); and substantial Si oxides distributed along particle boundaries constitute serious PPBs. The yield strength (YS), ultimate tensile strength (UTS) and total elongation (TE) of the sample are 723 ± 10 MPa, 1238 ± 16 MPa and 12.3 ± 0.4%, respectively. In master alloy route, the formation of local liquid disrupts the distribution of Si oxides along particle boundaries and promotes the coarsening of oxides, which results in dispersed distribution of oxides and significant reduction in oxide density. In addition, since the structural uniformity depends on element diffusion, a relatively uniform structure is obtained through the optimization of powder size. The structure of optimized sample mainly consists of GB and martensite; and good tensile properties with YS of 798 ± 7 MPa, UTS of 1310 ± 8 MPa and TE of 15.3 ± 0.6% are obtained. The higher tensile properties are mainly due to the improvement of PPBs, which avoids the initiation of microcracks along PPBs and further avoids premature failure of material.

Effect of annealing treatment on the microstructure and mechanical properties of bulk nanostructured Ti/AZ61 composites prepared by hot pressing

Magnesium alloys, as the lightest structural alloys, provide the possibility of solving environmental problems by improving energy efficiency. Developing and maintaining nanocrystalline matrix is a huge challenge, and the key to achieving ultra-high strength. The nanocrystalline Ti/AZ61 composite prepared by hot pressing was annealed, and its microstructural evolution was characterized via SEM, XRD, and TEM, including the changes in magnesium matrix, Ti dispersions, and nano-scale precipitates. The density and mechanical properties of annealed composites were tested and analyzed. After annealing treatment at 623 K for 80 h, the average grain size of nanocrystalline Ti/AZ61 composites was 102 nm. 97% of Ti dispersions were submicron sized, and Ti3Al precipitates remained nanometer-sized. Ti dispersions and Ti3Al precipitates were uniformly distributed in the magnesium matrix. Compared with hot pressed composites, the crack that originated from the prior particle boundary (PPB) was restrained for annealed composites. The improved density and PPB bonding strength of annealed Ti/AZ61 composites had a positive impact on strength and plasticity. After annealing treatment for 40 h, the density increased from ∼2.041 g/cm3 to 2.066 g/cm3. The hardness, yield strength, and FFS were 146 HV, 493 MPa, and 5.6%, respectively, with corresponding increasing rates of 7%, 12%, and 65%, respectively.

Hot oscillatory pressure preparing low-cost FGH4096 superalloys with high performance

The low-cost and high-performance FGH4096 superalloy was prepared by replacing the high-cost fine powder particles (<51 μm) with coarse particles (>120 μm) using the hot oscillatory pressure (HOP) sintering method. The effects of powder particle size on the grain size and mechanical properties of the superalloy were investigated. The results showed that the tensile strength is 1350 MPa and the elongation is 30.4% for the HOP samples of coarse powder, the mechanical properties are higher than that of the samples prepared by the hot-press (HP) with finer powder. In addition, HOP sintering shows good inhibitory effect for the prior particle boundaries (PPB) in the powder metallurgy superalloys, for the HOP sintering providing the great driving force of particles plastic deformation. The results suggest that the coarse powder particle size with HOP sintering can achieve the better mechanical properties for the FGH4096 superalloys, thus provides a strategy for the low-cost and high-performance powder metallurgy superalloys.

Improvement in tensile strength of GH3536-TiB2 composites fabricated by powder metallurgy

To further improve the tensile strength of the GH3536 alloy (Hastelloy X), the GH3536-1 wt%TiB2 composites were prepared successfully by powder metallurgy. The results show that numerous M3B2 and TiC particles precipitated in the GH3536-TiB2 composite system. Interestingly, M3B2 particles were distributed along the grain boundaries in the composites, while TiC particles were in-situ synthesized along the prior particle boundaries of the spherical GH3536 powders. In addition, the grain size of the composite matrix was decreased to 6.7 μm compared with that of the GH3536 alloy (9.5 μm) since the grain growth being hindered by numerous intergranular phases, which further caused the increase of twin boundary proportion (from 26.5% in the GH3536 alloy to 38.8% in the composites). Consequently, the yield strength of the prepared composites was improved to 361 MPa and 298 MPa, respectively, from 240 MPa and 200 MPa, representing a roughly 50% improvement over the GH3536 alloy at 700 °C and 815 °C. The improvement in high-temperature strength is mainly ascribed to grain refinement strengthening, twin strengthening, hetero-deformation-induced hardening, and the additional grain boundary strengthening effect induced by secondary phases.

Characterization of microstructural and precipitates evolution in cold sprayed AA7050 after in situ laser heat treatment

This research systematically studied the processing-microstructure relationship in cold sprayed AA7050 after laser heat treatment with different levels of heat input (HI). The ability to use in situ laser heating to modify the microstructure and properties of cold sprayed AA7050 is an important goal for repair applications. In this research, a diode laser was used to heat treat the cold sprayed AA7050 in situ, without sectioning samples that are required in the conventional furnace heat treatment. Four AA7050 deposits were cold sprayed with identical spraying parameters and geometry. Three of these deposits were then subsequently heat treated by the laser source with the following linear heat input: 20 J/mm (low-HI), 40 J/mm (medium-HI) and 80 J/mm (high-HI). Electron backscattered diffraction showed that laser heat treatment coarsened the ultra-fine grains in the prior particle boundary regions in the deposit, whereas the larger grains in the prior particle center regions generally remained the same. Scanning transmission electron microscopy revealed precipitates nucleation in the deposits after the laser heat treatment, and the high-HI deposit possessed the largest density of precipitates. In the substrates, a reduction in hardness was observed in the sample with high-HI. The decreased hardness is primarily attributed to the decreased precipitate density. This study shows that with a precise control of HI, laser heat treatment could be a practical approach evolving the microstructure and precipitates in cold sprayed AA7050 deposits without over-aging the precipitates in the underlying substrate.

Initiation and Propagation of Localized Corrosion on Cold-Sprayed Aluminum Alloy 2024 and 7075

This paper investigates the initiation and propagation of localized corrosion on cold-sprayed (CS) aluminum alloy 2024 and 7075 deposits. CS deposits on AA2024-T351 and AA7075-T651 substrates were produced with commercially available AA2024 and AA7075 powders using helium as carrier gas. Electrochemical and immersion tests in 0.6 M NaCl and ASTM G110 test were used to evaluate the corrosion properties of CS AA2024 and AA7075 deposits. For analysis, optical microscopy, scanning electron microscopy, energy dispersive x-ray spectroscopy, and x-ray diffraction were utilized before and after the experiment. The results indicated that the initiation and propagation of localized corrosion highly depend on microstructural characteristics, such as the distribution and chemical composition of intermetallic networks with the matrix and propagated along these networks and prior particle boundaries, where an ultrafine grain structure is present. Accelerated localized corrosion tests generated deep penetration as well as a large area of defects owing to the coalescence of fissures.

Anisotropic microstructure and mechanical properties of as-forged (Ti, Nb)B/Ti2AlNb composites

In situ boride reinforced Ti2AlNb-based composites are a new type of lightweight material that has excellent high-temperature performance exceeding that of Ti-based composites. Forging is one of the effective methods to improve the microstructure and mechanical properties of Ti2AlNb-based composites. After forging, the mechanical properties of the composites exhibit evident anisotropy, but the anisotropic mechanism remains unclear. In this study, the anisotropic mechanical properties of the as-forged (Ti, Nb)B/Ti2AlNb composite and its formation mechanism were investigated. It was found that the as-forged composite possessed higher yield strength along the radial direction (RD) and better plasticity along the compression direction (CD). The microstructural analysis results demonstrated that the O phase formed [100]O//CD fiber texture, and the (Ti, Nb)B reinforcement exhibited [010](Ti, Nb)B//RD fiber texture in the as-forged composite. The analysis based on In-Grain Misorientation Axes (IGMA) suggested that the {1 1‾ 0}<110> was the main activated slip system of the O phase during the deformation. The difference in both the Schmid factor of {11‾ 0}<110> slip system caused by the [100]O//CD texture and the strengthening efficiency induced by [010](Ti, Nb)B//RD texture led to the anisotropy of yield strength. The anisotropy of prior particle boundaries (PPBs) contributed to the different crack propagation modes of the as-forged composite under different loading directions, resulting in plastic anisotropy.

Microstructure and corrosion resistance of chromate conversion coating on cold sprayed aluminum alloy 2024

This paper examines the microstructure and corrosion resistance of chromate conversion coating (CCC) on cold sprayed (CS) aluminum alloy 2024. CCC is an effective surface treatment for aluminum aircraft alloy to improve paint adhesion as well as corrosion resistance. Although the studies of CCC on wrought aluminum alloy have been extensively conducted, understanding the interaction of CCC applied to CS deposits has not yet been addressed in the literature. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and focused ion beam (FIB) were employed for investigating the microstructure of CCC on CS2024 deposits. Potentiodynamic testing, electrochemical impedance spectroscopy (EIS), and immersion testing were performed to probe into corrosion performance of CCC on CS deposit. The results demonstrated that unique intermetallic networks inside the CS deposit formed CCC with more heterogeneous microstructure compared to CCC on wrought counterparts. Although wrought and CS 2024 presented similar corrosion resistance within a short exposure time (<48 h), CCC on CS deposits was more susceptible to corrosion damage, revealed by EIS with an extended exposure time (>48 h). Additionally, distinctive corrosion propagation of CCC on CS deposits along intermetallic networks and prior particle boundaries was seen from the immersion test.

The optimization of Cu6Y sourced ODS-Cu: Effect of Y2O3 content

To refine the dispersive Y2O3 particles and to explore the optimum Y2O3 content, reducing the Y2O3 addition from 1.5 wt% to 0.5 wt% for Cu6Y sourced Cu-Y2O3 was investigated. It was found that reducing the Y2O3 addition had little influence on the number density, while reducing the size of the Y2O3 particles. Compared with the Y2O3 addition, the nucleation point is expected to be more important for the number density of the Y2O3 by the in-situ fabrication method. Moreover, reducing the Y2O3 addition indeed thinned the prior particle boundaries with O enrichment and contamination, thereby improving the mechanical performance and thermal conductivity. It was calculated that dispersion strengthening made great a contribution to the increment of the yield strength for ODS-Cu. While, the large particles with contamination on the prior particle boundaries induced the dimples and the premature micro-cracks, deteriorating the tensile performance. Therefore, the performance of Cu6Y sourced ODS-Cu can be improved once further reduces the introduction of contamination and suppresses the growth of MA powders during MA process.

A novel strategy for preparing high-performance powder metallurgical low alloy ultrahigh strength steel

In this study, a combined process of hot isostatic pressing and hot rolling (HIP-rolling) is developed to prepare high-performance powder metallurgical (PM) low alloy ultrahigh strength steel. The HIP-rolled 30CrMnSiNi2A steel achieves ultrahigh ultimate tensile strength of 1823.6 MPa, high elongation of 17.6% and superior impact energy of 29.4 J. The formation of lower bainite/martensite duplex structure, the refinement of prior austenite grains and blocks, and the disruption of prior particle boundaries are the critical factors to achieve the excellent strength-toughness balance. This study provides a novel insight for the development and application of PM low alloy ultrahigh strength steel.

Comparative Study of Prior Particle Boundaries and Their Influence on Grain Growth during Solution Treatment in a Novel Nickel-Based Powder Metallurgy Superalloy with/without Hot Extrusion

The prior particle boundaries (PPBs), as one of the typical defects in the nickel-based powder metallurgy superalloy, largely affect the microstructure and thus properties/performance of alloys. However, the effect of PPBs on the microstructure evolution in nickel-based powder metallurgy superalloy during heat treatment is still unclear. In this paper, a comparative study of PPBs and their influence on grain growth during solution treatment in a novel nickel-based powder metallurgy superalloy FGH4113A (i.e., WZ-A3 from Shenzhen Wedge, China) with/without hot extrusion (HEX) was conducted. Firstly, through a combination of scanning electron microscope (SEM), electron probe microanalyzer (EPMA) and transmission electron microscope (TEM) techniques, PPBs in FGH4113A alloys were characterized to be Al2O3, carbides (TiC, M6C, M23C6) and large-size γ′ particles. After HEX, the oxides broke, carbides deformed, and γ′ phase redistributed. After solution treatment at 950 °C, the TiC decomposed to M6C and M23C6, while no such decomposition occurred in FGH4113A alloys after solution treated at 1050 °C and 1150 °C. Secondly, the evolution of grain size in FGH4113A alloys was analyzed using the electron backscattered diffraction (EBSD) technique. At 950 °C, the decomposition of carbide TiC resulted in the increase of PPBs and the enhancement of their pinning effect on grain boundaries, thus inhibiting grain growth. At 1050 °C, the nucleation rate due to recrystallization is comparable to the grain growth rate, leading to the stable distribution of grain size. While at 1150 °C, the higher temperature can induce a higher content of PPBs. However, the driving force for grain growth surpassed the pinning force of PPBs, making the grains quickly coarsen. Finally, it was concluded that the HEX process is an effective method to modify the microstructure of powder metallurgy superalloy after HIP that can heavily refine the grains in the powder metallurgy superalloys. Furthermore, based on the present experiment and analysis, an appropriate solution treatment mechanism (i.e., 1050 °C for 2 h) was proposed for FGH4113A alloys.

Effect of Volume Fraction of Reinforcement on Microstructure and Mechanical Properties of In Situ (Ti, Nb)B/Ti2AlNb Composites with Tailored Three-Dimensional Network Architecture.

The mechanical properties of (Ti, Nb)B/Ti2AlNb composites were expected to improve further by utilizing spark plasma sintering (SPS) and inducing the novel three-dimensional network architecture. In this study, (Ti, Nb)B/Ti2AlNb composites with the novel architecture were successfully fabricated by ball milling the LaB6 and Ti2AlNb mixed powders and subsequent SPS consolidation. The influence of the (Ti, Nb)B content on the microstructure and mechanical properties of the composites was revealed by using the scanning electron microscope (SEM), transmission electron microscopy (TEM) and electronic universal testing machine. The microstructural characterization demonstrated that the boride crystallized into a B27 structure and the α2-precipitated amount increased with the (Ti, Nb)B increasing. When the (Ti, Nb)B content reached 4.9 vol%, both the α2 and reinforcement exhibited a continuous distribution along the prior particle boundaries (PPBs). The tensile test displayed that the tensile strength of the composites presented an increasing trend with the increasing (Ti, Nb)B content followed by a decreasing trend. The composite with a 3.2 vol% reinforcement had the optimal mechanical properties; the yield strengths of the composite at 25 and 650 °C were 998.3 and 774.9 MPa, showing an 11.8% and 9.2% improvement when compared with the Ti2AlNb-based alloy. Overall, (Ti, Nb)B possessed an excellent strengthening effect and inhibited the strength weakening of the PPBs area at high temperatures; the reinforcement content mainly affected the mechanical properties of the (Ti, Nb)B/Ti2AlNb composites by altering the α2-precipitated amount and the morphology of (Ti, Nb)B in the PPBs area. Both the continuous precipitation of the brittle α2 phase and the agglomeration of the (Ti, Nb)B reinforcement dramatically deteriorated the mechanical properties.

Microstructure and high-temperature tensile behavior of superalloy prepared by hot oscillatory pressing

Powder metallurgy (PM) superalloys have been widely used in hot-end components of turbine engines. However, the occurrence of prior particle boundaries (PPBs) during superalloys consolidation detriments its mechanical properties, and additional hot working is required to eliminate PPBs. In this article, a novel hot oscillatory pressing (HOP) method was proposed to prepare PM superalloys. Microstructure and high-temperature tensile behavior of HOPed superalloys were investigated using scanning electron microscopy (SEM), electron probe micro-analyzer (EPMA), electron backscatter diffraction (EBSD), and compared with those of superalloys fabricated by hot pressing (HP) and hot isostatic pressing (HIP). The result shows that the oscillatory pressure can effectively inhibit the formation of prior particle boundaries (PPBs). Compared to the HPed sample, the HOPed sample exhibited improved mechanical properties. The ultimate tensile strength and elongation of the HOPed sample at 700 °C are 1084 MPa and 19.5%, respectively, which is markedly better than those of the HPed sample and comparable to those of the sample prepared by hot isostatic pressing (HIP) at higher pressure and longer time. These findings suggest that HOP is likely a promising technique for making superalloys.