Variant selection and texture evolution of 18%Ni M350 maraging steel produced by laser-based directed energy deposition (DED) additive manufacturing (AM) technology are investigated in this study. The effects of heat treatment and AM processing parameters, including energy area density (EAD), powder feed rate, and laser power, on variant selection and texture evolution are discussed. Theoretically, 24 martensite variants tend to be formed during a cubic-to-cubic transformation based on the Kurdjumov–Sachs orientation relationship. Identifying the preference of variants in a thermo-mechanical process can pave the way for tailoring the desired product texture. Grain graph and local neighbor voting algorithms using measured electron backscatter diffraction (EBSD) were employed to reconstruct the parent austenite grain structure in this study. Even though no evidence of global variant selection is found, the variant selection tends to take place locally in small parent grains of heat-treated samples. Strong rotated Copper 112110γ and Z 111110γ components are observed in the parent austenite phase of the low EAD sample after heat treatment due to a high degree of recrystallization and second-order twins. A Goss texture 110001α in child martensite transitioned from these strong components is also observed to be strongly derived from packet 3 of this sample. This finding indicates that some low-misorientation variants of packet 3 are more energetically favorable to develop in a heat-treated M350 alloy produced by a low EAD.

AlSi10Mg, as-built by laser powder bed fusion, includes performance-enhancing refined eutectic cells invariably paired with performance-impairing melt pool boundaries, where the cell network coarsens and breaks down. Eliminating melt pool boundaries (and their associated anisotropy) is a motivation for conventional heat treatments but at the cost of removing the AM-specific eutectic cell strengthening. This work evaluates the advantages and disadvantages of retaining the AlSi10Mg cell network (with melt pool boundaries) via direct aging, in contrast to a conventional (T6) approach, which relies primarily upon precipitation strengthening. We compare both heat treatment approaches and their impacts on microstructure, tensile properties, fracture, strain-hardening, and thermal stability. Direct aging achieves superior thermal stability and performance by preserving the as-built cell network and its exceptional strain-hardening capacity, despite the retained melt pool boundaries. As-built eutectic cells (which closely depend on build parameters and do not require post-processing) offer unique promise as potential enablers of voxel-by-voxel AM property control.

A novel high nitrogen hot-work die steel 3Cr5Mo2SiVN was manufactured by pressurized induction melting, possessing excellent hardness-strength-toughness balance under conventional heat treatment process. The results indicated that 3Cr5Mo2SiVN steel exhibited better hardness-toughness synergy compared with H13 steel. Meanwhile, its impact energy and elongation were improved by ca. 50% and 13% compared with H13 steel with similar hardness. The achievement of superior toughness of 3Cr5Mo2SiVN steel could be attributed to the following reasons: firstly, the higher fraction of operators 1 and 2, more uniform and refined close-packets and Bain groups as well as smaller effective grain size obtained a higher ratio of high angle grain boundaries, which resulted in the more changes of crack propagation direction and thus an increase of energy absorption. Secondly, the smaller size and number density of undissolved M(C, N) carbonitrides distributed along the prior austenite grain boundaries required greater tensile stress and then reduced grain boundary brittleness. For the strengthening mechanism, the precipitation strengthening increment was mainly provided by M(C, N) carbonitrides and M3C carbides for 3Cr5Mo2SiVN steel, and MC carbides for H13 steel, respectively. The smaller effective grain size and higher interstitial atom (C and N) content in the martensitic laths of 3Cr5Mo2SiVN steel were crucial factors in enhancing its strength.

This study explicates the microstructural evolution and mechanical response of the joints of 304 stainless steel developed through micro-deformation diffusion bonding. The diffusion-bonded interface comprised the refined grains embedded with the intergranular M23C6 carbides and complex oxides, which was induced by dynamic recrystallization at 950 °C. The refined grains persisted at the bonding temperature of 950 °C due to the dynamic equilibrium of dislocations and pinning effect of intergranular particles. As the bonding temperature was raised to 1000 °C, the combination-grow up of grains was initiated caused by sufficient thermal activation and complete dissolution of M23C6 carbides. The interface was entirely migrated at 1050 and 1100 °C, and notably, the migration of interfacial grain boundaries played a greater role in diffusion bonding with an increase of bonding temperature. The interface with refined grains permitted a high joint strength (or impact load), while was incompetent to enhancing plasticity and particularly impact toughness. The migrated interface, in contrast, was provided with exceptional plasticity and impact toughness, attributed to its enhanced resistance to crack propagation. The joint prepared at 1050 °C for 60 min exhibited the optimum combination of ultimate tensile strength, fracture elongation, and impact toughness, and the ductile fracture was observed passing through the substrate instead of along the interface. In addition, the comprehensive performance of joints was degraded when the grains were excessively coarsened at 1100 °C for 60 min.

In this study, the Computational Fluid Dynamics (CFD)-Finite Element Method (CFD-FEM) were used to simulate the transient stress response of the structure surface under thermal striping environment, and a uniaxial random cyclic loading (RCL) method was used to reveal the mechanical properties and initial microstructure evolution of 316H austenitic stainless steel (SS) under random stress induced by thermal striping. The experimental results showed that 316H exhibited a lower saturation ratcheting strain rate at 334.44 MPa, along with stronger cyclic hardening effects and resistance to plastic accumulation at loading rate of 100 Hz. Random loading tests with different cycles were performed at σpeak=334.44MPa and f=100Hz, and ratcheted specimens were characterized to investigate the effect of the micro-mechanism on cyclic softening/hardening. The geometrically necessary dislocation (GND) density increases and then decreases with cyclic loading, which is mainly attributed to the deformation twin (DT) and multi-dislocation slip mechanisms. The deformation mechanism during the ratcheting cycle from 1000 to 3000 cycles is mainly dominated by the twinning reduction, where the dislocation migration accelerated the transformation from dislocation walls to subgrain boundaries; the twinning increase dominates from 3000 to 9000 cycles, where the subgrain boundary grew into boundaries with high angle misorientations.

The accelerating effect of oxides on TiN formation to promote the nucleation of equiaxed ferrite was investigated under simulated gas tungsten arc (GTA) welding conditions. This revealed that the formation of oxides such as Ti2O3, (Ti, Al)2O3, and MgAl2O4 is essential for the solidification of equiaxed δ-ferrite. MgAl2O4 significantly accelerates the formation of TiN and produces a fine-grained equiaxed structure. Minor alloying with Ti, Al, and Mg significantly influenced the composition and crystal structure of the oxides. Ti2O3 and (Ti, Al)2O3 were identified as corundum structures with the same orientation relationship as TiN on the {0001} crystal plane. Ti2O3 and (Ti, Al)2O3 could act as heterogeneous nuclei for TiN because low lattice misfits of 0.6% and 0.6–8.1% could be reached for Ti2O3 and (Ti, Al)2O3 for {0001}oxide//{111}TiN and [10–10]oxide//[110]TiN, respectively. Spinel type MgAl2O4 was found to have a fully paralleled orientation relationship with TiN, resulting in a 4.8% misfit with TiN in each crystal plane. This indicates that all the crystal planes of MgAl2O4 could accelerate the formation of TiN. Baker-Nutting orientation relationship was confirmed for TiN and δ-ferrite. Therefore, the TiN formed by oxide acceleration could nucleate δ-ferrite. The multiple accelerating planes of MgAl2O4 and a high quantity of MgAl2O4-TiN as nucleation sites showed an optimal effect on equiaxed solidification under a high-temperature gradient.

To meet the increasing requirement of high-temperature die attach for power module on the excellent thermal conductivity and reliability, a novel Ag paste modified with AlN nanoparticles was designed. The thermal conductivity of the joint sintered by this modified Ag paste during long-term thermal aging was tested. It was found that with 1 wt% addition of AlN nanoparticles, the thermal conductivity of the joint was increased from 164.3 to 273.0 W/(K·m) due to the significant decrement of porosity. But the thermal conductivity of sintered joints was decreased slightly with the excessive addition of AlN nanoparticles. Meanwhile, the increasing porosity and pore size were observed in all joints during the following thermal aging and hence the thermal conductivity was decreased. But the pore growth in the joint sintered by Ag paste consisting of 1.0 wt% AlN nanoparticles was quite limited, which still had a high thermal conductivity of 249.4 W/(K·m) after 1008 h thermal aging, showing great thermal conductivity and reliability.

The carbide formation and evolution of a new Ni-26 W-6Cr (wt%) superalloy for the 800 °C molten salt reactor (MSR) was investigated by homogenization heat treatment tests and the effect of carbide characteristics on the plastic deformation behavior was further investigated by isothermal simulated tensile tests. The plastic deformation properties of Ni-26 W-6Cr superalloys are associated with the morphological characteristics and distribution of carbides. During homogenization at 1220 °C, a unique phenomenon was discovered, that is, the precipitation of an acicular carbide. The XRD, TEM and EBSD results show that this acicular carbide is distributed along the particular crystal plane of the γ matrix. Based on crystallographic theory, there is no obvious orientation relationship between the carbide and the γ matrix, which is prone to cracking along the carbide/γ interface during deformation. After homogenization at 1220 °C for 48 h, the acicular M6C carbides dissolved and the discrete granular M6C carbides distributed diffusely at the grain boundaries can relieve stress concentration and strengthen the grain boundaries, reducing the deformation resistance of the alloy, effectively inhibiting grain boundary slip and thus inhibiting crack propagation, and improving the tensile strength and plasticity of the material.

The segregation of Bi phase during current stressing deteriorates the mechanical property of Sn-58Bi solder joint. To improve the reliability of Sn-58Bi solder joint, a thorough understanding of the diffusion behavior of Bi elements is necessary. In this study, a Cu/SnBi/Sn-xCu/SnBi/Cu multi-layer solder joint structure was adopted to reveal the diffusion path of Bi element and the effect of Cu on the diffusion behavior of Bi during electromigration. The result showed that the Bi element diffused along the grain boundary. The presence of Cu had two competing mechanisms on the diffusion of Bi: it slowed the process by forming Cu6Sn5 intermetallic compounds that blocked and prolonged the diffusion path, yet also accelerated it by refining the Sn and creating more diffusion channels. A data processing method was used to estimate the diffusion rate of Bi and determine the dominant mechanism. The results indicated that the diffusion rate was accelerated by Cu doping, confirming that the dominant mechanism was the refining effect, which promoted the diffusion of Bi element.

Eutectic high entropy alloys (EHEAs) have garnered much research attention due to their excellent mechanical properties, superior castability, good oxidation resistance, and stable microstructure in a wide range of temperatures. Inspires from mature Ni and Co-based superalloys, a novel Al20Co36Cr4Fe4Ni36 EHEA was designed and prepared to obtain good mechanical properties at both room temperature and high temperatures in this study. The novel EHEA consists of a L12 phase and a B2 phase with a superior ultra-tensile strength of 1005 ± 40 MPa and a ductility of 8.8 ± 0.4% at room temperature. The engineering compressive stress–strain curves at 1073 K indicated that the current EHEA performs a high strength of 321 ± 9 MPa without fracture even at a high temperature of ~0.6 TE, which is even superior to some refractory high entropy alloys (RHEAs). The hot deformation behavior and microstructure evolution of the new EHEA was investigated at the temperature range of 1073 K to 1273 K and various strain rates from 10−3 s−1 to 10−1 s−1. The constitutive equation that describes the correlations between the flow stress, strain rate, and temperatures was constructed. Continuous dynamic recrystallization (CDRX) was the primary dynamic recrystallization (DRX) mechanism in both the L12 and B2 phases during hot deformation. However, the proportion of dynamic recrystallization grains in the L12 phase is higher than that in the B2 phase according to the high angle grain boundaries (HAGBs) under the same deformation condition, suggesting that the CDRX kinetics was faster and stronger in the L12 phase. The present study provided a new thought for designing EHEAs and an insight into the hot deformation behavior of EHEAs with lamellar morphologies.

In this paper, strain-controlled non-proportional creep-fatigue tests under three non-proportional loading paths were performed for type 304 stainless steel. The strain holding period at the peak axial or peak shear strains was introduced to investigate the effect of the holding position on the creep damage accumulation. Compared with the other non-proportional loading paths, the circle loading path generated the most detrimental effect on the material life. In addition, the introduction of the holding period reduced the material life, in which axial holding resulted in a greater creep damage than that of the shear holding. A post-examination method through the electron backscattered diffraction (EBSD) observations was conducted to reveal the damage mechanisms under various loading paths. The variations of six EBSD-based disorientation parameters showed the significant effect of the loading path and holding position on the damage mechanisms. Subsequently, a multiaxial damage factor (MDF)-involved strain energy density exhaustion (SEDE) model and a non-proportional energy parameter (NPEP)-involved Ostergren model were collaborated to provide a precise prediction within a factor of 1.5 for non-proportional creep-fatigue life distributions.

Deformation-based friction rolling additive manufacturing (FRAM) is a promising solid-state additive manufacturing (AM) approach for producing non-fusible, weldable AlLi alloys. Along with the thermal cycles of conventional AM methods, the materials deposited during FRAM are affected by the repeated force of a tool head. However, the effect of repeated thermal–mechanical cycles on the microstructure and mechanical properties of a deposited material remains unclear. This study attempts to develop mechanisms by characterizing the microstructure and local mechanical properties of FRAM-ed AlLi alloys, with different deposition heights and those that underwent a different number of thermal–mechanical cycles. A recrystallized microstructure is successfully obtained and fine equiaxed grains are observed in the deposited material without pores and cracks. The value of ultimate tensile strength (UTS) reaches 98–91% of the base metal from the top to bottom regions. Under repeated thermal–mechanical cycles (from the top to bottom region), the grain size increases from 2.0 μm to 6.5 μm. The texture strength and diameter of δ’ precipitate also increase, while the volume fraction decreases. Together, they lead to a decrease in local UTS and elongation, from 450.7 MPa to 415.1 MPa and from 10.4% to 6.3%, respectively. The relationship between thermal–mechanical cycles and microstructure established in this study provides the foundation for subsequent research on FRAM thermal balance and microstructure regulation.

This study investigated the synergistic effect of transpassive film and γ″ phase on the electrochemical dissolution behavior of Inconel 718 manufactured by laser directed energy deposition. The investigation was based on samples after homogenization and aging with (γ + γ″) phases and homogenization with γ phase. The compositions of the transpassive films for two kinds of heat-treated samples were similar, mainly comprising porous Ni hydroxides. The incompact transpassive film induced the local preferential dissolution of the γ matrix and generated multiple dispersive pits on the surface. Compared with the sample after homogenization, the spatial distribution of the γ″ phase in the sample after homogenization and aging restricted the expansion of pits in the γ matrix, leading to superior surface quality. The detachment of the γ″ phase during electrochemical dissolution provided extra mass loss and higher current efficiency. A qualitative model was proposed to demonstrate the synergistic effect of the transpassive film and γ″ phase on the dynamic electrochemical dissolution process.

Multi-principal element alloys (MPEAs), as a novel alloy design idea, have been proved to be promising materials and gained increasing attention recently. In this study, the single-phase (CoFe)50Si50 multi-principal element intermetallics (MPEIs) was synthesized and subject to rapid solidification via the melt fluxing technique. A maximum undercooling of 314 K was achieved and the microstructure of as-solidified samples with different undercooling was characterized. An ordered B2 phase was found to dominate the phase constitution despite the formation of minor Fe-rich BCC metastable phase in highly undercooled samples. The microstructure of undercooled (CoFe)50Si50 MPEIs shows a clear morphological evolution from dendrites to equiaxed grains and then to refined dendritic seaweed-like morphology with undercooling. At low undercooling, spontaneous grain refinement can be described by the dendrite fragmentation and dendrite remelting mechanisms. At high undercooling, detailed EBSD analysis reveals that the formation of subgrains and subsequent recrystallization play a significant role, but the inherent characteristics of (CoFe)50Si50 MPEIs (such as high stacking fault energy) prevent the occurrence of complete recrystallization and hence complete grain refinement. This study could provide a reference for controlling the non-equilibrium microstructure of MPEIs and understanding the microstructure evolution from binary intermetallic compounds to MPEIs.

In this paper, the effects of cooling rate after casting and stirring process on γ’ phase, γ / γ’ eutectic phase, MC type carbide morphology and grain structure of K447A alloy turbine blisk were studied systematically. The quantitative relationships between cooling rate, stirring process and γ’ phase, γ / γ’ eutectic phase, MC type carbide and grain size were described in detail. The results show that with the increase of cooling rate, the distribution region of γ / γ ‘eutectic phase from the center of blisk to blade decreases gradually and MC type carbide changes from skeleton to block due to the shorter formation time of eutectic and carbide in the later stage of solidification. Besides, with the decrease of cooling rate, the size of γ’ phase increases under the influence of interfacial strain energy and elastic strain energy, and the morphology of γ’ phase gradually evolves from cuboid to concave cuboid, octet and dendritic. At the same time, due to the synergistic effect of cooling rate and stirring process, the blade part is columnar grain and the central part of the blisk is equiaxed grains. In this paper, the evolution mechanism of γ ‘phase, γ / γ’ eutectic phase, MC type carbide and grain size with cooling rate and stirring process was proposed. The research results would play a key supporting role in precise control during solidification for microstructures and properties of dual-performance superalloy blisk.

Nimonic 263 superalloy is primarily being used in gas turbines, high-pressure pipelines, steam heaters, etc. due to its capability to withstand high temperatures and pressure for a longer duration. In this work, an experimental investigation on Nimonic 263 alloy has been carried out to improve its mechanical and microstructural properties using constrained groove pressing (CGP) at various temperatures. CGPed specimens have been characterized through mechanical tests (tensile test and hardness test) and microstructural studies (XRD, SEM fractography, and optical microscope). The results demonstrated that the CGP process had a significant impact on the alloy's properties. Compared to the as-received material, the CGPed specimens exhibited substantial improvements in yield strength (YS), ultimate tensile strength (UTS), and micro hardness, with average values of 234.58%, 43.79%, and 110.31%, respectively. The average grain size gets reduced due to CGP from 220.70 μm for the as-received material to 106.02 μm, 117.09 μm and 94.01 μm for CGP at room temperature, 250o C, and 450o C, respectively. An increase in the peak intensity is observed in the XRD after the CGP process due to a decrease in crystal size and an improvement in dislocation density with an average value of 31.68%. Fractography studies have revealed a mixed-mode (ductile and brittle) failure for the CGPed specimens compared to the ductile failure mode in the as-received material. Using Abaqus 6.14 software, the finite element analysis of the CGP process has been performed and the results have been found in good agreement with the analytical results. Thus, the CGP process can effectively improve the mechanical and microstructural properties of Nimonic 263 alloy for high-temperature applications.

During the laser melting deposition (LMD) additive manufacturing process, titanium alloys undergo rapid solidification, which induce grain growth into coarse columnar grains, resulting in lower mechanical properties of the material than conventional forgings. This study proposes the incorporation of hydrogen into titanium alloys to refine columnar grains, reduce deformation resistance, and improve machinability. In this work, the hot deformation behavior of hydrogenated Ti6Al4V alloy was analyzed via high-temperature compression experiment. The experimental results revealed the nucleated grains generated by the intense deformation and the twins brought by the hydrogen element led to massive dynamic recrystallizations, which effectively improved the hot workability. When the alloys are deformed above phase transition temperature, the flow stress is positive related with H content. Meanwhile, when the alloys are deformed below the phase transition temperature, the flow stress initially experiences a decrease with an increase in hydrogen content. Adding only 0.27 wt% H can reduce the flow stress by 34.4%–54%, but when the H content continues to increase, the flow stress still increases subsequently. Moreover, when the strain rate is constant, the flow stress of Ti6Al4V alloy can decrease by about 80% with an increase in deformation temperature from 820 °C to 940 °C. When the deformation temperature is constant, the flow stress of Ti6Al4V alloy with different hydrogen contents can decrease by around 65% as the strain rate reduces from 10s−1 to 0.01 s−1.

The contemporary investigation focusses on the development of a welding technique to join AA5083 alloy plates using equimolar medium entropy alloy (4E-MEA - Fe0.25Al0.25Ni0.25Cu0.25) and high entropy alloy (5E-HEA - Fe0.2Al0.2Ni0.2Cu0.2Ti0.2) filler wires. A man-made combined cable wire method, was chosen to prepare the filler wires and the gas tungsten arc welding was used to conjoin the AA5083 plates. This study evaluated the metallurgical characteristics and mechanical performance of welded joints using 4E-MEA and 5E-HEA filler weldments and compared them with the standard ER5356 filler weldment. XRD analysis showed that both 4E-MEA and 5E-HEA weldments had evolved BCC and FCC phases, whereas microstructural analysis revealed a fine grain distribution without intermetallic phases. SEM analysis of the weldments revealed a heterogeneous microstructure (equiaxed and columnar grains) owing to the influence of the constituent elements. Solid solution formation, structural refinement, and high configurational entropy effects promoted grain refinement in the weldments as evidenced by EBSD analyses. The microhardness of 4E-MEA and 5E-HEA weldments increased by 18.3% and19.89%, respectively, compared with the standard filler weldment, while the tensile strength of 4E-MEA and 5E-HEA weldments increased by 24.3% and 27.9%, respectively and also obtained work-hardening rates were higher. It was observed that the heterogeneous microstructure in the weldment led to complicated plastic deformation and improved the mechanical responses. The presence of Fe and Ni increased ductility and inhibited the formation of intermetallic. The fractography results showed that the 4E-MEA and 5E-HEA filler weldments exhibited a combination of ductile and brittle fractures. Overall, the researchers demonstrated that the developed equimolar 4E-MEA and 5E-HEA filler wires could produce weldments with improved mechanical properties and can be utilized in automobile and shipbuilding applications.

The stability of the metastable β'-Cu4Ti phase is a crucial factor for the electrical conductivity of CuTi alloys. Determining the factors that influence the stability of the β'-Cu4Ti phase and the relationship between the β'-Cu4Ti phase and the electrical conductivity of CuTi alloys is a prerequisite to improve their electrical conductivity. In this study, the stabilities of the β'-Cu4Ti and β-Cu4Ti phases were investigated by first-principles calculations combined with experimental characterization. When the Ti content of the β'-Cu4Ti phase is greater than or equal to 19 at.%, its stability is lower than that of the β-Cu4Ti phase, resulting in instability of the β'-Cu4Ti phase and dissolution of Ti atoms back into the Cu matrix. This is the main reason why it is difficult to improve the electrical conductivity of CuTi alloys when the β'-Cu4Ti phase is the main strengthening precipitate.

Joining ceramics and metals is important to expand their application scope in the nuclear industry. The brazing of SiC and Mo was conducted using a Nb0.74CoCrFeNi2 eutectic high-entropy alloy filler. The microstructure and mechanical properties of the joints were examined at different brazing temperatures. As the temperature increased, the eutectic structure gradually disappeared, and a layered structure was formed at the brazed joint. An increase in the MoNi phase content can improve joint strength owing to differences in the thermal expansion coefficient and lattice mismatch between phases. The joints exhibited a maximum shear strength of 62 MPa at 1300 °C. Finite element analysis results demonstrated that the presence of a Cr0.46Mo0.4Si0.14 phase causes the concentration of residual stress. Brittle fracture occurred mainly in the Cr0.46Mo0.4Si0.14 phase, causing the rupture of the joint. This study provides valuable insights into the use of eutectic high-entropy alloys as fillers to establish a strong and reliable connection between ceramics and metals.