Coarsening mechanism of precipitations was investigated in a weld metal of Alloy 617 during long-term aging at 750 °C, and its effect on impact toughness was clarified distinctly. The needle-like M6C phases at the grain boundary nucleated and coarsened at 2000 h and then presented a stable size with aging to 8000 h. Spherical γ′ phase grew rapidly with the rate of 0.0121 nm/h when aged at 1000 h; then, its ripening rate (RR) reduced to 0.0033 nm/h at 8000 h and stabilized around it. The coarsening of M6C and γ′ was, respectively, controlled by interface diffusion and volume diffusion with the coarsening rate constant of 7.865 × 10−20 m2/s and 1.519 × 10−27 m3/s. Interaction of M6C and γ′ could facilitate their coarsening and cause dramatic decrease in toughness at the early stage. At aging to 8000 h and more, the lower RR of needle-like M6C phases and γ′ phases helped to form stable toughness at a later stage.

To improve the high-temperature properties of tool steel, a microstructure-dense and crack-free Fe5Cr5Co5SiTiNbMoW high-entropy alloy (HEA) coating was successfully fabricated by laser cladding. And its microstructure and hardness evolution after various annealing temperatures of 800 °C, 850 °C, 950 °C, and 1050 °C for 4 h were carefully investigated by OM, scanning electron microscope, energy dispersion spectrum, X-ray diffraction, and microhardness tester, respectively. The experimental results show that the HEAcoating was mainly composed of body-centered cubic and (Nb, Ti)C plus few Laves phase. The high-temperature annealing processing has little influence on the phase composition. The dendrites and matrix are decomposed with the annealing temperature increasing. While annealing at 950 °C, a eutectic microstructure appeared in the coating. Moreover, the thickness of the diffusion layer of HEA coating increased with the increasing of annealing temperatures. Surprisingly, the HEA coating after annealing at 850 °C possessed ultra-high average hardness, about 1050 HV0.2, huge improvement compared with as-cladding HEA coating (∼780 HV0.2). Therefore, it might reveal that the HEA coating exhibits excellent annealing strengthening ability.

The impact toughness and related microstructure of the dissimilar 9Cr/NiCrMoV welded joint fabricated by narrow-gap submerged arc welding were systematically investigated in the paper. Results indicated that the fracture appearance transition temperature (50% FATT) for weld metal was −11 °C, while low and scattered absorbed energies determined by different crack growth paths for the heat affected zone of 9Cr were gained which could not satisfy the requirement of service. However, a dramatically enhanced impact toughness was obtained by optimizing the post-weld heat treatment (PWHT) process. Microstructure characterization revealed that the microstructure evolution from martensitic laths in the previous PWHT metals to a softer ferrite matrix with the supersaturated carbon precipitating from the matrix led to higher toughness in the optimized PWHT materials. In addition, the observation of the fracture morphology found that the fractography varied from brittle fracture to a fracture mode with both brittle and ductile fracture feature with the change of crack growth paths in 9Cr-HAZ.

The purpose of this work is, based on CAFE method, to study the microstructure evolution and optimize the quality of the large-scale titanium slab ingot during EBCHM. The nucleation parameters of the microstructure simulation of titanium ingot are determined based on one of the actual experimental results. For the determined parameters, our theoretical results are agreement with other experimental results. The effects of pouring temperature and pulling speed on the microstructure are presented based on CAFE method. The quantitative analyses of the simulated results show that with the pulling speed increasing, the number of grains decreases, whereas the mean grain radius increases under identical thermal condition; with the pouring temperature increasing, the mean grain radius increases under the given pulling speed. Our results are very important to obtain the optimal structure of the ingots by controlling pulling speed and pouring temperature.

Isothermal compression testing of Ti–5.8Al–3Sn–5Zr–0.5Mo–1.0Nb–1.0Ta–0.4Si–0.2Er titanium alloy is performed on a Gleeble-3500 thermal simulator, and the corresponding microstructures are analyzed to clarify the softening mechanism and participates evolution. A constitutive equation compensated by strain has been established to describe the hot deformation behavior of the alloy. The deformation activation energies are calculated to be 369760.93–699310.86 J/mol in α + β two-phase region and 268030.03–325800.41 J/mol in β single-phase region. At a temperature of 880 °C, the main softening mechanism is the continuous dynamic recrystallization of lamellar α colony, controlled by the mechanical rotation of the sub-grain followed by dislocation climbing and annihilation by diffusion. Meanwhile, the dominant softening mechanism is the discontinuous dynamic recrystallization of β phase during the deformation at temperatures of 920 °C–1080 °C. Silicide containing Er with an average diameter of 20 nm is formed during the water quenching.

Hot deformation is an effective way to tackle major problems in powder metallurgy, i.e., inferior mechanical properties and low relative density. To characterize the hot deformation behavior of NiAl-based alloy manufactured by hot pressing sintering, the isothermal compression tests were performed in the deformation temperature range of 1100–1300 °C with strain rate of 0.001–1 s−1. The result indicates that calculated hot activation energy Q is 326.31 kJ/mol. The processing efficiency maps and instability maps of NiAl-based alloy were established to optimize deformation parameters on the basis of dynamic material model. They were validated through microstructure evolution. The microstructure observation revealed that fine grains, dislocation pile-up, cracks appear in high efficiency, low efficiency, and instability domains, respectively. According to effective processing window revealed by processing maps, hot forging of sintered billets was performed. The elevated temperature elongation increases from 17.86% to 74.87% after forging. The stripping feature is found on fracture surface after forging.

The deformation behaviors (flow behavior, power dissipation, dynamic recrystallization, and microstructure evolution) of a typical powder metallurgy nickel-based superalloy were investigated in compression tests at temperatures range of 1020–1140 °C and strain rates range of 0.001–1.0 s−1 with the true strains of 0.3, 0.5, and 0.7, respectively. The efficiency of power dissipation can be shown by the power dissipation maps at different true strains. The results showed that true strain had a great effect on the power dissipation. Besides, the deformed microstructures were investigated. The processes of microstructure evolution at different deformation temperatures and strain rates are different. The continuous dynamic recrystallization takes place at the deformation condition of 1080 °C/0.1 s−1. The fine and uniform dynamic recrystallized grains gradually replace the pre-existing grains with the increase of true strain. The discontinuous dynamic recrystallization takes place at the deformation condition of 1110 °C/0.001 s−1. The fine dynamic recrystallized grains grow up and a part of new fine grains appear in the dynamic recrystallized grains because of the periodic dynamic recrystallization.

Inconel 617 alloy (IN 617) is an important candidate material of advanced ultrasupercritical power unit above 700 °C. However, there are some issues in welding of IN 617 such as constitutional liquation and hot cracking. Tungsten inert gas (TIG) is considered as an effective welding method to join IN 617 because of low heat input and high quality. Investigation of the microstructure variation of TIG welded joint and its correlation with properties is helpful in deep understanding the stability and reliability of IN 617 welded joint. In this paper, the microstructure evolution and element segregation of IN 617 welded joint were investigated systematically. It is found that the base metal (BM) with significant banded structure is characterized by austenitic grains and some secondary phases distribute along the grain boundaries and inside the grains. The fine secondary phases are determined as M23C6 enriched with Cr and Mo elements. A few large polygon phases are identified as Ti(C, N) with a size of about 10 μm. The coarsened secondary phases are observed in the heat affected zone (HAZ) close to BM whilst the lamellar structure enriched with Cr and Mo is present along grain boundaries in the HAZ near the fusion line. The weld metal (WM) is fully austenitic with a dendritic structure and contains particles dispersing in the matrix. The element segregation on grain boundaries of IN 617 welded joint was analyzed by energy dispersive spectrometer. No obvious element segregation was observed in HAZ. In WM, the area in the vicinity of solidification grain boundaries and solidification subgrain boundaries (SSGBs) has a local depletion of Ni and Co while the Cr and Mo have no obvious segregation. Microhardness and high temperature tensile test of BM and WM were conducted. The WM has a little bit larger hardness value than BM and HAZ because of the strengthening effect of SSGBs. The fracture position is determined in the middle of WM, which is attributed to the grain boundary failure in the center of WM. The high temperature tensile properties of the welded joint are close to BM. In this investigation, the constitutional liquation in HAZ and solidification in WM have little effect on the high temperature tensile properties. TIG welding method is proved to be a suitable welding method to join IN 617.

The effect of high-energy electropulsing treatment (EPT) on the microstructure evolution, mechanical properties, and fracture behavior of as-treated Ti–6Al–4V alloy strips was investigated. EPT was found to accelerate phase transition and microstructure evolution of quasi-single-phase titanium alloy strips at a relatively low temperature, and obtain characteristic duplex microstructure and Widmanstatten microstructure. The EPT-induced microstructural changes increased elongation-to-failure remarkably with a slight decrease in tensile strength. Fracture surface observation and three-dimensional analysis showed that transition from small-shallow dimple colony to big-deep colony fracture took place with an increase in frequency of EPT. The rapid phase change of the Ti–6Al–4V alloy strip under EPT was attributed to the enhancement of nucleation rate and atomic diffusion resulting from the coupling of the thermal and athermal effects. It is supposed that EPT can provide a highly efficient method for the intermediate-softening annealing of titanium alloy sheet/strips.