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Grain-boundary (GB) precipitation has a significant adverse effect on plasticity of alloys, which easily leads to catastrophic intergranular failure in safety-critical applications under high external loading. Herein, we report a novel strategy that uses the local stress concentration induced by GB precipitates as a driving force to trigger phase transformation of preset non-equiatomic high-entropy solid-solution phase at GBs. This in situ deformation-induced phase transformation at GBs introduces a well-known effect: transformation-induced plasticity (TRIP), which enables an exceptional elongation to fracture (above 38 %) at a high strength (above 1.5 GPa) in a GB precipitation-hardened high-entropy alloy (HEA). The present strategy in terms of “local stress concentration-induced phase transformations at GBs” may provide a fundamental approach by taking advantage of (rather than avoiding) the GB precipitation to gain a superior combination of high strength and high ductility in HEAs.

The microstructure evolution and tensile properties of laser directed energy deposition (LDED) 300 M steel with isothermal heat treatment at 320 ℃ for different holding time were investigated in this paper. The results showed that the microstructure of the as-deposited LDED 300 M steel consisted of martensite and bainite. After heat treatment, the microstructure of the LDED 300 M steel is still the mixture of martensite, bainite and retained austenite, while the size of the lower bainite significantly decreased. As the increasing holding time, the content of martensite gradually decreased, while the content of bainite and austenite gradually increased. When the holding time of LDED 300 M steel increased from 20 min to 40 min, the number and size of austenite increased slightly. However, when the holding time was 60 min, the number and size significantly increased. As holding time continuously increase, it contributes little to the number and size of austenite. With regard to the hardness of the LDED 300 M steel, it was 464 HV when the holding time was 20 min. With the increment of holding time, the hardness gradually decreased until 428 HV when the holding time was 80 min. While holding time had less effect on the tensile strength and yield strength. To be specific, the tensile strength decreased from 1348 MPa to 1332 MPa, and yield strength decreased from 853 MPa to 759 MPa. The elongation gradually increased, and it increases to 33 % when the holding time was 80 min, which was higher than that of the forged 300 M steel standard (15 %). Effect of various factors, like microstructure, dislocation density and e-carbides precipitated in the bainite on the ductility of the LDED 300 M steel with isothermal heat treatment was also studied.

Laser remelting can be used not only to improve the metal surface structure, but also to repair metal surface defects. 34CrNiMo6 belongs to European standard high strength alloy structural steel. It has been widely used in the manufacture of large-size shaft parts which are prone to defects such as cracks and wear during service. This study proposed a repair method for these defects by laser remelting the filled block and then performed heat treatment. The microstructure, m9icrohardness and tensile strength of the repaired sample and heat-treated sample were analyzed using scanning electron microscope (SEM), Vickers hardness tester and universal tensile tester. The final results showed that the metallurgical bonding between the substrate and the filler was good. The microstructure of repaired sample could be divided into three regions: I, II, and III. The main microstructure in region I and II was tempered martensite, and in region III was bainite and martensite. Different regions of repaired sample presented uneven hardness distribution, and the fracture mode was quasi-cleavage fracture. However, after heat treatment, the microstructure of the sample was tempered martensite. Its hardness became uniform, and the fracture mode of the heat-treated sample was ductile fracture. The average hardness value of the heat-treated sample was 296.4HV0.2, which was slightly higher than the substrate. The tensile strength value of the heat-treated sample reached 972.91MPa, which was about 99.3 % of substrate, and its elongation δ was 14.25 %. Overall, the mechanical properties were close to those of the forging.

In order to achieve the microstructure homogeneity at a lower temperature and improve the mechanical properties, a novel post heat treatment was introduced to Inconel 718 fabricated by laser direct energy deposition (LDED), by means of δ aging treatment + solution treatment. The microstructure analysis shows that the fully precipitation of δ phase in LDED Inconel 718 superalloy can “cut” the long strip Laves phase into small pieces. According with the δ phase solution treated at 1020 °C for 30 min, only tiny particle Laves phase was remained. The equilibrium volume fraction of Laves phase after δ phase solution treatment is about 1%, which is much lower than that of the as-deposited sample or the sample only solution treated at 1020 °C. Due to the solution strengthening and precipitation of γ″ phase during aging treatment, the tensile strength of δ aging + δ solution + double aging treated samples was 10.27% higher than that of directed solution + double aging treated samples, as well as a 31.38% and 52.71% increases in elongation and the reduction of area during tensile tests.

The forming mechanism of anomalous eutectic structure (AES) in undercooled eutectic alloys has been a controversial issue. In this work, we conducted in situ observation of the AES forming process in an Ni-18.7 at.% Sn eutectic alloy using a high-temperature laser scanning confocal microscope. Our results provide an evidence of the AES formation induced by remelting of primary eutectic lamellae constituted by Ni3Sn and α-Ni lamellae. During recalescence, a pronounced remelting of the Ni3Sn phase into liquid occurs, while the α-Ni lamellae subject to a rapid morphological transition into granules. Eventually, the completion of solidification leads to the AES formation.

Uniaxial tensile tests and stress-controlled low-cycle fatigue (LCF) and creep–fatigue interaction (CFI) tests of Inconel 625 alloy manufactured by selective laser melting (SLM) were performed at 815 °C in air environments. The microstructure was characterized by optical microscopy and scanning electron microscopy after testing. The results confirmed that significant embrittlement and large scatter in LCF life are resulted from manufacturing defects. The CFI life is decreased sharply to approximately dozens of cycles with the accumulated creep strain; however, the selected dwell time (i.e., 60 s and 300 s) exhibits low sensitivity to the fracture time and elongation to failure. The embrittlement of SLM Inconel 625 was proposed to be due to the low grain uniformity and precipitation of carbides at the grain boundaries. Due to the quality of the SLM process, the accelerated initiation and propagation of fatigue crack are caused by the present unmelted powder particles, which result in the large dispersion of LCF life. Meanwhile, due to the accumulation of creep damage, cracks in the CFI test are initiated along the grain boundaries and then linked together, contributing to a significant decline in fatigue life.

In the process of laser-based direct energy deposition, the particle concentration distribution and geometric characteristics of powder flow play an important role in laser–powder interaction and powder utilization, and they affect the forming quality and accuracy. In the current study, based on the geometry information of a powder nozzle and the divergence angles of a powder jet at the nozzle outlet, the geometric profile of a powder stream is analyzed. A set of formulas for calculating the geometric characteristics of the powder stream is derived based on an analytic geometry method. The influence of each parameter on the geometric characteristics of the powder stream is further studied using single-factor and sensitivity analyses. Validation is performed by comparing the results from the presented analytical expressions with those from experiments and/or simulations in published papers. The analytical formulas provided in this paper are simple and practical, providing a theoretical foundation for the control of powder flow and related processes in the forming process.

AZ91D magnesium alloy matrix composites reinforced with nano SiC particles (n-SiCp) was fabricated by Friction Stir Processing, and the effects of n-SiCp on the β-Mg17Al12 phase precipitation at 180 °C as well as the age hardening response of AZ91D were investigated by X-ray diffraction, optical microscopy, scanning electron microscopy, transmission electron microscopy, and micro Vickers hardness measurements. The results show that, with the addition of n-SiCp, the microstructure of AZ91D after FSP was greatly refined, the texture strength was decreased, and the age hardening efficiency became inferior. As n-SiCp can efficiently block the migration of grain boundaries and grain growth, the composite exhibited a higher microstructure stability at elevated temperatures, and the discontinuous precipitation of β-Mg17Al12 phase that dominated the age process of AZ91D at 180 °C was remarkably restricted in the composite. Besides, n-SiCp promoted the continuous precipitation of β-Mg17Al12 phase and the precipitation rate in the composite was faster at the initial aging stage, but on further aging the precipitation rate became gradually slower than that in the AZ91D. The underlying mechanisms for the different age precipitation behavior and age hardening response were also discussed.

Phase change materials (PCMs) are highly promising for the digital-storage technology based on the fast phase transition between the crystal and amorphous states. In order to optimize the properties of PCMs, the doping of PCMs has become research hot points nowadays. In this paper, effects of Sn doping (0%, 10%, 30%) on the crystallization behavior of Ge2Sb2Te5 (GST) films induced by a picosecond pulsed laser with Gaussian energy profile were investigated experimentally. As compared to the previous study irradiated by a nanosecond pulsed laser, such factors as melting, ablation and crystallization thresholds dropped remarkably by a picosecond laser irradiation, and with the increase of Sn content those factors were further decreased. Due to the replacement of Ge atoms by Sn atoms, the Sn incorporation expanded the lattice constant so as to introduce the mechanical stress which, together with the thermal stress generated after laser irradiation, significantly affected the atomic movement during the crystallization process. High-resolution transmission electron microscopy (HRTEM) observations showed that GST with a lower Sn content of 10% tended to form sub-grain boundaries by local atomic shear to ensure long-range ordering of crystals, but with the increase of the Sn doping to 30%, the long-range ordering structure was broken, instead of some atoms arranged randomly, which were mainly Ge atoms, separating out from the lattice because of the replacement by Sn atoms. These extra Ge atoms were not involved in the crystallization process, proved by the Raman spectroscopy. The present study is fundamental for the design of advanced storage device based on PCMs.

In this paper, crystallization characteristics of amorphous Ge2Sb2Te5 (a-GST) films induced by a Gaussian picosecond laser with different laser fluence were carried out using transmission electron microscopy (TEM), Raman spectra and ab initio molecular dynamics (AIMD) simulations. TEM observations presented a solid-state phase transition with nanocrystalline microstructure at lower fluence, while a liquid-solid phase transition accompanied by an ingot-like crystalline microstructure was found at higher fluence, including a central coarse and an outer fine equiaxed regions as well as a columnar crystal region between them. In spite of the remarkable difference in microstructure, the composition of different regions kept constant from the Energy Dispersive Spectrometer (EDS) measurements. Different phase change behavior and laser fluence affected the shift of Raman peaks due to the phase change stress and thermal stress. A laser irradiation coupled AIMD simulation was then developed to study the preferential nucleation possibility. AIMD simulation results indicated that a thermal process in the central coarse equiaxed regions, with a higher cooling rate (10 k/ps) generated by the Gaussian picosecond laser irradiation, reduced the nucleation rate remarkably, causing crystal nucleus have enough space to grow up even to the micrometer scale. This is different from the morphology induced by nano- or femtosecond pulsed laser.

Because of the excellent mechanical properties of 34CrNiMo6 steel, it is widely used in high-value components. Many conventional approaches to strengthening-steels typically involve the loss of useful ductility. In this study, 34CrNiMo6 Steel having high strength and ductility is produced by laser solid forming (LSF) with a quenching-tempering (QT) treatment. Tempering of bainite is mainly by solid phase transformation in the previous LSF layers during the LSF process. The stable microstructure of LSF consists of ferrite and fine carbides. The microstructure transfers to tempered sorbite after heat-treatment. The tensile properties of the LSF steel meet those of the wrought standard. The UTS and elongation of LSF sample at 858 MPa, 19.2%, respectively, are greater than those of the wrought. The QT treatment enhanced the ultimate tensile strength and yield strength of the LSF sample. The ultimate tensile strength, yield strength, reduction in area, and elongation of the LSF+QT sample at 980 MPa, 916 MPa, 58.9%, and 13.9%, respectively, are greater than those of the wrought standard. The yield strength of the LSF+QT sample is approximately 1.27 times that of the wrought. The LSF samples failed in a ductile fracture mode, while the LSF+QT samples showed mixed-mode failure. The defects have only a small effect on the tensile properties owing to the excellent ductility of the LSF sample.

Long-striped Laves phases are considered to be crack sources in laser repaired Inconel 718 during stress rupture test. In order to obtain fine Laves phases, Nd:YAG pulse laser is used to fabricate Inconel 718 superalloy on a wrought Inconel 718 substrate in this study. The microstructures and stress rupture properties of pulse laser repaired Inconel 718 superalloy after DA treatment (720 °C × 8 h/furnace cooling at 50 °C/h to 620 °C × 8 h/air cooling) and 900STA treatment (900 °C × 20 min/air cooling+720 °C × 8 h/furnace cooling at 50 °C/h to 620 °C × 8 h/air cooling) are investigated by using SEM and TEM. The results shows that granular Laves phases can be obtained when pulse laser is used. The formation of granular Laves phase is related to high cooling rate (G·R) and low ratio of temperature gradient to growth rate (G/R). The critical values of G·R and G/R are 104 and 107, respectively. After 900STA treatment, δ phase precipitates around the granular Laves phases. But there is no δ phases existing in the DA samples. The surrounding δ phases can induce stress concentration, resulting in that the elongation of the 900STA samples is worse than that of the DA samples. However, the regions denuded in Nb element around the δ phases can delay the formation of the vacancies, which leads to that the stress rupture life of the 900STA samples is better than that of the DA samples.

This work investigated the microstructure and its influence on room-temperature fatigue crack growth (FCG) behavior of Inconel 718 (IN718) alloy fabricated with laser-based directed energy deposition (L-DED). As-deposited specimens were heat-treated by direct aging (DA), solution treatment + aging (SA) and homogenization + solution treatment + aging (HSA) respectively to investigate the effect of post-heat-treatment. On comparison with as-deposited microstructure, which mainly consists of γ dendrite and Laves phases in the interdendritic regions. Dendritic segregation still appears after DA heat-treatment, which is certified to alter the precipitation kinetics of γ′′/γ′ precipitations of IN718 and induce the formation of more abundant γ′′/γ′ precipitations in interdendritic Nb-rich regions versus the dendritic trunk regions. After SA heat-treatment, Nb released from the dissolution of Laves phase is prone to diffuse to the ambient interdendritic regions, which facilitates the formation of δ-phase and the homogeneous precipitations of γ′′/γ′ phases in dendritic trunk region. After HSA heat-treatment, the Laves phases are eliminated completely, the dispersive γ′′/γ′ precipitates (~30 nm) are distributed within the bimodal recrystallized grains. The results of FCG testing reveal that as-deposited specimen possesses the lowest threshold for fatigue-crack initiation (ΔKth) and the highest FCG rate (FCGR). The ΔKth and FCGR of DA specimen do not have the notable improvement versus those of as-deposited specimen, due to its inferior strength-plasticity mainly caused by heterogeneous γ′′/γ′ phases and Laves particles. The fatigue-crack propagates easily along the precipitate-free zone around short-acicular δ-phase, which causes the crack branching and deflection, enhances the resistance to FCG of SA specimen to some extent. The ΔKth of HSA specimen increases by ~2.6 times compared with as-deposited specimen and reaches 10.8 MPa∙m1/2, due to its decent strength-plasticity (~1130 MPa, ~26.5%). Especially, the ΔKth and FCGR of HSA specimen are comparable to that of the wrought IN718. The FCG behaviors of all specimens predominantly occur in the transgranular ductile fracture mode. Overall, the present study demonstrates that a judicious post-heat-treatment can enhance the damage tolerance of L-DED IN718 by increasing ΔKth and decreasing FCGR.

Phase-change materials (PCMs) have been widely used in optical and electronic applications for their extraordinary properties. However, the procedure of fast transition about PCMs is still not clear for the time limit. Pulse laser, especially ultra-short pulse laser, is an excellent tool to study the crystallization characteristics of PCMs for its high heating rate. In this paper, picosecond pulsed laser was employed to study the crystallization of amorphous Ge2Sb2Te5 (a-GST) thin films via the evolution of morphology and microstructure. The threshold for inducing crystallization was 7.9 mJ/cm2 and amorphization occured at 17.2 mJ/cm2. With the aid of high-resolution transmission electron microscopy (HRTEM), the growth of grains were observed to own a preferable growth direction of [100]. The grain size and laser fluence showed an exponential relationship under the ps laser pulse, and the maximal grain reached to ~5 μm when approaching the melting point. The morphology and grain distribution of GST film under 18.8 mJ/cm2 indicated a solid-state phase transition. Besides, the matrix of a-GST film changed into crystalline state even though the surface melted into amorphous state when irradiated by high laser pulse. Finally, the reason for fast transition of a-GST film under ultra-short laser pulse was discussed in terms of heating rate and temperature. The present study is fundamental for the understanding of the fast phase transition of PCMs.

Second-phase particle pinning has been well known as a mechanism impeding grain boundary (GB) migration, and thus, is documented as an efficient approach for stabilizing nanocrystalline (NC) materials at elevated temperatures. The pinning force exerted by interaction between small dispersed particles and GBs strongly depends on size and volume fraction of the particles. Since metallic oxides, e.g. Al2O3, exhibit great structural stability and high resistance against coarsening at high temperatures, they are expected as effective stabilizers for NC materials. In this work, NC composites consisting of NC Fe and Al2O3 nanoparticles with different amounts and sizes were prepared by high energy ball milling and annealed at various temperatures (Tann) for different time periods (tann). Microstructures of the ball milled and annealed samples were examined by X-ray diffraction and transmission electron microscopy. The results show that the addition of Al2O3 nanoparticles not only enhances the thermal stability of NC Fe grains but also reduces their coarsening rate at elevated temperatures, and reducing the particle size and/or increasing its amount enhance the stabilizing effect of the Al2O3 particles on the NC Fe grains.

The coarse columnar grains and special interface in laser solid formed (LSFed) Inconel 718 superalloy workpieces seriously affect their mechanical properties. To improve the microstructure and mechanical properties of LSFed Inconel 718 superalloy, electromagnetic stirring (EMS) was introduced to alter the solidification process of the molten pool during LSF. The results show that EMS could not completely eliminate the epitaxially growing columnar grains, however, the strong convection of liquid metals can effectively influence the solid–liquid interface growing mode. The segregation of alloying elements on the front of solid–liquid interface is inhibited and the degree of constitutional supercooling decreases correspondingly. Comparing the microstructures of samples formed under different process parameters, the size and amount of the γ+Laves eutectic phases formed in interdendritic area decrease along with the increasing magnetic field intensity, resulting in more uniformly distributed alloying elements. The residual stress distribution is proved to be more uniform, which is beneficial to the grain refinement after recrystallilzaiton. Mechanical properties testing results show an improvement of 100 MPa in tensile strength and 22% in elongation was obtained after EMS was used. The high cycle fatigue properties at room temperature was also improved from 4.09 × 104 cycles to 8.21 × 104 cycles for the as-deposited samples, and from 5.45 × 104 cycles to 12.73 × 104 cycles for the heat treated samples respectively.

Ni-Sn anomalous eutectic is obtained by single track laser cladding with the scanning velocity from 1 mm/s to 10 mm/s using the Ni-32.5 wt.%Sn eutectic powders. The microstructure of the cladding layer and the grain orientations of anomalous eutectic were investigated. It is found that the microstructure is transformed from primary α-Ni dendrites and the interdendritic (α-Ni + Ni3Sn) eutectic at the bottom of the cladding layer to α-Ni and β-Ni3Sn anomalous eutectic at the top of the cladding layer, whether for single layer or multilayer laser cladding. The EBSD maps and pole figures indicate that the spatially structure of α-Ni phase is discontinuous and the Ni3Sn phase is continuous in anomalous eutectic. The transformation from epitaxial growth columnar at bottom of cladding layer to free nucleation equiaxed at the top occurs, i.e., the columnar to equiaxed transition (CET) at the top of cladding layer during laser cladding processing leads to the generation of anomalous eutectic.

In this study, 20 mm thick AA7075-T6 alloy plates were joined by friction stir welding. The microstructure and mechanical properties of the nugget zone along the thickness direction from the top to the bottom was investigated. The results showed that the microstructure including the grain size, the degree of dynamic recrystallization, the misorientation angle distribution and the precipitation phase containing its size, type and content exhibited a gradient distribution along the thickness direction. The testing results of mechanical properties of the slices showed that the nugget was gradually weakened along the depth from the top to the bottom. The maximum ultimate tensile strength, yield strength and elongation of the slice in the nugget top-middle are obtained, which are 415 MPa, 255 MPa and 8.1%, respectively.

Nanocrystalline (NC) materials are expected to exhibit excellent irradiation resistant property because of their high volume fraction of grain boundaries. However, the NC materials are usually suspicious to grain coarsening at elevated temperatures. This strongly restricts their applications as practical irradiation resistant materials which are expected to serve at relatively high temperatures. In this work, the irradiation resistant property of a thermally stable NC Fe-1 at.% Zr alloy was examined. The NC alloy was prepared by ball milling method. The irradiation experiments were carried out in a Helium Ion Microscope by 28.4 keV He+ ions to 1 × 1017, 2 × 1017 and 4 × 1017 ions cm−2 at room temperature. The microstructures of the specimens were examined in a Transmission electron microscope. No He bubbles were observed in the irradiated NC Fe-1 at.% Zr alloy, whereas high number density of He bubbles were found to appear in the coarse grained Fe-1 at.% Zr alloy. The results indicate that the NC Fe-1 at.% Zr alloy exhibits an excellent irradiation resistance.

In this paper, creep-fatigue behaviors of the thin-walled tubular specimen with film holes were experimentally studied to address the influence of cyclic stress interruptions on creep strength and creep-fatigue life of a directionally solidified superalloy. The specimens with and without multiple holes were designed and manufactured to consider the effect of stress concentration produced by film cooling holes. The creep life of the specimen with holes has reduced significantly with the introduction of fatigue. An interesting result was found that the creep strength and life was enhanced on the stress concentrated specimens even if the creep loading was interruption by fatigue loadings, while creep fatigue life will be commonly shortened for notched specimen. Failure mechanisms related with cooling holes were analyzed on the basis of microscope observation.

The aim of this study was to elucidate cyclic hardening behavior and deformation mechanisms of the strain-hardened zones of friction-stir-welded (FSWed) dissimilar AA5083-to-AA2024 aluminum alloy joints (with a heterogeneous microstructure) after cyclic deformation via quaasi-in-situ electron backscatter diffraction (EBSD) technique and transmission electron microscopy (TEM) examinations. The extent of cyclic hardening was observed to decrease with increasing number of cycles at a constant total strain amplitude. Strain localization occurred mainly in the stir zone (SZ) and heat-affected zone (HAZ) on the AA5083 side, accompanied by an increasing density of dislocations during cyclic deformation identified via both EBSD and TEM. The strain hardening capacity of AA5083 SZ was significantly higher than that of AA5083 HAZ based on the density of geometrically-necessary dislocations (GNDs). The {111}-oriented grains exhibited a higher strain hardening capacity than other grains. Cracks initiated and propagated in the AA5083 HAZ along with the presence of shear bands. The evolution of GNDs played a predominant role in the AA5083 SZ, while the evolution of statistically-stored dislocations (SSDs) was dominant in the AA5083 HAZ during cyclic deformation.