The powder-bed laser additive manufacturing (AM) process is widely used in the fabrication of three-dimensional metallic parts with intricate structures, where kinetically controlled diffusion and microstructure ripening can be hindered by fast melting and rapid solidification. Therefore, the microstructure and physical properties of parts made by this process will be significantly different from their counterparts produced by conventional methods. This work investigates the microstructure evolution for an AM fabricated AlSi10Mg part from its nonequilibrium state toward equilibrium state. Special attention is placed on silicon dissolution, precipitate formation, collapsing of a divorced eutectic cellular structure, and microstructure ripening in the thermal annealing process. These events alter the size, morphology, length scale, and distribution of the beta silicon phase in the primary aluminum, and changes associated with elastic properties and microhardness are reported. The relationship between residual stress and silicon dissolution due to changes in lattice spacing is also investigated and discussed.

A systematic investigation of influence of the laser hatch style on densification behavior, microstructure, and tribological performance of aluminum parts’ preparation by selective laser melting (SLM) was implemented in this study. The scans with checker board (CB) style left better processing quality of surface morphology and few metallurgical defects to SLM parts in comparison with single fill and cross fill styles, hence leading to a relatively high densification level (99.42%). The CB style of shorter scan length left higher undercooling degree in small checker areas compared with other longer scan lengths, leading to finer equiaxed grains to the solidification microstructure. Accordingly, an enhanced mean microhardness of 129.7 HV0.1 was obtained in this hatch style, due to the grain refinement strengthening effect. The lowest coefficient of friction of 0.49 and wear rate of 2.43 × 10−4 mm3/(N m) were obtained. The improved densification level and formation of refined equiaxed grain and evenly distributed ring-shaped Si particles formed in CB parts changed the mechanism of material removal during sliding from the abrasion to adhesion of the tribolayer, significantly improving the wear resistance of SLM aluminum parts.

To obtain a fine-grained Mg matrix, the (submicron + micron) bimodal size SiC particle reinforced AZ91 (SiCp/AZ91) composite was subjected to forging followed by the extrusion process first. Then, the fine-grained bimodal size SiCp/AZ91 composite was compressed at 270–370 °C with 0.1–0.001 s−1. The result indicated that the refinement of the Mg matrix contributed to its deteriorated strength at high temperature. However, the grain size is not the only factor influencing flow stress but the SiCp also plays an important role. The effect of SiCp on the fine grained Mg matrix depends on grain size and dislocation density, both of which strongly depend on temperature and strain rate. As compared with the fine grained Mg matrix reinforced by single size SiCp, the one with bimodal size SiCp unusually exhibit lower flow stress during hot compression. The calculated activation energy of the bimodal size SiCp/AZ91 composite is higher than the micron SiCp/AZ91 composite; however, nearly the same as the submicron SiCp/AZ91 composite, and the deformation of which was thought to be controlled by ∼1 vol% submicron SiCp.

Boron nitride nanotubes (BNNTs) have been utilized to strengthen various engineering materials especially metal matrix composites thanks to their extraordinary high tensile strength, elastic modulus, and failure strain. In this paper, single- and multi-walled BNNTs were therefore used to combine with aluminum (Al) metal matrix. Mechanical characteristics and deformation mechanism of nanocomposites reinforced with long (continuous) and short (discontinuous) BNNTs were then investigated for different loadings including uniaxial tension and compression and different boundary conditions based on molecular dynamics simulations. It was found that long BNNTs remarkably improved tensile mechanical properties of the matrix and effectively enhanced elastic modulus and strength of the nanocomposites by 82% and 79.4%, respectively. They could provide effective barriers to propagation path of dislocations formed inside the matrix. Diameter and wall number of the reinforcement did not leave considerable impacts on the nanocomposite behavior while its atomic fraction remarkably influenced the material response.

The three-dimensional microstructures of two conventional 316L stainless steels and a grain boundary (GB)-engineered version of the same steel have been characterized by using serial sectioning and electron backscatter diffraction mapping. The morphologies, area fractions, and number fractions of twin boundaries (TBs) were measured and compared, and the random boundary connectivity was evaluated. Although two-dimensional observations suggest that TBs are planar, occluded twin-grains and tunnel-shaped TBs were also observed. In addition, some large and morphologically complex TBs were observed in the GB-engineered sample, and these TBs were responsible for the increase in the twin area fraction that has been reported in past studies. While GB engineering increased the boundary area fraction, the TB number fraction was almost unchanged. Because the GB engineering process changed only the area fraction and not the number fraction, the connectivity of random boundaries was not disrupted.

Strain path changes during clock rolling cause more serious interaction between adjacent grains, resulting in the occurrence of interactive regions (IRs) with random orientations. Furthermore, plenty of new grains with relatively random orientations are introduced by the subsequent annealing of these IRs. The morphology of the IR and the origin of random orientations were therefore investigated in this study, and the electron backscatter diffraction technique was used to characterize crystallographic orientations of nuclei and deformed matrices. A short-time annealing was imposed on a specimen to catch the transient nucleation behaviors. The results indicate that the orientations of nuclei are similar to their surrounding deformed matrices, especially the points with larger local-misorientation. Additionally, the shape of new grains depends on where it forms, and it is suggested that this fact mainly results from the great difference in stored energies between deformed matrices with {111} and {100} orientations.

The microstructure evolution of high nitrogen austenitic steel wires under various annealing times and drawing temperatures was carefully characterized. Special attention was paid to the widely distributed twins and the nanoprecipitates at twin boundaries (TBs) in high nitrogen stainless steels (HNSSs). The results of microhardness indicated that the traditional Hall–Petch (H–P) equation, which only took the role of grain boundaries into account, was unsuitable. A new H–P equation that connected grain size, twin density, precipitates at TBs, and microhardness in HNSS was established for the first time and showed to be in good agreement with the experimental results. By analyzing the strained regions near TBs, a model describing the precipitation of nano-M23C6 carbides on coherent twin boundaries and incoherent twin boundaries was proposed. In addition, the influence mechanism of the nano-M23C6 at TBs on microhardness was discussed.

Al–12.6Si was annealed at both 500 and 560 °C for different lengths of time in this study. Additionally, the effects of annealing treatment on the spheroidization of eutectic Si and the mechanical properties of the Al–Si alloy have been investigated. The morphology of these particles was described using surface shape factor (φ), and it was found that the optimal annealing time of Al–12.6Si at 500 and 560 °C is seven hours and five hours, respectively. The average size of the Si particles in the Al–Si alloy annealed at 500 °C is less than that of the particles at 560 °C. The roundness of the Si particles within the Al–Si alloy annealed at 500 °C is slightly better than that at 560 °C. The elongation of the alloy apparently increases, while the tensile strength of the Al–Si alloy decreases. The tensile strength and elongation of the eutectic Al–Si alloy annealed at 500 °C is higher than that at 560 °C.

An Al–3% B master alloy has been subjected to equal channel angular pressing (ECAP). The grain refining performance and fading resistance of an Al–3% B master alloy on a commercial purity Al (CPA) have been evaluated. The effect of the number of ECAP passes on the size and the distribution of the AlB2 particles, the grain size of CPA ingots with and without adding the Al–3% B master alloy subjected to ECAP have been investigated. The mean size of AlB2 particles was significantly reduced from ∼34 to ∼12 μm after four ECAP passes. Fine blocky AlB2 particles were uniformly distributed in the Al matrix. It has been revealed that when it was inoculated by the Al–B master alloy subjected to ECAP, the grain size of α-Al was decreased from ∼1200 to ∼180 μm after four ECAP passes, beyond that, the grain size tends to be saturated. It has been proved that grain refinement efficiency and fading resistance of the Al–3% B master alloy subjected to ECAP in CPA ingots was enhanced.

AZ91 magnesium plates with a thickness of 6 mm were subjected to one- and two-pass friction stir processing (FSP). Microstructures and mechanical properties of the experimental materials were investigated. The results show that FSP can significantly refine the microstructures of magnesium alloys, and two-pass FSP can prepare slightly finer grains in comparison with one-pass FSP. Some coarse β-Mg17Al12 phases existed in the first pass FSP break and dissolve into the matrix under the action of the second pass FSP. Microhardness distribution of the two-pass FSP AZ91 alloy exhibits no too much difference with that of the one-pass FSP AZ91 alloy. Due to further finer microstructures, the tensile properties of the two-pass FSP alloy are slightly higher than those of the one-pass FSP alloy. Both FSP AZ91 alloys show typical ductile fracture characteristics, while the dimples on the two-pass FSP specimen are much deeper and increase in quantity.

The microstructure evolution and mechanical properties of Mg–10Gd–3Y–xZn–0.6Zr (x = 0.5, 1, and 1.5 wt%) alloys in the as-cast, solution-treated, and peak-aged conditions have been investigated systematically. The results indicate that the microstructure of the as-cast alloy with 0.5% Zn consists of α-Mg, (Mg,Zn)3RE and Mg24(RE,Zn)5 phases, while the alloy with 1.0 and 1.5% Zn consists of α-Mg, (Mg,Zn)3RE and some stacking faults. Moreover, 18R-LPSO phases are observed in the as-cast alloy with 1.5% Zn. The formation of LPSO phases involves not only stacking sequence ordered but also chemical composition ordered. After solution treatment, the Mg24(RE,Zn)5, (Mg,Zn)3RE, stacking faults, and 18R-LPSO phases transform into 14H-LPSO phases. The 14H-LPSO phase plays an important role in the improvement of mechanical properties, especially for the ductility. The β′ phase with a bco structure precipitates in the peak-aged alloys results in precipitation hardening, significantly improving the tensile strength, but it leads to poor ductility.

As a new functional composite material, functionally graded shape memory alloy (FG-SMA) holds particular properties of both functionally graded materials and shape memory alloys. By bringing in a new concept of conical surface assumption, considering the axial deformation, a macroconstitutive model which can describe the thermal mechanical properties of a FG-SMA cylinder subjected to pressure and graded temperature loads is established in this work. Furthermore, a new layered finite element model (FEM) which can avoid the direct assumption of the macroproperties of the FG-SMA cylinder is provided. The theoretical results display a good agreement with the FEM results, which indicates that both the macroconstitutive model and the FEM provided here are valid. The obtained results show that the stress in the cylinder distributes complexly, and it decreases remarkably as a result of the martensite transformation. This research can provide a base for the design and in-depth investigation of FG-SMA materials.

In this work, a high-temperature low-cycle fatigue (LCF) behavior of a newly developed austenitic oxide dispersion strengthened (ODS) steel is investigated. The LCF tests were performed in air at 650 °C under three different strain amplitudes (±0.4, ±0.5, and ±0.7%) with a nominal strain rate of 10−3 s−1. The measured cyclic stress response showed four distinct stages which include short initial stable cyclic response followed by a prolonged hardening with subsequent short saturation and finally crack initiation and growth stage. The rate of hardening and the duration of stages are a function of applied strain amplitude. Microstructural investigations were carried out to shed light on the deformation mechanisms. After cycling, the overall microstructure appears stable without any modifications in grain shape and size. In addition, twinning and stacking fault fractions remain unchanged. However, cyclic hardening is an aftermath of dislocation multiplication whose rate is also a function of applied strain amplitude. Furthermore, oxide particles, as well as fine grains, inhibit strain localization by restricting three-dimensional dislocation structure formation that are associated with the development of extrusions and intrusions and are readily observed in conventional austenitic non-ODS steels.

The effect of equal-channel angular pressing (ECAP) at various temperatures (310, 330, and 350 °C) on precipitations and strengthening mechanisms of Mg–9Al–1Si alloys was investigated. The results indicated that the average grain size decreased gradually with decreasing of ECAP temperature. The distribution of the Mg2Si phase changed a little when the ECAP temperature increased. However, the different morphologies of β-Mg17Al12 phase were observed, including continuous and uncontinuous precipitation of particles at 310 and 350 °C. The continuous β-Mg17Al12 phase was hardly found and the refined β-Mg17Al12 phase was distributed dispersedly in the matrix at 330 °C. Thus, the mechanical properties of the Mg–9Al–1Si alloy was optimum: ultimate tensile strength and elongation were ∼350.8 MPa and ∼14.77%, respectively. It can be deduced that both grain refinement strengthening and precipitation strengthening play significant roles in strength increment of the alloy during the ECAP process. However, precipitation strengthening is the predominant mechanism.

The effect of aging time on the corrosion behavior of 6005 Al alloys has been investigated in aerated 3.5% NaCl aqueous solution. The corrosion resistance of the alloy with different aging times is analyzed by measuring potentiodynamic polarization, electrochemical impedance spectroscopy. The surface morphology is examined by scanning electron microscopy. The results demonstrated that the corrosion resistance of the alloy in the peak-aged condition is worse than the other conditions. Accordingly, corrosion rate and the corrosion current density of the alloy reach its maximum value. An Fe-rich phase is identified as the β-Al4.5FeSi phase by atomic-resolution high angle annular dark field scanning transmission electron microscopy and energy dispersive X-ray spectroscopy mapping analyses. The β-Al4.5FeSi is wrapped slowly by the precipitates of Mg2Si from the process of the peak-aged condition to the over-aged condition. It is hypothesized that the change of corrosion behavior of the alloy may be attributed to the β-Al4.5FeSi wrapped slowly by the precipitates of Mg2Si.