In the present study, 3D atom probe was used to study the effect of increased Mo (1–3 wt%) on clustering in secondary hardening ultra-high strength steels. Clusters have been classified into three categories, namely, Type I, Type II, and Type III with the (Cr + Mo)/C ratio of <1.5, 1.5–3.25, and >3.25, respectively. Cluster evolution suggests that size and volume fraction (Vf) of Type II clusters increase continuously from as-quenched to aged samples, while the number density (Nv) increases in 400 °C aged sample and decreases in 450 and 500 °C samples. On the other hand, Nv and Vf of both Type I and Type III clusters decrease on aging. This work clearly suggests that on aging, Type II clusters, which are close to M2C stoichiometry, become most stable, which may eventually either become M2C precipitates upon prolonged aging or act as potential nuclei for the precipitation of equilibrium M2C precipitates.

In this work, a Ti–29Nb–13Ta–4.6Zr–xO Gum Metal with two significantly different oxygen levels (388 and 3570 ppm) was investigated during deformation. The alloys were compressed during in situ high-energy X-ray diffraction using three different strain rates, 10−4, 10−3, and 10−1 s−1, in order to evaluate their influence on phase stability and mechanical properties. The influence of oxygen on the deformation process was also studied. Deformation takes place by twinning, stress-induced, and reverse martensitic transformation and was observed, for some samples, a spinodal decomposition of the β-phase during elastic deformation. The mechanical properties were similar for the different rates employed when considering the same oxygen level. The alloy with a higher amount of oxygen, however, showed a substantial increase in mechanical strength, with a yield strength of around 680 MPa, which is more than three times higher than for the specimen with 388 ppm of oxygen.

The fracture toughness of 617 Ni-based weld metal (WM) under different elevated temperatures was tested with a novel method and its fracture mechanism was investigated in this paper. It was found that the fracture toughness of WM was lower than that of base metal (BM) at the same temperature, which was mainly due to the coarse columnar structure, differences in misorientation, and precipitated phases. For both BM and WM, the fracture toughness was lower at elevated temperature due to decreased strength. Much more micro-voids caused by Ti(C, N) and M23C6 inside grains of BM could be observed adjacent to the crack path, which accounted for the dramatically decreased fracture toughness of BM at elevated temperature. In comparison, fewer micro-voids could be observed in WM due to the lack of those second particles. As a result, the J0.2 value and propagation path morphology both showed that the WM had more stable microstructure even though possessing lower toughness.

This study investigates the effect of C on the deformation mechanisms in Fe–C alloys by molecular dynamics simulations. In uniaxial tensile simulations, the face-centered-cubic (fcc) structures of Fe–C alloys undergo the following deformation processes: (i) fcc→body-centered-cubic (bcc) martensitic transformation, (ii) deformation of bcc phase, and (iii) bcc→hcp martensitic transformation, which are significantly influenced by the C concentration. For the low C concentrations (0–0.8 wt%) fcc phase, the fcc→bcc phase transformation accords a two-stage shear transformation mechanism based on the Bain model, the deformation mechanism of the bcc phase is the first migration of twinning structures and then elastic deformation, and the bcc→hcp phase transformation follows Burgers relations resulting from the shear of the bcc close-packed layers. However, for the fcc phase with high C concentrations (1.0–2.0 wt%), the fcc→bcc phase transformation follows a localized Bain transformation mechanism impeded by the C atoms, the bcc phase only experiences elastic deformation, and the bcc→hcp phase transformation also conforms to Burgers relations but become localized due to the addition of more C atoms. Because of the different phase transformation mechanisms between the high C and low C supercells, the dislocation generation mechanism is also different.

The microstructure evolution, dynamic recrystallization (DRX) and precipitation of the ZM61 alloy sheets prepared with different rolling conditions were studied. The DRX grain sizes (dDRX) at four high strain rate rolling (HSRR) temperatures (275–350 °C) are 1.9, 2.3, 2.6 and 3.1 μm, respectively, while the DRX volume fractions (fVDRX) are 69, 73, 76 and 82%, respectively. 300 °C is selected as the optimal HSRR temperature. The dDRX and fVDRX of the alloys prepared by pre-rolling (PR) at 300 °C + HSRR are 1.0 μm and 91%, respectively. The PR treatment does not change the types of the precipitates but promotes the precipitation. The tensile strength (UTS) of 369 MPa and yield strength (YS) of 261 MPa can be achieved by HSRR at 300 °C, while a further increase in both UTS and YS can be obtained by PR treatment.

In this research, the mechanical alloying (MA) technique was used to study solid solubility in the immiscible Zr–Cr alloy system. At first, Zr and Cr powders were milled, and then, the phase evolution, alloying mechanism, microstructural change, and mechanical properties of the milled powders were investigated by X-ray diffraction technique, scanning electron microscopy along with energy dispersive spectroscopy, transition electron microscopy, and microhardness measurements. Moreover, the solubility limit of Zr in Cr lattice was obtained by Vegard's law. The results showed that the MA was significantly enhanced the solubility of Zr in Cr up to about 21.6 wt% after an optimum milling time of 32 h and led to form an amorphous/nanocrystalline composite of Zr-reach and Cr-reach supersaturated solid solutions with the microhardness value of 503 Hv approximately. Also, the thermodynamic analysis indicated that the Gibbs free energy changes for the amorphous and solid solution were positive, which were provided by the MA process.

The γ″ phase (hexagonal structure with space group ${ P\bar{6}}2{ m}$) plays an important role in the strengthening of Mg–Gd–Y–Ag–Zr alloy. In this study, Cs-corrected high-angle annular dark-field scanning transmission electron microscopy was applied to characterize the Mg–Gd–Y–Ag–Zr alloy in different conditions (as-cast, solution-treated, and isothermally aged at 200 °C). The nucleation, growing process, and transformation behavior of the plate-shaped γ″ phase were systematically investigated on the atomic scale. We found that the nucleation sites of the γ″ phase were separated by close-packed planes of the Mg matrix and the γ″ phase developed in two perpendicular directions of $\langle 10\bar{1}0 \rangle$ and ⟨0001⟩. The growing process of the γ″ phase on the atomic scale was captured. The γ″ phase was thermodynamically stable at room temperature, and no transformation behavior of the γ″ phase was observed up to 200 h during isothermal aging at 200 °C.

Polyglycolide (PGA) materials have been widely used in the medical field, but the degradation mechanism in the natural environment is still unclear. High-viscosity PGA was prepared by using twin-screw reaction extrusion polymerization. The mass and intrinsic viscosity of PGA samples, the pH of the solution surrounding the PGA samples in water, and the number of degradation products resulting from the degradation of the PGA samples were studied under different conditions and at different temperatures. PGA does not degrade at 70 °C in either dry air or in a vacuum. Infrared spectroscopy (FTIR) and differential spectroscopy revealed that the PGA samples in water at 70 °C for 40 days had a substantially reduced mass and substantially altered thermal behavior when compared with the control sample (undegraded PGA sample). The degradation of the PGA samples in humid conditions at 70 °C was similar to the degradation of the samples in water at 70 °C. The results of this study indicate that water and water vapor (moisture) in the natural environment are the main causes of PGA degradation, and higher temperatures accelerate the degradation process, which shortens the shelf life and life of PGA.

In this paper, we use finite element analysis (FEA) to study the linear viscoelastic response of polyurea, a type of hard–soft block copolymer. A Niblack's algorithm-based technique employed on atomic force microscopy images provides geometry inputs for the FEA model, while the viscoelastic master curves of the soft matrix are obtained via a combination of dynamic mechanical analysis data and molecular dynamic (MD) estimations. In this microstructural image-based FEA framework, we introduce an interphase area of altered properties between the hard and soft domains. Both spatial and property distributions of this interphase area affect the viscoelastic response of the copolymer system. To quantitatively investigate the impact of structural and property features of the interphase on the energy storage and dissipation of a system during linear perturbation, we develop a statistical descriptor representation of the interphase region related to physical parameters. Utilizing decision-tree and random forest concepts from machine learning, we apply a ranking algorithm to identify the most significant features for four different mechanical response descriptors. Results show that the total interphase volume fraction and shifting factor distributions in the interphase area dominate the magnitude of the tan δ peak, whereas the magnitudes of the shifting factors primarily affect the tan δ peak location in frequency space. This method allows us to readily identify the dominant features impacting individual properties and paves the way for material design of hard–soft block copolymer systems.

The aim of this research was to develop the UV-cured epoxy/carbon composites. The rheological properties of the uncured neat epoxy and epoxy composite with graphite, graphene, and multi-walled carbon nanotube (MWCNT) were evaluated to observe the macroscopic flow behavior and the microstructure by shear force. The results showed that epoxy/carbon composites at high filler content exhibited shear-thinning behavior with a high yield stress value and epoxy/MWCNT at 30 phr showed this characteristic obviously. The fractured surface and particle dispersion in the epoxy matrix were evaluated by scanning electron microscopy and transmission electron microscopy, respectively. Epoxy/carbon composites at high filler content displayed rough fracture surface with particle agglomeration, thus the electrical conductivity increased. The result showed that the epoxy/MWCNT composites had high potential to use as a conductive adhesive with a 3D printing process due to high electrical conductivity with high viscosity that could be formed easily during processing.

Thermal conductivity behaviors are one of the most important evaluations of carbon fiber-reinforced carbon matrix (C/C) composites in the field of thermal protective structures. In order to deepen the understanding of the thermal conductivity behaviors of C/C composites, the out-of-plane thermal conductivity of C/C composites is studied by considering voids and the fiber volume fractions. The representative volume element (RVE) models of microscale and mesoscale are proposed. The parameters of the RVE models are captured by X-ray micro-computed tomography. The carbon matrix equivalent models and fiber volume fraction models along the z-direction were established. The effects of the porosity and fiber volume fraction along the z-direction on the thermal conductivity were analyzed. The proposed model was validated by experimental results at room temperature. Further, the numerical methods developed in this study can provide guidance for predicting the thermal conductivity of C/C composites with complex structures.