Stress concentration around nanosized defects such as cavities always leads to plastic deformation and failure of solids. We investigate the effects of depth, size, and shape of a lotus-type nanocavity on onset plasticity of single crystal Al during nanoindentation on a (001) surface using a quasicontinuum method. The results show that the presence of a nanocavity can greatly affect the contact stiffness (S_c) and yield stress (σ_y) of the matrix during nanoindentation. For a circular cavity, the S_c and σ_y gradually increase with the cavity depth. A critical depth can be identified, over which the S_c and σ_y are insensitive to the cavity depth and it is firstly observed that the nucleated dislocations extend into the matrix and form a y-shaped structure. Moreover, the critical depth varies approximately linearly with the indenter size, regarding the same cavity. The S_c almost linearly decreases with the cavity diameter, while the σ_y is slightly affected. For an ellipsoidal cavity, the S_c and σ_y increase with the aspect ratio (AR), while they are less affected when the AR is over 1. Our results shed light in the mechanical behavior of metals with cavities and could also be helpful in designing porous materials and structures.

The effect of microstructure on the onset strain and rate of deformation-induced martensitic transformation (DIMT) in Q&P steel is studied by a mean-field micromechanics model, in which the residual austenite (RA) and primary martensite (M) phases are treated as elastoplastic particles embedded into the ferrite (F) matrix. The results show that when the volume fraction of the RA increases with a constant fraction of the M, the onset strain of DIMT increases and transformation rate decreases, in contrast to the case of the RA fraction effect with a fixed F fraction. Increasing the volume fraction of the M postpones the DIMT, regardless of the corresponding change from the RA or F fraction, which is similar to the effect of the RA fraction with the constant M but to a higher degree. Conversely, when increasing the fraction of the matrix F, the onset strain of DIMT increases and the rate decreases, and the effect is greater when the corresponding fraction change comes from the M rather than from the RA. Moreover, when the aspect ratio of the RA increases, the onset strain of DIMT decreases with a gradual increase in transformation rate, in agreement with the experimental observation that the equiaxial austenite is more stable in Q&P steels. However, the aspect ratio effect of the M is opposite to that of the RA, indicating that the lath-shaped primary martensite could protect the austenite from DIMT.

Interface has a significant effect on mechanical properties of graphene reinforced metal composites. Taking graphene nanosheet reinforced iron composite (Gr/Fe) as an example, the interfacial characteristics of Gr/Fe (110), (111), ($11\overline{2}$), and (001) interfaces have been studied using molecular dynamics (MD) simulations. Two types of interfacial bonding have been examined: physical and chemical bonding. The results show that when the graphene and iron form a physical adsorption (weak-bonded) interface, the interactive energy of the graphene and Fe (110), (111), ($11\overline{2}$), and (001) interface is −1.00 J/m², −0.73 J/m², −0.82 J/m², and −0.81 J/m², respectively. The lengths of the Fe-C bonding are distributed in the range of 2.20–3.00 Å without carbide formation, and no distinct patterns of atomic structure are identified. When the graphene and iron form a chemical (strong-bonded) interface, the corresponding interactive energy is −5.63 J/m², −4.32 J/m², −4.39 J/m², and −4.52 J/m², respectively. The lengths of the Fe-C bonding are mainly distributed in the ranges of 1.80–2.00 Å and 2.30–2.50 Å, which the carbides such as Fe₃C and Fe₇C₃ are formed at the interface. Moiré patterns are observed at different-oriented interfaces, because of the lattice geometrical mismatch between graphene and different-oriented iron crystal structures. The pattern of diamond stripe is at the (110) interface, which is in good accordance with the experiment. Other patterns are the hexagonal pattern at the (111) interface, the wavy stripe pattern at the ($11\overline{2}$) interface, and the chain pattern at the (001) interface. These moiré patterns are formed through the competition and coordination of the three binding sites (Hollow, Bridge, and Top) of graphene with Fe atoms.

Bimodal grain structure (BGS) Mg alloys containing a high fraction of fine grains (FGs) and a low fraction of coarse grains (CGs) show a good combination of strength and plasticity. Here, taking the ZK60 alloy as an example, the influences of CG size, volume fraction, and texture intensity on mechanical properties and the hetero-deformation-induced (HDI) effect were examined using the Mori–Tanaka mean-field method combined with strain gradient theory of plasticity. The results indicate that the overall mechanical properties decrease with an increase in CG size because the limited HDI effect cannot compensate for the strength and plasticity decrease derived from larger CGs. A higher aspect ratio of CG along the loading direction can weaken the HDI effect and subsequently reduce the overall mechanical properties. Optimal comprehensive mechanical properties can be achieved when the CG volume fraction is approximately 30%. Furthermore, an increasing basal texture intensity in CG results in higher yield strength and lower ultimate tensile strength, while the uniform elongation reaches a maximum value when ~60% of CGs possess hard orientations with Euler angles of (0~30°, 0°, 0°).