X.Y. Zhang

Work softening and annealing hardening of deformed nanocrystalline nickel

We reported that work softening takes place during room-temperature rolling of nanocrystalline Ni at an equivalent strain of around 0.30. The work softening corresponds to a strain-induced phase transformation from a face-centered cubic (fcc) to a body-centered cubic (bcc) lattice. The hardness decreases with increasing volume fraction of the bcc phase. When the deformed samples are annealed at 423 K, a hardening of the samples takes place. This hardening by annealing can be attributed to a variety of factors including the recovery transformation from the bcc to the fcc phase, grain boundary relaxation, and retardation of dislocation gliding by microtwins.

Non-classical Twinning Behavior in Dynamically Deformed Cobalt

We performed transmission electron microscopy studies on dynamically deformed pure cobalt. The results show that the twinning plane of the most common twinning mode in hexagonal close-packed metals is non-coincident, that is, the twinning plane of the parent and the twin does not coincide. Statistical measurements show that only 44% of the twins have a misorientation angle close to the theoretical value of 86.3°, and 79% of the twin boundaries (TBs) deviate from the twinning plane. Thirty-two percent of the TBs deviate from the twinning plane by more than 40°. These results indicate that the twinning significantly departs from the classical twinning behavior.

HREM investigation of twin boundary and interface defects in deformed polycrystalline cobalt

Twin boundary (TB) structure and interface defects in a twin in deformed polycrystalline cobalt were studied by using high-resolution electron microscopy (HREM). The TB tends to be flat and a mirror plane. Two interface defects with Burgers vectors b −2/−2 and b 17/17 were found. The core structures of the observed interface defects were relatively compact.

Growth of deformation twins in room-temperature rolled nanocrystalline nickel

Deformation twinning has been observed in room-temperature rolled nanocrystalline Ni. The growth of the deformation twins via the emission of partial dislocations from a grain boundary has been examined in detail. Partial dislocations on neighboring slip planes may migrate for different distances and then remain in the grain interior, leading to the formation of a steplike twin boundary (TB). With continued twin growth, the TBs become gradually distorted and lose their coherent character due to accumulated high stresses. Moreover, we propose that microtwins may form near such TBs due to the emission of partial dislocations from the TB.

Characterization of basal-prismatic interface of twin in deformed titanium by high-resolution transmission electron microscopy

Disconnections in the basal-prismatic (BP) interfaces of twin have been investigated by many computer simulations. In this paper, we report experimentally that the disconnection in the BP interface of twin in deformed titanium can be observed by high-resolution TEM (HRTEM). This disconnection can be characterized as using a coherent dichromatic complex. Correspondingly, the migration process of the basal-prismatic plane resulting from glide is described.

Effects of erbium on microstructure and mechanical properties of as-cast Mg-7Zn-3Al alloy

Abstract Mg-7Zn-3Al- x Er ( x =0.1, 0.4, 0.7) magnesium alloys were prepared by permanent mould casting. The effects of rare earth element of erbium on the microstructure and mechanical properties of as-cast Mg-7Zn-3Al alloy at both room temperature and elevated temperatures were investigated with optical microscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy, differential scanning calorimetry, and tensile testing. The results show that the quasi-continuous grain boundary networked τ(Mg 32 (Al, Zn) 49 ) phases are changed into discontinuous globular particles due to the addition of Er. Spherical Al-Er compounds are also identified in the matrix. The mechanical testing reveals that, with Er addition, the elevated temperature tensile properties can be remarkably improved. The microstructure evolution during tensile deformation under both temperature and stress and its effects on the mechanical properties were further discussed.

Microstructure evolution and hardness variation of pack-rolled nanocrystalline nickel

The microstructure evolution and hardness of nanocrystalline nickel during pack rolling at room temperature have been investigated. It was found that the roll-bonding side (R) and non-roll-bonding side (NR) behaved quite differently. The hardness of side R is higher than that of side NR. No obvious work softening was observed in either side R or side NR until the strain reached ~ 0.611. Quantitative X-ray diffraction analysis indicated that the grain size in side NR increases faster than that in side R, a result confirmed by transmission electron microscopy. Texture analysis showed that (2 0 0) preferred orientation first strengthens but then weakens in both sides NR and R, while a strong (2 2 0) preferred orientation emerges, particularly in side R. Further texture analysis suggests that dislocation slip is responsible for the texture discrepancy between side NR and side R. The dislocation activity, grain rotation and grain growth are discussed based on the experimental results.