Hong Youshi

Experimental and theoretical study on numerical density evolution of short fatigue cracks

Fatigue testing was performed using a kind of triangular shaped specimen to obtain the characteristics of numerical density evolution for short cracks at the primary stage of fatigue damage. The material concerned is a structural alloy steel. The experimental results show that the numerical density of short cracks reaches the maximum value when crack length is slightly less than the average grain diameter, indicating grain boundary is the main barrier for short crack extension. Based on the experimental observations and related theory, the expressions for growth velocity and nucleation rate of short cracks have been proposed. With the solution to phase space conservation equation, the theoretical results of numerical density evolution for short cracks were obtained, which were in agreement with our experimental measurements.Fatigue testing was performed using a kind of triangular shaped specimen to obtain the characteristics of numerical density evolution for short cracks at the primary stage of fatigue damage. The material concerned is a structural alloy steel. The experimental results show that the numerical density of short cracks reaches the maximum value when crack length is slightly less than the average grain diameter, indicating grain boundary is the main barrier for short crack extension. Based on the experimental observations and related theory, the expressions for growth velocity and nucleation rate of short cracks have been proposed. With the solution to phase space conservation equation, the theoretical results of numerical density evolution for short cracks were obtained, which were in agreement with our experimental measurements.

ANALYSIS OF THE THERMAL STABILITY OF COPPER SPECIMENS DEFORMED BY HIGH–PRESSURE TORSION

The thermal stability of Cu specimens subjected to high-pressure torsion (HPT) defor- mation with varying strains was studied by optical microscope (OM), differential scanning calorimetry (DSC) and transmission electron microscope (TEM). It is found that cellular subgrains with high dis- location densities are firstly formed at a low strain level, some of the cellular subgrains are transformed into dislocation-free equiaxed grains at larger strains. A single exothermal peak between 150 and 250 is shown in DSC curves, corresponding to the heat release due to recrystallization and subse- quent grain growth. With the increase of strain, the peak position is shifted to a lower temperature and then is leveled off, but the stored energy of cold work, calculated according to the area under a peak, increases with strain at relatively low strain level and reaches its maximum value of 0.91 J/mol at strain of 13. Further deformation induces the stored energy of cold work to decrease due to the dynamic recovery of microstructure. A large drop in hardness appears in as-deformed samples at a temperature 45 lower than the start temperature of the exothermal peak after isochronal annealing, indicating that the recrystallization and grain growth process is closely relatine to annealing time and temperature.

Deformation Twinning in Nanocrystalline Ni during Cryogenic Rolling

Deformation twinning is evidenced by transmission electron microscopy examinations in electrodeposited nanocrystalline (nc) Ni with mean grain size 25 nm upon cryogenic rolling. Two twinning mechanisms are confirmed to operate in nc grains, i.e. heterogeneous formation via emission of partial dislocations from the grain boundary and homogeneous nucleation through dynamic overlapping of stacking faults, with the former being determined as the most proficient. Deformation twinning in nc Ni may be well interpreted in terms of molecular dynamics simulation based on generalized planar fault energy curves.