Z. G. Wang

Effect of reinforcement size on the elevated-temperature tensile properties and low-cycle fatigue behavior of particulate SiC/Al composites

The cyclic stress response characteristics and low-cycle fatigue endurance of powder-metallurgy-processed commercially pure aluminum composites reinforced with SiC particles of different sizes and of the unreinforced matrix were studied under a range of cyclic plastic strains at 441 K. Tensile properties of these materials were also examined. At elevated temperature the composites and unreinforced aluminum all definitely show cyclic softening. Under the same plastic strain amplitudes, the cyclic response stresses of the composites are higher than that of their unreinforced counterpart. Reinforcing particle size has no apparent influence on the cyclic stress/strain features of the composites The amount of cyclic softening is similar except that the cyclic response stress is slightly higher for the composite reinforced with small particles than for the large-particle-reinforced composite at a given cyclic strain amplitude. At high plastic strain ranges, the two composites display shorter fatigue lives than the unreinforced matrix material and of the two composites, the large-particle-reinforced composite exhibits slightly superior fatigue resistance

Cyclic deformation behaviour of a copper bicrystal with single-slip-oriented component crystals and a perpendicular grain boundary: cyclic stress-strain response and saturation dislocation observation

The cyclic deformation behaviour of a [(5) over bar 913]-[(5) over bar 79] copper bicrystal with a grain boundary (GB) approximately perpendicular to the stress axis was investigated under constant-plastic-strain control at room temperature in air. In particular, the bicrystal contains two single-slip-oriented component crystals. The results showed that the cyclic stress-strain curve (CSSC) of the bicrystal exhibited two plateau regions in the axial plastic strain range of 1.8 x 10(-4)-1.35 x 10(-3) and 2.05 x 10(-3)-2.56 x 10(-3) respectively. The corresponding plateau axial saturation stresses were about 62-64 MPa and 70-71 MPa. During cyclic deformation, only the primary slip system B4 (111)[(1) over bar 01] within the two component crystals was activated including the vicinity of the GB at all the applied strain amplitudes. Meanwhile, it is found that the plastic strains carried by the [(5) over bar 913] and [(5) over bar 79] grains are obviously different owing to the difference in their orientations. By means of the electron channelling contrast technique in scanning electron microscopy, the surface dislocation patterns of the bicrystal were observed. The dislocation patterns in the two grains also showed an apparent difference. Based on the experimental results above, the effect of crystal orientation on the CSSC, surface slip patterns and saturation dislocation configurations within two grains was discussed.

Microcrack initiation and dislocation structures in an aluminium bicrystal under cyclic loading

Tension-tension cyclic deformation of a [(1) over bar 23]-[(4) over bar 55] aluminium bicrystal with the stress axis parallel to the grain boundary (GB) was performed under load control at room temperature. At a lower stress amplitude, in the early stage of fatigue the dislocation configuration in both component crystals was characterized by an irregular cell-like structure. After 2 x 10(6) cycles, at the same lower stress amplitude, the dislocation density in some ceilings or floors of irregular cell-like structure increased, and these dislocations lined up in straight lines that corresponded to the persistent slip lines on the surfaces of the two component crystals. At an intermediate stress amplitude the irregular cell-like structure changed to a regular cell-like structure in the single-slip component [(1) over bar 23], while the irregular cell-like structure still remained in the double-slip component [(4) over bar 55]. At a higher stress amplitude the regular cell-like structure turned to a]adder-like structure in the single slip component. The microcracks were approximately parallel to the GB, and they were 3-5 mu m apart from the GB and emerged preferentially in the single slip component. A possible mechanism of microcrack initiation by voiding was suggested.

Quantitative study of correlation between fracture surface roughness and fatigue properties of SiC/Al composites

Quantitative measurements were carried out on the fatigue fracture surface of the SiC/Al composite by a sectioning method. It was shown that the cyclic plastic strain amplitude, SiC volume fraction and particle size have effects on fracture surface roughness R(S). The measured fracture surface roughness R(S) is closely related to the fatigue-crack propagation path and may corresponds to the fatigue life. Moreover, it was found that there is an obvious difference in the R(S) values for fatigue fractures which are due to different fracture mechanisms. These results show that it is possible to reflect the fracture mechanism using fracture surface roughness and relate it to the fracture properties of materials.

The strength of amorphous and nanocrystalline FeMoSiB alloys

The nanocrystalline FeMoSiB alloys with different grain sizes were prepared during the crystallization of their amorphous solids. The ultimate tensile strength (UTS) and fatigue strength of amorphous and nanocrystalline materials were measured and compared with each ether. It is found that the ratio of fatigue limit over UTS of amorphous materials reaches the minimum and the maximum fatigue limit can be obtained in one of these nanocrystalline materials. Abnormal Hall-Fetch relation was also observed in these nanocrystalline materials.

Effect of reinforcement on cyclic stress response of a particulate SiC/Al composite

The cyclic stress response of SiC particulate-reinforced pure aluminium and its unreinforced matrix material, in the as-extruded condition, was investigated utilizing plastic strain-controlled testing at room temperature. The unreinforced aluminium exhibited initial cyclic hardening, cyclic stability and secondary hardening, while the composite showed gradual softening over most of the fatigue life. Transmission election microscopy examination has been made of the dislocation substructure developed during cyclic deformation. The differences in the dislocation substructure obtained from processing and its developments under cyclic strain loading were considered to give rise to the observed phenomena.

FRACTAL CHARACTERISTICS OF PITTING UNDER CYCLIC LOADING

Abstract Specimens of A537cLl offshore structural steel were tested under cyclic loading and saltwater spray conditions. After the test, photographs were taken of the specimen surfaces where many pits were distributed randomly and then analyzed by using the slit island approach to determine their fractal dimensions. The result reported here reveals that the irregularity of the surfaces with many pits can be described very well by the fractal approach. Moreover, the fractal dimensions D f of the specimen surfaces increased with the increase in the number of cycles and the decrease in the residual life of those specimens.

Crystallographic cracking behavior in 〈110〉 oriented Ni3Al alloy single crystals under cyclic loading

Fatigue cracking behavior was studied on compact tension specimens of Ni3Al alloy single crystals having [110] tensile axis. Crystallographic cracking in the (110)[1(1) over bar0

Brief review and reconsideration of fatigue crack closure effect in materials

] oriented specimen occurred on alternating {111} planes, and the whole fracture was almost parallel to the loading axis. The preferentially activated slip systems ahead of the growing crack tip and their interaction with each other resulted in the fracture characteristics in that oriented specimen. The experimental findings could be well explained by calculating the shear stresses on all possible octahedral slip planes around the growing crack tip.