Z.G. Wang

The cyclic deformation behavior of Cu single crystal oriented for double slip

Copper single crystals oriented for double slip ([034] and [(1) over bar 17]) and single slip ([(1) over bar 25]) were cyclically deformed at constant plastic shear strain amplitude in the range 8 x 10(-5) less than or equal to gamma(pl) less than or equal to 1.4 x 10(-2). The initial cyclic hardening of the [034] crystal was found to be similar to that of the [(1) over bar 25] crystal, while the initial cyclic hardening rates of the [(1) over bar 17] crystal are significantly higher than those of [(1) over bar 25] and [034] crystals. The cyclic stress-strain (CSS) curve of the [034] crystal exhibits a plateau similar to that of the [(1) over bar 25] crystal, but the plateau ends at a relatively lower strain amplitude (gamma congruent to 4.3 x 10(-3)). The CSS curve of the [(1) over bar 17] crystal shows almost no plateau. The saturation shear stresses increase monotonously with Increasing gamma(pl). The above mechanical behaviors are in accord with the strength of dislocation interactions between different slip systems. Surface observation revealed that for the double slip oriented crystals, secondary slip is inactive for gamma(pl) below 10(-3), and two types of deformation bands may form for gamma(pl) greater than 10(-3), which are different from those previously reported.

Cyclic deformation behavior of copper bicrystals

Abstract Cyclic deformation was applied to three kinds of copper bicrystals with different geometries designated as [35]/[35], [35]/[–35] and [–35]/[–35] as well as a [35] copper single crystal at a plastic shear strain amplitude in the range of 1.66 × 10 −4 to 9.1 × 10 −3 . Experimental results showed that the four kinds of crystals all demonstrate a plateau in the cyclic stress-strain (CSS) curves in the range of approximately 1.66 × 10 −4 ⩽ γ pl ⩽ 5.0 × 10 −3 except for the [35]/[35 ] bicrystal, which has a slightly smaller upper plateau value. The initial hardening rate and the cyclic hardening curves for the [35]/[35] bicrystal were found to be similar to those of the [35] single crystal, and the CSS curve shows a plateau stress of about 29 MPa, very close to that of the [35] single crystal. The [–35]/[–35] bicrystal shows rapid cyclic hardening behavior, and the CSS curve exhibits a high plateau stress of about 36 MPa. For the [35]/[–35] non-isoaxial bicrystal, the cyclic hardening rate and the plateau stress are between those of the [35]/[35] and [–35]/[–35] bicrystals; the plateau stress is approximately the mean of the above, about 32.5 MPa. The above results are in accord with the surface observations. Activation of secondary slip due to grain boundary (GB) constraints and different degrees of dislocation interaction may be responsible for the above results. Slip heterogeneity, slip transfer across the GB and deformation bands were observed on the surfaces of the bicrystals.

Analysis of fractal characteristics of fractured surfaces by secondary electron line scanning

Abstract Fractal characteristics of impact-fractured surfaces at room temperature for CK45 steel under different heat treatment conditions were investigated by means of secondary electron line scanning, and the results were compared with those obtained by the profilometric method. It was found that the value of the fractal dimension Dse obtained from secondary electron line scanning analysis is larger than that from vertical section analysis (Dvs) for the same fractured surface. However, Dse, Dvs and the impact toughness of the material vary with tempering temperatures in a similar manner. The profile of the secondary electron scanning line can reflect the angular distribution of the scanned surface but cannot directly express the outline of its vertical section.

Low cycle fatigue behaviour of new type of stainless steel

Specimens of an Fe-Cr-Mn-N dual-phase stainless steel were solution treated at 1050 and 1250-degrees-C, yielding grain sizes of 10 and 32-mu-m respectively. Low cycle fatigue tests at constant total strain amplitude ranging from 4 x 10(-3) to 1.2 x 10(-2) were carried out at room temperature in air. The cyclic stress response, hysteresis loss per cycle and dislocation structure were investigated in the course of the cyclic deformation. No significant differences were noted in either the cyclic behaviour or the dislocation structure between the two heat treatment conditions. Cyclic hardening occurred in the very early stage of fatigue life. After passing through a maximum of hardening, cyclic softening set in until fracture. The strain amplitude affects the initial hardening rate and hysteresis loss. Dislocation tangles and planar arrays of dislocation were observed after the first few cycles in the initial hardening stage, while the cyclic softening stage is characterized by the formation of irregular bands or wall structures. In addition, grain boundaries were found to be effective barriers to slip dislocations during the initial hardening stage, while increased relaxation activities at grain and twin boundaries are evident during the cyclic softening stage. The present observations suggest that the dislocation activities within the grains and at or near the grain boundaries are both responsible for the observed cyclic hardening and softening of the material under study. The failure mode for the specimens solution treated at 1050-degrees-C was transgranular while that at 1250-degrees-C was intergranular. The former has a fatigue life about twice that of the latter under identical total strain amplitude.

Effect of frequency on cyclic creep of polycrystalline aluminium at room temperature

A study was made of the effect of cyclic frequencies from zero to 1 Hz on the cyclic creep behaviour with different extents of unloading in commercial purity aluminium. There exists an optimum frequency corresponding to the lowest steady state creep rate, the value of which exhibits the trend of decreasing with attenuating stress amplitude at a given cyclic peak stress. Both the cyclic frequency and the stress amplitude strongly affect the threshold stress for the transition from cyclic creep acceleration to retardation but do not change the average internal stress. Expression for the threshold stress were derived as a function of thermal activation parameters. When the value of V(c) is near that of V(s) (V(c) and V(s) are the apparent activation volume of cyclic and static creep, respectively) the threshold stress varies inversely as (V(c)/V(s)-1); otherwise, it is inversely proportional to 1-V(s)/V(c). The effect of grain size on cyclic creep is also discussed.

Mixed-mode I and II fatigue threshold and crack closure in dual-phase steels

Fatigue threshold under mixed-mode I and II loading and elastic plane-strain conditions has been studied in dual-phase steels (DPS) of two types of volume fraction of martensite (Vm) in laboratory air at room temperature. Near-threshold mixed-mode (I and II) fatigue crack growth occurs mainly by two mechanisms: shear mode, and tensile mode. Particular emphasis was placed on the influence of the mode II component. The mixed-mode threshold is controlled not only by mode I displacement but also by the mode II component. Apparent- and effective-bound curves (corrected closure) are obtained for the threshold condition and discussed in terms of the shape and size of the plastic region of crack tip; crack surface rubbing; and especially, roughnessinduced closure and shear resistance of crack surface that resulted in an extremely high extrinsictoughening contribution to the mixed-mode fatigue threshold values. The ratio of the threshold value of pure mode II to that of pure mode I (ΔKthII/ΔKththI) attained highly to 1.9 times; the maximum hoop direction stress-intensity factor range of pure mode II branch crack tip is 2.2 times that of pure mode I. Obviously, the resistance of pure mode II crack growth here is far larger than that of pure mode I; the former is just to introduce the shear resistance of crack surface, the latter, to reduce the driving force of crack tip for crack closure. It is proposed that the apparent- and effective-bound curves are nonconservative risky and too conservative for design purposes, respectively. So, the threshold data should be obtained under the specific conditions found by concrete mechanical, microstructural, and environmental factors.

Effect of dislocation substructure of crack tip on near fatigue threshold in dual-phase steels

Abstract The effects of stabilized subgrain cell and wall structures of the near-threshold of physical short cracks in dual-phase steels have been found for the first time in the work reported in this paper. These effects result from both the larger number of cycles in the original position and the higher stress-strain field of the crack tip. For the near-threshold of long cracks, there was a tendency for dislocation cells to form. These behaviours illustrate that the dislocation morphologies of crack tips are the products of strain history. It is proposed for the first time in this paper that threshold values computed by the subgrain model should be related to effective fatigue threshold values: the dislocation cell is one of the intrinsic toughening factors and also has an effect on the microstructure.

Influence of plastic deformation upon the half-width of engineering metallic materials in hard state

The half-width values of the X-ray diffraction profiles are frequently used to characterize the static strength of a strengthened surface, or the depth distribution of this mechanical parameter, in a strengthened surface layer, especially in a shot-peening affected layer. However, for the unpeened surface and the base material of the shot-peened specimen of an alloy steel treated in hard state, the experimental results shown in this article indicate that uniaxial tensile or compressive plastic deformation increases the yield strengths while it decreases the half-width values. The half-width values of the shot-peened surface and surface layer greatly decrease, whereas the yield strength of this surface remarkably increases. Accordingly, in the authors’ opinion, the half-width values could not correctly describe the static strengths of hard metallic materials, and, contrary to the viewpoint put forward by a lot of researchers, the shot-peened surfaces of such materials are work hardened instead of work softened. A model demonstrating that plastic deformation reduces the half-width values by decreasing the second kind internal stresses is developed.

Structural characterization and fatigue behaviour of a carbon-implanted pure polycrystalline nickel☆

Abstract Pure polycrystalline nickel was implanted with 150 keV carbon ions to fluences of 2 × 10 17 ions cm −2 at room temperature. Surface layer structural characterization for carbon-implanted and post-implantation heat-treated specimens, and its effect on fatigue behaviour, were investigated. The microstructural changes by the direct implantation of carbon into nickel were examined by transmission electron microscopy. It was found that the implantation results in the formation of amorphous phase, Ni 3 C compound and some other metastable phases corresponding to different depths below the surface. The crystallization temperature of amorphous Niue5f8C is about 500°. The specimens were tested under load-unload tensile fatigue. A 22% increase in endurance limit was found after implantation. Fatigue behaviour of post-implantation heat-treated specimens is the same as before implantation. The fatigue surface was examined by scanning electron microscopy. Grain boundary cracking is another crack nucleation mode of the implanted specimens besides slip band cracking which is the only crack initiation mode of pure nickel. Fatigue cracks of post-heat-treated specimens nucleate at large Ni 3 C particles. Transmission electron micrographs show that the cyclic loading leads to recrystallization of carbon-ion-induced amorphous Niue5f8C.

Comments on some of the fractal equations

Abstract The fractal equations or relationships used to measure the fractal dimension of a fractured surface are discussed and several suggestions are made to modify them.