Gu Haicheng

Bauschinger effect and residual phase stresses in two ductile-phase steels: Part I. The influence of phase stresses on the Bauschinger effect

The Bauschinger effect (BE) in dual-phase steels has been computationally simulated, and the influence of phase stresses, developed due to nonhomogeneous deformation during preloading, on the BE has been investigated. Isotropic-and anisotropic-hardening models were used in finite-element method calculation to produce the reverse flow stress-strain curves (compression) of dual-phase steels from the reverse stress-strain curves of single-phase materials. Aspects of the Bauschinger effect, including the rounding of the reverse flow curve, yielding at low reverse stresses, high initial work-hardening rates, and the absence of permanent softening,etc., were elucidated by the variation in phase stresses in the constituent phase.

Fatigue crack initatiin in high-purity titanium crystals

Abstract Fatigue crack initation has been studied in high-purity titanium monocrystals, bicrystals and polycrystals. Concerning persistent slip bands (PSBs), two fatigue damage modes were identified: holes and cavities along the bands, and microcracks perpendicular to the bands. The interactions of grain boundaries with slip bands or cyclic twins were observed to induce fatigue cracks. In addition to the twin-matrix interfaces, impinging between twins and extrusion ribbons within twins were also found to be preferential sites for crack nucleation. The mechanisms of fatigue crack initiation are discussed.

Bauschinger effect and residual phase stresses in two ductile-phase steels: Part II. The effect of microstructure and mechanical properties of the constituent phases on Bauschinger effect and residual phase stresses

A finite-element method has been employed to calculate the effect of the prestrain, particle size, volume fraction, and yield strength ratio of the constituent phases on the Bauschinger effect (BE) and residual phase stresses for ferrite-martensite two-phase steels. The relationships be-tween the BE parameters and residual phase stresses are given, and the basics of inelastic un-loading and the effect of reverse deformation on the BE and residual phase stresses are also discussed. Based on the decaying of the residual phase stresses (commonly called “back stresses”) during reverse loading, the relationship between back stresses and permanent softening has been elucidated. Some of the calculated results are compared with experimental ones, and good agree-ment between them is found.

Orientation dependence of cyclic deformation in high purity titanium single crystals

Abstract Cyclic deformation was studied in a series of high purity titanium single crystals with different orientations. The multiple step test method was employed in low cycle fatigue with a closed-loop electro-hydraulic servo-controlled Instron machine and cyclic stress-strain curves were obtained. The surface morphology of the fatigued specimens was observed with scanning electron microscopy, and dislocation structure was analysed with transmission electron microscopy. The results show that the cyclic responses and microstructures of titanium single crystals are sensitive to orientation. In the crystals oriented near the center of the (0001) stereographic projection, single slip and/or cross slip occur, planar dipole arrays and dislocation loops exist, and the cyclic strain hardening rate is low; whereas for crystals, in which orientations are favorable for double and multiple slip, cell structure is found and the cyclic strain hardening rate is high. Cyclic twins with different morphologies and fine structures are observed in almost all specimens, and the higher the content of twins, the higher is the strain hardening rate. Stacking faults are found in fatigued titanium especially in the region of twins in spite of the high stacking fault energy.

Low cycle fatigue properties and microscopic deformation structure of Zircaloy-4 in recrystallized and stress-relieved conditions

Abstract Low cycle fatigue (LCF) tests have been performed on Zr-4 in recrystallized (RZ) and stress-relieved (SR) conditions at RT and 400°C. Results show that Zr-4/RZ displays cyclic hardening at RT and 400°C, whereas Zr-4/SR exhibits cyclic hardening at RT, but cyclic softening at 400°C. Fatigue lifetime curves as a function of plastic strain range imply that Zr-4/RZ has higher lifetime than that of Zr-4/SR at RT and in the low-cycle region at 400°C, and the LCF properties at 400°C are superior compared to those at RT for Zr-4 in both RZ and SR conditions. TEM examination shows that typical dislocation configuration contains some parallel dislocation lines in Zr-4/RZ, and some dislocation lines are visible between the channels of elongated original grain boundaries for Zr-4/SR at RT. Elongated dislocation cells plus dislocation dipole perpendicular to the cell boundary are observed in Zr-4/RZ, whereas rectangle cells in Zr-4/SR at 400°C. Trace analysis indicates that prismatic and pyramidal slips are possible cyclic deformation modes for Zr-4 in different heat treatment conditions and at different temperatures. Finally, the relationships between LCF properties and microscopic deformation mechanisms are discussed.

Plastic Energy Dissipation Model for Lifetime Prediction of Zirconium and Zircaloy-4 Fatigued at RT and 400°C

Zirconium and zircaloy-4 are generally used as fuel tubes in pressurized heavy water reactors because of their low neutron absorption cross-section, excellent corrosion resistance, good strength and ductility. Low cycle fatigue properties of zirconium and zircaloy-4 were investigated at RT and 400 C. The microscopic structure was determined using scanning electron microscopy and transmission electron microscopy techniques. On the basis of analyses of fatigue damage mechanism, it is believed that fatigue is an irreversible energy dissipation process. Thus, the plastic dissipation energy per cycle is selected as a fatigue damage variable. The accumulated plastic dissipation energy is calculated at the condition of considering cyclic hardening, saturation and softening characters of zirconium and zircaloy-4 during cycling. The testing results show that they present a power law between the plastic dissipation energy and fatigue lifetime.

Loading mode dependence of deformation microstructure in a high-purity titanium single crystal oriented for difficult glide

Abstract A special high-purity titanium single crystal was prepared to study the effect of loading mode on deformation microstructure. Six rectangularly sectioned specimens were cut from the same single crystal. The loading axis of the specimens was parallel to the C-axis of the crystal, and the wide surface was parallel to the prism plane {10 1 &}, with the narrow surface {1 2 10}. Mechanical tests were conducted within the chamber of a scanning electron microscope JSM35C using a specially designed apparatus with closed-loop computer control. Five loading modes were used: monotonic tension, tension-tension fatigue, and tension-compression fatigue with three different stress amplitudes. The surface appearance of the specimens was registered with SEM periodically, and the cyclic twins as well as persistent slip bands were identified using trace analysis technique. The substructures of the tested specimens were examined with a transmission electron microscope on foils cut from the specimens parallel to the wide surface. The dislocation structures of specimens tested in different modes were observed, and the slip systems and twinning modes for each case are discussed.

High cycle fatigue properties and microstructure of zirconium and zircaloy-4 under reversal bending

Abstract Fatigue lifetime curves of a commercial-purity zirconium and zircaloy-4 were measured under reversal bending. Scanning electron microscopic examination on surfaces of fatigued specimens shows that zirconium displays planar slip, whereas zircaloy-4 displays wavy slip. Fracture surface analysis shows that many fatigue striations with a considerable number of micro-cracks are the primary characters in zirconium and zircaloy-4. Fatigue crack generally nucleates in persistent slip bands and twin–matrix interfaces. One set of stacking fault clusters was detected in zirconium. The stacking fault interface is the feeble barrier to dislocation motion. The typical fatigue dislocation configuration is the dislocation walls apparently parallel to the trace of the prismatic plane, which are separated by long straight screw dislocations perpendicular to walls in the commercial-purity zirconium, and also in zircaloy-4, whereas, zircaloy-4 is easier to cross-slip in comparison with zirconium. Several twins and their fine structures were detected in zirconium and zircaloy-4. The relations between twinning, cracking and slip are discussed.

Investigation on monotonic and cyclic stress–strain characteristics of Ti–2Al–2.5Zr alloy

Abstract The monotonic and cyclic deforming regularity of Ti–2Al–2.5Zr alloy rod, tube and profiled tube (PT) specimen was determined at room temperature (RT). The effects of tension speed, V , cycle number, N , strain ratio, R , strain amplitude, e a and specimen shape on tensile and cyclic deforming behavior and characteristics were analyzed. The stress–strain relation equations under monotonic and cyclic loading, and the fatigue life prediction equations of Ti–2Al–2.5Zr alloy were established. The structure and fracture were also observed with optical and scanning electron microscope (OM, SEM). The tensile properties of Ti–2Al–2.5Zr alloy are related with V and specimen shape. At R =0.1, the cyclic stress–strain curves of tube and rod appear as cyclic softening. But the rod (at R =−l) and PT (at R =0) behave as cyclic softening at low e a value (≤9.0%), whereas they appear as cyclic hardening at high e a value (>2.0%). The fatigue life of Ti–2Al–2.5Zr alloy determined with tube is lower than that with rod. The twinning is closely related to e a , N value, etc. The tensile fractures show typical dimple features. There are many irregular crystalline convexes in fatigue fractures where many secondary cracks, fatigue striations and holes also exist.

Hydrostatic Stresses and Their Effect on the Macroflow Behavior and Microfracture Mechanism of Two-Phase Alloys

The aspects of the hydrostatic pressure and tension stresses developed as a result of the interaction between hard and soft phases to maintain compatibility are represented for two-phase alloys with different microstructures. It has been theoretically proved that the stress triaxiality, defined as the ratio of the hydrostatic stress to the effective stress (ΣH/Σe), causes hardening or softening of the component phases. The average hydrostatic tension and pressure stresses in soft and hard phases developed during monotonic loading make the soft phase harden and hard phase soften. The extent of the hardening and softening increases with the strength ratio of the hard phase to soft phase and the size of the particles and decreasing the phase content. The hardening or softening effect of thein situ constituents has important influence on the flow stress of the composites and is one of the important reasons for deviation from the law of mixtures in prediction of the flow stress of composites based on that of the component phases in bulk. The stress triaxiality distributions in microstructure scale also provide an explicit physical picture of the microfracture mechanisms of the two-phase alloys.