Qualitative And Quantitative Investigations Of Deformation Damage And Fracture Features Of Several Kinds Of Typical Metallic Materials

With the variations in loading type, specimen size, service environment, interior defects, etc., the mechanical performances, the deformation behavior and the fracture mechanisms of metal materials would exhibit different characteristics. Systematically qualitative and quantitative investigations on these characteristics are of significant importance for the material application and design reliability, and for revealing the deformation and fracture failure mechanism of metal materials. At present, the qualitative characterizations on the microstructural changes and fracture morphologies during the process of fracture failure of materials have been well-improved; however, the relevant studies on quantitative characterizations still need to be constantly developed. In particular, the quantitative assessments on the deformation and fracture characteristics related to the material size effect remain less. Therefore, in the present work, several kinds of typical metallic materials with different crystalline structures are selected as the target materials, such as face-centered cubic (fcc) AL6XN super austenitic stainless steel, body-centered cubic (bcc) pure iron and hexagonal close-packed (hcp) pure titanium. With the aid of fractal quantitative analysis method, the mechanical properties, deformation damage, fracture behavior and corresponding dislocation structures of these materials are systematically studied under the conditions of different specimen thicknesses, loading modes, temperatures and initial micro-defects, respectively.The specimen size has an effect, to some extent, on the tensile and fatigue properties, fracture behavior and micro structures of submillimeter-scale AL6XN super austenitic stainless steel. As the specimen thickness increases, the ultimate tensile strength gradually increases but the yield stress changes little, meanwhile, the surface deformation becomes more serious and the size of dimples on fracture surface gradually increases. Correspondingly, the fractal dimensions of the lateral surfaces and fracture surfaces also tend to increase. Under this circumstance, the deformation micro-mechanisms transforms from the prominence of planar dislocation slip into the occupation of deformation twining. At the same stress amplitude, the thicker the specimen, the higher the fatigue life and the greater the capability of plastic deformation is. Such differences in the plastic deformation capability has a significant effect on the fatigue fracture morphologies, namely, with increasing thickness, the micro-features in the fatigue crack source zone changes gradually from cleavage to quasi-cleavage, the plastic features of fatigue striations become more evident, and the size and depth of dimples in the finial rapid fracture areas increases slightly; simultaneously, the fractal dimensions of the scanning profiles corresponding to the each zones of fatigue fracture surface increase. TEM observations demonstrated that, with increasing thickness, the fatigue microstructures change from pure planar/wavy slip dislocation morphologies into deformation twins of an increasing amount in addition to normal dislocation structures.SEM observations on fatigue fracture features of the AL6XN super stainless steel under different stress amplitudes show that the fatigue fracture features are more or less related with the applied stress amplitude. With increasing Δσ/2, the micro-features with brittle cleavage steps and brittle fatigue striations in the fatigue crack source zone changes gradually into features with regular fatigue striations and the formation of secondary cracks, the spacing of the plastic fatigue striations in the fatigue crack growth zones slightly increases, and the micro-features for all samples in the final rapid fracture areas are similarly featured by the formation of dimples.A quantitative relationship between the total fatigue life and the average fractal dimension of scanning profile of fracture surface is obtained for fatigued A16XN steels under constant stress amplitude control. The greater the fractal dimension, the shorter the fatigue life is. A similar change of fractal dimension with the applied Aa/2 in the fatigue crack source and growth zones implies that crack initiation and propagation lives might contribute comparably to the total fatigue life.Fatigue deformation behavior of the commercially pure iron containing micro-voids at grain boundaries (GBs) under total strain amplitude control and fractal analyses about the fatigue fractures indicated that the cyclic response curves of the CP iron show an initial softening stage within early several cycles followed by a continuous cyclic hardening. No stress saturation phenomenon was found. Pre-existence of micro-voids at GBs spurs intergranular cracking to become a common damage mode besides transgranular cracking along slip bands. Nearly independent on the applied strain amplitude, the measured value of fractal dimension D of the scanning profile in the crack growth zone of the whole fracture surface is the largest as compared to those in the crack source zone and final rapid fracture area, indicating that the majority of total fatigue life is occupied by the fatigue crack propagation life for the present CP iron containing GB micro-voids. At a relatively high total strain amplitude, the dislocation microstructure is primarily characterized by the formation of dislocation cells.CP titanium has a high temperature sensitivity of mechanical properties, deformation and damage behavior. With increasing temperature, the tensile yield strength, the ultimate tensile strength and the strain hardening exponent decreases, but the elongation increases with increasing temperature, the number and size of microvoids formed along shear bands (SBs) on the deformed specimen surface increases. The diameter and depth of dimples on the fracture surfaces of CP Ti increase significantly with increasing temperature, giving rise to a higher fracture surface roughness reflected by a higher fractal dimension. TEM observations demonstrated that the plastic deformation of CP Ti is gradually occupied by dislocation slipping rather than twinning with increasing temperature. This is in good agreement with the fractal analyses of the deformation and fracture features.