Papers by Keyword: Intergranular Fracture

Abstract: The 7000 series aluminum alloys suffer from intergranular fracture (IGF) that limits the use of the alloys, although they have highest strength among aluminum alloys. The types of IGF can be classified into two categories: (i) with smooth fracture surface showing practically no plastic deformation that takes place in hydrogen embrittlement and stress corrosion cracking, and (ii) with shallow and fine dimples on the fracture surface showing localized plastic deformation inside precipitate free zones. In this study, attempts have been made to suppress the IGF of both types by (a) controlling precipitate microstructure on grain boundaries by quench control and (b) controlling grain boundary morphology by strain induced boundary migration. The IGF of type (i) (hydrogen embrittlement) was successfully suppressed both by the two controlling processes.

Abstract: Fatigue crack growth resistance of ultrafine grained Cu processed by equal channel angular pressing (ECAP) was investigated. Particular emphasis was devoted to the effects of microstructure evolution on fatigue crack growth in the near-threshold regime. The ultrafine grained Cu exhibits a lower fatigue threshold than coarse-grained Cu at stress ratios of 0.1 and 0.7. Fatigue induced coarsening of the UFG structure near the fatigue crack and intergranular fatigue crack growth are observed.

Abstract: Mixtures of ZrO₂-Al₂O₃ ceramic composite were prepared using zirconia powder, ZrO₂-Al₂O₃ composite powder synthesized by sol-gel process as main starting materials and zirconia gel as a binder. Then the mixtures were pressed into the mold, and then sintered at 1650°C for 3h. Finally, the ZrO₂-Al₂O₃ ceramic composite samples were prepared. Then the mineral phases of ZrO₂-Al₂O₃ ceramic composite were analyzed by X-ray diffraction (XRD), and the fracture model of ZrO₂-Al₂O₃ ceramic composite was analyzed by press, scanning electron microscopy (SEM) and energy dispersive X-ray Spectroscope (EDS). The results show that, zirconia is the main phase and alumina is the secondary phase mainly present in the matrix. The fracture model of ZrO₂-Al₂O₃ material was jointly composed by intergranular fracture and transgranular fracture, of which the coarse particle shows the intergranular fracture model and the fine particle shows the transgranular fracture model.

Abstract: The effect of post-weld heat treatment (PWHT) on carbide precipitation and impact properties of coarse-grained heat-affected zone (CGHAZ) of Q690 Steel was studied in this paper. Carbide particles precipitated primarily at prior austenite grain boundaries and martensitic lath boundaries. When the PWHT temperature is 520–570 °C, temper embrittlement occurs. This temperature range is also where the number of carbide particles per unit area at grain boundaries reaches its maximum. The high number of particles per unit area increases the rate of crack initiation at grain boundaries under rapid loading; linking of microcracks along grain boundaries which are already weakened by impurity segregation results in TE and intergranular fracture.

Abstract: This paper deals with fracture of neutron irradiated austenitic Ti-stabilized stainless steel 08Ch18N10T. The steel had been tested in air and in water environment (320°C) using several tests representing different stress strain conditions for crack initiation and growth; Slow Strain Rate and Crack Growth Rate tests were performed in the water. Without irradiation the steel did not suffer from stress corrosion cracking in the water, but on irradiated specimens appeared areas of intergranular fracture mixed with transgranular cleavage-like facets and secondary cracks typical for IASCC phenomenon. The differences between fracture of irradiated and non-irradiated specimens in air and in water are documented and discussed.

Abstract: The sensitivity to liquid sodium embrittlement (LME) of T91 martensitic steel, one of the selected structural materials for future sodium fast reactors has been investigated. The study took into account the role of microstructure. Small punch tests and three points bending tests were carried out in a purified and controlled atmosphere. Precipitation state and dislocations structure resulting from a tempering at 550° C provoked LME of the T91 steel between 200 and 550 °C. Secondary Ions Mass Spectroscopy investigation suggested that sodium penetration at prior austenitic grains boundaries promoted by plastic deformation occurred and caused brittle crack initiation. Brittle cracks propagated in sodium preferentially along martensitic laths-boundaries. J integral calculations confirmed a drop in toughness of T91 tempered at 550°C by liquid sodium up to 80 %.

Abstract: The present work is an effort to provide experimental results focusing on segregation behavior of phosphorus at grain boundary and the intergranular fracture behavio under low tensile stresses. AES (Auger electron spectroscopy) experiments and dynamic analyses on the non-equilibrium grain-boundary segregation (NGS) of phosphorus and the SEM photos of intergranular fracture in Auger specimens in 12Cr1MoV steel were carried out in this paper. The variation of phosphorus segregation level in grain boundary under different low tensile stresses and at different temperature were obtained. Results show that NGS of phosphorus occurred in the experimental steel while subjected to low tensile stresses. Maximum values of phosphorus segregation level were obtained at the critical times. SEM photos of intergranular fracture in Auger specimens of the test steel show that the intergranular fracture rate increased with increasing concentration of phosphorus. The intergranular fracture behavior is accordant with the segregation behavior of phosphorus at grain boundary.

Abstract: The relationship between texture and secondary work embrittlement of interstitial-free steels was studied by X-ray analysis technique and scanning electron microscopy analysis technique. The results indicated that the stronger the γ fiber texture was, the higher the plastic strain ratio (r-value) was, and Secondary work embrittlement temperature rose as r-value increased or γ fiber texture intensified. The mode of secondary work embrittlement fracture was a blend of cleavage fracture and intergranular fracture, and the fraction of cleavage fractures decreased gradually as secondary work embrittlement temperature rose.

Abstract: Quantification of characteristics that govern intergranular fracture initiation and propagation of this fracture micromechanism in competition with cleavage one is main aim of the paper. A NiCr steel of commercial quality and the same steel with an increased content of impurity elements, Sn and Sb, were used. Step cooling ageing was applied in order to induce intergranular embrittlement. Standard and pre-cracked Charpy type specimens were both tested in three-point bending to determine fracture toughness characteristics. In order to characterise the quantitative differences in fracture surfaces roughness a fractal analysis was applied. A boundary level of fractal dimension has been determined to be 1.12: fracture surface roughness with a higher value reflects high level of intergranular embrittlement and thus fracture resistance degradation.

Abstract: Elevated temperature brittleness (ETB) of spheroidal graphite (s-g) cast iron has been referred to as reduced ductility within an elevated temperature range and has been related to grain boundary brittleness. The phenomenon of ETB has not been yet clearly understood. In this study, the factor affecting on ETB was studied in terms of strain rate and chemical composition. A study was carried out on the influence of phosphorus on ETB by using laboratory-made heats containing different phosphorus contents. ETB indicated the marked decrease in ductility at around 673K. S-g cast iron containing low phosphorus content manifested ETB at temperatures between 650K and 700K. There was a consistent correlation between the fractional increases in intergranular fracture appearance and the decrease in elongation. The increase of phosphorus suppressed ETB and s-g cast irons containing phosphorus exceeding 0.030 % were found to be immune to ETB. It was found that ETB could be suppressed by reducing the ratio of magnesium and phosphorus to less than 1.5.