Papers by Keyword: Intergranular Fracture

Abstract: The mechanism of toughness degradation during slow cooling in the austenite range was studied in CA6NM stainless steel, 13% Cr-4% Ni soft martensitic stainless steel. The variation of toughness, fracture mode and microstructural features were examined by means of cooling rate and isothermal heating in the austenite range together with chemical composition. Toughness degradation was referred to as the increases of FATT and intergranular fracture when those steels were cooled slowly after austenitizing and isothermally heated in the austenite range. The embrittlement was found to be related the intergranular fracture and the precipitation of carbide along prior austenite grain boundaries. Its fracture surface was characterized by mosaic-like markings when the carbide precipitation got to increase. Reducing carbon, silicon and phosphorus and increasing molybdenum improve the toughness degradation.

Abstract: The Fe-25Cr-1N alloy produced by solution nitriding possesses extremely high yield strength owing to the solid solution strengthening by nitrogen. However, it was found that the steel exhibited an insufficient elongation because of the brittle intergranular fracture caused during the uniform tensile deformation. This is due to the marked stress concentration at grain boundaries, which is derived from the grain coarsening caused during long time solution nitriding and the development of planar dislocation structure characteristic of high nitrogen austenitic steels. The most effective way to reduce the stress concentration at grain boundary during deformation should be grain refinement. In this study, grain refinement was attempted by the two-step heat treatment for the Fe-25Cr-1N(-Mn) alloy, and then the mechanical properties were investigated by means of tensile tests and fatigue tests. The two-step heat treatment resulted in the grain refinement of austenite to 20 microns in diameter. The intergranular fracture was greatly suppressed from 70% (as-solution-nitrided) to 10% (grain-refined) in area fraction by the grain refinement. In addition, elongation was markedly increased with local necking. The yield stress and tensile strength were also increased, and thus, the fatigue limit is also raised by more than 30%.

Abstract: By means of finite element method which is based on the conventional theory of mechanism-based strain gradient plasticity, cohesive interface model is used to study the intergranular fracture in polycrystalline metals with nanoscale and ultra-fine grains. A systematical study on the overall strength and ductility of polycrystalline aggregates which depend on both grain interiors and grain boundaries for different grain sizes is performed. The results show that the overall strength and ductility of polycrystalline aggregates with nanoscale and ultra-fine grains are strongly related to the competition of grain boundaries deformation with that in grain interiors. Finally, the deformation and failure behavior of nanocrystalline nickel are described by using the computational model.

Abstract: Using molecular dynamics (MD) simulation, we have investigated the mechanical properties and the microstructural evolution of nanocrystalline tantalum (NC-Ta, grain size from 3.25 nm to ~13.0 nm) under uniaxial tension. The results show the flow stress at a given offset strain decreases as the grain size is decreased within the grain size regime studied, implying an inverse Hall-Petch effect. A strain rate sensitivity of ~0.14, more than triple that of coarse-grain Ta, is derived from the simulation results. Twinning is regarded to be a secondary deformation mechanism based on the simulations. Similar to nanocrystalline iron, stress-induced phase transitions from body-centered cubic (BCC) to face-centered cubic (FCC) and hexagonal close-packed (HCP) structures take place locally during the deformation process, The maximum fraction of FCC atoms varies linearly with the tensile strength. We can thus conclude that a critical stress exists for the phase transition to occur. It is also observed that the higher the imposed strain rate, the further delayed is the phase transition. Such phase transitions are found to occur only at relatively low simulation temperatures, and are reversible with respect to stress.

Abstract: The γ-TiAl intermetallic compounds were produced at the temperature ranging from 850°C to 1050°C by the Spark Plasma Sintering (SPS) process. The effects of sintering temperature and holding time on the mechanical properties of γ-TiAl intermetallic compounds were investigated. The γ-TiAl intermetallic compounds sintered at 1050°C for 10 min showed a high relative density more than 98%, and had the best three-point bending strength of 643MPa, fracture toughness of 12 MPa·m1/2 and microhardness of 560MPa. The microstructural observations indicated typical characteristics of intergranular fracture, which meant the poor ductility of γ-TiAl intermetallic compounds.

Abstract: The objective of this study was to investigate and compare the influence of Mn and Fe additions on the fracture behaviour of AA6000 series alloys in under, peak and overaged conditions. Testing was completed under uniaxial tension and the microstructures of the alloys were observed using optical and Transmission Electron Microscopy. Alloys with very low levels of both Mn and Fe underwent a transition from transgranular to intergranular fracture and a reduction in strain-tofracture when heat treated from the under aged (UA) to peak aged (PA) condition. Increasing Mn and Fe content prevents this transition in fracture mode such that the stain-to-fracture is similar in the UA and PA states. Despite the change in fracture mode, when comparing the strain-to-fracture for a given ageing condition, increasing Fe systematically reduces the strain-to-fracture. Conversely, increases in Mn systematically increase the strain-to-fracture for a given ageing condition.

Abstract: Observation of fracture surfaces in ceramics is useful for improving their mechanical properties. In this study, fracture surfaces of polycrystalline alumina were observed using scanning-probe microscopy (SPM) on a nanoscale, also called “nano-fractography.” The average grain size of polycrystalline alumina specimen used in this study was 4.5µm, and the fracture toughness was 3.0MPa･m-1/2. The fracture mode was found to be a mixture of intergranular and transgranular fractures. The fracture surface of intergranular fractures consisted of smooth and rough areas composed of very small steps, whose detection was impossible using scanning electron microscopy. Cleavage and non-cleavage fractures were observed in transgranular fracture grains. The fracture surface of single-crystalline alumina, which is the typical model of the transgranular fracture, was also observed by SPM. The cleavage plane of alumina macroscopically exhibited a very smooth, glass-like surface. However, sub-nano meter steps can be observed on the cleavage fracture surface and appear to be formed by plastic deformation during crack propagation because the size of the step nears that of the Burgers vector.

Abstract: This paper discusses micropstructural aspects of brittleness fracture of polycrystalline materials caused by intergranular fracture. Structure-dependent intergranular brittle fracture in bicrystals and polycrystals are discussed and predicted theoretically. Experimental evidence for the structure-dependent intergranular fracture is shown and some general features are discussed to demonstrate the relationship between grain boundary structure/character, grain boundary energy and intergranular fracture strength. Theoretical prediction of the fracture toughness based on the strongest-link theory is introduced for polycrystals with different grain boundary microstructures, primarily defined by the grain boundary character distribution, grain boundary connectivity. Finally recent achievements of successful control of intergranular brittleness by grain boundary engineering based on the strongest-link theory are introduced for different materials.

Abstract: Grain boundary (GB) segregation has been measured in Ca-doped MgO by examining intergranular fracture surfaces with Auger electron spectroscopy. The measurements reveal several interesting features. The composition of any given GB on the fracture surface is almost uniform, except for small variations due to deviations from planarity. There is a strong anisotropy of GB composition, which can amount to as much as a factor of six between low and high segregation GB's. Finally, although the compositions of opposite sides of a GB fracture are uniform, there are sometimes significant differences between the two sides, in agreement with a recently formulated model of GB composition as a function of GB character.