Papers by Keyword: Intergranular

Abstract: In this paper a cohesive formulation is proposed for modelling intergranular and transgranular damage and microcracking evolution in brittle polycrystalline materials. The model uses a multi region boundary element approach combined with a dual boundary element formulation. Polycrystalline microstructures are created through a Voronoi tessellation algorithm. Each crystal has an elastic orthotropic behaviour and specific material orientation. Transgranular surfaces are inserted as the simulation evolves and only in those grains that experience stress levels high enough for the nucleation of a new potential crack. Damage evolution along (inter-or trans-granular) interfaces is then modelled using cohesive traction separation laws and, upon failure, frictional contact analysis is introduced to model separation, stick or slip. Moreover some physical consideration based on cohesive energies were made, in order to guarantee the cohesive model in consideration was appropriate for the purpose of this work. Finally numerical simulations have been performed to demonstrate the validity of the proposed formulation in comparison with experimental observations and literature results.

Abstract: Al₂O₃–ZrO₂ ceramic composite samples were prepared using Al₂O₃–ZrO₂ composite powder synthesized by sol-gel processing as the main starting material and zirconia gel as the binder. Then the mineral phases were analyzed by X-ray diffraction (XRD) and the crystal bonding mechanism was analyzed by scanning electron microscopy (SEM). The results show that: 1) the mineral phases were monoclinic zirconia, tetragonal zirconia and corundum. The microstructure shows an alumina-zirconia diffusion mosaic structure and the grain boundary interface with direct bonding state. 2) The crystal growth model was the Ostwald dual growth model of the alumina and zirconia grains, and the crystal bonding mechanism was that the intragranular zirconia grain exited within the alumina grain and the intergranular zirconia grain exited during the alumina grain boundaries.

Abstract: The influence of precipitated phases in Ni-based alloys during solid solution aging treatment on the performance of Ni-based alloys was investigated by means of metallographic observation, scanning electron microscope (SEM), and transmission electron microscope (TEM). The variation of microstructure and resultant phases as a result of solid solution aging treatment at different temperatures was discussed. The results show that the heat treatment temperature has significant influences on the type as well as quantity of precipitation phases. Lots of phases precipitated at grain boundaries, the distribution of precipitated phases are characterized by mesh-like structure. The corrosion tests results indicate that there is a potential difference between grains and grain boundaries due to the precipitation of chrome carbide at grain boundaries, resulting in pitting corrosion occurred preferentially at grain boundaries, consequently, the corrosion resistance of Ni-based alloys is reduced. In order to enhance the corrosion resistance of Ni-based alloys, it is expected to control the carbon content in a lower range and proper heat treatment process to avoid large amount precipitation of chrome carbide.

Abstract: Intergranular corrosion can lead to significant problems such as sub-critical crack growth or loss in section strength, potentially leading to failure, as well as a substantially increased maintenance burden. This type of corrosion is found in most types of aluminium alloys, but is a particularly significant problem in aerospace aluminium alloys. The form of intergranular corrosion can vary widely, and may depend on alloy composition, product form, environmental conditions and the presence or otherwise of local or global stresses. One notable example is the occurrence of intergranular corrosion due to atmospheric corrosion, in which salts and deposits deliquesce on the surface forming discrete corrosion cells. Intergranular corrosion of aluminium alloys is usually most rapid in the rolling or extrusion direction of wrought alloy. The reasons for this are not fully understood, and may include texture effects that produce highly susceptible grain boundaries, the inhomogeneous distribution of noble constituent particles, and stresses acting at a microscale. This paper will review and discuss the evidence for and against for the different effects mentioned.

Abstract: Three types of corrosion attack frequently suffered by type Cr18Ni10 stainless steel in the heat-affected zone of welds were studied – knife-line, fusion-line, and fissure corrosion. Possible mechanisms and selected preventative measures were discussed in the light of microstructural findings.

Abstract: In this paper, intergranular microfracture evolution in polycrystalline brittle materials is simulated using a cohesive grain boundary integral formulation. A linear cohesive law is used for modelling multiple microcracking initiation and propagation under mixed mode failure conditions, encountering the stochastic e=ects of the grain location, morphology and orientation. Furthermore, in cases where crack surfaces come into contact, slide or separate, fully frictional contact analysis is performed.