Unified mechanism of intergranular embrittlement based on non-equilibrium grain boundary segregation

Abstract

The review is aimed at presenting a unified approach in understanding the mechanism of nonequilibrium grain boundary segregation, which can satisfactorily describe the three types of intergranular embrittlement, namely, reverse temper embrittlement of steels, intergranular corrosion embrittlement of stainless steels and intermediate temperature embrittlement of metals and alloys. The review starts with a broad perspective of non-equilibrium grain boundary segregation, including thermally induced non-equilibrium grain boundary segregation and stress induced non-equilibrium grain-boundary segregation. Next, it focuses on the recent progress made in the non-equilibrium grain boundary segregation, including (1) critical time, (2) segregation peak temperature, (3) segregation peak temperature movement for thermally induced and stress induced non-equilibrium grain boundary segregation, and (4) the effect of temperature difference on thermally-induced non-equilibrium grain boundary segregation. Next, the attention is focused on the grain boundary coverage of elements and intergranular embrittlement phenomena. Three types of intergranular embrittlement is analysed in terms of (1) the ductility healing effect induced by the critical time, (2) embrittlement peak or ductility trough induced by the segregation peak temperature, (3) embrittlement peak or ductility trough movement induced by the segregation peak temperature movement and (4) widening and deepening of ductility trough induced by differences in temperature. These experimental phenomena concerning the three types of intergranular embrittlement are consistent with the models of thermally induced and stress induced non-equilibrium grain boundary segregations of impurities, instead of precipitation or equilibrium grain boundary segregation. Towards the end, we visit the subject of grain boundary segregation and associated embrittlement process from the viewpoint of fracture resistance and briefly discuss different perspectives that are of practical significance. List of symbolsA constant b the Burger's vector B Norton coefficient C b (t c ) grain boundary concentration at critical time C b(s50) equilibrium concentration of solute at the grain boundaries C g concentration of solute within the grains C v s~0 ð Þ equilibrium vacancy concentration at grain boundaries C v s~s ð Þ grain boundary vacancy concentration induced by the tensile stress D b the diffusivity D c diffusion coefficients for complexes under or in absence of applied tensile stress D i diffusion coefficients for solute atoms under or in absence of applied tensile stress E the elastic or Young's modulus -E the embrittlement sensitivity in K/at-% of solute at the grain boundary E b formation energy of vacancy solute atom complex International Materials Reviews 2013 VOL 58 NO 5263 E f vacancy formation energy E gb the elastic or Young's modulus of grain boundary region F v the formation energy of a vacancy in the boundary region H Z intergranular Auger peak to peak height of an element normalised with the ...

Segregation-induced changes in grain boundary cohesion and embrittlement in binary alloys

“…We calculate DE GBC by assuming random mixing in each of the bulk and interfacial regions and introducing the resulting numbers of bonds into Eqs. (6) and (7), to obtain…”

“…To the authors' knowledge, this is the only study to provide a framework for the quantitative treatment of embrittlement in the presence of equilibrium segregation using readily measureable materials parameters. However, the developments of Lejcek are analytical and parametric, and use only equilibrium properties of interfaces, and are thus not readily applicable to changes in interfacial cohesion due to non-equilibrium segregation [5,6], or to the chemically-constrained equilibrium of fast fracture. Additionally, the models in [37,38] are limited to open systems, and are therefore less relevant for, e.g., fine-grained or nanocrystalline alloys, which must be treated as closed systems because segregation may appreciably change the concentration of solute remaining in the bulk.…”

“…Fractographic analyses reveal that such failures often occur along grain boundaries [2,3], where embrittlement via segregation or wetting of tramp elements has occurred. Such impurity-induced embrittlement has been shown to commonly occur in most metallic base metals of interest, such as Cu- [4], Ni- [5], Fe- [6], and Al-based [7][8][9][10][11] systems. A grain boundary is considered embrittled when boundary cleavage (i.e.…”

The measurement uncertainty of reduction in area of metals in tensile testing system

“…Ref. [7] pointed out for the first time that intermediate temperature embrittlement is produced because of performance of technical standard of tension testing system.…”

“…3-5, its reduction in area varies with the temperature and there is a temperature at which the reduction in area will reach a minimum (maximum embrittlement). That is the intermediate temperature embrittlement in metals [7]. Hence intermediate temperature embrittlement is a kind of measurement uncertainty of reduction in area relative to temperature variation in tensile test system, suggested by the technical standard ISO6892-2-2011.…”