Papers by Keyword: Grain Boundary Wetting

Abstract: This study characterizes the mechanical behavior of an advanced multiphase high strength steel by means of high temperature tensile testing. The results show a drastic reduction of the maximum tensile elongation from around 700 °C up to 950°C. Scanning electron microscopy investigations show that the temperature range for embrittlement is correlated with the total wetting of steel grain boundaries. Under external strain, crack propagates along the grain boundaries according to a mechanism that leads to the presence of nanometer-thick films of Zn at the crack tip, as shown by fine X-ray spectroscopy analyses. The effective temperature range for embrittlement is discussed. Mechanisms of i) external stress-free wetting, and ii) atomic-scale crack propagation, are today under discussion in the light of the literature, regarding in particular recent experimental results and theory about grain boundary wetting, intergranular penetration, and the correlation between surface energy and crack propagation rate.

Abstract: The thin layers of a second phase (also called complexions) in grain boundaries (GB) and triple junctions (TJs) are more and more frequently observed in polycrystals. The prewetting (or premelting) phase transitions were the first phenomena proposed to explain their existence. The deficit of the wetting phase in case of complete wetting can also lead to the formation of thin GB and TJ phases. However, only the phenomenon of pseudopartial (or pseudoincomplete, or constrained complete) wetting permitted to explain, how the thin GB film can exist in the equilibrium with GB lenses of a second phase with non-zero contact angle.

Abstract: Mechanical properties, microstructure of the Sn–38wt. %Pb eutectic and the development of deformation - induced diffusion processes on interphase boundaries (IB) were investigated. Experiments were carried out both in deformed and annealed states of eutectic using micro- and nanoindentation, SEM, AFM and optical microscopy techniques. It was found that the deformation of the annealed alloy is localized at the Pb/Sn interphase boundaries and occurs by grain boundary sliding (GBS) accompanied by sintering micropore processes under the action of the capillary forces on the Pb/Sn IB. During severe plastic deformation of Sn-Pb eutectic phase transition in the Sn grain boundary occurs. This deformation-induced process takes place due to the wetting of tin with Pb. These diffusion accommodation processes (sintering and wetting) are facilitated by the low values of the Pb/Sn interphase energy (0.07 J/m2). Wetting is thermodynamically favourable because the condition γgb > 2 γib is satisfied and it is also kinetically allowed due to the relatively high homologous temperature (> 0.5•Tm). The obtained values of the nanohardness and elastic modulus evidence that the IBs in the Sn–Pb eutectic have to be considered as a separate quasi-phase with its own properties.

Abstract: The wetting behavior of grain boundaries is affected by temperature, pressure and misorientation of grain boundaries. However, the influence of quenching baths on liquid state grain boundary wetting is rarely reported. In this work, this effect in the Sn−25 at% In alloy was investigated. The Sn−In alloy was prepared by smelting of In and Sn at 300°C for 6 hr in Ar atmosphere. Samples were annealed at temperature between 130 and 165°C and quenched in two kinds of baths: −10°C salt-saturated water and liquid nitrogen. The results from X-ray diffraction show a difference in preferential orientations between samples quenched in these two baths. Metallographic analysis reveals that the percentage of grain boundaries fully wetted in samples quenched in −10°C salt-saturated water is lower than that quenched in liquid nitrogen. It is pointed out that a proper quenching bath is necessary for preserving the initial microstructure of grain boundary wetting.

Abstract: The grain boundary (GB) wetting was investigated in the Sn – 25 at.% In alloy. It was found that the portion of GBs wetted by the melt depends on the annealing temperature. No GB completely wetted by melt was observed at 140°C, while all GBs were fully wetted after annealing at 180°C. Between 140°C and 180°C the portion of wetted GBs increases with increasing temperature. The tie-lines of GB wetting phase transition were constructed in the Sn–In bulk phase diagram.

Abstract: Thermodynamic conditions for spontaneous grain boundary wetting (GBW) and stress driven liquid metal embrittlement (LME) are related to each other. Kinetic mechanism responsible for fast GB penetration (GBP) under small stress is described. Dissolution–condensation mechanism of LME and linear fracture mechanics for calculation of crack profile are applied to the classical system “Al-liquid Ga”. The results tend to support the idea that the recently observed fast linear penetration of Ga along 150° tilt <110> GB of Al should be considered as propagation of LME crack under small residual stress rather than as spontaneous GBW. With the residual tensile stress σ≈ 0.5MPa acting normal to the GB plane, all major findings reported for this model system are explained in semi quantitative way assuming that GB spreading coefficient is extremely small by its absolute value, i.e. that the system is near the threshold of spontaneous GBW.

Abstract: The consequences of the contact between liquid bismuth and a copper bicrystal are investigated at 500°C. Atoms of bismuth are shown to penetrate and embritlle the copper grain boundary. Grain boundary concentration profiles of bismuth are obtained on fracture surfaces by both Auger electron spectroscopy and He4+ Rutherford backscattering spectroscopy. The maximum bismuth intergranular concentration is calculated from experimental data to be about 1.7 monolayers (near the liquid bismuth / solid copper interface). The overall profiles are significantly different from typical erfc profiles and an interpretation is proposed, based on the coupling effect between grain boundary diffusion and non-linear segregation. These results allow us to conclude on the absence of grain boundary wetting for the Cu / Bi system at 500°C.

Abstract: A model Ni-Bi system has been used to investigate intergranular penetration (IGP) phenomenon. All experiments have been done on Ni 26°<110> bicrystal at 700°C using bismuth vapour condensation as a source of liquid bismuth. Such a procedure results at room temperature in either partial or total Liquid Metal Induced Embrittlement (LMIE) of a unique grain boundary, depending on the duration of liquid Bi / solid Ni contact at 700°C. Auger Electron Spectrometry (AES) and Rutherford Backscattering Spectrometry (RBS) have been used to measure the Bi concentration profile between the source of liquid bismuth and the penetration front. Two zones have been clearly identified : the first one of almost constant Bi concentration called nanometrethick film which is interpreted in terms of Fowler-Guggenheim multi-layer segregation under local equilibrium conditions and the second one with a progressive decrease of Bi concentration over a distance of the order of 20-200µm. Such a long transition zone, together with parabolic diffusion kinetics indicates diffusion-based mechanism of intergranular penetration as opposed to the direct grain boundary wetting.