The “Solidification” of Grain Boundaries with Increasing Temperature (original) (raw)
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Grain Boundary Wetting by a Second Solid Phase in the High Entropy Alloys: A Review
Materials, 2021
In this review, the phenomenon of grain boundary (GB) wetting by the second solid phase is analyzed for the high entropy alloys (HEAs). Similar to the GB wetting by the liquid phase, the GB wetting by the second solid phase can be incomplete (partial) or complete. In the former case, the second solid phase forms in the GB of a matrix, the chain of (usually lenticular) precipitates with a certain non-zero contact angle. In the latter case, it forms in the GB continuous layers between matrix grains which completely separate the matrix crystallites. The GB wetting by the second solid phase can be observed in HEAs produced by all solidification-based technologies. The particle chains or continuous layers of a second solid phase form in GBs also without the mediation of a liquid phase, for example by solid-phase sintering or coatings deposition. To describe the GB wetting by the second solid phase, the new GB tie-lines should be considered in the two- or multiphase areas in the multicomp...
Grain Boundary Phase Transitions and their Influence on Properties of Polycrystals
Interface Science, 2004
Grain boundary (GB) phase transitions can change drastically the properties of polycrystals. The GB wetting phase transition can occur in the two-phase area of the bulk phase diagram where the liquid (L) and solid (S) phases are in equlibrium. Above the temperature of the GB wetting phase transition a GB cannot exist in equlibrium contact with the liquid phase. The experimental data on GB wetting phase transitions in numerous systems are analysed. The GB wetting tie-line can continue in the one-phase area of the bulk phase diagram as a GB solidus line. This line represents the GB premelting or prewetting phase transitions. The GB properties change drastically when GB solidus line is crossed by a change in the temperature or concentration. The experimental data on GB segregation, energy, mobility and diffusivity obtained in various systems both in polycrystals and bicrystals are analysed. In case if two solid phases are in equilibrium, the GB "solid state wetting" can occur. In this case the layer of the solid phase 2 has to substitute GBs in the solid phase 1. Such GB phase transition occurs if the energy of two interphase boundaries is lower than the GB energy in the phase 1.
Grain Boundary Wetting by a Second Solid Phase in Ti-Fe Alloys
Journal of Materials Engineering and Performance, 2018
The microstructure of Ti-Fe polycrystals has been studied between 595 and 815°C in the concentration interval between 1 and 9 wt.% Fe. In these conditions, two phases, namely hexagonal a(Ti, Fe) and cubic b(Ti, Fe), are in equilibrium. The a(Ti, Fe) phase forms either continuous or discontinuous layers in the b(Ti, Fe)/b(Ti, Fe) grain boundaries (GBs). Continuous layers correspond to the complete wetting of b(Ti, Fe)/b(Ti, Fe) GBs by a second solid phase a(Ti, Fe). Discontinuous layers correspond to the incomplete (or partial) GB wetting by a second solid phase. The temperature dependences of the portion of completely wetted GBs as well as that of the thickness of continuous GB layer of a(Ti, Fe) phase have been measured. Both values monotonously increase with increasing temperature.
Grain-boundary melting phase transition in the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline">mml:mrow<mml:mi mathvariant="normal">Cumml:mo−<mml:mi mathvariant="normal">Bisystem
Physical Review B, 2005
Copper embrittlement by Bi atoms is a typical example of a severe detrimental phenomenon in materials science. It has recently been proposed that the strong Bi segregation in Cu can cause a liquidlike film at the grain boundaries ͑GBs͒, when the alloy is in the single-phase ͑solid solution͒ region of the equilibrium phase diagram ͓L.-S. Chang, E. Rabkin, B. B. Straumal, B. Baretzky, and W. Gust, Acta Mater. 47, 4041 ͑1999͔͒. However, a direct experimental confirmation of a liquidlike state of GBs in such a case is missing. If a liquidlike GB phase is indeed formed, the GB diffusivity should dramatically be enhanced. Radiotracer GB diffusion of 64 Cu and 207 Bi radioisotopes were measured in a set of well-characterized Cu-Bi alloys in the single-phase ͑solid solution͒ region of the equilibrium phase diagram as well as in the two-phase ͑solid + liquid͒ region. An abrupt increase of the GB diffusivities of both Cu and Bi by about two orders of magnitude was observed at certain Bi contents which are unequivocally in the single-phase region and are definitely less than that associated with the bulk solidus concentration. This critical Bi concentration was the same for selfand solute GB diffusion. With further increase of the Bi content and after exceeding the bulk solidus concentration, only marginal changes in the diffusivities of Cu as well as Bi were observed. The present results convincingly showed the occurrence of the premelting phase transition in GBs of the Cu-Bi system.
Grain boundary wetting transition in Al–Mg alloys
A B S T R A C T The melting of coarse-and fine-grained Al–Mg alloys was studied by the differential scanning calo-rimetry (DSC). The transition from incomplete to complete wetting of grain boundaries (GBs) takes place in binary Al–Mg alloys. The heat effect of GB wetting manifests itself in the asymmetric shape of the DSC melting curve. By decreasing grain size (and increasing specific area of GBs) the GB " shoulder " in DSC curve becomes more pronounced. Basing on DSC data, the position of GB solidus (or premelting) line for Al–Mg alloys has been estimated. Contrary to the Al–Zn alloys, the difference between bulk and GB solidus does not exceed 2 °C.
Grain-boundary melting phase transition in theCu−Bisystem
Physical Review B, 2005
Copper embrittlement by Bi atoms is a typical example of a severe detrimental phenomenon in materials science. It has recently been proposed that the strong Bi segregation in Cu can cause a liquidlike film at the grain boundaries ͑GBs͒, when the alloy is in the single-phase ͑solid solution͒ region of the equilibrium phase diagram ͓L.-S. Chang, E. Rabkin, B. B. Straumal, B. Baretzky, and W. Gust, Acta Mater. 47, 4041 ͑1999͔͒. However, a direct experimental confirmation of a liquidlike state of GBs in such a case is missing. If a liquidlike GB phase is indeed formed, the GB diffusivity should dramatically be enhanced. Radiotracer GB diffusion of 64 Cu and 207 Bi radioisotopes were measured in a set of well-characterized Cu-Bi alloys in the single-phase ͑solid solution͒ region of the equilibrium phase diagram as well as in the two-phase ͑solid + liquid͒ region. An abrupt increase of the GB diffusivities of both Cu and Bi by about two orders of magnitude was observed at certain Bi contents which are unequivocally in the single-phase region and are definitely less than that associated with the bulk solidus concentration. This critical Bi concentration was the same for selfand solute GB diffusion. With further increase of the Bi content and after exceeding the bulk solidus concentration, only marginal changes in the diffusivities of Cu as well as Bi were observed. The present results convincingly showed the occurrence of the premelting phase transition in GBs of the Cu-Bi system.
Acta Metallurgica et Materialia, 1991
Tin and zinc penetration along the tilt grain boundary 43 ° [100] in b.c.c. Fe-5 at.% Si alloy is studied in the temperature range from 652 to 975°C. Wetting transition of grain boundary by the tin-rich melt at T w = 810 + 5°C is observed. Above T w there is a thin wetting film at grain boundary. With zinc penetration along the grain boundary a wetting film has been observed at all temperatures studied. Behind that film there is a region with an unusually high diffusivity of zinc, and below that region there is a region of "ordinary" grain boundary diffusivity. Such a phenomenon may be explained in terms of the phase transition "grain boundary-thin wetting film on the boundary", which is commonly known as a premelting phase transition. A model is proposed which explains the form of the temperature dependence of the concentration cat , at which such transition occurs, and, in particular, the influence of the "paramagnet-ferromagnet" transition in the bulk on the premelting transition. The influence of the temperature dependence of the volume solubility limit, co, on the cat (T) dependence is also discussed. In critical region below Curie point Tc critical exponents d of magnetic part of activation free energy of bulk and grain boundary diffusion are calculated. Critical index d for grain boundary diffusion by premelting layer, as well as activation energy in paramagnetic region, lies in the interval between bulk values of d and estimation of d for truly two-dimensional grain boundary diffusion.
Dynamics of grain boundary premelting
Scientific Reports, 2020
The mechanical strength of a polycrystalline material can be drastically weakened by a phenomenon known as grain boundary (GB) premelting that takes place, owing to the so-called disjoining potential, when the dry GB free energy \sigma _{gb}σgbexceedstwicethefreeenergyofthesolid–liquidinterfaceσ gb exceeds twice the free energy of the solid–liquid interfaceσgbexceedstwicethefreeenergyofthesolid–liquidinterface\sigma _{sl}σsl.WhilepreviousstudiesofGBpremeltingarealllimitedtoequilibriumconditions,weuseamulti−phasefieldmodeltoanalyzepremeltingdynamicsbysimulatingthesteady−stategrowthofaliquidlayeralongadryGBinaninsulatedchannelandtheevolutionofapre−meltedpolycrystallinemicrostructure.Inbothcases,ourresultsrevealthecrucialinfluenceofthedisjoiningpotential.AdryGBtransformsintoapre−meltedstateforagrain−size−dependenttemperatureintervalaroundσ sl . While previous studies of GB premelting are all limited to equilibrium conditions, we use a multi-phase field model to analyze premelting dynamics by simulating the steady-state growth of a liquid layer along a dry GB in an insulated channel and the evolution of a pre-melted polycrystalline microstructure. In both cases, our results reveal the crucial influence of the disjoining potential. A dry GB transforms into a pre-melted state for a grain-size-dependent temperature interval aroundσsl.WhilepreviousstudiesofGBpremeltingarealllimitedtoequilibriumconditions,weuseamulti−phasefieldmodeltoanalyzepremeltingdynamicsbysimulatingthesteady−stategrowthofaliquidlayeralongadryGBinaninsulatedchannelandtheevolutionofapre−meltedpolycrystallinemicrostructure.Inbothcases,ourresultsrevealthecrucialinfluenceofthedisjoiningpotential.AdryGBtransformsintoapre−meltedstateforagrain−size−dependenttemperatureintervalaroundT_mTm,suchthatacriticaloverheatingofthedryGBsoverT m , such that a critical overheating of the dry GBs overTm,suchthatacriticaloverheatingofthedryGBsoverT_m$$ T m should be exceeded for the classical melting process to take place, the liquid layer to ach...