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Grain Boundary Phase Transitions and Their Influence on Properties of Polycrystals 37 Mater.Phys.Mech.2 (2000) 37-41 Grain Boundary Phase Transitions and Their Influence on Properties of Polycrystals 37 GRAIN BOUNDARY PHASE TRANSITIONS AND THEIR INFLUENCE ON PROPERTIES OF POLYCRYSTALS B. B. Straumal1 and M. Friesel2 1 Institute of Solid State Physics, Chernogolovka, Moscow District, RU-142432, Russia 2 SIMS Laboratory, Chalmers University of Technology, S-41296 Gothenburg, Sweden Received: June 25, 2000 Abstract. 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. The GB wetting tie line appears in the L+S area. Above the temperature of the GB wetting phase transition a GB can not exist in equlibrium with the liquid bulk phase. The liquid phase has to substitute the GB and to separate both grains. The experimental data on GB wetting phase transitions in the systems Al– Sn, Al–Ga, Al–Sn–Ga, Cu–In, Cu–Bi, Fe–Si–Zn, Mo–Ni, W–Ni, Zn–Sn and Zn–In 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 the GB solidus line is crossed by a change in the temperature or concentration. The experimental data on GB segregation, energy, mobility and diffusivity in the systems Cu–Bi, Al–Ga, Al–Pb and Fe–Si–Zn obtained both in polycrystals and bicrystals are analysed. 1. INTRODUCTION by Cahn [11] the study of wetting phase transitions has been of great experimental and theoretical in- The properties of modern materials, especially those terest, primarily for planar solid substrates and fluid of superplastic, nanocrystalline or composite ma- mixtures [12–14]. Particularly, it was experimentally terials, depend critically on the properties of inter- shown that the wetting transition is of first order, nal interfaces such as grain boundaries (GBs) and namely the discontinuity of the surface energy was interphase boundaries (IBs). All processes which measured and the hysteresis of the wetting behav- can change the properties of GBs and IBs affect ior was observed [15, 16]. The important difference drastically the behavior of polycrystalline metals and is that in case of GB wetting only two phases ceramics [1]. GB phase transitions are one of the coexist, namely the liquid (melt) phase and the solid important examples of such processes [2]. one containing the boundary between the Recently, the lines of GB phase transitions be- misoriented grains. Therefore, the contact angle θ gan to appear in the traditional bulk phase diagrams also depends only on two different surface energies [2–7]. The addition of these equilibrium lines to the σ (the GB energy GB and the energy of the solid/liquid bulk phase diagrams ensures an adequate descrip- σ interphase boundary SL) instead of three ones in tion of polycrystalline materials. σ σ θ σ the usual experiments: GB = 2 SLcos ( /2). If GB< σ 2 SL, the GB is incompletely wetted and the contact 2. GB WETTING PHASE TRANSITION θ angle >0 (Fig.1a). At the temperature Tw of the GB σ σ ≥ In this work GB wetting, prewetting and premelting wetting phase transition GB = 2 SL, and at T Tw the phase transitions are considered. The GB melting, GB is completely wetted by the liquid phase and θ GB faceting transition and the “special GB – random = 0 (Fig.1b). If two GBs have different energies the GB phase transitions” are analyzed elsewhere [8– temperatures of their GB wetting transitions will also σ 10].One of the most important GB phase transitions differ: the lower GB, the higher Tw (Figs.1c and 1d). is the GB wetting transition. Since their prediction If the GB wetting phase transition is of first order, Corresponding author: B.B. Straumal, e-mail: [email protected] © 2000 Advanced Study Center Co. Ltd. 38 B. B. Straumal and M. Friesel Fig. 1. (a) Scheme of the equilibrium contact between the grain boundary in the solid phase S and the liquid phase L (incomplete σwetting). (b) Complete GB wetting. (c) Scheme of the temperature dependenceσ for the GB energy GB (for two different GBs) and the energy of the solid-liquidθ interface boundary SL. (d) Schemeσ of the σtemperature dependence of the contact angle for two grain boundaries with energies GB1 and GB2. Tw1 and Tw2 are the temperatures of the GB wetting phase transition. there is a discontinuity in the temperature derivative the two-phase region (S+L) of the bulk phase dia- ∂σ ∂ σ of the GB energy at Tw, which is equal to [ GB/ T– gram. Above this tie line GBs with an energy GB ∂ σ ∂ (2 SL)/ T ] [11, 16]. If the GB wetting phase transi- can not exist in equilibrium with the liquid phase. ∂σ ∂ ∂ σ ∂ tion is of second order, GB/ T = (2 SL)/ T at Tw. The liquid phase forms a layer separating the The theory predicts also the form of the tempera- crystals. θ → ture dependence (T ) at T Tw: it must be convex First indications of the GB wetting phase transi- for a first order wetting transition [θ~t1/2, where t= tions were found by measuring of the contact angles (Tw–T)/Tw] and concave for a second order wetting in polycrystals [17]. Correct measurements were transition: θ~t 3/2 [12]. Nowdays, the GB phase tran- later perfomed using metallic bicrystals with sitions of the second order were not observed ex- iundividual tilt GBs in the Al–Sn (Fig. 2), Cu–In (Fig. perimentally. 3) [4], Al–Pb–Sn [3, 18, 19], Al–Ga, Al–Sn–Ga [20, Consider the contact between a bicrystal and a 21], Cu–Bi [5, 22, 23], Fe–Si–Zn [24–27], Mo–Ni σ liquid phase L. If the GB energy GB is lower than [28], W–Ni [29] and Zn–Sn [7] systems. The tie- σ the energy of two solid/liquid interfaces 2 SL, the lines of the GB wetting phase transition were GB is not completely wetted and the contact angle constructed basing on the experimental data [3, 4, θ σ σ >0 (Fig.1a). If GB > 2 SL the GB is wetted 7, 18–29]. The difference in the GB wetting phase completely by the liquid phase and θ = 0 (Fig.1b). If transition temperature was experimentally revealed σ σ the temperature dependences GB(T) and 2 SL(T) for GBs with different energies [4, 18]. The precize intersect, then the GB wetting phase transition pro- measurements of the temperature dependence of ceeds at the temperature Tw of their intersection the contact angle revealed also that the GB wetting (Fig.1c). The contact angle θ decreases gradually phase transition is of the first order [18]. The indica- with increasing temperature down to zero at Tw. At tions of presence of the liquid-like phase along the θ T > Tw the contact angle =0 (Fig.1d). The tie line of dislocation lines were also found [23]. the GB wetting phase transition appears at Tw in Grain Boundary Phase Transitions and Their Influence on Properties of Polycrystals 39 Fig. 2. The AlSn phase diagram. Thick solid Fig. 3. The CuIn phase diagram. Thick solid lines represent the bulk phase transitions. Thin lines represent the bulk phase transitions. Thin solid lines are the tie lines of the GB wetting solid lines are the tie lines of the GB wetting phase transitions. Thin dotted represent the phase transitions in the (Cu)+L two phase area. estimation for the GB wetting phase transiton for the GB with highest possible energy. 3. GB PREWETTING AND PREMELTING [24–27]. It was found that the penetration profiles of PHASE TRANSITIONS Zn along GBs consist of two sections, one with a small slope (high GB diffusivity) at high Zn It was pointed out by Cahn [30] that, when the critical concentrations and one with a large slope (low GB consolution point of two phases is approached, GBs diffusivity) at low Zn concentrations. The transition of one critical phase should be wetted by a layer of from one type of behavior to the other was found to another critical phase, and in the one-phase region occur at a definite Zn concentration cbt at the GB, of a phase diagram there should be a singularity which is an equilibrium characteristic of a GB and connected with an abrupt transition to a microscopic depends on the temperature. The GB diffusivity in- wetting layer. We distinguish two possible situa- creases about two orders of magnitude which is an tions: the first one, when a layer of the new phase indication of a quasi-liquid layer present in the GBs is formed on the GB (prewetting transition), and the at high Zn concentration. The line of GB premelting second one, when the GB is replaced by a layer of phase transition in the one-phase area of the bulk the new phase (premelting phase transition). At the phase diagram continues the line of the GB wetting prewetting transition the difference between two phase transion in the two-phase L+S area: by pres- phases must be small, while at the premelting tran- sure increase both the GB wetting and the GB sition the wetting phase may differ from that of the enhanced diffusivity disappear together at the same bulk dramatically. The lines of the GB prewetting or pressure value (Fig. 5) [27]. The GB mobility was premelting phase transitions appear in the one-phase studied for two tilt GBs in bicrystals grown of high areas of the bulk phase diagrams where only one purity 99.999 wt.% Al and of the same material bulk phase can exist in the thermodynamic equilib- doped with 50 wt.
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