Structural-peritectic transformations in Cr-C and Fe-P ...

Structural-peritectic transformations in Cr-C and Fe-P melts Larisa V. Kamaeva, Irina V. Sterkhova, and Vladimir I. Lad’yanov Citation: AIP Conference Proceedings 1673, 020016 (2015); doi: 10.1063/1.4928270 View online: http://dx.doi.org/10.1063/1.4928270 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1673?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Electronic spectroscopy and electronic structure of diatomic CrC J. Chem. Phys. 133, 034303 (2010); 10.1063/1.3456178 First principles investigation of chromium carbide, CrC J. Chem. Phys. 123, 014302 (2005); 10.1063/1.1926247 Local atomic structure in amorphous Fe‐P alloys J. Appl. Phys. 63, 4124 (1988); 10.1063/1.340516 Structures and composition before and after annealing of coatings in the Cr–C binary system produced by reactive physical deposition J. Vac. Sci. Technol. A 3, 2378 (1985); 10.1116/1.572885 Concentration dependences of electrical resistivity and magnetoresistance of Fe‐P amorphous alloys J. Appl. Phys. 53, 8254 (1982); 10.1063/1.330299

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Structural-peritectic transformations in Cr-C and Fe-P melts Larisa V. Kamaeva, Irina V. Sterkhova, Vladimir I. Lad`yanov Physical-Technical Institute Ural Branch of RAS, 132 Kirov Str., Izhevsk 426000, Udmurtia, Russia. Abstract. Undercooling and the solidification processes of Fe-P and Cr-C melts with eutectic composition have been studied in this paper. It is shown that the equilibrium eutectics, α-Fe/Fe3P (for Fe83P17 melt) and α-Cr/Cr23C6 (for Сr86С14 melt), are formed at cooling the eutectic Fe-P and Cr-C liquid alloys from the melt temperatures lower than t* (t*= 1125ºС for Fe83P17 and t*= 1610ºС for Cr86C14) with the cooling rates interval from 20 to 100ºС/min. The nonequilibrium eutectics, α-Fe/Fe2P (for Fe83P17) and α-Cr/Cr7C3 (for Сr86С14), are formed at cooling from the melt temperatures higher than t* and decomposed to equilibrium phases on further cooling. Keywords: undercooling, crystallization of melts, eutectic PACS: 72.15.Cz, 64.70.dg, 81.30.-t.

INTRODUCTION Most amorphizing alloys including bulk-amorphous alloys are eutectic systems with a peritectic [1-3]. The eutectic melts have the quasieutectic structures at temperatures slightly higher than the liquidus temperature [4, 5]. As a rule, the components of such quasieutectic structures are atom microgroups with the short-range composition ordering based on crystal phases which form a eutectic in solid state [4]. However, structural transformations in the melts can be observed as the melt temperature changes [6-8]. Sharp structural transformations in melts on heating cause the difference of the short-range ordering in amorphising melts from the short-range ordering in equilibrium crystal phases which provide high melt undercooling at high cooling rates [9]. In this connection structural changing in a liquid phase of eutectic alloys which include chemical compounds resulted from the peritectic reaction is of great interest. It is also of great interest to study the effect of such structural changes on melts undercooling. The FeP and Cr-C systems have been used as the subject of investigation. Alloys based on the Fe-P system are easily amorphizing alloys of metal- metalloid type. A simple eutectic-like diagram is characteristic of the Fe-P system in metal-rich areas [10]. The Fe3P phosphide is formed in accordance with the peritectic reaction at 1166ºC: L+Fe2P→ Fe3P. It forms a eutectic with α-Fe in the vicinity of 17at.%P at 1048°С [10]. The alloys with the phosphor concentration of 15-20at.% is characterized by the larger undercooling values [11]. In these conditions the metastable eutectic, α-Fe/Fe2P with the melting point equal to 945ºC is formed. A Cr-C binary system has also a simple eutectic-like phase diagram in chromium-rich areas [12]. A eutectic of the Cr-C system contains a Cr23C6 phase which is formed in accordance with the peritectic reaction at 1612ºС [12]. The Cr atoms in the Cr23C6 carbide can be easily displaced by the atoms of transition metals. Consequently, the M 23C6 complex carbide family, which is referred to as a τ- phase, is formed. A τ- phase is the “phase-glassformer” for the group of bulk amorphous alloys based on 3-d transition metals [8, 13]. It has been shown by the viscosity investigations that quasieutectic structures are realized in the Fe-P [6] and CrC [14] melts with the component concentration close to the eutectic concentration. It has been shown that thermal structural transformations are observed to occur in these melts. Therefore, the processes of solidification of the eutectic melts, Fe83P17 and Cr86C14 at cooling from various temperatures have been studied in this paper.

EXPERIMENTAL PROCEDURES The crystallization processes were investigated by the methods of differential thermal analysis, X-ray structural analysis and metallography. The differential thermal analysis was carried out using a high-temperature thermal analyzer in pure helium. The investigations have been carried out under cycling conditions with the meltingcrystallization interval. In each following cycle the temperature was successively increased by 10ºC from the liquidus temperature (TL) to Tmax (Tmax is equal to 1600ºC for the Fe-P melts and 1680ºC for the Cr-C melts («heating» regime), then it was step-by-step decreased from Tmax to TL («cooling» regime). The heating rate was 100 °С /min. After being exposed for the given temperature for 20 minutes the melt was cooled. The cooling rates were Proceedings for the XV Liquid and Amorphous Metals (LAM-15) International Conference AIP Conf. Proc. 1673, 020016-1–020016-4; doi: 10.1063/1.4928270 © 2015 AIP Publishing LLC 978-0-7354-1320-7/$30.00

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from 20 to 100°С/min. The melting and crystallization temperatures were determined by the DTA thermograms. The undercooling values were defined by to the solidus and the liquidus temperatures as the difference of these temperatures at heating and cooling i.e. ΔТS=Tsheting-TScooling and ΔТL=TLheating-TLcooling, correspondingly. The X-ray structure investigation was carried out using a DRON–6 diffractometer (Cu-Kα radiation with graphite monochromator).

EXPERIMENTAL RESULTS Fig. 1 presents the undercooling dependences on the melt temperature obtained for a Fe-P alloy, Fe83P17, in the “heating” (black circles) and cooling (while circles) regimes. At the superheating temperature of the melt, t*=1125°С, both in the “heating” regime and “cooling” regime a sharp increase of ΔТS takes place. As seen from the thermograms (Fig. 1b) the crystallization process depends on the superheating melt temperature. On the superheating of the melt lower than 1125 оС only one maximum of heat release is evidenced. In the case of superheating higher than 1125оС two exothermal peaks are observed, the peak which is larger in area (this peak corresponds to the crystallization eutectic) being sharply shifted to the lower temperature range. Thus, with thermograms changing the values of undercooling which is one of the conditions for the beginning of the eutectic crystallization increase dramatically.

FIGURE 1. The dependences of undercooling defined by to the solidus temperatures (ΔТS) on the melt superheating temperature obtained for Fe83P17 in the “heating” (●) and cooling (○) regimes (a); the microstructures of the Fe83P17 alloy after cooling from 1100°С (с) and 1550°С (d).

A microstructure of the one-step crystallized sample cooled from 1100оС mainly consists of the eutectic structure (Fig. 1c). Small crystals of Fe3P which border the α-Fe dendrites are also marked on the surface of the metallographic section. At considerably magnification it is seen that the entire eutectic structure is a typical cell eutectic which is characteristic of the metal-metalloid systems. A microstructure of the same melt cooled from 1550оС with two peaks in the thermogram (Fig. 1b) differs from that of mentioned above. A special net of the primary dendrites of α-Fe (black lines on Fig. 1d) intergrows throughout the entire body of the ingot. A various dispersion structure which can not be referred to as a eutectic occupies the areas between the dendrites (Fig. 1d). It is assumed that such a structure is formed due to the decomposition of the eutectic which consists of α-Fe and a phosphide, the phosphide being richer in phosphorus than Fe3P.


The temperature dependence of the Cr86C14 melt undercooling, the thermograms of its cooling from 1605 and 1680°С as well as the microstructures of the ingots obtained after cooling from these temperatures are shown in Fig. 2. It is seen from Fig. 2a-d that with temperature increasing higher than 1610°C the crystallization character sharply changes similar to that of described for Fe83P17. At cooling the melt from 1605°C there are two exothermal effects (Fig. 2b) in the thermograms which are poorly separated. A microstructure consisting of eutectic colonies with the internal structure typical of metal-metalloid systems forms due to such cooling of the melt (Fig. 2c). A lager structural compound consisting of the mixture of the Cr23C6 crystals and dendrites of α-Cr solid solution is distributed along the boundaries of the eutectic colonies (Fig. 2c). At cooling from 1680°С three peaks of thermal release are present in the thermograms. A microstructure of the ingot cooled from 1680°С takes the form characteristic of hypoeutectic alloys contained of large primary dendrites of α-Cr. The area between the dendrites is occupied by the two-phase structure enriched in carbide which is obtained due to the peritectoid decomposition.

FIGURE 2. The dependences of undercooling defined by to the solidus temperatures (ΔТS) on the melt superheating temperature obtained for Cr86C14 in the “heating” (●) regime (a); the microstructures of the Cr86C14 alloy after cooling from 1605°С (с) and 1680°С (d).

According to the X-ray structural analysis all the ingots after DTA regardless of the cooling rate are equilibrium in phase composition. The Fe-P alloys consist of the solid solution based on α-Fe and Fe3P. The Cr-C alloys contain α-Cr and Cr23C6.

DISCUSSION It has been supported by the experimental data that at the cooling of the Fe83P17 melt from 1100оС the melt crystallization starts at undercooling value of ~50°C with the α-Fe formation due to the Fe3P formation delay. Along with this the remained liquid phase is enriched by the phosphorus. This results in the precipitation of the Fe3P crystals. In the process of the α-Fe/Fe3P eutectic crystallization Fe3P is the leading phase. Its formation results in the formation of the equilibrium eutectic throughout the remained volume. On cooling from 1550оС as well as on cooling from 1100°С under slight undercooling conditions, ~50°C (Fig. 1b), the process of crystallization starts with the precipitation of primary α-Fe dendrites. However, crystallization of α-Fe from the liquid alloy which is eutectic in composition does not give rise to Fe3P. The dendrites of the solid solution intergrowth freely throughout the ingot. On further cooling the nonequilibrium eutectic, α-Fe/Fe2P, is formed only if the undercooling value reaches ~200 оС. Fe2P decomposes according to α-Fe+Fe2P→Fe3P on further cooling. In Cr86C14 the crystallization behavior is


observed to change at 1610°C. With the temperature higher than 1610°C the Cr23C6 phase fails to form from the melt which leads to the growth of the metal dendrites, the nonequilibrium eutectic α-Cr/Cr7C3 formation and subsequent decomposition to the equilibrium phases. Thus the changes of the number of the crystallization stages and the alloys microstructures which take place at cooling from t* (t*= 1125оС for Fe-P and t*=1610оС for Cr-C) are caused by the change of the crystallization mechanism (from equilibrium eutectic formation to nonequilibrium eutectic formation). Taking into consideration the microinhomogeneous structure of the Fe-P and Сr-С melts it has been concluded that the peculiarities of nucleation and growth of the crystal phases point to the fact that the change of the crystallization type relates to the change of the phosphide and carbide types of the clusters in the liquid phase from Fe3P to Fe2P (for Fe-P system) and Cr23C6 to Cr7C3 (for Cr-C) at melt superheating over the critical temperature. The latter are close to the peritectic transformation temperatures in the phase equilibrium diagrams of the investigated systems. The transformations described above are considered as a manifestation of the structural-peritectic transformations in a liquid phase caused by the change of the short-range composition ordering at the specified temperatures. The relationship between the structures and properties of the liquid and solid phases in the process of crystallization reveals that the structural features of the undercooled melt are formed depending on the structural elements of the initial equilibrium liquid phase. In this case its structural transformation results to the change of undercooling, nucleation conditions and the growth of equilibrium and nonequilibrium carbides and phosphides including the morphology of the primary crystals and eutectic structures.

CONCLUSIONS The investigations carried out by differential thermal, X-ray structural analyses and metallography have been shown that the equilibrium eutectics, α-Fe/Fe3P (for Fe83P17 melt) and α-Cr/Cr23C6 (for Сr86С14 melt), are formed at cooling the eutectic Fe-P and Cr-C liquid alloys from the melt temperatures lower than t* (t*= 1125ºС for Fe83P17 and t*= 1610ºС for Cr86C14) with the cooling rates interval from 20 to 100ºС/min. The nonequilibrium eutectics, αFe/Fe2P (for Fe83P17) and α-Cr/Cr7C3 (for Сr86С14), are formed at cooling from the melt temperatures higher than t* and decomposed to equilibrium phases on further cooling.

ACKNOWLEDGMENTS This work has been carried out thanks to the financial support of the RFFR grant № 12-03-31798.

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