metals Article
Theoretical Study of Particle Dissolution during Homogenization in Cu–Fe–P Alloy Spyros Papaefthymiou 1, * 1
2
*
ID
, Marianthi Bouzouni 1,2 and Evangelos Gavalas 1,2
ID
Laboratory of Physical Metallurgy, Division of Metallurgy and Materials, School of Mining & Metallurgical Engineering, National Technical University of Athens, 9, Her. Polytechniou str., Zografos, 15780 Athens, Greece;
[email protected] (M.B.);
[email protected] (E.G.) Department of Physical Metallurgy and Forming, Hellenic Research Centre for Metals S.A.—ELKEME S.A., 56th km Athens-Lamia Nat. Road, 32011 Oinofyta, Greece Correspondence:
[email protected]; Tel.: +30-210-772-4710
Received: 18 May 2018; Accepted: 12 June 2018; Published: 14 June 2018
Abstract: The effect of temperature, soaking time and particle size on the dissolution of particles (Fe3 P and Fe) during homogenization was simulated employing Thermocalc® and DICTRA software. The initial precipitate size was determined through metallographic evaluation on industrial as-cast Cu–Fe–P alloy. The particle sizes vary from submicron (C o, there is a flux Fe atoms from the Fe particle andbe diffusional be this indicates that the Fe content in the Cu matrix should reach C composition. In order to achieve α achieved. When Cα > Co, this indicates that the Fe content in the Cu matrix should reach Cα this, Fe atoms In diffuse the Fe particle Cu matrix of Fe particles canand be composition. orderfrom to achieve this, Fe towards atoms diffuse fromand thethe Fe dissolution particle towards Cu matrix achieved. The different solubility particles andThe Cu different matrix is solubility attributedof toparticles the chemical the dissolution of Fe particles canofbe achieved. and composition Cu matrix is and crystal structure. Fe P and Fe particles were considered to have a BCC (body centered cubic) attributed to the chemical3 composition and crystal structure. Fe3P and Fe particles were considered structure the Cu matrixcubic) has anstructure FCC (facewhereas centeredthe cubic) solute atoms to have awhereas BCC (body centered Cu structure. matrix hasThe an flux FCCof(face centered towards the interface is described by an error function introduced by Whelan [16]. The diffusional cubic) structure. The flux of solute atoms towards the interface is described by an error function distance thatbythe atoms[16]. haveThe to move ∆x can be calculated the overall concentration of mass is introduced Whelan diffusional distance that thefrom, atoms have to move Δx can be calculated given (2) for isothermal transformation [23]: from, by theEquation overall concentration of mass is given by Equation (2) for isothermal transformation [23]: 11 Cβ − Co x = (Co − Ca )∆x ∆ 22
(2) (2)
where CC stands stands for for concentration concentrationand andxxfor forparticle particlesize. size where
Figure 11. 11. Plot of Fe profiles versus Cu matrix (a) growth of Fe particles, Figure particles, (b) dissolution dissolution of Fe particles. particles. The diagrams diagrams are are concentrated concentratedon onthe theinterface interfacethus thusCβ isis9090wt. wt.%. %. The
It can can be be observed observed that that in in alloying alloying element-enriched element-enriched regions, regions, particles particles had had even even increased increased their their It size. The increase of particle size can also be explained from the Ostwald ripening theory where large size. The increase of particle size can also be explained from the Ostwald ripening theory where large particles are are more more energetically energeticallystable stablethan than smaller smaller particles. particles. Atoms Atoms of of the the small small particles particles diffuse diffuse and and particles attachto tothe thelarge largeparticles particlesresulting resulting large particles growing in size small particles shrinking attach inin large particles growing in size andand small particles shrinking [24]. [24].
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6. Conclusions Particle dissolution has been investigated during the homogenization process in a Cu–Fe–P alloy via Thermocalc® and DICTRA software. The phase diagrams indicate that the iron mass percentage does not influence the dissolution temperature. However, higher phosphorus concentration stabilizes the Fe3 P loop and increases the dissolution temperature. According to the simulation results, the particle dissolution depends on four critical parameters influencing each other: (a) the size of phosphides; (b) soaking time; (c) the temperature of homogenization; and (d) the local chemical composition of the base metal. In alloying element-depleted regions dissolution was realized in approximately 1 h for the small particles of 1 µm in the temperature of 1273 K (1000 ◦ C). Larger particles are more difficult, requiring longer, or are even impossible to dissolve. Chemical heterogeneity due to segregation caused local phenomena of a higher element concentration in the base metal, resulting in particles either remaining undissolved or in many cases increasing in size due to the Ostwald ripening phenomena. Metallographic evaluation showed that the experiments are consistent with the simulation. Microsegregation is evident in the initial microstructure situated mainly in grain boundaries and interdendritic regions. The homogenization heat treatment created in the microstructure patches of precipitation-free regions and other regions with a small number of enlarged particles due to a different local chemical equilibrium. For the optimal homogenization of the specific material, a high temperature of 1273 K (1000 ◦ C) is required. Homogenization time of 1.5–2 h will result in the dissolution of the small particles without affecting the larger ones. Longer soaking time will result in no further dissolution. Author Contributions: Conceptualization, S.P.; Methodology, S.P., E.G. and M.B.; Validation, E.G., M.B. and S.P.; Writing–Original Draft Preparation, M.B.; Writing–Review and Editing, S.P. & E.G.; Supervision, S.P.; Project Administration, E.G. Funding: This research received no external funding. Acknowledgments: The authors express their gratitude to the Hellenic Research Centre for Metals—ELKEME S.A., the R&D company VIOHALCO S.A., for their overall support to this research. The authors sincerely thank Athanasios Vazdirvanidis and Alexandros Antonopoulos for carrying out SEM and OM. Conflicts of Interest: The authors declare no conflict of interest.
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