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First-Principles Modeling of the Temperature Dependence for the Superlattice Intrinsic Stacking Fault Energies in L12 Ni75x Xx Al25 Alloys J.D.T. ALLEN, A. MOTTURA, and A. BREIDI Stronger and more resistant alloys are required in order to increase the performance and efficiency of jet engines and gas turbines. This will eventually require planar faults engineering, or a complete understanding of the effects of composition and temperature on the various planar faults that arise as a result of shearing of the c0 precipitates. In the current study, a combined scheme consisting of the density functional theory, the quasi-harmonic Debye model, and the axial Ising model, in conjunction with a quasistatic approach is used to assess the effects of composition and temperature of a series of pseudo-binary alloys based on the ðNi75x Xx ÞAl25 system using distinct relaxation schemes to assess observed differences. Our calculations reveal that the (111) superlattice intrinsic stacking fault energies in these systems decline modestly with temperature between 0 K and 1000 K. https://doi.org/10.1007/s11661-018-4763-4 Ó The Minerals, Metals & Materials Society and ASM International 2018

I.

INTRODUCTION

IN precipitation-strengthened alloys, the shearing of particles is often one of the active deformation mechanisms. Superalloys are no exception to this, and their complex shearing mechanisms are indeed partly responsible for their superior mechanical properties at high temperatures. Over the last few decades, increasing focus has been spent on understanding these shearing mechanisms, which change with composition and temperature. The crystal structure of the matrix (c, fcc) and precipitate (c0 , L12 ) phase is such that a full dislocation in the matrix results in the introduction of an anti-phase boundary (APB) in the precipitate phase. Other partial dislocations can also shear these precipitates, leading to a diverse range of faults: superlattice intrinsic stacking faults (SISFs), superlattice extrinsic stacking faults (SESFs), complex stacking faults (CSFs), which can themselves be intrinsic or extrinsic, twin structures and more complicated planar defects. The energies of these planar faults are extremely important as they determine the nature of the complex dislocation structures shearing the precipitates, as well as the segregation of solute elements to the fault

J.D.T. ALLEN and A. MOTTURA are with the School of Metallurgy and Materials, University of Birmingham, Edgbaston B15 2TT, UK. A. BREIDI is with the UK Atomic Energy Authority, Culham Science Centre, Oxfordshire OX14 3DB, UK. Contact e-mail: [email protected] Manuscript submitted March 18, 2018.

METALLURGICAL AND MATERIALS TRANSACTIONS A

energies, which in turns can affect the motion of dislocations through the precipitates. As a result, a number of mechanical properties, such as minimum grain size due to milling, strain hardening and yield stress depend on planar fault energies. Creep resistance is also affected by the planar fault energies.[1] As microstructure and