Enhanced strength and ductility in a friction stir processing engineered dual phase high entropy alloy

www.nature.com/scientificreports OPEN Received: 19 October 2017 Accepted: 14 November 2017 Published: xx xx xxxx Enhanced strength and ductility in a friction stir processing engineered dual phase high entropy alloy S. S. Nene1, K. Liu1, M. Frank1, R. S. Mishra 1 , R. E. Brennan2, K. C. Cho2, Z. Li 3 & D. Raabe3 The potential of high-entropy alloys (HEAs) to exhibit an extraordinary combination of properties by shifting the compositional regime from the corners towards the centers of phase diagrams has led to worldwide attention by material scientists. Here we present a strong and ductile non-equiatomic HEA obtained after friction stir processing (FSP). A transformation-induced plasticity (TRIP) assisted HEA with composition Fe50Mn30Co10Cr10 (at.%) was severely deformed by FSP and evaluated for its microstructure-mechanical property relationship. The FSP-engineered microstructure of the TRIP HEA exhibited a substantially smaller grain size, and optimized fractions of face-centered cubic (f.c.c., γ) and hexagonal close-packed (h.c.p., ε) phases, as compared to the as-homogenized reference material. This results in synergistic strengthening via TRIP, grain boundary strengthening, and effective strain partitioning between the γ and ε phases during deformation, thus leading to enhanced strength and ductility of the TRIP-assisted dual-phase HEA engineered via FSP. High-entropy alloys (HEAs) represent a special class of materials that were orginaly designed to obtain a single-phase massive solid solution devoid of any secondary phases1. The approach provides high solid solution strengthening and may suppress formation of brittle intermetallic phases1,2. Significant efforts have been devoted to develop new HEAs for overcoming the strength-ductility trade-off inherent in most conventional materials. A prominent example is an equiatomic HEA with composition Fe20Mn20Ni20Co20Cr20, which showed excellent strength, ductility, and fracture toughness at cryogenic and room temperatures2. However, the limitation of the equi-atomic HEA approach is that it represents a single point within a huge compositional phase space. Thus, a number of non-equiatomic HEAs such as Al1.5CoCr0.5FeNi0.5 and different variants of the FeMnNiCoCr system were introduced, showing in part promising property profiles3–6. HEAs have shown a strong potential in tuning their primary deformation mechanisms by adjusting their chemical composition and by engineeering the microstructure via processing. Recent work by Li et al.6 demonstrates tunability of deformation mechanisms such as dislocation slip, twin induced plasticity (TWIP) and trnasformation induced plasticity (TRIP) by varying the Mn content from 45 to 30 at % in the Fe-Mn-Co-Cr system. This led to the developement of another class of HEAs known as dual phase Fe50Mn30Co10Cr10 HEA which utlizes the TRIP effect as the primary strain accomodation mechanism during plastic deformation. In this material a simultaneous increase in strength and ductility was obtained due to the engineered fraction and thermodynamic stability of the face-centered cubic (f.c.c., γ) and hexagonal close-packed (h.c.p., ε) phases in the microstructure through well designed composition and thermomechanical processing3,4,6. While such microstructures and their effects associated with conventional thermomechanical processing on the deformation behavior in HEAs, including TRIP HEAs, have been investigated in detail, studies on severe plastic deformation, such as equal channel angular pressing (ECAP), high-pressure torsion (HPT) and friction stir processing (FSP), of HEAs and their effects on the mechanical properties7–9 have been limited so far. Among these techniques, FSP is the most industrially feasible process for bulk products and has applicability for solid state joining. Earlier work by Kumar et al.9 showed that FSP of Al0.1CoCrFeNi HEA resulted in substantial improvement of strength and ductility when compared with the as-cast condition, due to enhanced grain refinement and a greater fraction of high angle grain boundaries. The difference between FSP and ECAP/HPT lies in the pathway of microstructural refinement. The 1 Center for Friction Stir Processing, Department of Materials Science and Engineering, University of North Texas, Denton, Texas, 76203, USA. 2Weapons and Materials Research Directorate, U.S. Army Research Laboratory, Aberdeen Proving Grounds, MD, 21005, USA. 3Max-Planck-Institut für Eisenforschung, Max-Planck-Str. 1, 40237, Düsseldorf, Germany. Correspondence and requests for materials should be addressed to R.S.M. (email: Rajiv. ) SCiENTiFiC REPOrTS | 7: 16167 | DOI:10.1038/s41598-017-16509-9 1 www.nature.com/scientificreports/ Figure 1. (a–c) Phase maps for the material in the as-homogenized condition; after additional 350, and after 650 RPM processing, (d1–d2) corresponding EDS results for 350 RPM processed sample, (e) corresponding XRD results for the as-homogenized and FSP samples, (f) microstructural properties including the fraction of high angle grain boundaries, low angle grain boundaries, and kernel average misorientation values after FSP. AH: as-homogenized; FSP: friction stir processing; RPM: rotations per minute; LAGB: low angle grain boundary; HAGB: high angle grain boundary. fine grain size obtained from FSP results from dynamic recrystallization and limiting subsequent grain growth10. Grain refinement during ECAP and HPT occurs by deformation-driven grain fragmentation and patterned dislocation storage leading to a high fraction of low angle grain boundaries11,12. Li et al.3,4 had shown that the strength-ductility profile of this material depends not only on grain size but also on the fraction of the h.c.p. ε phase and the density of stacking faults4. FSP leads to a highly transient microstructure type, as the material is deformed at a high temperature and individual grains experience different stages of straining. Here we used FSP to engineer the microstructure of the TRIP Fe50Mn30Co10Cr10 HEA along two strands of motivation. The first is that grain size reduction enhances the stability of the f.c.c. γ-phase grains against deformation-driven transformation. This effect should lead to a more uniform distribution of the f.c.c. γ → h.c.p. ε transformation zones, thereby maximizing the dispersion and hence the TRIP effect3. Second, joinability of any advanced novel alloy is an essential precondition for using it in engineering applications. FSP results provide a first overview of the alloy’s suitability for friction stir welding. Results and Discussion Microstructure evolution after FSP. Figure 1a–c show electron back scattered diffraction (EBSD) maps for the as-homogenized and 350 and 650 rotations per minute (RPM) treated FSP samples, highlighting the drastic reduction in average grain size from ~100 µm to 6.5 and 5.2 µm, respectively. Since FSP is a high temperature, severe plastic deformation process, it also changed the fraction of f. (...truncated)