Abstract
Among equiatomic alloys of the Cr-Mn-Fe-Co-Ni system, MnFeNi was shown to exhibit a strong anti-invar behavior but little is known regarding its mechanical properties. The objective of the present study is to investigate Hall–Petch strengthening by grain and annealing twin boundaries in MnFeNi. For this purpose, seven different grain sizes between 17 and 216 µm were produced. Mean grain sizes (excluding annealing twin boundaries) and crystallite sizes (including them) were determined using the linear intercept method. Overall, 25% of the boundaries were found to be annealing twin boundaries regardless of the grain size. In some cases, two twin boundaries can be present in one grain forming an annealing twin, which thickness represents one quarter of the mean grain size. Based on a comparison of the mean twin thickness of different alloys with different stacking fault energy (SFE), we estimated an SFE of 80 ± 20 mJ/m2 for MnFeNi. Compression tests of MnFeNi with different grain sizes were performed between 77 and 873 K and revealed a parallel shift of the Hall–Petch lines with temperature. The interaction between dislocations and boundaries was investigated by scanning transmission electron microscopy (STEM) in a deformed specimen. It was found that a large number of dislocations are piling up against grain boundaries while the pile-ups at annealing twin boundaries contain much fewer dislocations. This indicates that annealing twin boundaries in this alloy are less effective obstacles to dislocation motion than grain boundaries.
Highlights
Yeh et al [1] first introduced the concept of high-entropy alloys (HEAs) which represent a new class of metallic materials composed of at least five elements in approximately equiatomic concentrations
Equiatomic face centered cubic (FCC) alloys from the Cr-Mn-Fe-Co-Ni system have attracted a lot of interest in the scientific community due to their very interesting mechanical properties, namely, a good combination of ductility and ultimate tensile strength [8,9], a high work-hardening rate [10,11], an impressive fracture toughness [12], and an excellent resistance to fatigue-crack growth [13]
Mn were immersed in 500 mL of deionized water, (ii) a solution of nitric acid (38 vol.% in distilled water) was added stepwise under constant manual stirring until a shiny surface was visible, (iii) 500 mL of deionized water were subsequently added to dilute and neutralize the acid, and (iv) Mn was further rinsed with ethanol and dried using a hair dryer
Summary
Yeh et al [1] first introduced the concept of high-entropy alloys (HEAs) which represent a new class of metallic materials composed of at least five elements in approximately equiatomic concentrations. Yeh et al [2] further classified alloys comprising two to four elements in near equiatomic proportions as medium-entropy alloys (MEAs). Several of these alloys may form disordered solid solutions with simple crystallographic structures, e.g., face centered cubic (FCC) [3,4]. Wu et al [14] were the first to investigate the phase stability and mechanical properties of several equiatomic MEAs of the Cr-Mn-Fe-Co-Ni system. Most of these MEAs were found to be single phase FCC after casting and homogenization as well as after cold rolling and recrystallization [14]
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