Cooperative deformation in high-entropy alloys at ultralow temperatures

Abstract

High-entropy alloys exhibit exceptional mechanical properties at cryogenic temperatures, due to the activation of twinning in addition to dislocation slip. The coexistence of multiple deformation pathways raises an important question regarding how individual deformation mechanisms compete or synergize during plastic deformation. Using in situ neutron diffraction, we demonstrate the interaction of a rich variety of deformation mechanisms in high-entropy alloys at 15 K, which began with dislocation slip, followed by stacking faults and twinning, before transitioning to inhomogeneous deformation by serrations. Quantitative analysis showed that the cooperation of these different deformation mechanisms led to extreme work hardening. The low stacking fault energy plus the stable face-centered cubic structure at ultralow temperatures, enabled by the high-entropy alloying, played a pivotal role bridging dislocation slip and serration. Insights from the in situ experiments point to the role of entropy in the design of structural materials with superior properties.

Relation between Intergranular Stress of Austenite and Martensitic Transformation in TRIP Steels Revealed by Neutron Diffraction

“…The formation of α′ in Steel C might occur via the γ →α′ process directly and/or via the γ →ε→α′ process indirectly; however, the martensitic transformations in the three alloys was concluded to start from the instability of γ. values of the γ -(111)//LD, γ -(200)//LD, and γ -(220)//TD grains increase, and oppositely, those of the γ -(111)//TD and γ -(220)//LD grains decrease with progressive tensile loading. 23,30) The decreases in the I hkl J rel meas , values in Fig. 3(a) were therefore not always due to the occurrence of martensitic transformation; they included contributions from both texture and martensitic transformation.…”

“…In a previous report involving commercial 304-type stainless steel, 23) stresses in γ -(hkl)// LD grains evaluated from the ε γ − hkl values by multiplying by the Young's moduli of γ-[hkl] orientations showed the largest values for γ -(200)//LD grains and the smallest values for γ -(111)//LD grains; i.e., intergranular stress partitioning was observed. At Stage III, the ε γ − hkl values increase almost linearly with increasing true stress as long as no phase transformation occurs; 15,23,30,36) however, the ε γ − hkl responses to the true stress change, exhibiting smaller slopes above a true stress of ~650 MPa. This finding suggests that another phase formed as a result of the martensitic transformations bearing larger stress.…”

“…and ε γ − hkl to the true stress can be divided to three stages: 23) Stage I, elastic deformation; Stage values that show an increasing trend should monotonically increase, whereas those that show a decreasing trend should monotonically decrease during tensile loading until the fracture point. 23,30) The responses of I hkl J rel meas , values for γ -(200)// LD and γ -(111)//LD grains changed, exhibiting smaller slopes that subsequently became negative, indicating that martensitic transformations occurred in these grains under progressive tensile loading. Figure 8(b) shows the responses of ε γ − hkl with respect to the true stress during tensile loading of Steel C. Upon entering Stage II, the slopes of the ε γ − hkl responses started to change, where the γ -(200)//LD slope increased and the γ -(111)//LD slope decreased.…”

Manipulation of Microstructure and Mechanical Properties in N-Doped CoCrFeMnNi High-Entropy Alloys

“…For example, at 293 K, the elongation of CoCrFeMnNi HEA can reach 80% [6]. Furthermore, the ductile-brittle transition will not occur even at 15 K, and the elongation can remain about 60% [7,8]. Therefore, the CoCrFeMnNi HEA has an important engineering application prospect in structural materials, especially in the cryogenic field.…”

Correlating work hardening with co-activation of stacking fault strengthening and transformation in a high entropy alloy using in-situ neutron diffraction

“…Recently a number of studies have investigated low SFE high entropy alloys using in-situ neutron diffraction, demonstrating the ability to relate macroscopic stress-strain response with multiple deformation mechanisms 5,[23][24][25][26][27][28] . Thus, to gain a better understanding of the deformation behavior in both phases, the strengthening in the γ-f.c.c.…”