Strain Rate Dependence of Flow Stress in Single-Crystal Micropillars of bcc FeCrNi Solid Solutions

Multi-component alloys with a single-phase microstructure (generally known as medium/high entropy alloys) have recently emerged as a new class of alloys. In particular, body-centered cubic (bcc) solid solution alloys exhibit superior strength retention up to high temperatures but low toughness at ambient temperature. This issue is closely related to the ductility control of conventional ferritic stainless steels with FeCrNi solid solutions at various temperatures. To fundamentally understand the roles of solute elements in plastic deformation of multi-component bcc FeCrNi alloys, we investigated the strain rate dependence of flow stress in single-crystals of bcc Fe-Cr-Ni alloys with different solute Cr and Ni contents by micropillar compression tests in terms of thermal activation of dislocation motion.<br/>Fe-Cr binary alloys (with 18% and 40% Cr) and Fe-18%Cr-Ni ternary alloys (with 1% or 2% Ni) were used in the present study. These alloy ingots were hot-rolled to approximately 4 mm thickness and solution-treated at 1300 °C for 1800 s, followed by water quenching. The solution-treated alloy samples exhibited fully recrystallized microstructures with the bcc single-phase. High-index orientations for the favorable activation of a single slip system were selected for the compression directions (corresponding to the crystallographic directions normal to the sample surface). We used a Focused Ion Beam (FIB) to fabricate cylindrical micropillar specimens with different diameters (<i>d</i>) of approximately 2 and 5 µm on the sample surface of the solution-treated alloys. We performed the micropillar compression tests at a wide range of initial strain rates (~10^-5s^-1 to ~10^-2s^-1) using a load-controlled mode at ambient temperature. The measured values of flow curves were used to calculate the strain rate sensitivity (<i>m</i>). Afterward, the slip traces of the compressed micropillars were observed to identify the activated slip system. Transmission electron microscope (TEM) to evaluate the dislocation morphologies before and after the micropillar compression test.<br/>In the Fe-18%Cr binary alloy, single-crystal micropillars with <i>d</i> = 2 µm exhibited a relatively high strain-rate sensitivity (<i>m</i> = 0.12) of stress required for the slip initiation. The <i>m</i> value became lower (0.04) in the large-sized micropillars with <i>d</i> = 5 µm. The <i>m</i> value was equivalent to one of pure iron. The added solute Ni element in bcc solid solutions reduced the <i>m</i> value in both specimen sizes. The Fe-18%Cr-2%Ni ternary alloy exhibited low <i>m</i> values below 0.01. All compressed micropillars showed the predominant activation of the [111] (12-3) single slip system. These results suggested the sluggish kinetics of kink-pair nucleation and motion of screw dislocations interacted with solute Ni atoms. The Fe-40%Cr binary alloy showed higher yield strength and much lower <i>m</i> values than the Fe-18%Cr alloy, regardless of the specimen size. The compressed micropillars of the Fe-40%Cr alloy micropillars exhibited the slip activation on multi-planes (from (110) to (11-2)). These results suggest the interaction of dislocations on multi-planes might be enhanced by high-concentrated Cr solute element in bcc solid solutions. The roles of solute Ni and Cr elements in the thermal activation process of dislocation motion will be discussed in terms of the activation volume of plastic deformation.