Microstructural signatures of dislocation avalanches in a high-entropy alloy

Here, we trace in situ the slip-line formation and morphological signature of dislocation avalanches in a high-entropy alloy with the aim of revealing their microstructural degree of localization. Correlating the intermittent microplastic events with their corresponding slip-line patterns allows defining two main event types, one of which is linked to the formation of new slip lines, whereas the other one involves reactivation of already existing slip lines. The formation of new slip lines reveals statistically larger and faster avalanches. The opposite tendency is seen for avalanches involving reactivation of already existing slip lines. The combination of both these types of events represents the highest degree of spatial avalanche delocalization that spans the entire sample, forming a group of events that determine the truncation length scale of the truncated power-law scaling. These observations link the statistics of dislocation avalanches to a microstructural observable.

Figure 1

(a) Typical stress-strain curves obtained from the in situ compression experiments on the high-entropy alloy (HEA) microcrystals. Each sudden displacement jump corresponds to one resolvable dislocation avalanche that may range between a few tenths of a nanometer to >1 μm. (b) Image capture process during the experiment. At the start of each experiment, an initial picture is taken; after each recorded mechanical event, the experiment is stopped, and a new picture is taken. This picture represents the “after” image associated with that event, as well as the “before” image for the next, future event. Inset: detail of force-depth curve showing a few small precursor events that could not always be caught during in situ experimentation.

Figure 2

Categories #1–#6 of slip morphology observed during in situ experimentation. For each category, subpanel (a) refers to an image taken before the event was recorded, and subpanel (b) shows the image taken immediately after the event.

Figure 3

Cumulative distribution functions (CCDFs) of all measured intermittent events and their truncated power law (TPL) fit. In addition, a much smaller dataset from Ref. [13] is shown for comparison. Despite the different experimental setup and conditions, the exponent and general form obtained when fitting both datasets is very much comparable.

Figure 4

(a)–(h) Distributions of size range for each event category, with the percentage of the total dataset ($n=817$) indicated. Data points highlighted in green are the largest 10% of the event sizes recorded in each category. (i) Example of a scanning electron microscope (SEM) image with a single new slip line (indicated by white arrow) linked to a 3 nm displacement jump. The other visible slip lines in (i) were assigned to previous events.

Figure 5

Cumulative distribution functions (CCDFs) of subcategories #2–#6, using the fitting results listed in Table 2. Each fit is only defined for the relevant size range of its category. Categories #2, #3, and #6, involving newly activated slip planes, distribute shallower than the subsets that only involve reactivation of existing slip lines.

Figure 6

Histogram of log value of recorded peak velocities summarized in two datasets for events corresponding to new slip lines or the reactivation of an existing slip line, respectively. While both datasets show a peak in the range $log\left(v_{\mathrm{peak}}\right)\approx 1.8-2.6\to v_{\mathrm{peak}}\approx 60–400\mathrm{nm}{\mathrm{s}}^{-1}$, the peak velocities involving the formation of new slip lines clearly populated higher velocities.

Figure 7

Displacement-time and velocity-time trace for (a) an avalanche leading to new slip lines, and (b) an avalanche involving the reactivation of an already existing slip line. Events leading to a new slip line show sharp velocity profiles, whereas events involving the reactivation of slip lines have an in-time extended fluctuating velocity trace.