Probing the small-scale plasticity and phase stability of an icosahedral quasicrystal i-Al-Pd-Mn at elevated temperatures

Quasicrystalline materials possess a unique structure that is ordered yet not periodic. Despite their special configuration and many useful properties, they are typically very brittle at temperatures below ∼75% of their melting points, rendering them difficult to process and often unsuitable for practical implementations. Micro-compression offers an opportunity to unveil the fundamental mechanisms of quasicrystal plasticity. Here, we study the mechanical behavior of a typical icosahedral quasicrystal (i-Al-Pd-Mn) using microthermomechanical techniques over a temperature in the range 25–500 °C. We observe a few interesting phenomena, including micropillar shrinkage, phase transformations, grain refinement, and thermally induced transitions in deformation behavior (from brittle fracture at room temperature to serrated plastic flows and then to homogeneous flows at elevated temperatures). Furthermore, we discuss the multiple underlying mechanisms on the mechanical behavior of the quasicrystal in this temperature regime, exploring surface evaporation/diffusion, diffusion-enhanced plasticity, dislocation activities, and grain boundary rotation/sliding. Our study bridges the gap between room-temperature and high-temperature plasticity in quasicrystals, demonstrating an opportunity to study complex intermetallic phases in broad size and temperature regimes.

Figure 1

TEM electron diffraction patterns for the i-Al-Pd-Mn crystal along (a),(b) twofold and (c) fivefold symmetry axes [48]; (d) powder diffraction (Cu, $K$α1) pattern for the as-cast i-Al-Pd-Mn ingot. SEM images of (e) the array matrix of FIB-milled pillars produced in one large grain; and (f) a micropillar before compression (approximately 1 μm in diameter).

Figure 2

Representative SEM (SE: secondary electron) micrographs of i-Al-Pd-Mn micropillars after compression experiments at (a) 25, (b) 100, (c) 200, (d) 300, (e) 400, and (f) 500 °C. Panels (g)–(l) show the corresponding stress-strain curves for the pillars deformed in this temperature range. An initial constant strain rate of ${10}^{-3}{\mathrm{s}}^{-1}$ was used, and compression was carried out at 200–500 °C at strain rate jumps of $3.3\times {10}^{-4}{\mathrm{s}}^{-1}$ and $3.3\times {10}^{-3}{\mathrm{s}}^{-1}$.

Figure 3

SEM micrographs showing the morphologies of undeformed pillars after different testing stages: (a) as-milled, (b) annealed at 300 °C, and (c) annealed at 500 °C. (d) Corresponding diameters and volumes of the undeformed pillars after annealing. The average decrease of the pillar volume is $0.14\mu {\mathrm{m}}^3$ (4.5%) and $0.42\mu {\mathrm{m}}^3$ (13.8%) for 300 and 500 °C, respectively. The melting point ($T_{\mathrm{m}}$) of the i-Al-Pd-Mn is about 950 °C (1223 K).

Figure 4

(a),(b) Representative TEM and (c) HR TEM micrographs, and (d) corresponding elemental maps acquired from compressed pillar loaded just up to the point of fracture at room temperature. The highly localized slip band in (b) and (c) is at about 45 ° from the loading axis. The uniformly elemental distribution indicates no phase separation in the specimen.

Figure 5

(a),(c) HAADF STEM, (b),(d),(e) BF TEM, and (f),(g) HR TEM micrographs of the undeformed pillar (annealed at 500 °C for about 3 h). Diffraction patterns acquired from the large pillar area in (b) and the upper part of the pillar in (d) do not show an icosahedral structure. The fast Fourier transform (FFT) patterns (Supplemental Material Fig. S1) acquired from the HR TEM regions in (f) and (g) confirm the existence of the ${\mathrm{Al}}_3{\mathrm{Pd}}_2$ (hP5) phase.

Figure 6

HAADF STEM micrographs of (a) the whole undeformed pillar annealed at 500 °C and (b) a higher magnification region [square region in Fig. 6], with the corresponding EDS maps showing elemental content distribution.

Figure 7

(a) HAADF STEM, (b) BF TEM with SAED, and (c),(d) enlarged BF TEM views of a pillar compressed at 500 °C, showing the phase separation and grain refinement; (d) dislocations array contrast.

Figure 8

HAADF STEM micrographs of (a) the deformed pillar annealed at 500 °C and (b) a higher magnification region [the square region in Fig. 8] with the corresponding EDS color maps showing compositional distribution.

Figure 9

(a) The calculated volume change rates attributed to the evaporation and surface diffusion of all three elements at different temperatures. (b) The cumulative volume change with the corresponding effects of evaporation and diffusion for 2.5 h testing at each temperature from 100 to 500 °C, composing the volume change due to evaporation, diffusion, and the value of the volume changes in the previous annealing steps. The experimental results for the pillar volume change at 300 and 500 °C are also shown.

Figure 10

The grain size distribution of (a) undeformed (annealed) and (b) compressed i-Al-Pd-Mn pillars at 500 °C. The average grain sizes are 240 ± 83 nm and 145 ± 75 nm for undeformed and compressed pillars, respectively.

Figure 11

Statistical analysis of the serration behavior during micropillar compression: (a) illustration of the effect of temperature on the flow behavior. The segments of the stress-strain curves are extracted from Fig. 2. (b) The serration frequency distribution is plotted on logarithmic scales; α refers to the scaling exponent in Eq. (8).

Figure 12

(a) The flow stress at 5% strain as a function of strain rate, (b) corresponding strain rate sensitivity ($m$) and activation volume ($V_{\mathrm{a}}$) for the pillars deformed at 200, 300, 400, and 500 °C, and (c) apparent activation energy ($Q$) calculated from (flow stress) vs (1000/T) plots. The stress exponent $n$ equals 22 and 4.8 for the deformation temperature below and above 300 °C, respectively.

Figure 13

Yield strength as a function of temperature for the micropillars analyzed in this study (pillar diameter ∼1 μm), previous studies (pillar diameter ∼500 nm at RT) [48], and the bulk materials reported in the literature [40, 56]. Three temperature regimes are identified: (i) brittle fracture, (ii) serrated plastic flow, and (iii) homogeneous plastic flow. Phase transformations occur in small-scale samples in the temperature range of approximately 300–500 °C, in which the sample is no longer fully quasicrystalline.