Plastic deformation of polycrystals of Co₃(Al,W) with the L1₂structure

Abstract

The plastic behaviour of Co 3 (Al,W) polycrystals with the L1 2 structure has been investigated in compression from 77 to 1273 K. The yield stress exhibits a rapid decrease at low temperatures (up to room temperature) followed by a plateau (up to 950 K), then it increases anomalously with temperature in a narrow temperature range between 950 and 1100 K, followed again by a rapid decrease at high temperatures. Slip is observed to occur exclusively on {111} planes at all temperatures investigated. The rapid decrease in yield stress observed at low temperatures is ascribed to a thermal component of solid-solution hardening that occurs during the motion of APB-coupled dislocations whose core adopts a planar, glissile structure. The anomalous increase in yield stress is consistent with the thermally activated cross-slip of APB-coupled dislocations from (111) to (010), as for many other L1 2 compounds. Similarities and differences in the deformation behaviour and operating mechanisms among Co 3 (Al,W) and other L1 2 compounds, such as Ni 3 Al and Co 3 Ti, are discussed.

Tensile behaviour of Co–Al–W–Ta–B–Mo alloys with a coherent γ⧸γ′ microstructure at room and high temperatures

“…In addition, strengthening of the grain boundaries by precipitates of borides or intermetallic phases has been found to be critical to the mechanical properties of the γ 0 -strengthened Co-base superalloys [14,15], which exhibited even better creep strength that was comparable to the Ni-base alloy IN100. The microstructural evolution of the high-temperature compressed samples has revealed that a slip of paired 1/2 o1 1 04 superpartial dislocations on the {0 0 1} planes in addition to the {1 1 1} planes in the γ 0 precipitates [2,3,16] results in flow stress anomalies at 700-800 1C. This dislocation movement mode is the same as that of the stress anomalies mechanism found in the γ 0 -Ni 3 Al compound [17,18].…”

Micropillar compression deformation of single crystals of Co3(Al,W) with the L12 structure

“…For many different metals with the fcc structure, on which the L1 2 structure is based, the critical size has been reported to be in the range of 20-Scripta Materialia 121 (2016) 28-31 30 μm [12,17,18], while Uchic et al reported that the critical size for L1 2 -Ni 3 (Al,Ta) is 42 μm at room temperature [12]. However, this critical size value obtained for L1 2 -Ni 3 (Al,Ta) may not be used directly for L1 2 -Co 3 (Al,W), in view of the fact that the onset temperatures for yield stress anomaly are quite different for these two L1 2 compounds (77 K and 950 K respectively for Ni 3 (Al,Ta) [21] and Co 3 (Al,W) [4]). Cross slip of superpartial dislocations from (111) to (010) planes frequently occurs so as to give rise to yield stress anomaly even at room temperature for Ni 3 (Al,Ta), while such cross slip is unlikely to occur at room temperature for Co 3 (Al,W).…”

“…Indeed, these γ + γ′ two-phase alloys called Co-based 'superalloys' have demonstrated superior high-temperature strength when compared to conventional Co-based alloys strengthened by solid-solution hardening and carbide precipitation without cuboidal γ′ precipitates [1-3]. We have recently investigated the deformation behavior of polycrystals of Co 3 (Al,W), the constituent phase of Co-based alloys, revealing that while the yield stress rapidly decreases with the increase in temperature at low temperatures, it increases anomalously with temperature only in a narrow temperature range at high temperatures (950-1100 K) [4,5]. TEM analysis has indicated that the yield stress anomaly is due to cross slip of superpartial dislocations with b (Burgers vector) = 1/2[ 101] from (111) to (010) planes [6,7] as in many other L1 2 intermetallic compounds based on Ni 3 Al, although these Ni 3 Al-based L1 2 intermetallic compounds are known to exhibit the yield stress anomaly in a much wider temperature range so as to start from a lower temperature such as liquid nitrogen temperature, especially in ternary alloys [8][9][10].…”

High-temperatureγ(FCC)/γ′(L12) Co-Al-W based superalloys

“…Therefore, the measured APB energy is 71 mJ m −2 . For comparison, the {111} APB energy of Co 3 (Al,W) has been found to be 146 mJ m −2 [16], and in pure Ni 3 Al, 180 mJ m −2 [17]. In both systems, Ti and Ta are suspected to substantially raise the APB energy [17,18], but values of as low as 118 mJ m −2 are found in commercial superalloys, presumably because other alloying elements such as Cr decrease the APB energy.…”