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

Abstract

a b s t r a c tThe compression deformation behavior of single-crystal micropillars of L1 2 -Co 3 (Al,W) has been investigated at room temperature as a function of pillar size and crystal orientation. The critical resolved shear stress (CRSS) obeys an inverse power-law scaling against pillar size and is independent of orientation. With the bulk CRSS value of Co 3 (Al,W) at room temperature estimated by extrapolating the size dependence of CRSS for micropillars, comparison of high-temperature strength is made on the CRSS values for Co 3 (Al,W) and Ni 3 Al-based compounds. Co 3 (Al,W) has turned out not to be strong enough to provide excellent high-temperature strength for Co-based superalloys, at least for ternary composition.The discovery of the γ′-Co 3 (Al,W) phase with the L1 2 structure in the Co-Al-W ternary alloy system by Sato et al.[1] has launched a new era of the development of high-temperature structural materials. The γ′-Co 3 (Al,W) phase coherently precipitates in the γ-Co (face-centered cubic (fcc) structure) solid-solution phase, resulting in a γ + γ′ two-phase microstructure with a regular array of cuboidal γ′ precipitates, which resembles the typical microstructure of Nibased superalloys strengthened by the γ′-Ni 3 Al phase [1]. 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]. Since the occurrence of yield stress anomaly in the constituent γ′ phase is important for the strength of γ + γ′ two-phase alloys at high temperatures, comparison of the strength of Co 3 (Al,W) with those with Ni 3 Al-based L1 2 compounds should be made to see how significantly Co 3 (Al,W) contributes to the strength of Co-based superalloys especially at high temperatures. However, the critical resolved shear stress (CRSS) has not been available for Co 3 (Al,W) due to the difficulties in preparing single crystals of the γ′-Co 3 (Al,W) single-phase large enough for the study of plastic deformation [11].In recent years, however, a method based on compression testing of s...

Microcompression of brittle and anisotropic crystals: recent advances and current challenges in studying plasticity in hard materials

“…Selection of hard crystals tested by microcompression. ZrB 2 , [87] Mo 2 BC, [85] WC, [21] Fe 3 Al, [88] Co 3 (Al,W), [89] CMSX-4, [36] Si, [31] GaAs, [2] InSb, [90] Al 2 O 3 (unpublished), MgO, [25] (Fe,Ni) 2 Nb, [30] (Mg, Al) 2 Ca (courtesy of C. Zehnder), Nb 2 Co, [34] AlN, [91] GaN, [92] doped ZrO 2, [93] LiF, [94] Al 7 Cu 2 Fe, [95] FeZn 13 , [96] Cu 6 Sn 5 , [24] either conventional or high resolution, is easily performed after compression by the site-specific FIB milling and where necessary further thinning of a transparent membrane from the micropillar (see Figs. 3, 4, 6, and 10, for examples).…”

The microstructure and hardness of Ni-Co-Al-Ti-Cr quinary alloys

“…In the ten years since this discovery, considerable work has been performed to gain an enhanced understanding of the behaviour of these materials and develop commercially viable alloys. This research has included studies of the phase equlibria and the effect of alloying [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27], evaluation of their deformation behaviour [22,[28][29][30][31][32][33][34][35][36][37][38][39][40][41], and assessment of their environmental resistance [42][43][44]. However, despite exhibiting several beneficial attributes, further development is still required before any of these alloys can compete with existing Ni-based superalloys.…”

On the Critical Resolved Shear Stress and Its Importance in the Fatigue Performance of Steels and Other Metals with Different Crystallographic Structures

“…Namely, the investigated pillar exhibited low hardening rates and discrete strain bursts (see Figure 3 for the two effects), and a power-law relationship between CRSS and pillar diameter, or the so-called size effect (see Figure 4e) [8]. It has been noted that the CRSS values of both FCC and BCC pillars decrease as the pillar size increases, conforming with inverse power-law scaling [7,9,10]. A large amount of publications regarding small-scale plasticity are available today.…”

“…The small-scale yield (shear) stress, i.e. CRSS, for pillars is determined as the stress at which the first strain burst occurs [10]. the example of cylindrical Ni micropillars, which at that time was an entirely new behavioral regime.…”

“…The small-scale yield (shear) stress, i.e. CRSS, for pillars is determined as the stress at which the first strain burst occurs [10]. [11].…”