February AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y / M A R C H 2 0 1 9 2 3 precipitation of MoB. Figure 2a is a field emission scanning electron microscope (FESEM) micrograph of synthesized MoAlB particulate where the Mo:Al ra- tio is 1:1 [12,13] . Comparatively, MoAlB treated with 12 M HCl showed partially etched (PE) surfaces (PE MoAlB). Energy-dispersive x-ray spectros- copy ( EDS) analysis of the top surface compared with the cross section of PE MoAlB (note that [010] is perpendicular to the basal planes [14] ) showed that Al is preferentially etched from the cross section compared with the top surface (Fig. 2b, compositions A and B1, Table 2) as the basal planes are capped by MoB layers. The peaks of [0k0] also are dif- fused compared with other peaks, TABLE 1 – PROPERTIES OF DIFFERENT MAB PHASES Composition Compressive strength, MPa Relative density Hardness, GPa Electrical conductivity, × 10 6 S/m Thermal conductivity, W/m/K Space group Ref MoAlB 1420-1940 94 ± 1% 10.6 ± 0.3 2.0-2.8 35 Cmcm 8 1253-1620.2 96.73% 9.3 ± 0.4 2.91 ± 0.23 29.21 ± 0.01 Cmcm 11 Fe 2 AlB 2 2101 ± 202 96% 10.2 ± 0.2 2.27 ± 0.01 0.44 Cmmm 9 Mn 2 AlB 2 1240 ± 100 97.2 ± 0.2 8.7 ± 0.6 ∼ 0.2 x Cmmm 10 Ti 3 SiC 2 580 ± 20 >99 4 4.5 43 P6 3 /mmc 2-5 Ti 3 AlC 2 560 ± 20 ∼ 99 3.5 ∼ 2.85 40 P6 3 /mmc TABLE 2 – COMPOSITION OF DIFFERENT MICROCONSTITUENTS ID Composition A Mo 0.61±0.01 Al 0.39±0.01 O 0.1±0.012 B x B1 Mo 0.58±0.01 Al 0.42±0.01 O 0.09±0.012 B x C Mo 0.61±0.01 Al 0.39 O 0.12 B x D Mo 0.61±0.02 Al 0.39±0.012 O 0.03 B x E Mo 0.58±0.01 Al 0.42±0.01 O 0.11±0.01 B x F Mo 0.55 Al 0.45 O 0.08 B x G Mo 0.61±0.01 Al 0.39±0.01 O 0.15±0.01 B x H Mo 0.60±0.01 Al 0.40±0.01 O 0.13±0.01 B x I Mo 0.65±0.01 Al 0.35±0.012 O 0.11±0.01 B x J Mo 0.55±0.1 Al 0.45 O 0.05±0.01 B x K Mo 0.64±0.1 Al 0.36±0.03 O 0.13±0.03 B x Fig. 2 — FESEMmicrographs of synthesized MoAlB particulate (compositions in Table 2): (a) milled MoAlB [12,13] ; (b) MoAlB-12 M HCl (post sieving); (c) MoAlB-12 M HCl-0.1M LiF (post sieving); (d) MoAlB-12M HCl-0.37M LiF (post sieving and sonicated); (e) MoAlB-12 M; (f) MoAlB-12 M HCl-0.1M LiF; (g) MoAlB-12M HCl-0.37M LiF; (h) MoAlB-12M HCl-0.37M LiF (higher magnification); and (i) MoAlB-12M HCl-0.37M LiF (higher magnification). (a) (b) (c) (d) (e) (f) (g) (h) (i) which further indicates that Al is etched from the edges causing a high densi- ty of stacking faults [15] (Fig. 1). There is evidence of fragmentation of the grains into PE MoAlB slabs (Fig. 2b). Similarly, MoAlB particulates etched with differ- ent concentrations of LiF/HCl molar ra- tio also showed PE MoAlB surfaces with signs of grain fragmentation (Figs. 2c-d). Figures 2(e-i) show the morphology of treated particles after sonication. In all cases, well dispersed PEMoAlB particulates were observed. In addition, the PE MoAlB cross section (composi- tions E, G, I, and K, Table 2) was etched preferably compared with the top sur- face (compositions F and J, Table 2). Alameda et al. [15] observed signifi- cant corrosion while etching MoAlB single crystals with 5% and 10% HF. However, no corrosion of the MoAlB par- ticulates was observed in this study, although some grains broke down into PE MoAlB slabs, which indicates that both HCl and LiF/HCl can be used as etchants. The etched particles did not show peri- odicity, as the starting com- position was obtained from milled MoAlB particulates. It is also possible that the PE MoAlB slabs may have 2D par- ticles as well. The effect of du- ration on the etching of these particles is being studied. Figures 3(a1 and a2) are schematics of the fabrica- tion of PE MoAlB from MoAlB where Al is etched from edges to form particles with etched surfaces (Fig. 3a2). Figure 3(a3) shows a FESEM