Creep-deformation behavior of (Mo0.85Nb0.15)Si2 lamellar-structured C40/C11b two-phase crystals

Abstract

The creep-deformation behavior of (Mo 0.85 Nb 0.15)Si 2 crystals that were composed of C40 and C11 b two-phases with an oriented lamellar microstructure was examined over 1200-1400 °C under compression. The creep behavior of the crystals varied greatly depending on the orientation of the loading axis with respect to the lamellar interfaces. The steady-state creep strain rate (SSCR) when the loading orientation was parallel to the lamellar interfaces (0°-orientation) was approximately 2 orders of magnitude lower than that when the loading orientation was inclined by 45° (45°-orientation). Creep tests were performed also with single crystals of C40-single-phase (Mo 0.85 Nb 0.15)Si 2 , and proved that the C40-phase effectively acts as a strengthening phase in the creep behavior of the C40/C11 b duplex-phase crystals. The variant-1-type C11 b phase grains, which have a loading orientation parallel to [001], were also found to act as an effective strengthening component. The SSCR of the 0°-orientation was succeeded to be further decreased by controlling the lamellar microstructure via the addition of Cr. The SSCR of the 1 at.%-Cr-added lamellar-structured crystal under 300 MPa in the 0°-orientation was approximately a third of that of the non-added ternary crystal.

Outstanding compressive creep strength in Cr/Ir-codoped (Mo0.85Nb0.15)Si2 crystals with the unique cross-lamellar microstructure

“…The Schmid factors for the expected operative slip systems in the C11 b and C40 phases are listed in Table S1 in the Supplementary Information. In the graphs, the expected temperature dependencies of the yield stresses for the three C11 b variants and C40 phase, which are the predominant constituents in the lamellar crystal, were plotted according to the evaluation in the previous papers 5, 15, 24 . Note that the development of a lamellar microstructure means that in the 0°-oriented specimen, both of the C40 and C11 b phases must be plastically deformed for the specimen to be macroscopically deformed.…”

“…The yield (fracture) stresses were measured by compression tests at 1000 and 1400 °C in the ( a ) 0° and ( b ) 45° orientations. In the figures, the expected temperature dependencies of the yield stresses for the three C11 b variant grains and C40-matrix phase in the lamellar-structured crystals, according to the evaluation in the previous papers 5, 15, 24 , are also drawn for comparison. Open marks indicate that the specimen was fractured before reaching 0.2% plastic strain. …”

“…Based on these background, we have examined the high-temperature compressive deformation behavior of the CrIr-codoped (Mo 0.85 Nb 0.15 )Si 2 crystals (CrIr-added crystal) in this study. Results of the current study were compared to previous study of non-added crystal 24 and Cr-only-added and Ir-only-added crystals, and then the effectiveness for the improvement of the creep strength and its mechanism by the development of “cross-lamellar microstructure” were clarified.…”

“…Through the heat treatment of the (Mo 0.85 Nb 0.15 )Si 2 C40 single-phase single-crystals grown by the floating zone (FZ) method, C11 b phases with ~(Mo 0.94 Nb 0.06 )Si 2 composition were precipitated in the ~(Mo 0.79 Nb 0.21 )Si 2 C40 matrix phase. Notwithstanding the observed improvement in the mechanical properties of the silicide crystal due to the development of lamellar microstructure 15, 21, 24 , its room temperature fracture toughness and high-temperature creep strength were still insufficient for practical application. To further improve these properties, studies 22 aimed at controlling the lamellar microstructure by a minute amount of alloying element additions have been conducted.…”

Effect of additional elements on fracture toughness of (Mo0.85Nb0.15)Si2 C40/C11b lamellar-structured crystals

“…As a new class of ultra-high-temperature (UHT) structural materials that can tolerate temperatures above 1200°C, many researchers have studied duplex disilicide crystals combined with C40 and C11 b phases [1][2][3][4][5][6][7][8][9][10][11][12][13]. Among them, Nakano et al reported that as a result of the heat treatment of the (Mo 0.85 Nb 0.15 )Si 2 C40 single-phase single crystals, C11 b phases with composition of $(Mo 0.94 Nb 0.06 )Si 2 were precipitated in the $(Mo 0.79 Nb 0.21 )Si 2 C40-phase matrix phase with distinct crystallographic orientation relationships of (0001) C40 //{110) C11b and o¯1210 4 C40 //o1 10] C11b , called variant orientation relationships, resulting in the development of an oriented lamellar microstructure [4,6].…”