Anomalous Strengthening Mechanism in NbSi 2 -Based Silicide Single Crystals
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Anomalous Strengthening Mechanism in NbSi2-Based Silicide Single Crystals Yukichi Umakoshi, Takayoshi Nakano and Masafumi Azuma Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871, Japan ABSTRACT Anomalous strengthening behavior of NbSi2-based silicide single crystals with C40 structure was examined focusing on the effect of substitutional alloying elements and the formation of a solute dragging atmosphere. After pre-straining to 1% at temperatures between 600 and 1600Û& single crystals were deformed at 400Û&WRH[DPLQHWKHHIIHFWVRIGLVORFDWLRQVRXUFHVDQG segregation of solute atoms around dislocations. The stress amplitude of yield drop was derived by the segregation of solute atoms at the superlattice intrinsic stacking fault (SISF) between two superpartials. Introduction of dislocation sources by pre-straining remarkably improved fracture strain, but the segregation of solute atoms around dislocations during pre-straining at higher temperatures induced a rapid drop of fracture strain. Change in the SISF energy by static and dynamic aging was observed to obtain evidence of segregation of solute atoms at the SISF between two superpartials. INTRODUCTION NbSi2-based silicides with the C40 structure deform by the (0001) < 1 210 > slip and show anomalous strengthening with a peak at and above 1400Û&>@7KHVWUHQJWKDQGSHDN temperature could be remarkably improved by addition of substitutional alloying elements [2]. Although the limited slip system is unfavorable for industrial application, high strength and high anomalous peak temperature make the NbSi2-based phase attractive as a superior reinforcement phase in MoSi2 matrix silicides for a new refractory structural material operating at ultra high temperatures [3-5]. The critical resolved shear stress (CRSS) for the basal slip and the anomalous peak temperature showed no orientation dependence but they varied depending on the strain rate [1,2]. In addition, serrations in the stress-strain curves appeared at a certain temperature and strain rate. From these results, we have proposed an anomalous strengthening mechanism for NbSi2-based silicides based on the formation of dragging atmosphere around moving dislocations [2]. This paper describes evidence of the dragging atmosphere around dislocations and effect of this atmosphere on the SISF energy and plastic deformation behavior in NbSi2-based silicides by pre-straining and aging at high temperatures. EXPERIMENTAL PROCEDURE Binary NbSi2, ternary (Nb0.97Mo0.03)Si2, (Nb0.9Mo0.1)Si2, (Nb0.9W0.1)Si2, (Nb0.97Ti0.03)Si2 and Nb(Si0.97Al0.03)2 silicide single crystals were grown by a floating zone method at a rate of 5mmh-1 in a flowing argon atmosphere. Compression specimens with 2x2mm2x5mm and a loading axis N2.5.1
shown in figure 1 were cut from the crystals by spark machining and were mechanically polished with a diamond paste. Compression tests were carried out at a nominal strain rate of 1.7x10-4s-1 at temperatures between 1
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