Tensile Deformation of Si Single Crystals at High Temperatures

Czochralski silicon single crystals were deformed in tensile tests along the direction at between 1173 K and 1373. Yield point phenomenon were observed in the specimens deformed at below 1273 K while continues yield was observed in the specimens deformed at above 1323 K. It is due to the effect of dislocation starvation in the used crystals. Work-hardening rates in stage II were consistent with those reported in fcc crystals such as copper. The onset of stage II was found to be active before the Schmid factor of the second slip system becomes larger than that of the primary slip system. Electron backscattered diffraction images indicated clear kink bands near grips and in the parallel portion. The kink bands were formed at the middle of stage I, which suggest that the formation of kink bands is a trigger of stage II.

Crystal Plasticity Analysis of Change in Active Slip Systems of α-Phase of Ti-6Al-4V Alloy under Cyclic Loading

In this paper, we investigated changes in active slip systems of α-phase of Ti-6Al-4V alloy under a cyclic plastic loading using a crystal plasticity finite element method. In the analyses, a bicrystal model was employed, and the crystallographic orientations were set so as that prismatic <a> or basal slip system was the primary slip system in each grain. The results showed that there was a mechanism where the basal slip systems could reach the stage of activation under the cyclic plastic loading even though the condition was that the prismatic <a> slips initially operate. The reason for the activity changes was due to the changes in the incompatibility between the grains by the work hardening, and the effect of the incompatibility on activities of slip systems appeared even in the perpendicular arrangements of the grains to the loading direction.

Deformation Behavior of Fe-Al-Co Single Crystals Containing CoAl Precipitates

The effect of the CoAl precipitates on the deformation behavior of Fe-15.0Al-15.0Co (at.%) single crystals was examined. The spherical CoAl phase with the B2 structure was precipitated in the single crystals and was stable below 974 K. The bcc matrix and CoAl phase satisfied the cube-on-cube orientation relationship with a misfit strain of 0.25%. The single crystals showed a high yield stress up to 923 K while the stress dropped at 1023 K due to the dissolution of the CoAl phase into the matrix. Moreover, the activated sip system of the crystals containing the CoAl precipitates depended strongly on loading axis. At <149> orientation, {101} <111> slip favorable for the bcc matrix and the CoAl precipitates were sheared by a pair of 1/2<111> dislocations without forming Orowan loops. The CoAl single phase was known to hardly deform by <111> slip which resulted in high strength at <149> orientation. In contrast, {010} <001> or {hk0} <001> slip favorable for the CoAl precipitates was activated at <011> orientation, although the volume fraction of the CoAl phase was very small. <001> slip was generally impossible to take place in the bcc matrix, leading to the extreme hardening. Therefore, the difference in primary slip system between the bcc matrix and CoAl precipitates was responsible for the significant precipitation hardening.

New Creep Deformation Concept Based on Creep under Lower Stresses

Through the analysis of many creep rate-time or creep rate-strain curves of -single phase Ni-20mass% Cr alloy single crystals with various stress axes, it was clarified that the creep deformation manners at lower stresses are drastically different to those at higher stresses. These creep features at lower stresses are summarized into three ones as follows. (i) The fully extended transient stage occupies the considerable ratio of rupture life. (ii) The steady state stage disappears, because the transient stage directly connected with the accelerating stage. (iii) The origin of the onset of accelerating stage is regarded as the formation of the dynamic recrystallized grain. These difference in creep deformation manner were caused by the predominant operation of the primary slip system and then the homogeneous evolution of dislocation substructures.

Interaction of Slip Bands with Grain Boundary - in situ TEM Observation

ABSTRACTMechanism of the slip transmission across the grain boundaries was studied on Σ3 bicrystals of Fe-4at%Si by transmission electron microscopy. In situ straining experiments as well as post mortem observations were performed. Three distinct events were observed in dependence on the angle α between the slip and grain boundary planes, namely transformation of the tertiary slip system in one grain into the secondary slip system in the other grain for α ≍ 90°, cross slip of dislocations of the primary slip system into the {112} plane parallel to the grain boundary for α ≍21°, and an abrupt formation of a sub-grain boundary in one grain for α ≍49°. Reasons for these diverse phenomena will be discussed in terms of interactions between the slip dislocations and the grain boundary dislocations.

Deformation and Toughness of α-Silicon Nitride Single Crystals

ABSTRACTVickers and Knoop indentation methods were used to determine the deformation behavior and fracture toughness of single crystal α-Si3N4 at room temperature. The tests were performed on (0001), (1100) and (1120) faces; both hardness and toughness values were found to be independent of orientation within statistical variations. Thus, the mechanical properties of silicon nitride are essentially isotropic at room temperature.Single crystals of α-Si3N4 were also compressed at 1760 and 1820°C at a strain rate of The sample tested at 1760°C showed yielding at a stress of 200MPa but it fractured before indicating a significant amount of plastic deformation. On the other hand, the sample compressed at 1820°C deformed to a strain of 2.7% at a stress of 4OMPa before fracturing. A significant density of dislocations was observed by transmission electron microscopy. From conventional g.b analysis, the Burgers vector of the dislocations was determined to be 1/3<1120>. From the line direction of the dislocations, the primary slip system of α-Si3N4 is determined to be {1101}<1120>.