High-temperature structural change and microtexture formation of sillimanite and its phase relation with mullite

Synchrotron powder X-ray diffraction (XRD) experiments and transmission electron microscopy (TEM) observations of heat-treated sillimanite at various pressures were conducted to clarify the detailed phase relation between sillimanite and mullite. Under TEM, heat-treated sillimanite frequently showed anti-phase boundary (APB)-like textures with a displacement vector of ½[001]sil. Additional scanning TEM energy-dispersive X-ray spectroscopy analysis of regions with APB-like texture showed that they were clearly enriched in Al and accompanied by very fine, Si-rich glass inclusions, which indicates that the APB-like textures are composed of fine mullite. Moreover, synchrotron XRD patterns of these samples clearly showed double peaks of newly formed mullite and remnant sillimanite, indicating that the compositional transformation from sillimanite to mullite and glass is discontinuous. We separately determined the cell parameters of the sillimanite and mullite from the XRD pattern and found that the b axial length of the sillimanite increased with the treatment temperature, reflecting disordering of tetrahedral Al and Si in the sillimanite. In contrast, the positions of the deconvoluted mullite peaks indicated that the a axial length of mullite decreased as experimental pressure increased, owing to enrichment of the Si component. By projecting the cell parameters onto the a–b axial plane, the detailed changes in the crystallographic state of the sillimanite and mullite could be easily and comprehensively identified. On the basis of our results, we propose a new P-T diagram for the Al2SiO5 system that shows the transformation boundary between sillimanite and mullite + SiO2-rich melt and the contour of the Al/Si order parameter of sillimanite.

Deformation Behavior of Fe-Al Single Crystals Containing Fe2AlTi Precipitates

Fe-20Al-5Ti (at.%) single crystals composed of the bcc Fe-Al matrix and the Fe2AlTi precipitates with the L21 structure was examined. In the single crystals furnace-cooled (FC) from 1373 K to room temperature, coarse Fe2AlTi phase about 300 nm in diameter were precipitated in the bcc matrix. A misfit strain and a dissolution temperature of the L21 precipitates are +0.59% and 1151 K, respectively. The single crystals exhibited high yield stress above 600 MPa up to 973 K while further increase in temperature resulted in a decrease in yield stress due to the dissolution of the precipitates. In the FC crystals, 1/2<111> dislocations in the bcc matrix bypassed the coarse L21 precipitates due to their large misfit strain, resulting in high strength. In contrast, the fine L21 precipitates about 30 nm in diameter were observed in the crystals after solutionization and annealing at 873 K. The crystals with the fine L21 precipitates demonstrated high yield stress above 1100 MPa at and below 773 K. Uncoupled or paired 1/2<111> dislocations cut the fine L21 precipitates, leaving an anti-phase boundary (APB) inside the precipitates. The APB inside the precipitates was considered to be responsible for strong precipitation hardening.

Types and configurations of domain walls in ferroelectric Bi4Ti3O12single crystals

In the ferroelectric Bi4Ti3O12, the major componentPs(a)of spontaneous polarizationPslies along theaaxis; the componentPs(c)along thecaxis is small. The two switchable components are expected to make up various types of domain walls (DWs). According to group-theory analysis, 11 permissible types of DWs are predicted to exist theoretically, and they include five types of ferroelectric DWs, one type of anti-phase boundary (APB) and five types of APB-combined ferroelectric DWs. The five types of ferroelectric DWs arePs(a)-90° DWs,Ps(a)-180° DWs,Ps(c)-180° DWs,Ps(a)-90°/Ps(c)-180° DWs andPs-180° DWs. In Bi4Ti3O12single crystals, just the five types of ferroelectric DWs were observed using transmission electron microscopy, with no trace of APBs or APB-combined ferroelectric DWs seen. ThePs(a)-90° domains are lamellar and do not have even thickness. Both thePs(a)-90° DWs andPs(a)-90°/Ps(c)-180° DWs lie mainly on the (110) plane, but often fold to the (001) plane. ThePs(a)-180° domains are predominantly flaky. Both thePs(a)-180° DWs andPs-180° DWs lie mainly on the (001) plane. ThePs(c)-180° DWs observed are irregularly curved.