No abstract

The authors recently reported the very first radiometric plateau observation of high-temperature fixed points of metal-carbide carbon peritectics. These act in a similar way to the metal (carbide)-carbon eutectic points in the sense that they can be used at high temperature without being contaminated by their graphite crucibles. The performance seems similar in terms of repeatability and plateau shape. The peritectic transition temperatures are close to the transition temperatures of some of the metal (carbide)-carbon eutectics. In this article, results of further study to understand the melting and freezing involved in these fixed points are reported, with the focus on the Cr 3 C 2 -C peritectic point. Difficulty in producing an ingot without voids was encountered. To overcome this, a filling technique that takes advantage of the capillary effect was devised. Plateau shapes and microstructures observed with electron-probe microanalysis (EPMA) and back-scattered electron imaging (BSE) for various filling methods were compared. The observation of two fixed-point plateaux, one at the Cr 3 C 2 -C peritectic point and the other at a lower temperature of the Cr 7 C 3 -Cr 3 C 2 eutectic point, correlates to the presence of two kinds of domains in the observed microstructure. The graphite crucible is shown to play an essential role in realizing peritectic plateaux of good quality.

The microstructure and thermal analysis of the melt spun Al -5 wt-%Sr alloy have been investigated using X-ray diffraction, TEM, and differential scanning calorimetry (DSC). The experimental results show that rapid solidi cation makes the eutectic composition shift to a higher Sr content exceeding 5 wt-%Sr. The microstructure of the melt spun Al -5Sr alloy is hypoeutectic and composed of primary a-Al cells and the a-Al/Al 4 Sr eutectic; quite different from that of the ingot like alloy comprising coarse primary Al 4 Sr phase embedded in the eutectic. In the melt spun Al -5Sr alloy, some areas comprise equiaxed or elongated a-Al cells with intercellular irregular a-Al/Al 4 Sr eutectic. Moreover, some areas fully comprise coupled a-Al/Al 4 Sr eutectic. The eutectic Al 4 Sr phase is lamellar strip like or vermiform in morphology. The size of the eutectic Al 4 Sr phase is less than 50 nm in width. Furthermore, the very ne microstructure of the melt spun Al -5Sr alloy has a marked effect on the DSC trace in heating process. MST/5275Dr Zhang (zh_zhang@sdu.edu.cn)and Professor Bian are in the Key