Ａｌ‐０．６％Ｆｅ合金連続鋳造塊におけるもみの木組織の生成

Abstract

A study was made on the formation of 'fir-tree' structure during D. C. casting of Al-Fe alloys. Al-Fe alloys containing 0.58%Fe and less than 0.01%Si were unidirectionally solidified. The alloys were also D. C. cast. The cooling rate during unidirectional solidification was measured by thermal analysis, while the cooling rate during D. C. casting was estimated through measurements of the dendrite arm spacing of the ingots. Identification of constituent phases were carried out by optional microscopy, electron microscopy and X-ray diffraction, and some of their chemical properties were discussed.The results obtained are summarized as follows:1) Al3Fe, Al6Fe and AlmFe (a body-centered tetragonal Al-Fe phase8)) were identified as secondary constituents. They were found to be formed in the following ranges of cooling rate:Al3Fe: less than 2°C/secAl6Fe: from 2 to 20°C/secb.c.t.AlmFe: more than 20°C/sec.2) Al3Fe and b.c.t. AlmFe were oxidized during anodizing in 15%H2SO4 solution, while Al6Fe did not undergo oxidation. Al3Fe and b.c.t. AlmFe were etched to appear dark-brown by 0.5%HF solution, while in the case of Al6Fe, only a slightly grey tint was detected.3) The microstructure of the D. C. cast ingots was divided into the four regions corresponding to different sizes of dendrite cells and to different constituent phases. The first, these cond and the third regions were outside the 'fir-tree' structure, while the fourth region was inside. Body-centered tetragonal AlmFe and a small amount of Al3Fe were secondary constituents in the first region which solidified very rapidly. Al3Fe phase existed in the second and third regions which solidified more slowly. Al6Fe phase was an interdendritic phase in the fourth region which solidified fairly fast.4) The cause of formation of 'fir-tree' structure seems to be attributable to the fact that the cooling rate during solidification varies from the surface to the center of D. C. cast ingots. In other words, the cause is likely attributed to macroscopic segregation of different types of intermetallic compounds.