Temperature Dependence of Grain Boundary Structure and Grain Growth in Bulk Silicon-Iron.

Abstract

Grain boundary shapes and grain growth in bulk 2.61 wt% silicon-iron have been studied by heat-treating at temperatures between 700 and 1 200°C. Initial microstructure with fairly uniform fine grains has been obtained by recrystallization at 800°C for 5 min after deformation. When subsequently heat-treated at 700 and 800°C, a fraction of the grain boundaries have hill-and-valley shapes with several facet planes or kinks. Some of these facet boundary segments are expected to be singular. Abnormal grain growth occurs at 700 and 800°C and is attributed to step growth of the boundaries. When heat-treated at 1 000°C, all grain boundaries are defaceted with smoothly curved shapes, indicating that they are atomically rough. At temperatures above 1 000°C, normal grain growth occurs, because the rough grain boundaries move continuously. This correlation between grain boundary structure and grain growth is consistent with the earlier observations in other metals and oxides. It is thus shown that the abnormal grain growth in this alloy occurs at low temperatures because of the singular grain boundary structure.KEY WORDS: silicon-iron; silicon-steel; abnormal grain growth; grain boundary faceting; singular grain boundary.ing along the grain boundaries using an image analysis program. For the specimens with fine grains, about 500-2 000 grains were measured, and for those with large grains, about 100-300 grains were measured. The grain boundary shapes were also examined in a transmission electron microscope (TEM). Results and DiscussionWet chemical analysis of the heat-treated bar showed that the Si content was 2.61 wt% and the major impurities were P (0.015 wt%), Ni (0.06 wt%), and Cr (0.01 wt%). The C, Mn, and S contents were about 0.005 wt%. The specimens heat-treated at 800°C for 5 min after compressing to 66.7 % in height had typical primary recrystallization structures with fine grains of 8.7 mm in average radius as shown in Fig. 1. The X-ray pole figure and electron back-scattered pattern (EBSP) analysis of these specimens did not show any macroscopic or microscopic texture. Figure 1 is the initial microstructure for all subsequent heat-treatments at various temperatures to observe the grain growth behavior.The grain growth during the heat-treatment at 600°C was very slow. After 250 h at 600°C, the grains were only slightly larger than those shown in Fig. 1. During the heattreatment at 700°C, typical AGG behavior was observed as shown in the microstructures of Fig. 2, and grain size distributions in Fig. 3. After 3 h, some large grains began to appear as shown in Fig. 2(a), and after 24 h, some grains had grown to about 700 mm radius as shown in Fig. 2(d). In Fig. 3, those large grains that could be clearly identified as the abnormal grains are marked by arrows. The fine matrix grains grew slowly as can be seen in Figs. 2 and 3.When the heat-treatment temperature was increased to 800°C, distinct AGG behavior was again observed during 24 h, but the number density of the abnormal grains was larger than that observed at ...

Review: grain boundary faceting–roughening phenomena

“…In silver, the abnormal grain growth occurred during anneals at low temperatures (700, 600 and 500°C, when a certain portion of GBs was faceted), and at high temperatures (920 and 800°C when all GBs were rough or non-faceted), the normal grain growth took place [176]. Similar effect of temperature and resulting GB facetingroughening has been observed in stainless steel [177][178][179] and in Fe-Si alloys [180]. Thus, when commercial 316L stainless steel specimens are heat treated in a single-phase state at 1100°C, abnormal grain growth occurs, and some GBs are faceted with hill-and-valley structures in TEM.…”

“…If all GBs have a rough structure, then a nearly isotropic normal grain growth occurs. The abnormal grain growth is observed to occur when all or a fraction of the GBs are faceted [126,160,[173][174][175][176][177][178][179][180][181][182][183][184]. Below follows the detailed discussion of the works [126,160,[173][174][175][176][177][178][179][180][181][182][183][184].…”

Facet–Defacet Transition of Grain Boundaries in Alumina

“…G RAIN boundaries of hill-and-valley (h&v) shapes with alternating parallel segments or kinked shapes (as observed at cross sections) with flat or curved segments intersecting at joints have been observed in many metals (Au, [1][2][3] Ni, 4,5 Al, 2,6,7 Ag, 8 -10 Cu, 11 Zn, 12,13 316L stainless steel, 14,15 a Ni-base superalloy, 16 Si-Fe, [17][18][19] Cu-Bi, 20 -22 and Fe-Te 23,24 ) and some oxides (MgO, 25 MgO⅐2Al 2 O 3 , 26 NiO, 27 Al 2 O 3 , 28 -34 and BaTiO 3 35-39 ) with both special and general misorientation angles between the grains. Some flat segments of the h&v boundaries were parallel to low-index planes of one or both of the grain pairs 1,3,28,32,34 -39 and likely to be singular corresponding to cusps in the polar plot of the boundary energy ␥ against the normal direction.…”

Singular Grain Boundaries in BaTiO<SUB>3</SUB> with Excess TiO<SUB>2</SUB>

“…In polycrystalline Au, Ichinose and Ishida 8) reported that about 30% of the grain boundaries observed were low index grain boundaries but the macroscopic shapes of the boundaries were not described. In Ni, 17) 316L stainless steel, 18) a Ni-base superalloy, 19) Si-iron 20) and alumina, 16) grain boundaries with hill-and-valley (h&v) shapes were observed. Although the segments of these boundaries were likely to be singular, the orientations of their boundary planes were not determined.…”