Directional recrystallization and microstructures of an Fe–6.5wt%Si alloy

Abstract

Directional recrystallization of an Fe–6.5wt%Si alloy was investigated by changing hot zone temperatures and growth rates. Elongated (columnar) grains with an aspect ratio more than 10 can be produced when growth parameters are carefully adjusted. It was found that at a fixed growth rate, the grain length and aspect ratio increase with increased hot zone temperatures. At a fixed hot zone temperature, there is a critical growth rate at which columnar grains have the largest average aspect ratio. Below or above this growth rate, the aspect ratio decreases. Texture and grain orientation analysis showed that the preferentially selective growth to form columnar grains was favored by the formation of low-energy surfaces and grain boundaries.

Directional recrystallisation processing: a review

“…Both Baker et al [36,37] and Zhang et al [38,41,44] have found that in particle-free metals or alloys, there is a lower limit to the annealing temperature for columnar grain formation through directional secondary Si 1373 NS 3-30 [13] Ni-based superalloy PWA1123 NS NS NS [14] Fe-3wt-%Si 1323-1623 320-550 increased with increasing T 19 [15] Ni-base superalloy TMO-2 1563 25 50-100 [16] Ni-base superalloy MA6000 1123-1373 NS NS [17] Ni-base superalloy MA6000 1373-1573 NS 48-600 [18] Ni-base superalloy APK-6 1493 NS 2.0-20.0 [19] Ni-base superalloy MA6000 and MA760 1523 NS 2-200 K min −1 (heating rate) [20] Ni-base superalloy MA6000 1443 25 38 [21] Fe-based Superalloy MA957 1673 NS 20-75 [22] Ni-base superalloy MA6000 1623 40 38 [23] Ni-base superalloy MA6000 1223-1503 NS 15-400 [24] Ni-based superalloy PM3030 1503 100 NS [25] Ni-base superalloy MA6000 and MA760 1573 40 38 [26] Fe-based Superalloy MA 956 and MA957 1533-1613 50-140 24-75 [27] Ni-base superalloy MA6000 and MA760 1448 NS 38 [28] Ni-base superalloy MA760 and iron-based superalloy MA 956 ∼1523 NS 48-600 [29] Ni-base superalloy MA6000 1223-1273 NS 10-100 [30] Ni-base superalloy SRR99 ∼1533 NS 5 [31] Ni-base superalloy MA6000 1573 NS 38 [32] N i 3 Al 1698 NS 25-100 [33] Ni-base superalloy APK-6 1493 NS NS [34] TiAl-based alloy 1553-1593 NS 1.62 and 4.02 K min −1 (heating rate) [35] Cu 643-743 70-270 2-600 [36] Ni 1273 ∼1000 and 50 2-100 [37] Ni 1073-1273 ∼1000 and 50 2-100 [38] Iron 973-1123 ∼200 3.6-90 [39] Iron 1123-1473 ∼200 and 400 3.6-90 [40] Iron 1123 ∼200 0.36-90 [41] Iron 948-1173 ∼200 1.08-108 …”

“…Note that the temperature gradient along the drawing direction depends on the experimental set-up used for directional recrystallisation. Usually, the temperature gradient of air-cooled furnaces is tens to several hundreds of K cm −1 [7][8][9]15,26,35,36,45,47,57] while furnaces designed with a heat sink can provide a temperature gradient from 100 to 1000 K cm −1 [3][4][5][6]8,[35][36][37][38][39][40][41][42][43][44]46,48,49,52,53,55,56], depending on the furnace layout and cooling system. The heating source, cooling method and temperature gradient noted in various papers are listed in Table 2.…”

“…The heating source, cooling method and temperature gradient noted in various papers are listed in Table 2. For furnaces where the heating unit moves when the sample is static [6,18,[20][21][22][23]25,28,30,31,33,35,50], the temperature gradient often changes continuously during processing since the distance between the heating unit and heat sink changes; while for furnaces where the sample travels when the heating unit is static [8,[35][36][37][38][39][40][41][43][44][45][46]48,49,[52][53][54][55][56][57][58], the temperature gradient is usually constant as long as the sample is long enough. It is clear that static heating unit design is better for directional recrystallisation research since the temperature gradient is well controlled.…”

“…A transverse temperature gradient can be present in some directional annealing studies, especially with large samples and using induction heating, in which a transverse temperature gradient is induced by the skin effect. Using the finite element method, Liu et al [54] showed that for a five-turn induction coil furnace (cross-section of 60 × 60 mm 2 ), a transverse temperature gradient was observed when the iron sample diameter was larger than 20 mm and that it increased with increasing sample Air-impingement cooling 844 [15] Induction coil Ambient air cooling 25 [35] Focused light radiation Ambient air cooling 50 [35][36][37]43,53,53,55,56] Focused light radiation Water-cooled copper 1000 [38][39][40][41][42]44,48] Induction coil Liquid Ga-In cooling bath 200-350 [45,47] Resistance heating Ambient air cooling 82 [46] Induction coil Water cooling bath 900 [49] Induction coil Water-cooled chill block 200 [55] Induction coil Water cooling bath 680 [57] Induction coil Ambient air cooling 180 diameter. Thus, a transverse temperature gradient can undoubtedly affect the directional recrystallised structure in different parts of the material.…”

“…Several studies [35][36][37][38][39][40][41]43,44,53,55] have demonstrated that the lower hot zone velocity limit exists for directional secondary recrystallisation because normal grain growth occurs ahead of the hot zone if the hot zone velocity is too low, which reduces the driving force for columnar grain formation. In contrast, the upper hot zone velocity limit corresponds to the highest grain boundary velocity that can keep up with the moving hot zone.…”