The influence of melt convection on dendritic spacing of downward unsteady-state directionally solidified Sn-Pb alloys

Abstract

Microstructures are the strategic link between materials processing and materials behavior. A dendritic structure is the most frequently observed pattern of solidified alloys. The microstructural scales of dendrites, such as primary and secondary arm spacings, control the segregation profiles and the formation of secondary phases within interdendritic regions, determine the properties of cast structures. In this work, the influence of thermosolutal convection on dendrite arm spacings is experimentally examined in the downward vertical unsteady-state directional solidification of Sn-Pb hypoeutectic alloys. The experimental observations are compared not only with the main predictive theoretical models for dendritic spacings but also with experimental results obtained for Sn-Pb alloys solidified vertically upwards. Primary dendritic arm spacings have been affected by the direction of growth, decreasing in conditions of downward vertical solidification when compared with those grown vertically upwards. Further, the unsteady-state lambda1 predictive models did not generate the experimental observations

Microstructure and Microhardness of Directionally Solidified Al-Si Alloys Subjected to an Equal-Channel Angular Pressing Process

“…Over the years, regarding either vertical unidirectional solidification [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][27][28][29][30] or equal-channel angular pressing 19−26, 31, 32 experiments, sundry works have focused on the resulting microstructure. Since microstructure control is a key factor in generating excellent properties in metals and their alloys, determination of parameters which can affect said microstructures is of primary importance.…”

Effect of cross-section-change induced advective flow on the primary dendrite array morphology of hypoeutectic Pb-Sb alloys during directional solidification

“…The contradictory observations described above, larger primary dendrite spacing in the presence of convection due to shear-flow than that in its absence [29,30] and increased primary dendrite spacing in low gravity grown (reduced convection) samples as compared with those terrestrially solidified [35][36][37][38][39][40][41][42], suggest that the nature of convection may be quite different under the two circumstances; i.e., changes of the dendrite morphology resulting from convection due to shear flows do not represent those that are associated with the thermosolutal convection during terrestrial directional solidification. A true understanding of the effect of natural convection on the dendritic array morphology and its distribution cannot be attained by imposing only a transverse flow.…”

“…The influence of forced convection on the dendrite array morphology has been examined by superimposing a shear-flow (transverse to the growth direction) during solidification of transparent organic model "alloys" [24][25][26][27][28][29][30] and of Ga-25In [31,32]. The influence of natural convection on the dendrite array morphology has been studied by, (i) comparing morphologies observed in samples solidified in larger diameter ampoules with those grown in smaller diameter ampoules or within narrow-gaps in order to mitigate convection [33,34], (ii) selecting alloy compositions and growth conditions which promote buoyancy flows in the mushy zone either towards the tips of the primary dendrites or towards their base [35,36], (iii) comparing microstructures of alloys solidified on earth with those grown in the lowgravity environment of space [35][36][37][38][39][40][41][42][43][44], (iv) using magnetic fields during directional solidification to suppress [45] or enhance convection [46][47][48], and (v) by developing semi-empirical formulations [45] that qualitatively explain the discrepancy between the observed primary spacings and those predicted from models which are based on purely diffusive transport. Numerical simulation attempts, both two and three dimensional, have been made to study the relationship between convection and dendrite morphology [48][49][50][51][52].…”

Freeze‐cast honeycomb structures via gravity‐enhanced convection

“…Motivated by studies in directional solidification of metal alloys with convective flow [12][13][14] , this study focuses on the effect of the convective flow induced by gravity during freeze casting. In alloy systems, depending on the density of the composition in alloys, convective flow may be present during directional solidification 15,16 . For instance, in what has been labeled downward freezing (in the same direction as the gravitational force) if the solute is denser than the solvent, the segregated solute creates a denser fluid region ahead of the freezing front, and enhanced convective flow.…”