A tree-like crystal with arms branching regularly from a trunk. In cubic metals both the trunk and the arms grow along <100>, and most alloy castings solidify in this form.
Japanese name: デンドライト
Dendrite appears in 7 practice questions across 3 topics, including Solidification and cast structures, Phase diagrams and the lever rule, Computational materials science.
Answer: A. Richer in Ni than the average alloy composition
The solid that forms early in solidification has the composition at the end of the higher-temperature tie line, so it is rich in Ni. With rapid cooling, diffusion in the solid cannot keep up, and the outer layers that solidify later are progressively richer in Cu, leaving segregation in which the composition changes from the center of the dendrite outward (a cored structure, or coring). The center is therefore richer in Ni than the average composition. Diffusion in the solid is far slower than in the liquid, so the equilibrium composition is not necessarily reached. The last regions to solidify have a lower melting point and may melt locally on reheating, so a homogenizing anneal is used to reduce the segregation.
Answer: C. Microsegregation varies over short distances and can be reduced by homogenization annealing, while macrosegregation results from solute carried over long distances by liquid flow and the like, and heat treatment hardly removes it
Microsegregation is the uneven concentration in which a solute with k < 1 is lean in the dendrite cores and rich between the dendrites, over a scale of the arm spacing, some tens of micrometers. Once the diffusion distance √(Dt) reaches about half of this, the concentration evens out, so homogenization annealing at high temperature can reduce it. Macrosegregation forms when enriched liquid moves distances of several centimeters to several meters, through suction of interdendritic liquid by solidification shrinkage, convection driven by density differences, settling of crystals and so on. Centerline segregation in continuously cast slabs is a typical example; the distances are far too large to be undone by diffusion, so it is countered during solidification, for example by soft reduction near the end of solidification or electromagnetic stirring. The options that swap the causes and the ease of removal by heat treatment, or treat the two as the same mechanism, are wrong.
With rapid cooling, diffusion in the solid cannot keep up, and the outer layers that solidify later are progressively richer in Cu, leaving segregation in which the composition changes from the center of the dendrite outward (a cored structure, or coring).
When a Cu-Ni alloy is solidified with rapid cooling, as in …Microsegregation is the uneven concentration in which a solute with k < 1 is lean in the dendrite cores and rich between the dendrites, over a scale of the arm spacing, some tens of micrometers.
Segregation in castings and ingots includes microsegregatio…Because the interface need not be tracked explicitly, microstructures with complex changes of shape, such as dendrite growth, grain growth and precipitate coarsening, can be calculated.
Which statement about the phase-field method is correct?The first solid to form in an Al-4%Cu alloy contains only about 0.17 × 4 = 0.68% Cu, and the excess solute is rejected into the liquid, which gradually becomes richer, so the dendrite cores are lean and the interdendritic regions and grain boundaries that solidify last are rich.
In alloy solidification, the ratio k = Cs/CL of the solute …G×V, on the other hand, is the cooling rate (K/s) and sets the size of the structure: the faster the cooling, the less time solute has to diffuse and the finer the cell and dendrite spacings.
Which statement about how the shape of the solid–liquid int…Constitutional supercooling Columnar grains Macrosegregation Phase-field method Molecular dynamics Equilibrium partition coefficient Chvorinov's rule Dendrite arm spacing Eutectic modification Scheil equation