In the solidification of castings, elongated grains that grow inside the surface chill zone in the direction opposite to heat flow. Equiaxed grains form in the center.
Japanese name: 柱状晶
Columnar grains appears in 3 practice questions across 2 topics, including High-temperature materials and creep, Metals and alloys.
Field: Metals and alloys
Not every one of the fine chill grains formed at the mold wall grows inward. In cubic metals, dendrites grow fastest along <100>, and grains whose <100> lies closest to the direction opposite to heat flow (the direction of the temperature gradient) advance furthest. Poorly oriented grains are overtaken by their neighbors and stop growing, so the number of grains decreases with distance inward, leaving a zone of elongated grains with aligned <100>. The texture of the columnar zone, whose properties differ with direction, is a result of this competitive growth.
Whether columnar grains give way to equiaxed grains partway through solidification (the CET, columnar-to-equiaxed transition) is determined by the temperature gradient G and growth rate V at the solidification front and by the alloy composition. The smaller G and the larger V, and the more solute, the more the liquid ahead of the front is constitutionally undercooled and the more easily new nuclei form, favoring the switch to equiaxed grains. Conversely, raising the pouring temperature (increasing the superheat) remelts crystal fragments in the liquid, lengthening the columnar zone. Controlling the position of the CET is central to designing cast structures.
The columnar zone has mechanical properties that differ with direction, and solute, impurities and shrinkage cavities tend to collect on the central plane where columnar grains growing from both sides meet (centerline segregation). In continuous casting of steel, electromagnetic stirring is therefore used to move the liquid and break off columnar tips, increasing the fraction of equiaxed grains. In aluminum alloys, Al-Ti-B grain refiners are added to multiply heterogeneous nucleation sites, preventing columnar growth and giving fine equiaxed grains. If a casting is hot worked afterward, the columnar structure is converted into fine grains by recrystallization.
Columnar grains are not only a drawback. Jet engine turbine blades are made by directional solidification, in which the mold is cooled from the bottom so that heat flows in one direction and the blade consists only of columnar grains running along its length. At high temperature, grain boundaries perpendicular to the stress readily become sites of creep fracture, so eliminating them greatly extends life. Furthermore, if a helical passage (a grain selector) is used to select and grow a single grain, the result is a single-crystal blade with no grain boundaries at all. Here too, the fast growth along <100> is put to use.
Answer: C. From the outside: chill grains, columnar grains, equiaxed grains
When the melt touches the cold mold wall, the large undercooling nucleates many crystals at once, forming a layer of fine equiaxed chill grains at the surface. Next, grains whose orientation favors growth extend selectively in the direction opposite to heat flow toward the mold wall (inward, as heat flows outward), forming a zone of elongated columnar grains pointing toward the center. In the center the temperature gradient becomes small, and equiaxed grains with no preferred direction grow from crystal fragments drifting in the liquid. The choices that start from columnar or equiaxed grains on the outside rearrange this order. Columnar grains have direction-dependent properties, and impurities tend to collect where columnar grains meet, so castings are sometimes inoculated or stirred to increase the equiaxed zone.
Answer: C. At high temperature grain boundaries become fast diffusion paths and sliding sites, so grain boundary diffusion creep, grain boundary sliding and void formation on boundaries all become easier the more boundaries there are
At high temperature, diffusion of atoms along grain boundaries is fast and grain boundary sliding also occurs, so grain boundaries become weak spots. The temperature at which grain boundaries and grain interiors are equally strong is called the equicohesive temperature, and above it the boundaries are the weaker. Diffusion creep (especially Coble creep, inversely proportional to the cube of grain size), grain boundary sliding, and rupture by the nucleation and coalescence of voids on boundaries therefore occur more readily in fine-grained material with many boundaries. That is why coarse grains, columnar grains parallel to the stress, and even single crystals are used in high-temperature components. Young's modulus is set by interatomic bonding and hardly depends on grain size, so it is no advantage of coarse grains. The statement that boundaries become stronger than the interiors at high temperature reverses the relationship.
Columnar grains are elongated in one direction along the heat flow, while equiaxed grains are roughly the same size in every direction. Equiaxed grains show less directional variation in properties and spread segregation more evenly, so they are preferred in many castings.
Because in cubic metals, anisotropy in the solid–liquid interface energy and in how readily atoms attach makes dendrite trunks grow fastest along <100>. Grains with this direction close to the heat flow survive the competitive growth.
Yes. Heat in the weld pool escapes into the surrounding base metal, so columnar grains grow from the fusion boundary toward the weld centerline. Impurities collect on the plane where they meet at the center, which can cause solidification cracking.
Next, grains whose orientation favors growth extend selectively in the direction opposite to heat flow toward the mold wall (inward, as heat flows outward), forming a zone of elongated columnar grains pointing toward the center.
Which sequence of solidification structures is generally se…That is why coarse grains, columnar grains parallel to the stress, and even single crystals are used in high-temperature components.
At room temperature, refining the grains raises strength (t…Directional solidification produces columnar grains aligned with the stress, removing the perpendicular boundaries.
Why are directionally solidified columnar-grained or single…Hall–Petch relationship Diffusion creep Young's modulus Nickel-based superalloy Creep Dislocation Brass Recovery Recrystallization Solid-solution strengthening