Physical Metallurgy - Understand Solidification in Metals and Alloys

A: The crack path tracks solidification grain boundaries and lines up with the heat flow direction, pointing back to how the metal froze. Excessive columnar growth fits both observations and the timing. Hydrogen embrittlement sounds tempting because it also targets boundaries, but that mechanism needs a post-cast exposure step and usually shows delayed cracking. MnS stringers explain anisotropy in wrought product, not as-cast morphology. Martensite would drive bulk brittleness and hardness spikes, yet it doesn’t explain cracks confined to solidification features.

A: Grain size tracks the balance between how many nuclei form and how fast they grow. Slower cooling drops undercooling, so fewer nuclei form and growth dominates. Solute trapping needs high interface velocity, the opposite condition. Superheat can matter, but without a change in cooling rate it doesn’t automatically erase all nucleants. Oxide films affect surface finish and local heat flow, not bulk grain coarsening across the section.

A: The deviation sits in how the metal froze, not just what elements are present. Macro-etch directly exposes shrinkage and solidification pattern, so it answers the risk fastest. Hardness can miss casting defects entirely. PMI confirms chemistry but stays blind to feeding and grain morphology. Radiography finds voids yet won’t tell you whether columnar growth or segregation is sitting right at a pressure boundary.

A: Hot tearing happens when the last liquid films can’t feed shrinkage, so cracks track interdendritic paths. Oxide bifilms explain reduced fatigue life but not this crack morphology. Silicon content affects freezing range, yet without morphology it’s circumstantial. Uniform equiaxed grains actually reduce hot tearing risk, so that observation cuts against the diagnosis.