Grain boundary strengthening

Grain refining is a way to increase the strength of metallic materials. It involves the generation of a finer, smaller grain in the structure by means of appropriate heat treatment or treatment (vaccination ) of the melt.

The dependence of the yield strength of the grain size (or the average grain diameter ), the so-called Hall-Petch relation is valid:

Two constants which depend on the material condition and the test conditions:

• The starting voltage for the dislocation motion ( " friction stress " )
• K the grain boundary resistance.

The smaller the average diameter of the grains, the greater the strength difference ( see also: the stress-strain diagram ):

Practical significance

• The grain refinement increases the strength of metallic materials ( without loss of toughness) by lowering its transition temperature. A possible explanation for this is that the generation of dislocations is facilitated by the lowering of the shear modulus at grain boundaries. Another assumes that a crack at each grain boundary has to change its direction. This example reduces the tendency of the steel to the formation of hot cracks.
• In addition, rising through the finer grain size, the ductility of the materials, as a result, the likelihood consists of the finer grain that more favorable slip planes to the direction of shear stress are (45 ° angle, Schmid cal shear stress law ). It follows an increased number of possible sliding processes.
• Despite the many obstacles that arise within the material by the smaller grains, the material is very malleable. For these reasons, fine-grain steel sheets are eg used in the automotive industry. The higher strength makes it possible to use thinner plates ( and thus also to save weight).
• It should be remembered, however, that for smaller crystallites more grain boundaries occur and so a higher susceptibility to corrosion is. Each material is in fact seeks to establish an equilibrium state with the lowest possible energy content. However, a higher strength is a high energy content, the material degrades due to corrosion. Due to the strong susceptibility to corrosion, therefore, precautions must be taken to protect against corrosion. For this reason, car bodies are often galvanized.
• Especially in hard and very brittle materials is the grain refinement of great use, as they provide an opportunity to produce these materials less brittle.

Swell

• Materials Science