Grain refining

Grain refinement is one way of increasing the strength of metallic materials . This involves the creation of a finer, smaller grain in the structure through suitable heat treatment or treatment (inoculation) of the melt . Fine-grain hardening is the only hardening process based on lattice defects (in this case the grain boundaries ), in which both strength and ductility are increased.

meaning

Dislocations build up at the grain boundaries, since there is usually no suitable slip plane in the neighboring grain. Therefore the yield stress increases the more grain boundaries exist.

The grain refinement increases the strength of metallic materials (without reducing the toughness ) by lowering their transition temperature . One possible explanation for this is that the reduction in the shear modulus at grain boundaries facilitates the generation of dislocations. Another assumes that a crack must change direction at every grain boundary. This reduces the tendency of the steel to develop hot cracks, for example.

In addition, the rises through the finer grain size ductility of the materials, as a result consists of finer grain, the probability that more slip planes low to the direction of shear loading are (45 °, Schmid's Law ). This results in an increased number of possible sliding processes. Grain refinement is particularly useful for hard and very brittle materials, as it is a way of making these materials less brittle.

Despite the many obstacles that arise within the material due to the smaller grains, the material is easily deformable . For these reasons, fine-grain steel sheets z. B. used in the automotive industry. The higher strength makes it possible to use thinner sheets (and thus also to save weight).

It should be noted, however, that smaller crystallites result in more grain boundaries and thus a higher susceptibility to corrosion . Every material strives to achieve a state of equilibrium with the lowest possible energy content. However, a higher strength means a high energy content, which the material breaks down through corrosion. Due to the high susceptibility to corrosion, precautions must therefore be taken to protect against corrosion . For this reason, car be bodywork often galvanized . In addition, materials with a fine grain structure are not suitable for high-temperature applications where creep can occur; here, emphasis is placed on large grains.

calculation

The so-called Hall-Petch relationship applies to the dependence of the yield strength on the grain size (or the mean grain diameter) : ${\displaystyle R_{e}}$ ${\displaystyle d_{\rm {K}}}$

${\displaystyle R_{e}=\sigma _{0}+{K \over {\sqrt {d_{\rm {K}}}}}}$

with two constants that depend on the material condition and the test conditions:

• ${\displaystyle \sigma _{0}}$the starting stress for the dislocation movement ("frictional stress", yield point of the single crystal with favorable orientation)
• ${\displaystyle K}$the grain boundary resistance, a constant with the unit . ${\displaystyle \textstyle [{\mathpunct {MPa}}*{\sqrt {mm}}]}$

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

${\displaystyle \Delta \sigma ={K \over {\sqrt {d_{\rm {K}}}}}.}$

Individual evidence

1. ↑ ^{a b} Christoph Broeckmann, Paul Bite: Materials Science I . Institute for Material Applications in Mechanical Engineering at RWTH Aachen University , Aachen 2014.
2. ↑ Bleck: Materials testing in study and practice . Ed .: Bleck. 2011, p. 82 f .
3. ^ Gottstein: Material science and material technology . Ed .: Gottstein. S. 265 .
4. ↑ Yoshisato Kimuraa, David P. Pope, Ductility and toughness in intermetallics , Intermetallics, 6 (1998) 567-571, doi : 10.1016 / S0966-9795 (98) 00061-2
5. ↑ Institute for Material Applications in Mechanical Engineering at RWTH Aachen University : sample solution 9th exercise / material science I / WS 08/09 ( memo from February 16, 2016 in the Internet Archive ), accessed on February 16, 2016