New tests for complex metal alloys
Many reliable tests exist for testing the plastic deformation (breaking of bonds, in contrast to elastic deformation) of hard yet ductile materials such as metals. However, the situation is much more complicated when it comes to brittle materials. The EU-funded project HIGHTEMPPROP was the first study investigating the testing of a new class of brittle materials, complex metallic alloys (CMAs). CMAs are crystalline solids containing up to thousands of atoms per unit cell. Defects and deformation mechanisms in CMAs remain largely unknown in contrast to simple crystalline materials. CMAs show great industrial promise for friction reduction and surface protection applications, but suitable testing techniques are required for exploitation. Scientists set out to study the fracture behaviour of novel coating materials over a range of temperatures. Researchers chose two CMAs with different crystal structures. Exploiting world leadership in small-scale testing, scientists showed for the first time that these materials deform by movement of dislocations, defects in crystals where atoms are out of position. The dislocations occur below the ductile–brittle transition temperature where materials are much more likely to fracture than to bend or deform. Both CMAs demonstrated serrated flow that, as its name implies, is characterised by jumps or discontinuities in stress–strain curves under constant loading conditions. One demonstrated decreasing flow stress and hardness with increasing temperature, as expected. However, with the second, scientists showed anomalous yielding for the first time in a CMA. In other words, the stress necessary to initiate yielding increased with increasing temperature. Results have generated widespread interest within the CMA research community and led to preparation of several papers for peer-reviewed journals. Invited talks and a supply of crystals for deformation testing are among the benefits for project scientists. The much-needed novel deformation testing methodology stands to benefit not only the research community but also the numerous fields for potential applications of well-characterised CMAs.
Keywords
Complex metal alloys, metal alloys, deformation behaviour, coatings, plastic deformation, brittle materials, crystalline solids, deformation mechanisms, surface protection, fracture behaviour, serrated flow, loading conditions, anomalous yielding
