Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 56

GEOMETRICALLY CLOSE‐PACKED

TOPOLOGICALLY CLOSE‐PACKED

CARBIDE

The process of precipitation hardening relies on changes in a material's solid solubility with temperature to produce fine particles of an impurity phase within grain interiors, which impede the movement of dislocations. During precipitation hardening, alloys must be kept at elevated temperature for a significant amount of time, called aging, for precipitation to take place. However, not all precipitates are created equal, and engineers must be careful to design the system and processes to produce the desired precipitates. There are three general classifications for precipitates: (i) Geometrically close‐packed (GCP) precipitates are phases that are close‐packed in all directions and generally provide positive influences on mechanical properties. (ii) Topologically close‐packed (TCP) precipitates are not close‐packed throughout the whole structure. They often have close‐packed layers separated by relatively large interatomic distances due to sandwiching larger atoms (Bowman 2000). With fewer available slip planes, these precipitates tend to have brittle behavior. Moreover, nonideal TCP phases can grow at the expense of desired GCP phases and result in a depletion zone of the desired participate phase (Belan 2016). (iii) Carbide precipitates can have quite different phase structures than their surroundings and can have extremely important impacts – both desired and undesired – on properties.