An analysis of microhardness of single-quasicrystals in the Al–Cu–Co–Si system

Microhardness tests were conducted on single grain quasicrystals of the decagonal phase in a Al–Cu–Co–Si system by varying the load from 15 to 500 g on the ten-fold plane and two-fold planes. Ultrasonic tests were also performed on ten-fold plane and its perpendicular planes. No significant difference was found in the microhardness and elastic properties measured on these two planes. The microhardness was found to be load dependent, varying from 9.01 to 7.86 GPa. The variation of hardness i.e. the indentation size effect (ISE) was attributed to both elastic recovery (important at small loads) and to cracking. The cracks arising during indentation up to a 200 g load were observed to be the Palmqvist type. Lateral cracks other than Palmqvist were also observed at 300 and 500 g loads. At 15 and 25 g loads, cracks were not observed, indicating a limited amount of plasticity. At a 100 g load the four corners of the indentation yielded well-developed and symmetrical radial cracks without any lateral cracks. The fracture toughness (KIC) was calculated from the fracture mechanics relation developed for Palmqvist type of cracking. The measured value was 1.40±0.1 MPa √m, comparable to other data reported in the literature for quasicrystals. A simple model of cracking at indentations equating the release of stored elastic energy to the work of fracture, was used to explain the critical indentation size observed for crack nucleation.