The relationship between the fracture toughness and grain boundary character distribution in polycrystalline NiAl

This paper reports the results of experimental studies on the relationship between grain boundary character distribution and the fracture toughness in the polycrystalline NiAl. The fracture toughness has been investigated by the ring on ring disk bend test in conjunction with the acoustic emission (AE) measurement, and the grain boundary character distribution is measured by the electron back-scatter diffraction pattern (EBSD or EBSP) technique. The fracture toughness of the annealed specimen (1473 K, 20 h) is 6.42 +/- 0.88 (MPa m(-1/2)) and that of the forged (1473 K) and annealed (1473 K, 20 h) specimen is 5.08 +/- 0.36 (MPa m(-1/2)). The strong boundary and weak boundary were decided by comparing the distribution of grain boundary characteristics at the intergranular cracks to the distribution in general population. Sigma1, Sigma3 and Sigma5 boundaries have strong and the good crack resistance, but Sigma7, Sigma11, Sigma13, Sigma21 and Sigma23 boundaries are relatively weak boundary in the polycrystalline NiAl. Consequently, the annealing process makes the fraction of Sigma1 and Sigma5 boundaries higher than forging and annealing process. The high fraction of Sigma1 and Sigma5 boundaries that have the good crack resistance seem to affect the increase of fracture toughness in the annealed specimen. (C) 2003 Published by Elsevier Science Ltd.