A Acoustic Emission Investigation of the Ductile to Brittle Transition in Molybdenum.

The present investigation has shown the effectiveness of making careful acoustic emission measurements in conjunction with in-depth metallographic analysis, in order to fully investigate and characterize the cold embrittlement of polycrystalline molybdenum. Acoustic emission measurements were made during a series of tensile tests conducted in the ductile-to-brittle transition temperature range. The acoustic emission showed continuous emission and burst-type emission. The continuous emission was identified as being due to dislocation breakaway motion and the burst-type emission was shown to be due to brittle cracking phenomena. The principal cold embrittlement mechanism was believed to be the increasing dislocation pinning strength with decreasing temperature. The increasing dislocation pinning strength was acoustically detectable. Finally, the ability to differentiate between intergranular cracking and transgranular cleavage cracking, by studying the fast Fourier transformations of individual waveforms, was demonstrated.