No abstract

The crystallographic packet or microstructural unit was determined with 15 • and 5 • boundary grain misorientations in a quenched and tempered X38CrMoV5-1 steel, yielding results of 0.68 and 0.59 µm, respectively. Though both values are similar, the latter has been taken as the best to calculate the effective surface energy of cleavage fracture, γp, leading to a value of 15.8 J m −2. Thus it is demonstrated that the microstructural unit controlling crack propagation is that determined by EBSD with a 5 • misorientation angle. Other structural parameters, like the parent austenite grain size and the morphological martensitic packet, have been ruled out.

The effect of the nature of the second phases in extruded Mg-TM-Y-CeMM (TM refers to a transition metal) alloys reinforced by intermetallic MgRE (RE refers to a rare earth element) compounds and LPSO-phase on their superplasticity has been evaluated between 300 and 400°C at the strain rate of 10-4 s-1. The data have been compared with those of alloys containing a similar volume fraction of the LPSO-phase. The results evidence that no superplasticity below 350°C was found in the alloys containing exclusively the LPSO-phase while the alloys containing both MgRE compounds and LPSO-phase deform superplastically by grain boundary sliding from 300°C. These differences are related to the different behaviour of MgRE compounds and LPSO-phase in the course of superplastic regime. MgRE compounds assist to accommodate the deformation more easily than LPSO-phase, reducing tendency to develop cavities and extending the time for the occurrence of necking. The size and volume fraction of the respective phases are critical in order to promote enhanced superplastic behaviour. Maximum elongations are attained in the alloys combining similar volume fractions of MgRE compounds and LPSO-phase in which their size is reduced to the maximum. An increase in the particle size of the second phases, especially in the case of the LPSO-phase, hinders the grain boundary sliding mechanism in the alloys.