MECHANICAL PROPERTIES OF AL BASED AMORPHOUS/NANOCRYSTALLINE ALLOYS

摘要

During the last years, rapidly solidified Al-TM-RE (TM = transition metal; RE = rare earth) amorphous alloys have been studied because of enhanced properties in comparison with conventional light alloys. Studies have been recently done to improve knowledge not only on hardness, bending ductility, mechanical strength, wear resistance, but also on the mechanism of deformation. It is well established that glassy metals undergo inhomogeneous deformation at low temperature, as shown by the formation of vein patterns in fractured surfaces. The details of the deformation mechanism are not completely outlined yet. It is not clear which event induces the local nucleation of voids leading to failure. Moreover, after tensile and bending tests, the formation of Al nanocrystals was observed in TEM for some compositions, but not for others, both in the shear bands and within the vein patterns. For these reasons the mechanical properties of Al_(87)Ni_7RE_6 (RE = Ce, Nd, La) and Al_(88)Fe_9Nd_3 alloys are studied in this work. Ribbons were obtained by melt spinning and their structure was checked by X-ray diffraction (XRD), finding completely amorphous (Al_(87)Ni_7Nd_6 Al_(87)Ni_7La_6 e Al_(87)Ni_7Ce_6) and nanocrystalline (Al_(88)Fe_9Nd_3) samples. The stability of the amorphous phase was studied by differential scanning calorimetry (DSC). Al_(87)Ni_7La_6 and Al_(87)Ni_7Ce_6 present two transformation steps and the Al_(87)Ni_7Nd_6 shows three steps before reaching the stable crystalline phases. The first transformation is related, in the case of Al_(87)Ni_7La_6 and Al_(87)Ni_7Ce_6 alloys, to the primary crystallisation of metastable intermetallics finely dispersed in the remaining amorphous matrix. In the case of Al_(87)Ni_7Nd_6, nanocrystals of Al are produced during the first transformation step. Cold rolling was performed on amorphous samples obtaining for Al_(87)Ni_7Nd_6 elongations up to 10 percent without formation of cracks. Ribbon surfaces change after deformation and are characterised by flat areas and shear bands perpendicular and parallel to the rolling direction. This shape is due to the plastic deformation that involves the bulk of the ribbon, producing levelling of the cavities and undulations peculiar of the not deformed surfaces. No cracks were found after rolling, indicating that the tensile fracture strength is never exceeded locally. A reduction in ductility with respect to the amorphous samples was detected in nanocrystalline Al_(88)Fe_9Nd_3 where cold rolling to 2 percent of elongation causes cracks nucleating in the cavities of the ribbons where the thickness is reduced. Tensile fracture surfaces were examined by SEM, finding typical vein patterns for the amorphous samples. TEM analysis was performed orienting the deformed surfaces perpendicularly to the electron beam. In bright field TEM images the presence of protuberances or filaments emerging from the veins is evident. Filaments are variously bent and their tip is rounded; sometimes round particles were observed on them. In the images there is no evidence of crystallisation within the veins or the filaments and also the electron diffraction patterns are interpreted as due to an amorphous phase. When the fracture surface of the nanocrystalline sample is examined, instead of veins, dimples are found with diameters of the order of a hundred of nanometers. This behaviour can be explained by a reduced plasticity acting when the microstructure is characterised by nanocrystals dispersed in the amorphous matrix. The microhardness ofAl_(87)Ni_7La_6 and Al_(87)Ni_7Ce_6 was investigated by heating the alloys up to the end of the two transformation steps. The behaviour of the two alloys is very similar, showing that the change in rare earth element has no influence on this mechanical property. The partial crystallisation of the amorphous matrix causes an increment of the microhardness. The values find up to this step of crystallisation are similar to the ones find for the Al_(8