Tensile testing of cast and extruded binary NiAl was performed from 300 to 900 K at strain rates of 1.4 × 10−4 to 1.4 × 10−1 × s−1. The brittle-to-ductile transition temperature (BDTT) was dependent on strain rate, with a three order of magnitude increase in strain rate resulting in approximately a 200 K increase in transition temperature. Regardless of strain rate, at temperatures just above the BDTT the fracture strength increased significantly and the fracture morphology changed from mostly intergranular to predominantly transgranular. It was also determined that the mechanism responsible for the brittle-to-ductile transition in NiAl had an apparent activation energy of approximately 118 kJ/mol. These results support the argument that the mechanism for the brittle-to-ductile transition in NiAl is associated with the onset of a thermally activated deformation process. This process is probably dislocation climb controlled by short circuit diffusion.
NiAl has only three independent slip systems operating at low and intermediate temperatures whereas five independent deformation mechanisms are required to satisfy the von Mises criterion for general plasticity in polycrystalline materials. Yet, it is generally recognized that polycrystalline NiAl can be deformed extensively in compression at room temperature and that limited tensile ductility can be obtained in extruded materials. In order to determine whether these results are in conflict with the von Mises criterion, tension and compression tests were conducted on powder-extruded, binary NiAl between 300 and 1300 K. The results indicate that below the brittle-to-ductile transition temperature (BDTT) the failure mechanism in NiAl involves the initiation and propagation of cracks at the grain boundaries which is consistent with the von Mises analysis. Furthermore, evaluation of the flow behavior of NiAl indicates that the transition from brittle to ductile behavior with increasing temperature coincides with the onset of recovery mechanisms such as dislocation climb. The increase in ductility above the BDTT is therefore attributed to the climb of <001> type dislocations which in combination with dislocation glide enable grain boundary compatibility to be maintained at the higher temperatures.
As part of a study to investigate the fatigue behavior of the intermetallic compound NiAl near its monotonic brittle-to-ductile transition temperature (BDTT), prealloyed stoichiometric NiAl powders were hot extruded and fabricated into tensile and fatigue specimens. From the monotonic tests conducted at a nominal strain rate of lo4 sect, the BDTT was found to be approximately 650 K. Low cycle fatigue (LCF) tests were performed at 600, 675, and 700 K at plastic strain ranges of 0.005 and 0.01. The percentage of intergranular failure decreased in the fracture morphology of both the monotonic and fatigue tests. For all temperatures and strain ranges in this study, rapid hardening was observed for up-to 15 cycles, after which the peak stress increased only slightly for the remainder of the life. As test temperature was increased or strain range decreased, both the initial hardening rate and relative saturation stress decreased. Additionally, slip traces were observed on the gage surfaces of all LCF specimens. In fatigue specimens, secondary cracks were found in the gage section near the eventual fracture site. Transgranular surface cracks apparently resulted from slip intrusions/extrusions. Intergranular surface cracks were seen that could result from planar slip pileups at grain boundaries, however other mechanisms have not been ruled out. No significant improvement in life at a strain range of 0.005 was observed when the test temperature was increased from 600 to 700 K, a change which corresponds to an increase in monotonic tensile ductility from 2 to 18%. Apparently, a cyclic brittle-to-ductile transition did not occur in this temperature range. If thermally activated deformation affects fatigue lives as it does monotonic ductility, the cyclic transition temperature is higher than the monotonic BDTT. More work is necessary to precisely identify the deformation mechanisms that operate in low cycle fatigue of NiAl.
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