Observations on the brittle to ductile transition temperatures of B2 nickel aluminides with and without zirconium

Overview of NiAl Alloys for High Temperature Structural Applications

“…The high temperature strength of unalloyed NiAI needs improvement to be competitive with the superalloys [19,29,37,38]. There are five ways by which the high temperature strength of NiAl has been improved: 1) elimination of the grain boundaries, 2) solid solution strengthening, 3) metallic precipitate strengthening, 4) intermetallic precipitate strengthening, and 5) composite strengthening.…”

Tensile creep fracture of polycrystalline near-stoichiometric NiAl

“…In this case, fracture involves the nucleation, plastic growth and coalescence of voids without significant creep. Ductile fracture was observed in NiAl above the DBTT and a single datum point corresponding to the ductile fracture regime is shown in figure 15 [38,39]. Similar fracture morphologies observed under creep conditions are distinguished as transgranular creep fracture in figure 15 due to the fact that diffusional processes help reduce stress concentration at inclusions in the material thereby postponing the advent of void nucleation.…”

“…In this case, σ f has a similar form as equation (5) except that it now scales with grain size instead of the size of the pre-existing defect [29]. As demonstrated in previous investigations [38,39], near-stoichiometric NiAl exhibits only limited ductility, typically much less than 1%, and fails predominantly by brittle intergranular cleavage fracture below the ductile-to-brittle transition temperature (DBTT). The two low temperature datum points have been inserted in the slipinduced brittle intergranular cleavage fracture regime in figure 15.…”

“…In reality, their widths are much broader than shown in the figure since mixed failure modes were often observed. The fracture microstructures of Raj et al [38,39] obtained on powder-metallurgy (PM) extruded NiAl are also included in figure 15 in order to extend the range of observations to room temperature. The fracture microstructures of cast and PM NiAl are generally similar although it is expected that the positions of the boundaries are likely to be different.…”

Deformation in NiAl bicrystals

“…These authors show that a strong <I I 1> fiber texture is produced after extrusion of a cast Ni-45% Al alloy, and after extrusion of pre-alloyed powder of the same composition. Extruded material was used in each of the investigations reporting RT ductility for polycrystalline NiA1 [22][23][24]291, although this process did not guarantee RT plasticity [21,27]. Electron microscopy has not yet been conducted to determine if the observed texture plays a significant role in the deformation of the samples exhibiting RT ductility.…”