Effect of Cr on the ordering behavior and ductility of an Ni-Ni4Mo alloy

Abstract

Two approaches have been identified to improve the ductility of polycrystalline intermctallic compounds depending upon the embrittlement mechanism. Microalloying involving small additions of B has been proved effective in those cases where the embrittlement is associated with intrinsic weak cohesion along grain boundaries, as in the case of intermetallics based upon the L! ~ superlattice, such as Ni3AI m and Ni3Si. m Macroalloying is used when the embrittlement is caused by a restricted number of slip systems imposed by the crystal structure. For example, the crystal structure of Co3V could be changed from hexagonal (low ductility) into cubic (high ductility) by substituting Ni and Fe for some Co. TM In contrast to the cases of Ni3A1 and Co,V, most evidence points out that the embrittlement of Ni-Ni4Mo alloys is caused by a discontinuous grain boundary reaction. 14'~'61 Such a reaction can be described in terms of strain-induced recrystallization to minimize the strain energy associated with matrix ordering. 17'~} It is the objective of this communication to demonstrate that macroalloying involving an addition of Cr to a Ni-27Mo alloy which otherwise orders to Ni4Mo stabilizes a PhMo-type superlattice, producing about the same level of strengthening as with NiaMo ordering but with considerably improved ductility. Alloys examined included Ni-27Mo and Ni-27Mo-5Cr in all weight percent. Table 1 summarizes their chemical compositions. At the hypostoichiometric Ni-27Mo composition, Ni4Mo (tetragonal, D I , superlattice) precipitates from the parent face-centered cubic (fcc) lattice. Both alloys were prepared by vacuum melting and then processed into 1.5-mm-thick sheets by hotand coldrolling. Experimental specimens were solution annealed for 15 minutes at 1065 ~ in argon atmosphere and then water quenched. To examine the ordering behavior of the two alloys, annealed specimens were exposed at 700 ~ for up to 1000 hours. Mechanical properties were determined from roomand elevated-temperature tensile tests (50.8-ram gage length). Transmission electron microscopy (TEM) was used to characterize the ordered microstructure. Thin-foil specimens were prepared by the jet polishing technique in a solution consisting of 30 pet nitric acid in methanol at about 2 0 ~ Fractography was conducted in a scanning electron microscope. Both alloys were found to contain short-range order in the as-quenched condition, as was evidenced from the observation of {I I / 2 0} diffuse intensity maxima in