Characterizations of the B2-Ni-Al(B) ordered alloy produced by high-energy mechanical alloying process: microstructure and thermal behaviors

In general, Ni-Al alloys have significant commercial value throughout a range of industrial uses. As a nanostructured powder, these alloys offer an ideal balance of characteristics such as strength at high temperatures, oxidation resistance, and creep resistance. Uses may include heat exchangers, gas turbine parts, and other high-temperature structural elements that are commonly made of nickel-based superalloys. The current study details the high-energy mechanical alloying process that produces ordered B2-Ni-47Al-3B from elemental Ni, Al, and B powders in up to 40 h. Phase development, morphology, microstructure, and thermal properties were examined as a function of milling time. Transformations in the microstructure and phase were influenced by the milling time. When the milling time approached 18 h, the crystalline Al peaks disappeared. At this stage, a 12 nm-sized crystallite of partially ordered B2-Ni-Al(B) phase has been obtained. A high-energy milling process for 40 h produced a significant amount of lattice strain (ε∼0.75%). Through the use of XRD, changes in the degree of long-range order (LRO) during mechanical milling were studied. It may be concluded from the predicted values of S (0.35–0.55), even for extended milling times up to 40 h, that the complete ordering of the B2-Ni-47Al-3B intermetallic structure was not produced. The resulting state of nanostructure correlates with the concept of structural defects: after 40 h of milling, a density of dislocations of 8.56 × 10¹⁴ m^− 2 and a probability of stacking faults of 0.91% were estimated. Also, it was found that an orthorhombic Ni₃B intermetallic formed after 18 h of milling, which changed into tetragonal Ni₂B intermetallic during further milling up to 40 h. From the experimental heat-flows measured in DSC, it was determined that the reordering process is a result of the dislocation annihilation and growth of ordered domains.