Annealing Twinning in Boron-Doped Ni<SUB>3</SUB>Al

Abstract

Dependence of frequency of annealing twinning events on annealing temperature and composition, especially boron content, in polycrystalline Ni 3 Al alloys has been investigated by means of optical and transmission electron microscopy, and also by evaluation of coherent twin boundary energy via counting the wrong atomic bonds across twin boundaries. Microstructural observation found that after homogenization at temperatures from 1000 to 1360 • C both binary and boron-doped Ni 3 Al alloys without prestrain show annealing twins only on the Al-rich side with respect to stoichiometry below 1200 • C. Above 1200 • C annealing twinning may also take place on the Ni-rich side of stoichiometry in the binary alloy system but no twins were found in the γ + γ two-phase field. In contrast to plenty of annealing twins in binary alloys, addition of boron decreases greatly the number of twins. 0.2 at% addition significantly decreases the frequency of twinning events and no twins appear above 1200 • C. 0.5 at% boron almost cleans out all the twins from the microstructure. Annealing twinning is supposed to be completed by two processes including sweeping of 1/3 211 superpartial dislocations on successive (111) planes and simultaneous diffusion of boron atoms into the energetically favorable octahedral interstice that are surrounded by six Ni atoms. Boron acts as a strong obstacle to the movement of the superpartial dislocations and greatly increases difficulty in annealing twinning.

Effects of stacking fault energy and ordering energy on grain boundary character distribution of recrystallized L12-type ordered alloys

“…6 are not violated in the range of the first and second nearest neighbors [20,36]. However, the wrong atomic bonds across twin boundary are present if the atomic bonds exceeding the second and halfnearest neighbors are taken into account [34,36]. For example, there exist three excess A-A wrong bonds and one excess B-B wrong bond at second and half-nearest neighbor, and also six deficit A-A wrong bonds and six deficit A-B wrong bonds at third nearest neighbor.…”

“…Among possible two types of twin boundaries (i.e., complex stacking fault (CSF)-type and superlattice stacking fault (SISF)-type twin boundaries) shown in Fig. 6, the SISF-type twin boundary has been suggested to be stable (i.e., have the least energy) in L1 2 ordered structure because it does not involve wrong bonds in the first and second neighbors across twin boundary [20,34]. Based on this result, it has been suggested that the γ SFE (or γ twin ) of the stable (i.e., SISF-type) twin boundary for the L1 2 (A 3 B)-type ordered alloy can primarily be estimated as that of the major components, i.e., A atoms; consequently, it was deduced that the γ SFE for the Co 3 Ti and Ni 3 (Si,Ti) alloys is close to that of fcc Co-based alloys, e.g.…”

Characterization of Grain Boundaries in Recrystallized L1₂-Type Intermetallic Alloys

“…However, the values of γ SF for the Co 3 Ti and Ni 3 (Si,Ti) alloys are not available at the present time. In the previous paper [11], the value of γ SF for the Co 3 Ti and Ni 3 (Si,Ti) ordered alloys was estimated in terms of energetic and geometrical consideration for twin boundaries in L1 2 ordered structure, as follows; between possible two-types of twin boundaries (i.e., complex stacking fault (CSF)-type and superlattice stacking fault (SISF)-type twin boundaries), the SISF-type twin boundary has been suggested to be stable (i.e., have the least energy) in L1 2 ordered structure because it does not involve wrong bonds in the first and second neighbors across twin boundary [11,12]. Therefore, it has been suggested that the γ SF (or γ twin ) of the stable (i.e., SISF-type) twin boundary for the L1 2 (A 3 B)-type ordered alloy can primarily be estimated as that of the major components, i.e., A atoms.…”