Crystallographic analysis and observations of true twinning modes in the Ni{sub 2}Mo superlattice phase contained in a Ni-25Mo-8Cr alloy

Journal Article · Fri Aug 01 04:00:00 EDT 1997 · Acta Materialia

DOI:https://doi.org/10.1016/S1359-6454(97)00007-4· OSTI ID:532885

Kumar, M; Vasudevan, V K ^[1]

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1. Univ. of Cincinnati, OH (United States). Dept. of Materials Science and Engineering

Deformation twinning in a two phase Ni-25Mo-8Cr (in wt%) alloy containing Ni{sub 2}(Mo, Cr) domains/precipitates in a face centered cubic (f.c.c.) matrix has been studied by detailed crystallographic analysis, coupled with transmission electron microscopy and experimental and computer simulated electron diffraction. Correlation of crystallography with diffraction evidence indicates that all the twinning modes that lead to the formation of true twins (combined, types I and II) in the Ni{sub 2}Mo superlattice are in operation. Long range order within the domains/precipitates of the ordered phase is preserved during deformation by the reorientation of the sheared regions into anti-parallel or perpendicular twin variants, and the rationale of this, from crystallographic considerations, is provided. Defect analysis, through weak-beam dark field imaging, revealed that the twins in the Ni{sub 2}Mo domains form as a consequence of shearing by 1/6<112> Shockley partial dislocations with the same Burgers vector on adjacent {l_brace}111{r_brace} planes and eventual overlapping of intrinsic stacking faults. Twin nuclei are composed of three-layer or nine-layer stacking faults, depending on the geometry of the twinning plane within the NI{sub 2}Mo domains. A new dislocation model, based on the dissociation of a single set of {1/2}<011> unit dislocations--in slips bands at the precipitate matrix interface--to release the twinning Shockley partial dislocations, is able to satisfactorily account for twin formation within the Ni{sub 2}Mo precipitates.

• OSTI ID: 532885
• Journal Information: Acta Materialia, Journal Name: Acta Materialia Journal Issue: 8 Vol. 45; ISSN 1359-6454; ISSN ACMAFD
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
BURGERS VECTOR
CHROMIUM ALLOYS
DEFORMATION
DISLOCATIONS
FCC LATTICES
INTERMETALLIC COMPOUNDS
MOLYBDENUM ALLOYS
NICKEL ALLOYS
PRECIPITATION
STACKING FAULTS
TWINNING