Twinning in γ′ Cu–Al–Ni Martensite with Cu3Ti Type Ordered Structure
1974 Volume 15 Issue 2 Pages 109-113
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1974 Volume 15 Issue 2 Pages 109-113
The {121}γ′ and {101}γ′ twinnings in γ′ Cu–Al–Ni martensite with Cu3Ti type ordered structure, whose sublattice is HCP, have been analyzed by the theory of deformation twinning and the phenomenological theory of martensitic transformations. As a result the following conclusions have been reached:
(1) The twinning elements of {121}γ′ twinning consistent with the phenomenological theory are as follows. K1=(\bar1\bar21)γ′, η1=[1,‾0.7954,‾0.5907]γ′, K2=(1,‾1.5036,‾0.5036)γ′, η2=[\bar1\bar11]γ′, s=0.261.
(2) In spite of the ordered structure, the original structure can be restored by the above twinning mode and simple and small shuffles, thus providing a theoretical background for experimental evidences such as the mobile nature of twin boundaries and the absence of extra spots in electron diffraction patterns.
(3) {101}γ′ twins are considered to be variants of martensite rather than a lattice invariant strain in a variant of martensite.
(4) In spite of (3), {101}γ′ twinning can be treated by the theory of deformation twinning, and no structural change is introduced by the twinning with the required twinning shear and simple and small shuffles. This is consistent with the experimental findings.
(5) No distinction can be made between the mechanism of deformation twinning and that of transformation twinning in the present martensite, although the structure is ordered.
(6) It is suggested that the absence of {10\bar12} twinning as a lattice invariant shear in most hexagonal martensites is due to the fact that no solutions of the phenomenological theory exist for δ nearly equal to unity.
