Twinning in Hexagonal Close-Packed Materials: The Role of Phase Transformation

Abstract

Twinning is a major mechanism of plastic deformation in hexagonal close-packed (hcp) structures. However, a mechanistic understanding of twin nucleation and growth has yet to be established. This paper reviews the recent progress in the understanding of twinning in hcp materials—particularly the newly discovered phase transformation-mediated twinning mechanisms—in terms of crystallographical analysis, theoretical mechanics calculations, and numerical simulations. Moreover, the relationship between phase transformation-mediated twinning mechanisms and twinning dislocations are presented, forming a unified understanding of deformation twinning. Finally, this paper also reviews the recent studies on transformation twins that are formed in hcp martensite microstructures after various phase transformations, highlighting the critical role of the mechanical loading in engineering a transformation twin microstructure.

The Effect of Cooling Rate on Crystallographic Features of Phase Transformations in Zr-2.5Nb

“…At higher magnifications ( Figure 3 a), it can be clearly seen that large α′-platelets can be considered as a colony of alternating α′-lamellae of the same orientation, morphologically resembling annealing twins. Crystallographic analysis revealed that some of these platelets are characterized by a misorientation angle of ~60° around the a α -axis and can be classified as { }-compression twins [ 49 ]. The rest of the plates in the same colony are characterized by ~85° misorientation around the a α -axis with its neighbors and can be classified as { }-tension twins [ 49 ].…”

“…Crystallographic analysis revealed that some of these platelets are characterized by a misorientation angle of ~60° around the a α -axis and can be classified as { }-compression twins [ 49 ]. The rest of the plates in the same colony are characterized by ~85° misorientation around the a α -axis with its neighbors and can be classified as { }-tension twins [ 49 ]. Thus, α′-colonies consisting of relatively large alternating α′-platelets with twin misorientations were formed during cooling between the β-transus and the temperature of the invariant transformation.…”

“…It should be noted that the 60° and 90° misorientations are quite close to the { } and { } twins, respectively. Thus, one can conclude that both FC and WQ microstructures can be characterized as consisting predominantly of phase transformation twins [ 49 ].…”

“…At higher magnifications (Figure 3a), it can be clearly seen that large α'-platelets can be considered as a colony of alternating α'-lamellae of the same orientation, morphologically resembling annealing twins. Crystallographic analysis revealed that some of these platelets are characterized by a misorientation angle of ~60° around the aα-axis and can be classified as {101 � 1}compression twins [49]. The rest of the plates in the same colony are characterized by ~85° The regions with refined α -platelets are characterized by greater morphological inhomogeneity (fineness, sharpness of crystallites) compared to the structure of the ST + FC sample (Figures 1c,d, 2a and 3a), which, obviously, implied more martensite-type structure.…”