Spontaneous formation of the B2 phase from a decagonal quasicrystal under reduced constraint

Abstract

Since the discovery that vacancies can quasiperiodically order in a basic B2 structure to yield a one-dimensional quasiperiodicity, these phases were subjected to intense investigations for possible links with quasicrystal. The B2 phase was found to coexist extensively with the icosahedral quasicrystalline phase, particularly in Al–Cu–TM (TM = transition metal) systems where the two phase fields are often adjacent to each other. The two phase exhibit a well-defined orientation relationship. There exist several studies dealing with quasicrystalline to B2 transformation, particularly in decagonal quasicrystals Doblinger et al. has carefully explored different metastable states in Al–Co–Ni decagonal quasicrystals including nanodomained 1D quasicrystals and multiple twinned approximant phase. The decagonal to a cubic B2 ordered phase transformation can often be observed. The nanocrystals of the B2 phase could be observed on the surface of the decagonal quasicrystal. Zhang and Urban have studied the stability of the QC phase by employing electron irradiation in an electron microscope and observed the transformation of the decagonal quasicrystal (DQC) phase, in an Al–Cu–Co–Si alloy, first to a disordered phase [body centered cubic (BCC phase)] and then to an ordered B2 phase. Zurkirch et al. observed a decagonal to crystalline structural transformation occurring on a surface of Al–Co–Ni upon sputtering the surface. The annealing treatment was found to restores the decagonal structure. Shimoda et al. confirmed the above observation. A similar result has been reported recently by Fluckinger et al. who have reported more than one variant of the cubic crystal at the surface. These authors attributed the structural change to the change in the surface composition during sputtering. The annealing treatment allows the A1 atoms from the subsurface to diffuse back and restore the composition resulting in a reappearance of the decagonal quasicrystal. The formation of a BCC phase during ion milling has also been reported in Al–Cu–Co decagonal quasicrystals. Recently Muthy et al. has reported a quasicrystalline to $_\beta$ phase transformation in as-cast $A1_{65}Cu_{20}Fe_{15}, A1_{65}Cu_{20}Co_{15}$ and $A1_{72}Pd_{19:5}Mn_{8:5}$ alloys during high energy ball milling. They have correlated the phase change with a critical grain size of 20 nm.