QUANTIFICATION OF THE MODULATED STRUCTURES IN TIPDCR ALLOYS

Ti50Pd((50-x))Cr-x beta(2) alloys (B2, CsCl-type crystal structure) ha ve been investigated as part of an overall study of the systematics of phase transformation behaviour and also for the commercial applicabil ity of the shape memory effect at elevated temperatures. The phase tra nsformations are unique in the sense that electron diffraction and hig h-resolution transmission electron microscopy of Ti50Pd(50-x)Cr-x allo ys with x between 6 and 10 at.% reveal {110}(B2) modulated structures with periodicities between 3 and 4.5 planes that vary linearly with co mposition. Satellites along certain [110] lying at incommensurate pos itions are observed in electron diffraction patterns and are consisten t with transverse lattice displacements, High-resolution transmission electron microscopy (HRTEM) imaging usually reveals sinusoidal displac ements; however, a few images appear 'banded' at three- and four-plane intervals suggesting the possibility that three- and four-plane comme nsurate packets combine to give the overall incommensurate satellite s pacing, With the use of a newly developed technique for quantifying at omic displacements from HRTEM images, modulated phases in Ti50Pd43Cr7 and Ti50Pd42Cr8 alloys, where atoms are shifted from the cubic lattice sites by a transverse lattice displacement wave, have been studied. T he real space displacements of the modulated structures were determine d and compared to truly incommensurate and varying three- and four-pla ne wave packet combinations by examination of the goodness of lit as a ssessed by the correlation coefficient for a least-squares fit. In the cases of the Ti50Pd43Cr7 and Ti50Pd42Cr8 alloys, the incommensurate s ine wave exhibited much better matching than the commensurate packets, indicating that these modulations are probably incommensurate.