Characterization of the microstructure of deformed Al-Zn-Cu alloys by X-ray diffraction line profile analysis

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Characterization of the Microstructure of Deformed AI-Zn-Cu Alloys by X-Ray Diffraction Line Profile Analysis HIRANMAY PAL, S.K. PRADHAN, and M. DE The present study deals with the characterization of microstructure in the plastically deformed state of ternary a-phase A1-Zn-Cu alloys in the light of the recent investigations on the microstructure in the deformed state of some binary Al-base tL2~ and ternary Cu-base [3,4,51 alloys, employing the well-established technique of X-ray diffraction line profile analysis (XRDLPA). t6,7]The present study with A1-Zn-Cu alloys is perhaps the first such study with a ternary Al-base alloy system to investigate the occurrence of any stacking fault. Seven compositions of the alloys (Tables I and II) in the a-phase region ~81 have been prepared using spectroscopically pure (99.999 pct) metals by melting them at about 900 ~ in graphite crucibles vacuum-sealed in silica tubes, then homogenizing at 460 ~ for 10 days and finally quenching in liquid N2 to retain the crystal structure of the 460 ~ isothermJ 8J With the weight loss being very negligible, the nominal compositions were retained. Following earlier procedures, t3,4,Sj the X-ray diffraction patterns using CuK~ from freshly hand-filled, HIRANMAY PAL, Senior Research Fellow, S.K. PRADHAN, Research Associate II, and M. DE, Reader, are with the Department of Materials Science, Indian Associationfor the Cultivationof Science, Jadavpur, Calcutta-700 032, India. Manuscript submitted December 13, 1993.

"cold-worked," and annealed "standard" samples were recorded for all the alloy compositions. The patterns represented the face-centered cubic (fcc) Al-rich solid solution of a-phase A1-Zn-Cu and no evidence of phase transformations I9,~~ could be noticed, except for the appearance of a small hump (identified as that of (101) line of Zn) in the diffractogram. The resultant defect states were then characterized in terms of several microstructural parameters (Tables I and II) using standard XRDLPA techniques, I6,71 as adopted earlier, t3'4"5] As per faulting theory, 16,7~the resultant X-ray peak shift, calculated from the first three pairs of the neighboring peaks, is due to the composite effects of net deformation stacking faults of density (a) = (a' a") (where a ' and a" are the intrinsic and extrinsic types of deformation fault densities) and change in the lattice parameter, A a / ao (Table I), assuming the residual stress tr to be zero for a powder sample.t7] With the values of Aa/ao being very small, the resultant peak shift is considered primarily due to stacking faults (a) alone, an observation further supported by the plots of lattice parameter, ahk! VS extrapolation function, cos 0 cot 0 as per Paterson's theory of faulting. I1~1 From the foregoing study, it is apparent that stacking faults could be generated due to plastic deformation of ternary Al-base AI-Zn-Cu alloys unlike binary ones. tL21 The solute dependences of the generation of (a) showing increased propensity with increasing solute concentration are depicted in Figure