Detection of a metallic glass layer by x-ray diffraction
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D. H. Lowndes Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received 14 March 1986; accepted 28 July 1986) A simple, precise method for obtaining the average thickness of an amorphous layer formed by any surface treatment has been developed. The technique uses an x-ray diffractometer to measure the reduction in the integrated intensity of several diffracted x-ray lines due to the near surface amorphous layer. The target material for generation of x rays is selected so that the emitted x rays are strongly absorbed by the specimen. This method permits thickness measurements down to ~ 100 nm. It has been tested on a specimen of Fe80B20 on which an amorphous layer was produced by pulsed XeCl (308 nm) laser irradiation; the amorphous layer thickness was found to be 1.34 ( + 0.1) //m.
I. METHOD A method for detecting and precisely measuring the thickness of an amorphous layer on a crystalline substrate has been developed and used for analyzing amorphous layers formed by pulsed laser melting and rapid solidification. The method is also suitable for detecting massive structural changes that occur in the near surface region in layers of thickness of the order of 0.1 //m up to a few microns. An x-ray diffractometer is used, the target being selected so that the emitted x rays are strongly absorbed by the specimen. In order to fulfill this requirement, the wavelength of the target Ka radiation must be slightly shorter than the absorption-edge wavelength of the elements of the specimen. For instance, in iron the AT-edge wavelength is 0.1743 nm.1 Therefore, either nickel (AK = 0.166 nm) or copper AKa = 0.154 nm) are appropriate as target elements for that case. The use of a monochromator in the diffractometer suppresses all the background radiation coming from the specimen (such asfluorescenceand incoherent scattered radiation) that otherwise would be unacceptably high. Thus, using a sensitive counter, structural change in the near surface region can be detected. The integrated intensity of each diffracted beam for the specimen arrangement used in the diffractometer is given by2 dld = (I(fib /sin 0)exp( — 2/zx/sin &)dx,
(1)
where Io is the intensity of the incident beam, a is the specimen volume fraction having particles with the right orientation for diffraction, b is the fraction of /„ that is diffracted per unit volume, fi is the linear absorption coefficient, 0 is the Bragg angle, and x is the penetration distance normal to the specimen surface. For a thick and uniform specimen, Id = Ioab /2/n.
(2)
J. Mater. Res. 1 (5), Sep/Oct 1986 http://journals.cambridge.org
If there is an amorphous surface layer of thickness 8 present, containing a uniform dispersion of crystalline phase whose volume fraction is v, Eq. (1) can be integrated for each diffracted line i as
< = ^'.»5 a* exp
-
2fi'S sin©
(3)
where a1 and a5 are, respectively, the corresponding values of a in the fraction v and in the crystalline substrate. Equation (3) is derived in the Appendix. The ratio of intensities before
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