ISSN:
1662-9779
Source:
Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
Topics:
Physics
Notes:
A new characterization method, "Macroscopic Composition Gradient (MCG) Method" isproposed to investigate the phase transformations near the phase boundaries, such as the solubilitylimit, order/disorder line and so on. Since the macroscopic composition gradient in the alloy isprepared so as to step over the phase boundary, the morphological transition of critical phenomenaat the phase boundary can be observed by means of analytical transmission electron microscopy.By utilizing this method, the critical minimum size of stable precipitate in the vicinity of edge ofmiscibility gap is experimentally determined for the Ni3Si in Ni-Si, Ni3Al in Ni-Al, Cu4Ti in Cu-Tiand Co in Cu-Co binary alloy systems. The results are as follows: The critical nucleus size shows asteep increase up to several tens of nm in a very narrow composition range less than 0.3at% fromthe phase boundary. The Gibbs-Thomson relation and the conventional nucleation theorystatistically rationalize such the composition dependence of nucleus size change. However, thenucleus formation is kinetically never rationalized by the conventional nucleation theories. Thephase decomposition of supersaturated solid solution progresses by a mechanism of spinodal phasedecomposition, even in the composition range near the solubility limit, i.e. a so-called Nucleation-Growth region. Such the phase decomposition behavior is never rationalized by the Boltzmann-Gibbs free energy, which is based on the extensive entropy. The experimental facts obtained hereare explained by Tsallis's non-extensive entropy.It should be noted that the present experiments can systematically be conducted in thecomposition range very near the solubility limit where they has hardly been examined in the past.The MCG method proposed here is considered to open a new way to investigate the microstructureevaluation, particularly for the critical phenomena near the phase boundary
Type of Medium:
Electronic Resource
URL:
http://www.tib-hannover.de/fulltexts/2011/0528/02/24/transtech_doi~10.4028%252Fwww.scientific.net%252FSSP.138.43.pdf
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