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An X-Ray Diffraction Study of the Aging Reaction in Two Austenitic Alloys

Published online by Cambridge University Press:  06 March 2019

J. K. Abraham  and
T. L. Wilson
J. K. Abraham
Affiliation:
Republic Steel Corporation Research Center Cleveland, Ohio
T. L. Wilson
Affiliation:
Republic Steel Corporation Research Center Cleveland, Ohio
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Abstract

Aging processes exhibiting cluster to precipitate transitions were studied in polycrystalline line austenitic iron-base alloys with a Siemens' Guinier camera. This camera combines the Seemann-Bohlin focusing geometry with a curved-crystal monochromator arid thus maximizes the resolution of observed sidebands and the weak precipitate lines. Growth studies encompassing a cluster-size range of 15 to 70 unit cells were followed. For the systems of interest, this coincided with a variation from detectable hardness increase to a stage of maximum hardness immediately preceding precipitation, Cluster sizes were calculated on the basis of the Guinier model; variation with time and temperature permitted calculations of an apparent activation energy in the one system where decomposition was spontaneous. An iron-nickeltitanium alloy was used to study aging in a ternary system. Behavior was classic in that the cluster size present on quenching grew with aging coincident with a simultaneous hardness increase. Calculation of activation energies indicated strongly that transportation of nickel to, or iron from, the cluster was rate determining. Upon overaging, the nickel-titanium enriched clusters gave way to the hexagonal Ni2Ti phase. An iron-nickel-chromium-niobium quaternary, in addition to presenting a clustering system similar to the above ternary, showed two rather interesting phenomena. First, chromium was necessary for precipitation; the ternary ironnickel-niobium did not age. Secondly, a stable Pe2Nb Laves phase present upon quenching from 2200°F disappeared on aging in favor of nickel-niobium clusters; an incubation time for the formation of these clusters existed, and its duration was about 4 hr. An asymmetry was noted in the diffraction intensities about the (311)γ line in both systems. In the iron-nickel-titanium case, the asymmetry was only in intensity, whereas, with the iron-nickel-chromium-niobium alloys, the asymmetry existed in both intensity and position. Interpretation of these observations is made on the basis of anticipated variations in scattering factors, lattice spacings, and cluster sizes.

Type
Research Article
Information
Advances in X-Ray Analysis , Volume 11: Sixteenth Annual Conference on Applications of X-ray Analysis, August 9-11, 1967 , 1967 , pp. 434 - 446
Copyright
Copyright © International Centre for Diffraction Data 1967

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References

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