Spinodal decomposition during aging of Fe-Ni-C martensites
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INTRODUCTION
THIS paper
deals explicitly with aging phenomena that occur in Fe-Ni-C martensites prior to any carbide precipitation. The latter process, constituting the first or T1 stage of tempering, is the subject of a separate paper in this journal.[~l It has become increasingly clear that many types of change take place during the aging of ferrous martensites, t2,3] but there remains the need for a more complete understanding of the fundamental nature of these changes and their interrelationships. Prior work had demonstrated that virgin martensites (i.e., those without prior autotempering or aging) undergo significant changes in hardness t4J and strength [5~ on aging below, at, or somewhat above room temperat u r e - - w e l l before the onset of any carbide precipitation. Transmission electron microscopy indicates that aging is accompanied by a redistribution of carbon atoms in the martensite, producing diffuse electron scattering along (102)* lattice directions. [6,7j *Due to tetragonality of the martensites in this paper, it should be noted that the third Miller index (representing the c-axis) is not interchangeable with the other two indices when obtaining equivalent variants.
Detailed observations of the positions, shapes, and integrated intensities of (200), (020), and (002) martensitic X-ray peaks in an Fe-18Ni-1.0C alloy, via a pseudo singlecrystal technique, I81have led to the conclusion that, during
K.A. TAYLOR, formerly with the Massachusetts Institute of Technology, is Research Engineer with Bethlehem Steel Corporation, Bethlehem, PA 18016. L. CHANG, formerly with the University of Oxford, is with Materials Research Laboratories, Industrial Technology Research Institute, Chutung, Hsinchu 31015, Republic of China. G.B. OLSON, formerly with the Massachusetts Institute of Technology, is Professor with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208. M. COHEN, Institute Professor Emeritus, and J.B. VANDER SANDE, Professor, are with the Massachusetts Institute of Technology, Cambridge, MA 02139. G.D.W. SMITH, Lecturer, is with the University of Oxford, Oxford, United Kingdom. Manuscript submitted August 25, 1988. METALLURGICAL TRANSACTIONS A
aging, the carbon atoms undergo clustering but remain in a single interstitial sublattice, thus preserving a macroscopic tetragonal distortion. Aging studies by M6ssbauer spectroscopy, which is capable of probing the immediate environments of iron atoms in martensites affected by neighboring interstitial carbon atoms, have been interpreted to indicate the formation of a metastable Fe4C configuration within the martensite, t91 The kinetics of aging, particularly as a function of carbon concentration in Fe-Ni-C martensites, have been carefully explored by means of electrical resistivity measurements. [1~ An important feature of the aging process is the occurrence of a peak in the isothermal resistivity curves that increases in magnitude with carbon content. The attendant kinetics suggest that the underlying phen
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