Carbide precipitation during stage I tempering of Fe-Ni-C martensites
- PDF / 5,488,226 Bytes
- 17 Pages / 613 x 789 pts Page_size
- 106 Downloads / 250 Views
I.
INTRODUCTION
T H E so-called "first stage of tempering" of Fe-C martensites (hereafter referred to as " T I " ) involves the precipitation of a transition carbide, typically in the i00 ~ to 200 ~ temperature range. Jack m was one of the first to analyze the structure of the transition carbide in Fe-C alloys; his powder diffraction patterns were interpreted in terms of a phase possessing hexagonal symmetry. He could not determine the position of the carbon atoms in the structure but suggested that the phase might exist over a range of compositions from Fe2C to Fe3C. He named the transition carbide "e-carbide" and proposed an orientation relationship between e- and lowcarbon martensite which features parallelism between the closest-packed planes and directions of the two phases:
II (011). (10ill II (101). (0001)~
Electron microscopy by Wells [2j on an Fe-24Ni-0.5C alloy confirmed the Jack orientation relationship, as have a number of subsequent investigations. Extensive electron diffraction analyses of Fe-l.22C and Fe-Ni-C martensites reported by Hirotsu and Nagakura [3] and Hirotsu e t a l J 41 indicate that carbon atoms may assume an ordered arrangement within the transition carbide. Hirotsu and Nagakura's t31 proposed carbon ordering, depicted in Figure l, lowers the overall symmetry of the
K.A. TAYLOR, formerly with the Massachusetts Institute of Technology, is Research Engineer with Bethlehem Steel Corporation, Bethlehem, PA 18016. 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. Manuscript submitted August 25, 1988. METALLURGICAL TRANSACTIONS A
carbide phase from hexagonal to orthorhombic. Although the latter investigators distinguished their carbide from Jack's by referring to it as %7-carbide," the present authors suggest that the structural similarity between the two phases will be recognized more clearly by calling the ordered phase e'-carbide, rather than r/-carbide. This is the nomenclature to be adopted here. Note that, in Figure 1, the c-axis of the e'-carbide unit cell has been taken to be normal to the carbide " b a s a l " * *Basal is shown in quotes to indicate that the (001L. plane is not a close-packed plane having hexagonal symmetry, in contrast to the case for the (001) plane of a hexagonal lattice.
plane. This choice of crystal axes differs from previous investigations [3,5,6] but has the advantage of corresponding directly to the orthohexagonal crystal axes for e-carbide. Thus, the Miller indices given in this paper apply to both e' and e as long as one remembers that the e unit cell is defined to be orthorhombic (with be = V~a~). Various morphologies have been reported for the transition carbide that precipitates in ferrous martensites. Early TEM studies of Fe-Ni-C [2,71 and F e - C [3"81 alloys showed pl
Data Loading...
