The microstructure of discontinuously precipitated lamellae in an austenitic Fe-42.4 Wt Pct Ni- 4.15 Wt Pct AI-0.45 Wt P
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I.
INTRODUCTION
FOLLOWING the pioneering studies on age-hardening phenomena, t1'2] many investigators have investigated the microstructure and phase constitution o f austenitic (3,) Fe-Mn-A1-C and Fe-Ni-A1-C alloys. [3-13J Austenite decomposition in them follows somewhat different paths depending upon the degree o f supersaturation o f both interstitial and substitutional solute elements. [6A°,11~ With the high degree o f carbon supersaturation but with the relatively low concentration o f austenite stabilizing substitutional elements, spatial fluctuations o f carbon atoms along the (100)3/directions develop via the spinodal reaction, even in the rapidly solidified state. ~6,9.~°,121 These fluctuations bring about the characteristic linear m o d u lation image in transmission electron micrographs together with the sideband reflections around the main X-ray diffraction peak. As the amplitude o f composition modulation increases on annealing, the coupled ordering o f interstitial and substitutional atoms occurs concurrently in the carbon-rich regions. Subsequently, these regions become cubic carbide K phase with the chemicalformula o f (Fe,Mn)3A1Cx or (Fe,Ni)3A1Cx. [7,9] The crystal structure o f this phase is o f the L' 12 type, where carbon atoms are situated at the body-centered octahedral interstices in the ordered skeleton o f the substitutional lattice composed o f Fe(Mn/Ni) and A1, respectively, occupying the face-centered and corner positions o f the cube. In alloys o f low-carbon supersaturation o r high-austenite stabilizing element content, the K phase precipitates via the nucleation and growth mechanism rather than spinodal decomposition. [~0,~1] In this case, diffuse X-ray scattering is accompanied not by sidebands but by asymmetric line broadening. Besides the bulk precipitation, some observations on JEONG S. LEE, Postdoctoral Associate, and W O O N G K. CHOO, Professor, are with the Department of Electronic Materials Science and Engineering, K o r e a A d v a n c e d I n s t i t u t e o f S c i e n c e and T e c h n o l o g y , 373-i K u s o n g - d o n g , Y u s o n g - g u , T a e j o n , 305-701 Korea. Manuscript submitted October 31, 1991.
METALLURGICAL TRANSACTIONS A
the heterogeneous grain boundary precipitation have also been m a d e . [9,~4j In a rapidly solidified Fe-31.3 wt pct Mn-8.7 wt pct AI-2.0 wt pct C alloy aged at 923 K, colonies composed o f lamellae o f alternately arranged K and y phase platelets encroached upon the metastable matrix through the discontinuous precipitation process after a lengthy aging, f~4J In addition, the crystallographic orientation relationship between the two phases in the lamellae was the cube-to-cube correspondence. A fine lamellar structure was also reported in a rapidly solidified Fe-5 wt pct Ni-8 wt pct A1-2.0 wt pct C alloy on aging at 823 K.[9J The constituents o f the grain boundary precipitates in the latter alloy were cr (ferrite) and K, but no clearcut epitaxial relationship was established. In the meantime, the austenitic Fe-Ni-A1-C system has
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