A transmission electron microscopy study of copper precipitation in the cementite phase of hypereutectoid alloy steels
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INTRODUCTION
C O P P E R additions to low-carbon steels can produce strengthening by copper precipitation during aging, and such steels have been developed for naval and other applications, l~-4~ Aspects o f the precipitation o f copper and niobium have been examined in low-carbon copper bearing high-strength steels, and their application for structural and high-pressure pipelines has been reportedJ 5'6,7J The precipitation of copper in proeutectoid allotriomorphic ferrite has been examined in experimental carbonfree Fe-2Cu and Fe-2Cu-2-5Ni (wt pct) alloys, 18,91 and it has been reported that the precipitation o f the copper particles can occur in the form o f rows o r sheets in the ferrite, a characteristic o f so-called "interphase precipitation ''[l°j more usually found for alloy carbide precipitation in high-strength low-alloy (HSLA) and engineering steels.[~l ~7] Copper precipitation has also recently been studied in the ferrite phase o f duplex stainless steel, where it has been observed that copper precipitates initially in a spherical, body-centered cubic (bcc) coherent zone form and that, after reaching a critical size at a certain temperature, these zones lose their coherency and stable face-centered cubic (fcc) plates form by a shear transformation, f~sJ This is followed by the formation o f adjacent fcc twin-related plates resulting in a multitwinned particle. Copper particles were also found to refine the microstructure by pinning the interphase interfaces o f the duplex stainless steels. More recently, high-resolution F . A . K H A L I D , Postdoctoral Research A s s i s t a n t , and D.V. E D M O N D S , Lecturer, are with the Department of Materials, University of Oxford, Oxford OX1 3PH, United Kingdom. Manuscript submitted July 20, 1992. METALLURGICAL TRANSACTIONS A
transmission electron microscopy (TEM) studies 1~9~ have revealed that fine copper particles in two Fe-Cu and FeCu-Ni alloys were twinned 9R close-packed structures rather than fcc structures. The precipitation o f copper has also been studied in high-carbon Fe- 1.31 C-3.57Cu and Fe- 1.49C-4.90Cu alloys.12°1 In this investigation, copper particles were found within the proeutectoid allotriomorphic cementite phase. This work also reported that the kinetics o f proeutectoid cementite precipitation were not influenced significantly by copper. The morphology o f proeutectoid cementite observed was more o r less the same in both alloys studied, with and without copper additions. It was suggested that the copper particles formed at the cementite/austenite interphase boundaries and that their growth was accomplished by interphase boundary diffusion. Recently, copper precipitation has also been found to o c c u r in the proeutectoid and pearlitic cementite regions o f high-carbon steels.t2~.221 In this investigation, an attempt is made to document and examine the morphology and mechanism o f copper precipitation in association with the decomposition o f austenite to all the characteristic forms o f cementite in hypereutectoid steels, grai
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