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2/12/04

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Page 973

Effect of Aging and Deformation on the Microstructure and Properties of Fe-Ni-Ti Maraging Steel A. SHEKHTER, H.I. AARONSON, M.K. MILLER, S.P. RINGER, and E.V. PERELOMA The age-hardening behavior of Fe-25.3Ni-1.7 Ti (wt pct) alloy both in undeformed specimens and in specimens cold deformed by 10 or 20 pct prior to aging was studied. The microstructural changes during aging were observed using transmission electron microscopy (TEM) and atom probe analysis and there were related to the mechanical properties as measured by microhardness and shear punch testing. An excellent combination of hardness, strength, and ductility was achieved after only 5 seconds aging at 550 °C. We propose that this rapid strengthening is due to a dislocation friction effect arising from the formation of a fine dispersion of Ni-Ti atomic co-clusters during this short aging time. The concomitant effects of a reverse transformation of martensite to austenite during aging and a gradual increase in both size of the clusters and distance between them contributed to a decrease in strength after aging for 15 seconds. This decline proceeded until aging for 300 seconds and was followed by a secondary hardening reaction toward peak hardness (at 10,800 seconds) and subsequent overaging. This secondary hardening was associated with fine-scale precipitation of Ni3Ti and this process was accelerated by deformation prior to aging, leading to a reduction or elimination of hardness decline after the initial cluster hardening.

I. INTRODUCTION

MARAGING steels are low-carbon martensitic steels developed in the 1960s for applications requiring ultra-high strength combined with good fracture toughness.[1] These steels are used for heavy-duty structural applications after quenching and aging. Such properties are obtained by the precipitation of intermetallic phases in the martensite matrix.[2] The morphology and crystal structure of the precipitates depend on the composition of the alloy, aging temperature, and time. Typically, these steels are solution treated in the fully austenitic () region (900 °C), quenched to produce a completely martensitic () matrix, and then aged at temperatures within the range 400 °C to 600 °C. Although the precipitation process and the character of the precipitates in maraging steels with various compositions have been studied extensively,[1–7] details of their character and the strengthening mechanisms remain incomplete. Most of the work to date has involved isothermal aging for times of 180 seconds or greater.[8–11] However, indications of substantial strengthening taking place at earlier times have been reported in studies involving continuous heating.[3,4] In all

A. SHEKHTER, formerly Research Associate, School of Physics and Materials Engineering, Monash University, Victoria, 3800, Australia, is Research Scientist, Defence Science and Technology Organization (DSTO), Melbourne, Victoria, 3001, Australia. H.I. AARONSON, R.F. Mehl University Professor Emeritus, Department of Materials Science and En