Ageing behaviour of an Fe–20Ni–1.8Mn–1.6Ti–0.59Al (wt%) maraging alloy: clustering, precipitation and hardening E.V. Pereloma a, * , A. Shekhter a,1 , M.K. Miller b , S.P. Ringer c a School of Physics and Materials Engineering, Monash University, Vic. 3800, Australia b Oak Ridge National Laboratory, Metals & Ceramics Division, Tennessee, USA c Australian Key Centre for Microscopy & Microanalysis, The University of Sydney, NSW 2006, Australia Received 18 December 2003; received in revised form 11 August 2004; accepted 16 August 2004 Available online 25 September 2004 Abstract Changes in the solute distribution as well as the evolution of precipitation, microstructure and mechanical properties have been studied in an experimental maraging Fe–20Ni–1.8Mn–1.5Ti–0.59Al (wt%) alloy during ageing at 550 °C. An initial hardening reac- tion within 5 s is reported, which is remarkable in terms of extent and rapidity. This strengthening was caused by the formation of complex multi-component atomic co-clusters containing primarily Ni–Ti–Al as well as some Mn. This cluster strengthened condition produced the optimum toughness observed throughout the ageing sequence. After 60 s ageing, the appearance of discrete precipi- tation of needle-shaped g-Ni 3 Ti particles was associated with a second rise in hardness towards an eventual peak at 600 s. This pre- cipitation hardening was accompanied by an increase in tensile strength and a decrease in ductility. A reverse transformation of martensite to austenite occurs progressively during ageing and this contributes to the initial and secondary softening. Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Maraging steel; Cluster hardening; Three-dimensional atom probe; Transmission electron microscopy; Precipitation 1. Introduction With respect to precipitation from solid solutions, specialised techniques such as rapid quenching and mechanical alloying are currently used to achieve much higher supersaturations than are possible if the maxi- mum equilibrium solubility was to be employed. On the other hand, maraging steels are designed to exploit the martensite reaction in Fe–Ni-based alloys to achieve extremely high solute supersaturations, even in thick (20 cm) workpieces. Furthermore, the low homolo- gous temperatures (0.4T m ) of the martensite range en- sure the absence of competing diffusional modes of austenite decomposition. After ageing, the microstruc- tures contain very high precipitate number densities de- spite the low diffusivity associated with such low transformation temperatures. This is because the matrix possesses a dislocation density in the order of 10 11–12 / cm 2 and the fact that the volume diffusivities operative are sufficiently low that the regions adjacent to these dis- locations in which there is overlap of the diffusion fields of adjacent precipitates are quite small. The low trans- formation temperatures also inhibit coarsening. In addi- tion, austenite reversion during usual ageing times is delayed by hysteresis until temperatures approximately 300 °C above M s in Fe–20Ni (wt%) [1], 2 providing a 1359-6454/$30.00 Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2004.08.018 * Corresponding author. Tel.: +61 399 054 916; fax: +61 399 054 940. E-mail address: elena.pereloma@spme.monash.edu.au (E.V. Pereloma). 1 Present address: Defence Science and Technology Organisation, GPO Box 4331, Melbourne, Vic. 3001, Australia. 2 All alloy compositions are provided in wt%. Acta Materialia 52 (2004) 5589–5602 www.actamat-journals.com