Effect of solution treatment on spinodal decomposition during aging of an Fe-46.5 at.% Cr alloy

Jing Zhou,Joakim Odqvist,Wei Xiong,Gregory B Olson,John Ågren,Andrei Ruban,Peter Hedström,Mattias Thuvander

doi:10.1007/s10853-016-0333-6

Abstract

Spinodal decomposition is a key phase transition in advanced materials and a significant effort is paid to the quantitative modeling of the phenomenon. The initial materials condition is often assumed to be random during modeling, but this may be an oversimplification. In this work, the effect of solution treatment above the miscibility gap, on spinodal decomposition during subsequent aging, has been investigated for an Fe-46.5 at.% Cr alloy. By atom probe tomography (APT), it is found that different extents of quenched-in Cr clustering exist after solution treatments at different temperatures. The clustering is pronounced at 800 °C but decreases significantly with increasing temperature to 900 °C. Thermodynamic Monte Carlo simulations show that there is a difference in atomic short range order between the different solution treatment temperatures. By APT, it is, furthermore, found that the kinetics of spinodal decomposition at 500 °C, i.e., within the miscibility gap, is enhanced in the initial alloy condition, where Cr was less randomly distributed. These observations are supported by kinetic Monte Carlo simulations, predicting a similar but less pronounced qualitative effect on spinodal decomposition kinetics. Other possible reasons for the enhanced kinetics could be related to clustering of interstitial elements and/or sigma phase, but neither was found in the experiments. Nonetheless, the observations in this work suggest that it is necessary to implement a modeling strategy, where the initial structure is properly accounted for when simulating spinodal decomposition.

Highlights

Stainless steels are desirable in numerous engineering applications due to their high corrosion resistance and attractive mechanical properties
The phase separation induced during low-temperature aging will cause a demixing of the alloying elements Fe and Cr, which can occur via two different mechanisms, viz., spinodal decomposition or nucleation and growth [1, 2]
The hardening evolution of the two initial alloy conditions, i.e., solutiontreated at 800 and 900 °C, is rather similar initially, but after 1000 h of aging at 500 °C, there is a distinguishable higher hardness of the alloy that was initially solution-treated at 800 °C. This indicates that the kinetics of spinodal decomposition is faster for that alloy condition

Summary

Introduction

Stainless steels are desirable in numerous engineering applications due to their high corrosion resistance and attractive mechanical properties. The stainless steels containing the body centered cubic or tetragonal (bcc/bct) phase, i.e., ferrite or martensite, are susceptible to phase separation and a corresponding embrittlement phenomenon traditionally called ‘475 °C embrittlement’ This embrittlement, originating from the Fe–Cr binary system, limits the service temperature range and restricts the introduction of these alloys in certain applications, where fracture could lead to catastrophic consequences, e.g., within nuclear power generation. LaSalle and Schwartz [18] investigated two different solution treatment temperatures of 850 and 1200 °C and found that the lower temperature gave faster decomposition upon subsequent aging It has been found, both theoretically and experimentally, that there is a positive atomic short range order (SRO), i.e., clustering, above the miscibility gap in concentrated Fe–Cr alloys above about 10 at.% Cr [19,20,21,22,23].

Experiments

Results and discussion

Conclusions

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