Materials Science and Engineering A 527 (2010) 2079–2086 Contents lists available at ScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea Aging effects on the mechanical properties of alumina-forming austenitic stainless steels H. Bei ∗ , Y. Yamamoto, M.P. Brady ∗∗ , M.L. Santella Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6115, United States article info Article history: Received 5 October 2009 Received in revised form 18 November 2009 Accepted 19 November 2009 Keywords: Mechanical characterization Aging Intermetallics Steel Fracture abstract Isothermal aging and tensile evaluation were conducted for recently developed alumina-forming austenitic stainless steels (AFAs). Microstructural observation reveals that NiAl-type B2 and Fe 2 (Mo,Nb)- type Laves phase precipitates form as dominant second phases in the austenitic matrix during aging at 750 ◦ C. At room temperature these precipitates increase the strength but decrease the ductility of the AFA alloys. However, when tested at 750 ◦ C, the AFA alloys did not show strong precipitation hardening by these phases, moreover, the elongation to fracture was not affected by aging. Fracture surface and cross-sectional microstructure analysis after tensile testing suggests that the difference of mechanical behaviors between room temperature and 750 ◦ C results from the ductile–brittle transition of the B2 precipitates. At room temperature, B2 precipitates are strong but brittle, whereas they become weak but ductile above the ductile–brittle transition temperature (DBTT). © 2009 Elsevier B.V. All rights reserved. 1. Introduction Development of new structural materials with superior high- temperature capability is needed to permit increases in the operation temperature of energy-conversion systems in order to reduce emissions and to increase efficiencies [1,2]. The develop- ment of a family of alumina-forming austenitic (AFA) stainless steels for high-temperature use (600–900 ◦ C) in energy and chem- ical process applications was recently reported [3–11]. AFA alloys combine creep strengths in the range of advanced commercial austenitic stainless steels with superior high-temperature corro- sion resistance in many industrially relevant environments [4,6]. This superior corrosion resistance results from the formation of an alumina (Al 2 O 3 ) surface layer instead of the chromia (Cr 2 O 3 ) sur- face layer used to protect conventional austenitic stainless steels and Ni-based alloys from corrosion [3,4,6]. Studies to date of the new AFA alloy family have focused on alloy development, creep, and oxidation behaviors. For high- temperature application, another important issue that needs to be considered is the microstructural stability of the material, and how microstructural changes affect the properties. For example, the nature and distribution of precipitates play a very important role in determining mechanical properties [12]. In some cases, this ∗ Corresponding author. Tel.: +1 865 576 7196; fax: +1 865 574 7659. ∗∗ Corresponding author. E-mail addresses: BeiH@ornl.gov (H. Bei), Bradymp@ornl.gov (M.P. Brady). precipitation can result in decreased strength and ductility, which can lead to component failure [13–15]. Similar to high-temperature ultrafine precipitate-strengthened (HTUPS) steels (e.g. [16]) AFA alloys are strengthened by nano-scale MC precipitates (M pri- marily = Nb) (e.g. [3]). However, AFA alloys also contain high volume fractions of NiAl-type B2 and Fe 2 (Mo,Nb)-type Laves phase precipitates [3,6,8,9]. These precipitates are a consequence of com- positional modifications needed to achieve a balance of oxidation and creep resistance [3,4,6,8,9]. The effects of precipitation on creep resistance in austenitic stainless steels have been reviewed in Ref. [17]. For example, in cast PH13-8 Mo stainless steel, Hochanadel et al. [18] found that B2-NiAl precipitation increased strength and decreased the duc- tility when aging at low temperature (566 ◦ C and below). When samples are over-aged at high temperature (>566 ◦ C), the B2- NiAl phase becomes spherical, and the strength of the material decreases and ductility increases [18]. NiAl-type B2 precipitation- induced increasing strength and decreasing ductility were also observed in Fe–Ni–Cr–Al alloys [19]. Previous studies by the present authors showed that high Nb levels (0.6–3 wt.%) result in signifi- cantly improved oxidation resistance in AFA alloys [3–7]. However, these levels of Nb also result in Fe 2 (Mo,Nb) Laves phase precipi- tates in the austenitic matrix. In Fe–20Cr–30Ni–2Nb (at.%) alloys, fine (<1 m) Fe 2 Nb Laves phase dispersions within a gamma-Fe matrix resulted in only moderate creep resistance [20]. Further, the Fe 2 Nb Laves phase is brittle and its formation in Fe–15%Cr–15%Ni austenitic stainless steel was reported to decrease the tough- ness and ductility [21]. The goal of the present work was to 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.11.052