Materials Science and Engineering A 528 (2011) 7475–7483 Contents lists available at ScienceDirect Materials Science and Engineering A jo ur n al hom epage: www.elsevier.com/locate/msea Development of ultra high strength nano-Y 2 O 3 dispersed ferritic steel by mechanical alloying and hot isostatic pressing S.K. Karak a , T. Chudoba b , Z. Witczak b , W. Lojkowski b , I. Manna a,c,∗ a Metallurgical and Materials Engineering Department, Indian Institute of Technology, Kharagpur 721302, India b Institute of High Pressure Physics (Unipress), Polish Academy of Sciences, Sokolowska 29, 01-142 Warsaw, Poland c Central Glass and Ceramic Research Institute (a CSIR unit), Kolkata 700032, India a r t i c l e i n f o Article history: Received 21 November 2010 Received in revised form 10 April 2011 Accepted 15 June 2011 Available online 22 June 2011 Keywords: Oxide dispersion strengthening Ferritic steel Mechanical alloying Hot isostatic pressing Mechanical properties a b s t r a c t The present investigation aims to develop ultra high strength ferritic steels through consolidation of mechanically alloyed powders of 1.0 wt% nano-Y 2 O 3 dispersed 83.0Fe–13.5Cr–2.0Al–0.5Ti (alloy A), 79.0Fe–17.5Cr–2.0Al–0.5Ti (alloy B) , 75.0Fe–21.5Cr–2.0Al–0.5Ti (alloy C) and 71.0Fe–25.5Cr–2.0Al–0.5Ti (alloy D) alloys (all in wt%) by hot isostatic pressing (HIP) at 600, 800 and 1000 ◦ C using 1.2 GPa pressure for 1 h. Following this mechano-chemical synthesis and consolidation, extensive effort has been undertaken to characterize the microstructural evolution by X-ray diffraction, scanning and transmission electron microscopy and energy dispersive spectroscopy. Mechanical properties including hardness, compressive strength, Young’s modulus and fracture toughness were determined using nano-indentation and uni- versal testing machine. The present ferritic alloys record extraordinary levels of compressive strength (2012–3325 MPa), Young’s modulus (230–295 GPa), fracture toughness (4.6–21.8 MPa √ m) and hard- ness (15.5–19.7 GPa), and measure up to 2–3 times greater strength with a lower density (∼7.4 Mg/m 3 ) than that of other oxide dispersion strengthened ferritic steels (<1200 MPa) or tungsten based alloys (<2200 MPa). The novelty of these alloys lies in the unique microstructure comprising uniform disper- sion of 20–30 nm Y 2 O 3 (ex situ) or Y 2 Ti 2 O 7 (in situ) particles in higher volume fraction in high-Cr ferritic matrix. © 2011 Elsevier B.V. All rights reserved. 1. Introduction High-Cr ferritic steels are widely used for heat resistant struc- tural applications in nuclear and thermal power plants and fast breeder reactors due to the favorable combination of properties like body center cubic crystal lattice with good swelling resistance, low co-efficient of thermal expansion, high thermal conductivity, good oxidation and creep resistance and high tensile/compressive strength at ambient and elevated temperature. However, utility of ferritic steel is limited to temperatures up to 550 ◦ C due to lack of or inadequate creep strength above that limit. In order to over- come this limitation, dispersion of ultrafine oxides is considered a possible strategy, particularly for structural components in nuclear reactors [1–6]. Indeed, oxide dispersion strengthened ferritic steels record greater creep strength compared to that in traditional ferritic steels of similar compositions [7–13]. However, development of oxide dispersion strengthened steels by conventional melting and ∗ Corresponding author at: Metallurgical and Materials Engineering Department, Indian Institute of Technology, Kharagpur 721302, India. Fax: +91 33 24730957. E-mail address: imanna@metal.iitkgp.ernet.in (I. Manna). casting route is extremely difficult due to wide difference in melt- ing temperature, density and solubility limits of the constituent components or compounds. Among the probable powder metal- lurgy routes, mechanical alloying offers a convenient, inexpensive and flexible method of synthesizing complex alloys by high energy ball milling of an appropriate elemental/compound powder blend at room temperature [14–16]. The process involves repeated cold welding, fragmentation, dynamic recrystallization and mechani- cally driven interdiffusion to convert the initial powder blend into a single phase alloy [17]. The product, despite the novel microstruc- ture (nanostructured extended solid solution), is a powder mass that must be consolidated into bulk component for evaluation of mechanical properties and ultimate use. Several possibilities exist to consolidate mechanically alloyed powders like hot extrusion [18], cold compaction and pressure-less sintering [19], high pres- sure sintering [20,21], equi-channel angular pressing [22,23], laser sintering [24], pulse plasma sintering [25] and hot isostatic pressing [26]. The last option is ideal to develop isotropic and homogeneous solid body from powders with dispersion of ultrafine and light oxide particles. However, studies on hot isostatic pressing of nano-oxide dispersed mechanically alloyed ferritic steel are scarce. Further- more, composition of the ferritic steel itself needs optimization in 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.06.039