P.J. Maziasz

Tensile properties and fracture toughness of TiAl alloys with controlled microstructures

Abstract The objective of this study is to improve the mechanical properties by careful control of both microstructure and alloy additions in two-phase TiAl alloys based on Ti-47Al-2Cr-2Nb (at%). Hot extrusion at temperatures above Tα produces refined lamellar structures, whose microstructural features can be further controlled by subsequent heat-treatment at and above 900 °C. The mechanical properties of the alloys with lamellar structures depend on three factors: colony size, interlamellar spacing, and alloying additions. The tensile elongation at room temperature is strongly dependent on lamellar colony size, showing increasing ductility with decreased colony size. The strength at room and elevated temperatures is sensitive to interlamellar spacing, showing increasing strength with decreased colony spacing. The fracture toughness at room temperature can be substantially improved by heat-treatment at 1320 and 1350 °C. The tungsten addition at a level of 0.2% improves the tensile strength, whereas the silicon addition at a level of 0.3% reduces the castability of the TiAl alloys. The TiAl materials produced by hot extrusion are much superior to those produced by conventional thermomechanical treatments.

Recent advances in B2 iron aluminide alloys : deformation, fracture and alloy design

Abstract This paper reviews the deformation, fracture and alloy design of B2 iron aluminides based on FeAl. Moisture-induced environmental embrittlement is shown to be a leading cause of low tensile ductility and brittle cleavage fracture of Ferich FeAl alloys at ambient temperatures. With increasing Al concentration, two other factors, namely intrinsic grain-boundary weakness and quenched-in vacancies become important in limiting the tensile ductility of FeAl alloys. FeAl alloys show a yield-strength anomaly at intermediate temperatures. Recent work indicates that the anomaly is a result of hardening by thermal vacancies at elevated temperatures. The understanding of the deformation and fracture behavior has led to the development of FeAl-base alloys and composites with improved metallurgical and mechanical properties for structural applications. These FeAl-based alloys can be prepared by melting and casting or by powder processing. The unique combination of the excellent oxidation and carburization/sulfidation resistance coupled with relatively low material density and good mechanical properties at room and elevated temperatures has sparked industrial interest in FeAl alloys and composites for a number of applications.

Dose dependence of the microstructural evolution in neutron-irradiated austenitic stainless steel

Abstract Microstructural data on the evolution of the dislocation loop, cavity, and precipitate populations in neutron-irradiated austenitic stainless steels are reviewed in order to estimate the displacement damage levels needed to achieve the “steady state” condition. The microstructural data can be conveniently divided into two temperature regimes. In the low temperature regime (below about 300°C) the microstructure of austenitic stainless steels is dominated by “black spot” defect clusters and faulted interstitial dislocation loops. The dose needed to approach saturation of the loop and defect cluster densities is generally on the order of 1 displacement per atom (dpa) in this regime. In the high temperature regime (~ 300 to 700°C), cavities, precipitates, loops and network dislocations are all produced during irradiation; doses in excess of 10 dpa are generally required to approach a “steady state” microstructural condition. Due to complex interactions between the various microstructural components that form during irradiation, a secondary transient regime is typically observed in commercial stainless steels during irradiation at elevated temperatures. This slowly evolving secondary transient may extend to damage levels in excess of 50 dpa in typical 300-series stainless steels, and to > 100 dpa in radiation-resistant developmental steels. The detailed evolution of any given microstructural component in the high-temperature regime is sensitive to slight variations in numerous experimental variables, including heat-to-heat composition changes and neutron spectrum.

Atom Probe Tomography of Nanoscale Particles in ODS Ferritic Alloys

An atom probe tomography characterization of the microstructure of as-processed and crept mechanically-alloyed, oxide-dispersion-strengthened (MA/ODS) ferritic alloys has been performed. The significant enrichments of Cr, W, Ti, Y, O, C and B in the vicinity of dislocations and the presence of ultrastable 4-nm-diameter Ti-, Y- and O-enriched particles appears to be responsible for their improved high temperature mechanical properties.

Overview of microstructural evolution in neutron-irradiated austenitic stainless steels

Abstract Austenitic stainless steels are important structural materials common to several different reactor systems, including light water and fast breeder fission, and magnetic fusion reactors (LWR, FBR, and MFR, respectively). The microstructures that develop in 300 series austenitic stainless steels during neutron irradiation at 60–700°C include combinations of dislocation loops and networks, bubbles and voids, and various kinds of precipitate phases (radiation-induced, or -enhanced or -modified thermal phases). Many property changes in these steels during neutron irradiation are directly or indirectly related to radiation-induced microstructural evolution. Even more important is the fact that radiation-resistance of such steels during either FBR or MFR irradiation is directly related to control of the evolving microstructure during such irradiation. The purpose of this paper is to provide an overview of the large and complex body of data accumulated from various fission reactor irradiation experiments conducted over the many years of research on microstructural evolution in this family of steels. The data can be organized into several different temperature regimes which then define the nature of the dominant microstructural components and their sensitivities to irradiation parameters (dose, helium/dpa ratio, dose rate) or metallurgical variables (alloy composition, pretreatment). The emphasis in this paper will be on the underlying mechanisms driving the microstructure to evolve during irradiation or those enabling microstructural stability related to radiation resistance.

Tensile and Creep Properties of an Oxide Dispersion-Strengthened Ferritic Steel

The tensile and creep properties of two oxide dispersion-strengthened (ODS) steels with nominal compositions of Fe–12Cr–0.25Y2O3 (designated 12Y1) and Fe–12Cr–2.5W–0.4Ti–0.25Y2O3 (12YWT) were investigated. Optical microscopy, transmission electron microscopy, and atom probe field ion microscopy studies indicated that the 12YWT contained a high density of extremely fine Y–Ti–O clusters, compared to the much larger oxide particles in the 12Y1. The fine dispersion of particles gave the 12YWT better tensile and creep properties compared to commercial ODS alloys and ferritic/martensitic steels that would be replaced by the new ODS steel.

Microstructural control and mechanical properties of dual-phase TiAl alloys

Abstract This paper summarizes our recent work on the effects of microstructural features on the mechanical properties of TiAl alloys prepared by powder and ingot metallurgy. TiAl alloys based on Ti-47Al-2Cr-2Nb (at%) were alloyed with small amounts of Ta, W, and B additions for control of alloy phases and microstructure. The alloys were processed by hot extrusion above and below Tα, followed by short- and long-term heat treatments at temperatures to 1350 °C in vacuum. The microstructural features in the lamellar structures were characterized by metallography, SEM and TEM, and the mechanical properties were determined by tensile tests at temperatures to 1000 °C. The tensile elongation at room temperature is mainly controlled by the colony size, showing an increase in ductility with decreasing colony size. The yield strength, on the other hand, is sensitive to the interlamellar spacing. Hall-Petch relationships hold well for both yield strength and tensile elongation at room and elevated temperatures. TiAl alloys with refined colony size and ultrafine lamellar structures possess excellent mechanical properties for structural applications at elevated temperatures.

Defect and void evolution in oxide dispersion strengthened ferritic steels under 3.2 MeV Fe+ ion irradiation with simultaneous helium injection

Abstract In an attempt to explore the potential of oxide dispersion strengthened (ODS) ferritic steels for fission and fusion structural materials applications, a set of ODS steels with varying oxide particle dispersion were irradiated at 650°C, using 3.2 MeV Fe + and 330 keV He + ions simultaneously. The void formation mechanisms in these ODS steels were studied by juxtaposing the response of a 9Cr–2WVTa ferritic/martensitic steel and solution annealed AISI 316LN austenitic stainless steel under the same irradiation conditions. The results showed that void formation was suppressed progressively by introducing and retaining a higher dislocation density and finer precipitate particles. Theoretical analyses suggest that the delayed onset of void formation in ODS steels stems from the enhanced point defect recombination in the high density dislocation microstructure, lower dislocation bias due to oxide particle pinning, and a very fine dispersion of helium bubbles caused by trapping helium atoms at the particle–matrix interfaces.

Control of helium effects in irradiated materials based on theory and experiment

Helium produced in materials by (n,..cap alpha..) transmutation reactions during neutron irradiations or subjected in ion bombardment experiments causes substantial changes in the response to displacement damage. In particular, swelling, phase transformations and embrittlement are strongly affected. Present understanding of the mechanisms underlying these effects is reviewed. Key theoretical relationships describing helium effects on swelling and helium diffusion are described. Experimental data in the areas of helium effects on swelling and precipitation is reviewed with emphasis on critical experiments that have been designed and evaluated in conjunction with theory. Confirmed principles for alloy design to control irradiation performance are described.

Helium effects on void formation in 9Cr-1MoVNb and 12Cr-1MoVW irradiated in HFIR☆

Up to 2 wt% Ni was added to 9Cr-1MoVNb and 12Cr-lMoVW ferritic steels to increase helium production by transmutation during HFIR irradiation. The various steels were irradiated to ∼39 dpa. Voids were found in all the undoped and nickel-doped steels irradiated at 400°C, most of them at 500°C, but not in any of them at 300 or 600°C. Bubble formation, however, was increased at all temperatures in the nickel-doped steels. Maximum void formation was found at 400°C, but swelling remained less than 0.5% even with up to 440 appm He. Irradiation at 300 to 500°C caused dissolution of as-tempered M23C6 precipitates and coarsening of the lath/subgrain structure in the 9-Cr steels, whereas the microstructure generally remained stable in the 12-Cr steels. Irradiation in this temperature range also caused compositional changes in the as-tempered MC phase in all the steels, and produced combinations of fine M6C, G-phase, and M2X precipitates in various steels. The subgrain boundaries appear to be strong sinks that enhance resistance to void formation. Higher helium production during irradiation appears to shorten the incubation period for void formation. The effects of helium on steady state void swelling behavior, however, remain unknown.