Recent Progress in Developing and Qualifying Nanos

Authors: G R Odette

Publish Date: 2014/11/13

Volume: 66, Issue: 12, Pages: 2427-2441

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Abstract

This article summarizes the recent progress on developing a class of potentially transformational structural materials called nanostructured ferritic alloys which are leading candidates for advanced fission and fusion energy applications Here we focus on FeCrbased ferritic stainless steels containing a very high concentration of YTiO nanooxide features that enable a host of outstanding hightemperature properties along with unique irradiation tolerance and thermal stability Perhaps most notably these alloys have an unprecedented capability to manage very high helium concentrations pertinent to fusion service in a way that transforms this element from a severe liability to a potential asset In addition to providing some necessary background we update progress on I the character of the nanofeatures II some unifying insights on key mechanical properties III a quantitative model for nanofeature coarsening IV recent irradiation experiments of the effects of helium on cavity evolution and void swelling and V a powerful new mechanism controlling the transport fate and consequences of heliumThe success of nuclear fission and fusion as largescale sources of energy for the millennia requires new structural materials that provide and sustain a host of highperformance properties The challenges presented by irradiation effects are particularly daunting and in the case of fusion are exacerbated by high quantities of heliumThe objective of this summary review is to update the status of a transformational class of ironchromium–based ferritic stainless steels1 – 3 We use the nomenclature nanostructured ferritic alloys NFAs to distinguish NFAs from socalled oxidedispersionstrengthened ODS steels such as PM2000 which contain a variety of coarserscale oxides often associated with Al additions4 We also distinguish NFAs which typically contain 14Cr and more generally ≥12Cr along with small yttrium titanium and oxygen additions from transformable ODS steels which are alloyed with C and ≈9 Cr1 3 NFAs which are often designated by their percentage of Cr content followed by YWT as in 14YWT have many outstanding properties These include high tensile creep and fatigue strengths over a wide range of temperatures truly remarkable thermal stability up to 1000°C and unmatched irradiation tolerance especially with respect to managing high levels of helium1 2 5 There is a large and growing worldwide interest and literature on nanooxide dispersionstrengthened ironbased alloys that resulted in almost 150 Institute of Scientific Information Web of Science papers published in 2013 alone as well as a focus for several special symposia and journal issues in recent years Clearly this article cannot provide a comprehensive list of pertinent citations thus these are limited to representative examplesThe outstanding characteristics of NFAs result from the interrelated presence of an ultrahigh density of YTiO rich nanooxide features NFs fine grain sizes and high dislocation densities The NFs are multifunctional in that they1 – 3 I retard dislocation climb and glide thus increasing alloy strength II stabilize grain and dislocation structures and III act as very deep traps for helium resulting in the formation of tiny highpressure gas bubbles at their interface with the matrix1 2 5 The presence of nanometerscale bubbles adds to the irradiation tolerance of NFAs because they act as stable sinkrecombination centers that selfheal excess vacancy and selfinterstitial displacement damage defects Indeed the helium bubbles are much more effective in enhancing recombination than the oxidematrix interface itself1 2 5 The bubbles are also deep traps for additional helium Sequestering helium in bubbles reduces the accumulation of this bondweakening element on grain boundaries which otherwise can lead to degradation of both creep rupture and fast fracture toughness properties5 Helium trapped in a very high number density of NFinterface small bubbles also eliminates or greatly retards rapid void swelling1 2 5 Thus NFAs may turn high helium levels from a liability to an assetChallenges facing NFAs include I characterizing NF structures compositions oxidematrix interfaces and the various factors that control their nature II determining the role NFs play in providing high strength and irradiation tolerance over a wide range of service conditions III quantifying the thermal and irradiation stability of NFAs and NFs and IV alloy designs thermal–mechanical processing paths and joining methods that create sustainable optimized NFA microstructures and yield outstanding isotropic properties and defectfree product forms Other practical NFA challenges include corrosion and compatibility issues reducing costs improving alloy homogeneity and reproducibility establishing industrialscale supply sources and qualifying new alloys for nuclear service In this article we emphasize items I through III Other topics will be reviewed separatelyWe focus here on Fe14Cr3W alloys 14YWT microalloyed with titanium yttrium and oxygen solutes that are the primary constituents of the NFs1 – 3 In several cases we compare NFA properties with those of conventionally transformable 9Crtempered martensitic steels TMS However it is worth noting that the bodycentered cubic bcc NFAs are far superior to facecentered cubic fcc austenitic steels in terms of strength and especially irradiation toleranceThe first NFA processing steps are typically gas atomization of prealloyed metal 14CrFeTiW powders followed by ball milling them with yttria Y2O3 powders1 – 3 Proper milling effectively dissolves yttrium 01–03 wt and oxygen 01–02 wt solutes that interact with the alloy titanium 01–04 wt1 3 6 7 The titanium yttrium and oxygen precipitate as NFs during hot consolidation It is well established that titanium is needed to form small NFs at least at higher consolidation temperatures1 6 7 The milled powders are then canned degassed and consolidated by hot isostatic pressing HIP or hot extrusion More recently sparkplasma sintering has been used for consolidation8 NF precipitation kinetics are very rapid likely partly because of the high concentration of excess defects created by ball milling1 6 7 However the NF number densities N average diameters 〈d〉 and size distributions depend on the alloy composition and consolidation timetemperature history Precipitation manifests Ccurve type time–temperaturetransformation behavior1 2 6 7 As the consolidation temperature drops below 1150°C the NF N and volume fraction f increase the latter usually moderately whereas 〈d〉 decreases The maximum N for a 3 h isothermal consolidation time has a Ccurve nose at ≈675°C decreasing at both higher and lower processing temperatures Consolidation is typically carried out from 850°C to 1150°C yielding N = 1023 to 1024/m3 f = 05 to 1 and 〈d〉 ≈ 15 to 30 nmAsconsolidated alloys often have bimodal grain size distributions especially at higher processing temperatures1 9 Note that contrary to previous conclusions however NFs are also found in the larger grains9 10 The smaller grains are 1 µm whereas the larger grains range up to 10 µm or more10 11 Extrusions typically have a 〈110〉fiber texture with grain aspect ratios of ≈2 to 101 3 10 – 13 Dislocation densities typically range from ≈05 to 2 × 1015/m210 11 The scale of both grain and dislocation substructures is finer at lower consolidation temperatures10 NFA consolidation is usually followed by a series of thermal–mechanical processing steps needed to improve properties and fabricate product forms3

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