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All sourcesNASA NTRSDOE OSTI
NASA NTRS2023

Pervasive aqueous alteration in the early Solar System revealed by potassium isotopic variations in Ryugu samples and carbonaceous chondrites

C-type asteroids are the presumed home to carbonaceous chondrites, some of which contain abundant life-forming volatiles and organics. For the first time, samples from a C-type asteroid (162173 Ryugu) were successfully returned to Earth by JAXA’s Hayabusa2 mission. These pristine samples, uncontaminated by the terrestrial environment, allow a direct comparison with carbonaceous chondrites. This study reports the stable K isotopic compositions (expressed as δ 41 K) of Ryugu samples and seven carbonaceous chondrites to constrain the origin of K isotopic variations in the early Solar System. Three aliquots of Ryugu particles collected at two touchdown sites have identical δ 41 K values, averaged at -0.194 ± 0.038‰ (2SD). The K isotopic composition of Ryugu falls within the range of δ 41 K values measured on representative CI chondrites, and together, they define an average δ 41 K value of -0.185 ± 0.078‰ (2SE), which provides the current best estimate of the K isotopic composition of the bulk Solar System. Samples of CI chondrites with δ 41 K values that deviate from this range likely reflect terrestrial contaminations or compositional heterogeneities at sampled sizes. In addition to CI chondrites, substantial K isotopic variability is observed in other carbonaceous chondrites and within individual chondritic groups, with δ 41 K values inversely correlated with K abundances in many cases. These observations indicate widespread fluid activity occurred in chondrite parent bodies, which significantly altered the original K abundances and isotopic compositions of chondrules and matrices established at their accretion.

Asteroid Ryugu↗
NASA NTRS1992

Satellites of Mars - Geologic history

The small, irregularly shaped satellites of Mars, Phobos and Deimos, provide the most detailed view of the geomorphic forms and processes important on small solar system bodies. The satellites appear to be very similar in composition, strongly resembling carbonaceous asteroids; however, recent groundbased spectra suggest that their surfaces have little bound or interlayer water. Despite their similar compositions, sizes and environments, Phobos and Deimos have radically different surface features. Phobos is densely covered by craters that are nearly lunar in appearance; Deimos' craters are subdued and largely filled in by debris. Phobos shows only local downslope movement of regolith; Deimos has it on a global scale. Phobos is criss-crossed by linear depressions; Deimos has none. Crater ejecta appear to be retained near their sources on Phobos while the ejecta are widespread on Deimos. The reasons for the differences between the satellites are not known; imaging of asteroids should tell us which, if either, satellite is typical of the many small bodies that populate the asteroid belt.

Thomas, P.↗
NASA NTRS1993

Petrologic constraints on the surface processes on asteroid 4 Vesta and on excavation depths of diogenite fragments

The eucrite-howardite-diogenite meteorite groups are though to be related by magmatic processes. Asteroid 4 Vesta has been proposed as the parent body for these basaltic achondrite meteorites. The similarity of the planetesimal's surface composition to eucrite and diogenite meteorites and the large size of the asteroid (r = 250 km) make it an attractive source, but its position in the asteroid belt far from the known resonances from which meteorites originate make a relation between Vesta and eucrite-howardite-giogenite group problematic. It has been proposed that diogenites are low-Ca pyroxene-rich cumulates that crystallized from a magnesian parent (identified in howardite breccias), and this crystallization process led to evolved eucrite derivative magmas. This eucrite-diogenite genetic relationship places constraints on the physical conditions under which crystallization occurred. Elevated pressure melting experiments on magnesian eucrite parent compositions show that the minimum pressure at which pyroxene crystallization could lead to the observed compositions of main series eucrites is 500 bars, equivalent to a depth of 135 km in a 4 Vesta-sized eucrite parent body. Therefore, the observation of diogenite on the surface of 4 Vesta requires a post-crystallization process that excavates diogenite cumulate from depth. The discovery of diogenite asteroidal fragments is consistent with an impact event on 4 Vesta that penetrated the deep interior of this planetesimal.

Grove, T. L.↗
NASA NTRS1978

Physical properties of the satellites of Saturn

The photometric and bulk parameters of the known and suspected satellites of Saturn are presented in updated tables and are compared. The surface compositions of all the satellites are discussed in terms of modern photometry and spectroscopy; most if not all of the inner bodies have water frost surfaces, but the outer three satellites have surfaces of unknown composition. The few reliable mass values of some inner satellites, together with the best current values for the satellite radii, suggest mean densities representative of bulk compositions dominated by frozen volatiles, though Titan may have a substantial volume fraction of silicates. The special case of Iapetus is considered in the light of new studies of its two distinct faces and polar cap.

Cruikshank, D. P.↗
NASA NTRS1979

The surfaces and interiors of Saturn's satellites

The photometric and bulk parameters of the known and suspected satellites of Saturn are presented in updated tables and are compared. The surface compositions of all the satellites are discussed in terms of modern photometry and spectroscopy; most, if not all, of the inner bodies have water frost surfaces, but the outer three satellites have surfaces of unknown composition. The few reliable mass values of some inner satellites, together with the best current values for the satellite radii, suggest mean densities representative of bulk compositions dominated by frozen volatiles, though Titan may have a substantial volume fraction of silicates. The special case of Iapetus is considered in the light of new studies of its two distinct faces and polar cap.

Cruikshank, D. P.↗
NASA NTRS2011

The Regolith of 4 Vesta - Inferences from Howardites

Asteroid 4 Vesta is quite likely the parent asteroid of the howardite, eucrite and diogenite meteorites - the HED clan. Eucrites and diogenites are the products of igneous processes; the former are basaltic composition rocks from flows, and shallow and deep intrusive bodies, whilst the latter are cumulate orthopyroxenites thought to have formed deep in the crust. Impact processes have excavated these materials and mixed them into a suite of polymict breccias. Howardites are polymict breccias composed mostly of clasts and mineral fragments of eucritic and diogenitic parentage, with neither end-member comprising more than 90% of the rock. Early work interpreted howardites as representing the lithified regolith of their parent asteroid. Recently, howardites have been divided into two subtypes; fragmental howardites, being a type of non-regolithic polymict breccia, and regolithic howardites, being lithified remnants of the active regolith of 4 Vesta. We are in the thralls of a collaborative investigation of the record of impact mixing contained within howardites, which includes studies of their mineralogy, petrology, bulk rock compositions, and bulk rock and clast noble gas contents. One goal of our investigation is to test the hypothesis that some howardites represent breccias formed from an ancient, well-mixed regolith on Vesta. Another is to use our results to further understand regolith processing on differentiated asteroids as compared to what has been learned from the Moon. We have made petrographic observations and electron microprobe analyses on 21 howardites and 3 polymict eucrites. We have done bulk rock analyses using X-ray fluorescence spectrometry and are completing inductively coupled plasma mass spectrometry analyses. Here, we discuss our petrologic and bulk compositional results in the context of regolith formation. Companion presentations describe the noble gas results and compositional studies of low-Ca pyroxene clasts.

Mittlefehldt, D. W.↗
DOE OSTI2024

Selective phosphate removal with manganese oxide composite anion exchange membranes in membrane capacitive deionization

The discharge of excessive phosphorous into water bodies can lead to serious eutrophication threatening aquatic ecosystem. Membrane capacitive deionization (MCDI) is an effective platform for deionizing aqueous streams; however, conventional MCDI is unable to selectively remove targeted ions from a liquid mixture. Here, in this work, we fabricated manganese oxide composite anion exchange membranes (AEMs) for MCDI to enhance phosphate removal selectivity from sodium chloride-sodium dihydrogen phosphate (10:1 M ratio) aqueous mixtures. We systematically investigated several critical factors, such as constant current or voltage operation, applied voltage amount, process stream pH, and manganese oxide (Mn 2 O 3 ) content in the AEM, on phosphate removal efficiency and phosphate selectivity. A trade-off was observed between phosphate removal and selectivity when increasing the cell voltage. Under the best conditions, a MCDI unit with a 20 wt% Mn 2 O 3 composite AEM and a bipolar membrane facilitated high phosphate removal efficiency of ≥ 31.8 % and a phosphate over chloride selectivity of 1.1 while showing stability for at least 30 cycles. To help understand how Mn 2 O 3 composite AEM boosts phosphate selectivity, static electronic structure calculations were performed, and they revelated that hydrogen phosphate absorption on Mn 2 O 3 composite AEM was 314 kcal/mol more exothermic than that on pristine AEM while chloride adsorption on Mn 2 O 3 composite AEM was 2.2 kcal/mol less exothermic than that on a pristine AEM. Overall, this work presents an effective strategy for selectively removing phosphate from model wastewater solutions and the mechanistic understanding that governs ion selectivity in composite ion-exchange membranes used in MCDI.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1984

Rotational spectral variations of asteroid (8) Flora Implications for the nature of the S-type asteroids and for the parent bodies of the ordinary chondrites

The surface material and the surface material heterogeneities of the asteroid Flora are characterized using the best available data sets and the most sophisticated interpretive calibrations. Five spectrally derived mineralogic and patrologic properties of the surface assemblage of Flora which are relevant to whether this body is a differentiated or undifferentiated object are considered: bulk mineralogy, mafic mineral assemblage, metallic phase, pyroxene composition and structural type, and mineralogic variation. All of these properties indicate that Flora is a differentiated body. Flora is probably the residual core of an intensely heated, thermally evolved, and magnetically differentiated planetesimal which was subsequently disrupted. The present surface sample layers formed at or near the core-mantle boundary in the parent body.

Gaffey, M. J.↗
DOE OSTI2020

Samarium isotope compositions of uranium ore concentrates: A novel nuclear forensic signature

Uranium ore is mined and milled to produce uranium ore concentrate (UOC), a regulated product of the nuclear fuel cycle. Additionally, diversion of UOC from the fuel cycle into possible weapons production is a key concern in global nonproliferation efforts. As such, the ability to trace the origin of seized nuclear materials is imperative to law enforcement efforts. Although isotopic signatures of UOCs have proven fruitful to pinpoint sample provenance, new isotopic signatures are needed because most existing isotopic signatures are not indicative of the original ore body from which the U is derived. In this work, we developed a new method to separate samarium (Sm) from a U-rich sample matrix and report the first Sm isotope compositions of 32 UOCs derived from a variety of worldwide uranium mines. Relative to terrestrial standards, approximately half the UOCs have resolved and anticorrelated 149 Sm- 150 Sm isotope compositions, consistent with the capture of thermal neutrons by 149 Sm in the ore body. The UOCs with anomalous Sm isotope compositions tend to derive from older (~>1.5Ga) and higher-grade ore bodies, although other factors, such as the presence of neutron moderators like water, also play a role. Nonetheless, the Sm isotope compositions of UOCs directly reflects the neutron fluence over the history of the original ore body and can be used to discern different geologic conditions associated with that ore body. Overall, this work demonstrates the potential use of Sm isotopes as a novel nuclear forensics signature for origin assessment of UOCs.

58 GEOSCIENCES↗
NASA NTRS2026

Molecular Inventory and Comparative Organic Profiling of A Homogenized Aggregate Sample From Asteroid (101955) Bennu

Evaluating the distribution and diversity of solvent-soluble organic molecules in extraterrestrial materials provide information about the physicochemical environments where prebiotic chemistry occurred and is essential for assessing the inventory of prebiotic compounds available to the early Earth. In this work, we conducted a comprehensive molecular characterization of solvent-soluble organic compounds in a homogenized aggregate sample (ID: OREX-800107-128) of unsorted regolith from asteroid (101955) Bennu returned by NASA’s OSIRIS-REx mission. Using two-dimensional gas chromatography coupled with high-resolution time-of-flight mass spectrometry (GC×GC-HRMS), we identified 87 organic compounds across several molecular classes, including alkanes, polycyclic aromatic hydrocarbons, and nitrogen-, oxygen-, and sulfur-bearing species. Comparison with the organic inventory of CI (Orgueil), C2-ungrouped (Tagish Lake and Tarda), and CM2 (Murchison) carbonaceous chondrites revealed that Bennu sample OREX-800107-128 is most similar to Tagish Lake and Tarda yet compositionally distinct from all of these meteorites. This Bennu sample exhibits a relatively high abundance of low-molecular-weight aromatics (e.g., toluene), pyridine derivatives, and sulfur species such as dimethyl sulfite, consistent with episodic low-temperature aqueous alteration. The presence of these organic compounds suggests that this material experienced chemically diverse processes in the parent body conducive to complex organic synthesis, including aldol condensation reactions responsible for N- and O-bearing compounds. We also compared the molecular distributions of this Bennu sample and previously analyzed samples from (162173) Ryugu to assess potential common origins between these two carbonaceous asteroids. Bennu shows more chemically diverse and nitrogen-rich organic materials than Ryugu. While both asteroids share broadly similar primitive compositions, the observed chemical differences indicate distinct evolutionary histories within a likely common formation environment though whether Bennu and Ryugu derive from a single compositionally heterogeneous parent body or from multiple closely related progenitors remains unresolved. Further detailed comparative analyses of material from Bennu, Ryugu, and specific carbonaceous meteorites (e.g., Tagish Lake and Tarda) will provide new insights into how the N-rich molecular profile observed in Bennu samples was formed and evolved in the outer Solar System.

sample return↗
NASA NTRS2009

Impact Histories of Vesta and Vestoids inferred from Howardites, Eucrites, and Diogenites

The parent body of the howardites, eucrites and diogenites (HEDs) is thought to be asteroid (4) Vesta [1]. However, several eucrites have now been recognized, like NWA 011 and Ibitira, with major element compositions and mineralogy like normal eucrites but with different oxygen isotope compositions and minor element concentrations suggesting they are not from the same body [2, 3]. The discoveries of abnormal eucrites and V-type asteroids that are probably not from Vesta [see 4] raise the question whether the HEDs with normal oxygen isotopes are coming from Vesta [3]. To address this issue and understand more about the evolution of Vesta in preparation for the arrival of the Dawn spacecraft, we integrate fresh insights from Ar-Ar dating and oxygen isotope analyses of HEDs, radiometric dating of differentiated meteorites, as well as dynamical and astronomical studies of Vesta, the Vesta asteroid family (i.e., the Vestoids), and other V-type asteroids.

Scott, E. R. D.↗
DOE OSTI2024

Tarda and Tagish Lake: Samples from the same outer Solar System asteroid and implications for D- and P-type asteroids

Here, we report a comprehensive study of the ungrouped type 2 carbonaceous chondrite, Tarda, which fell in Morocco in 2020. This meteorite exhibits substantial similarities to Tagish Lake, Wisconsin Range 91600, and Meteorite Hills 00432, which are generally considered to have originated from a D-type asteroid(s). We constrain the compositions and petrologies of the materials present in a potential sample of a D-type asteroid by reporting the petrography, bulk chemical compositions, bulk H, C, N, Cr, and Ti isotopic compositions, reflectance spectra, and in situ chemical compositions of metals, sulfides, carbonates, and FeO-poor and FeO-rich chondrule silicates of Tarda. We also present new data for Tagish Lake. We then compare Tarda with the other Tagish Lake-like meteorites. Tarda and Tagish Lake appear to be from the same parent body, as demonstrated by their similar petrologies (modal abundances, chondrule sizes), mineral compositions, bulk chemical and isotopic compositions, and reflectance spectra. While the two other Tagish Lake-like meteorites, Wisconsin Range 91600 and Meteorite Hills 00432, show some affinities to Tagish Lake and Tarda, they also share similar characteristics to the Mighei-like carbonaceous (CM) chondrites, warranting further study. Similarities in reflectance spectra suggest that P-type asteroids 65 Cybele and 76 Freia are potential parent bodies of Tarda and the Tagish Lake-like meteorites, or at least have similar surface materials. Since upcoming spacecraft missions will spectrally survey D-type, P-type, and C-type Trojan asteroids (NASA's Lucy) and spectrally study and return samples from Mars' moon Phobos (JAXA's Martian Moons eXploration mission), which is spectrally similar to D-type asteroids, these meteorites are of substantial scientific interest. Furthermore, since Tarda closely spectrally matches P-type asteroids (but compositionally matches the D-type asteroid like Tagish Lake meteorite), P-type and D-type asteroids may represent fragments of the same or similar parent bodies.

58 GEOSCIENCES↗
NASA NTRS2024

Tarda and Tagish Lake: Samples From The Same Outer Solar System Asteroid and Implications for D- and P-Type Asteroids

We report a comprehensive study of the ungrouped type 2 carbonaceous chondrite, Tarda, which fell in Morocco in 2020. This meteorite exhibits substantial similarities to Tagish Lake, Wisconsin Range 91600, and Meteorite Hills 00432, which are generally considered to have originated from a D-type asteroid(s). We constrain the compositions and petrologies of the materials present in a potential sample of a D-type asteroid by reporting the petrography, bulk chemical compositions, bulk H, C, N, Cr, and Ti isotopic compositions, reflectance spectra, and in situ chemical compositions of metals, sulfides, carbonates, and FeO-poor and FeO-rich chondrule silicates of Tarda. We also present new data for Tagish Lake. We then compare Tarda with the other Tagish Lake-like meteorites. Tarda and Tagish Lake appear to be from the same parent body, as demonstrated by their similar petrologies (modal abundances, chondrule sizes), mineral compositions, bulk chemical and isotopic compositions, and reflectance spectra. While the two other Tagish Lake-like meteorites, Wisconsin Range 91600 and Meteorite Hills 00432, show some affinities to Tagish Lake and Tarda, they also share similar characteristics to the Mighei-like carbonaceous (CM) chondrites, warranting further study. Similarities in reflectance spectra suggest that P-type asteroids 65 Cybele and 76 Freia are potential parent bodies of Tarda and the Tagish Lake-like meteorites, or at least have similar surface materials. Since upcoming spacecraft missions will spectrally survey D-type, P-type, and C-type Trojan asteroids (NASA's Lucy) and spectrally study and return samples from Mars' moon Phobos (JAXA's Martian Moons eXploration mission), which is spectrally similar to D-type asteroids, these meteorites are of substantial scientific interest. Furthermore, since Tarda closely spectrally matches P-type asteroids (but compositionally matches the D-type asteroid like Tagish Lake meteorite), P-type and D-type asteroids may represent fragments of the same or similar parent bodies.

Tarda↗
NASA NTRS2007

Processes in Early Planetesimals: Evidence from Ureilite Meteorites

Ureilites are primitive ultramafic achondrites composed largely of olivine and pigeonite, with minor augite, carbon, sulphide and metal. They represent very early material in the history of the Solar System and form a bridge between undifferentiated chondrites and fully differentiated asteroids. They show a mixture of chemical characteristics, some of which are considered to be nebula-derived (e.g. a negative correlation between Mg/Fe and Delta O-17 that resembles that of the ordinary chondrites but at lower Delta O-17 values) whereas others have been imposed by asteroidal differentiation. Carbon isotope data show a striking negative correlation of delta C-13 values with mg# in olivine. delta C-13 also correlates positively with Delta O-17, and therefore this isotopic variation was probably also nebula-derived. Thus, oxygen and carbon isotope compositions and Fe-Mg systematics of each monomict ureilite were established before differentiation processes began. Heated by decay of short-lived radioactive isotopes, the ureilite asteroid started to melt. Metal and sulphide would have melted first, forming a Fe-S eutectic liquid, which removed chalcophile elements and incompatible siderophile elements, and basaltic melts that removed Al, Ca and the LREE. Several elements show different abundances and/or correlations with Fo content in olivine, e.g. carbon shows a positive correlation in ferroan ureilites, and a weak or even negative correlation in more magnesian compositions. HSE such as Os and Ir also show different distributions, i.e. ureilites with Fo < 82 have very scattered Os and Ir concentrations, which reach high values, whereas ureilites with Fo > 82 tend to have much less scattered and overall lower Os and Ir abundances. A similar change in elemental behaviour is shown by the Fe-Mn relations in ureilitic olivines: those with Fo contents < 85 show a good negative correlation, whereas those with Fo > 85 show much greater scatter. This suggests that a major change affected the parent body at a time when melting had reached relatively magnesian bulk compositions. We consider that this event may have been a hit and run collision in which the ureilite parent body collided with a larger object. During the collision, the ureilite mantle broke up catastrophically but re-accreted in a jumbled state around the still-intact core. Mg-rich basaltic melts that were in the process of being formed at the time of break-up were retained in part as melt clasts that re-accreted to the regolith and are found in polymict ureilites.

Mittlefehldt, David W.↗
NASA NTRS1975

Preliminary design and performance of an advanced gamma ray spectrometer for future orbiter missions

A knowledge of the composition of planets, satellites, and asteroids is of primary importance in understanding the formation and evolution of the solar system. Gamma-ray spectroscopy is capable of measuring the composition of meter-depth surface material from orbit around any body possessing little or no atmosphere. Measurement sensitivity is determined by detector efficiency and resolution, counting time, and the background flux while the effective spatial resolution depends upon the field-of-view and counting time together with the regional contrast in composition. The advantages of using germanium as a detector of gamma rays in space are illustrated experimentally and a compact instrument cooled by passive thermal radiation is described. Calculations of the expected sensitivity of this instrument at the Moon and Mars show that at least a dozen elements will be detected, twice the number which have been isolated in the Apollo gamma-ray data.

Metzger, A. E.↗
NASA NTRS1975

Preliminary design and performance of an advanced gamma-ray spectrometer for future orbiter missions

A knowledge of the composition of planets, satellites, and asteroids is of primary importance in understanding the formation and evolution of the solar system. Gamma-ray spectroscopy is capable of measuring the composition of meter-depth surface material from orbit around any body possessing little or no atmosphere. Measurement sensitivity is determined by detector efficiency and resolution, counting time and the background flux, while the effective spatial resolution depends upon the field-of-view and counting time together with the regional contrast in composition. The advantages of using germanium as a detector of gamma rays in space are illustrated experimentally and a compact instrument cooled by passive thermal radiation is described. Calculations of the expected sensitivity of this instrument at the moon and Mars show that at least a dozen elements should be measurable, twice the number which have been isolated in the Apollo gamma-ray data

Metzger, A. E.↗
NASA NTRS1988

Igneous history of the aubrite parent asteroid - Evidence from the Norton County enstatite achondrite

Numerous specimens of the Norton County enstatite achondrite (aubrite) were studied by optical microscopy, electron microprobe, and neutron-activation analysis. Norton County is found to be a fragmental impact breccia, consisting of a clastic matrix made mostly of crushed enstatite, into which are embedded a variety of mineral and lithic clasts of both igneous and impact melt origin. The Norton County precursor materials were igneous rocks, mostly plutonic orthopyroxenites, not grains formed by condensation from the solar nebula. The Mg-silicate-rich aubrite parent body experienced extensive melting and igneous differentiation, causing formation of diverse lithologies including dunites, plutonic orthopyroxenites, plutonic pyroxenites, and plagioclase-silica rocks. The presence of impact melt breccias (the microporphyritic clasts and the diopside-plagioclase-silica clast) of still different compositions further attests to the lithologic diversity of the aubrite parent body.

Okada, Akihiko↗
DOE OSTI2023

Dislocation Patterning in Deforming Crystals: Theory, Computational Predictions and Validation (Final Technical Report)

This project was awarded for an initial period of three years, followed by a no-cost extension for one year and a funded one-year renewal. During the first four years, the project focused on investigating the role of dislocation reactions and dislocation correlation in dislocation patterning in FCC metals. During the fifth year, the scope of research was expanded to investigate the effects of composition inhomogeneity on the mesoscale plastic response of Body-Centered Cubic (BCC) alloys. In addition to its impact on metal hardening during deformation, dislocation patterning provides the microstructure information required to understand phenomena like fracture and recrystallization in metals. The Continuum Dislocation Dynamics (CDD) framework was the methodology used during the first four years, followed by the use of Discrete Dislocation Dynamics (DDD) during the fifth year. CDD is a density-based formalism of dislocation dynamics in which the plastic deformation of crystals is predicted together with the mesoscale dislocation patterns by tracking the space-time evolution of dislocations as driven by the applied stress, short-range and long-range interactions of dislocations, and cross slip. CDD is a crystal mechanics approach in which the plastic constitutive part is replaced with the equations of dislocation dynamics, which is driven by the internal stress via a mobility laws, while the evolution of the density gives the eigenstrain required to update the internal stress itself. The governing equations are thus those of crystal mechanics cast as an eigenstrain problem and those of dislocation transport and reactions. Our investigations during the first four years were driven by the hypothesis that that dislocation patterning is triggered by spatiotemporal dislocation density and internal stress fluctuations, and that such fluctuations influence the collective dislocation dynamic through their effects on the short-range reaction rates and the collective dislocation mobility. This hypothesis was tested by modeling the influence of the dislocation reactions and dislocation correlations on collective dislocation dynamics. The main research components during the first four years were: 1) Reformulation of the CDD framework to integrate the dislocation correlations and dislocation reactions. 2) Simulation of the dislocation correlations and quantifying their contribution to the long-range stress of complex dislocation systems. 3) Computational implementation of the updated CDD model for FCC single crystals and development of an efficient CDD code. 4) Investigation of dislocation patterning in FCC crystals based on the updated CDD model. Some key findings from these investigations were: 1) The patterning of dislocations is initiated by cross slip and internal stress fluctuations, and the refinement of the pattern results from junction formation. 2) Patterning is more prominent under high stress. 3) The correlation stress was found to be a significant part of the mean field stress in continuum representations of dislocation dynamics. During the last year of the project, we have established a method for performing dislocation dynamics in inhomogeneous alloys and demonstrated the impact of composition inhomogeneity on the characteristics of initial yielding and dislocation character in BCC alloys. We also tested this method using an irradiated ferritic alloy in which the composition and dislocation loops effects are both present. In doing so, we have considered two aspects of the role of inhomogeneity, the creation of internal coherency stress due to the dependence of the lattice parameter on the local composition, and the dependence of the dislocation mobility on the local composition. These two aspects resulted in an unexpected behavior of the dislocation system, and, in turn, the yielding behavior of BCC alloys. Key findings include: 1) The composition inhomogeneity in BCC alloys alters the well-known role of screw dislocations by forcing a level of waviness on the average dislocation character, thus destroying the screw character dominance in the yielding observed in pure or homogeneous BCC metals. 2) In the case of irradiated alloys, while composition inhomogeneity by itself and dislocation loops by themselves result in some hardening, the superposition of the two mechanisms does not result in the sum of the two contributions via any known rule. The latter result contradicts the classical works related hardening via multiple mechanisms, which state that various hardening mechanisms can be added linearly or using some Pythagorean additive rule. We were able to rationalize this unexpected result by a closer loop at the origin of the hardening mechanisms themselves, which both involved long-range elastic interactions with dislocations. We reached the conclusion that the hardening resulting from the total stress associated with these two mechanisms (current work) differs from the sum of the effects of the individual stress fields (classical literature). We have also found a major flaw in the classical theory of spinodal hardening. The latter theory never considered the impact of composition undulation on the solute hardening. We have built a new general theory accounting for the effect of solute friction together with the composition undulations on the dislocation configuration at Critical Resolved Shear Stress (CRSS), which is yielding a new definition of the spinodal hardening and new different results. This work is the first to recognize the lack of the role of solute in spinodal hardening theory of alloys.

36 MATERIALS SCIENCE↗