Kester D. Clarke

Proton Radiography Peers into Metal Solidification

Historically, metals are cut up and polished to see the structure and to infer how processing influences the evolution. We can now peer into a metal during processing without destroying it using proton radiography. Understanding the link between processing and structure is important because structure profoundly affects the properties of engineering materials. Synchrotron x-ray radiography has enabled real-time glimpses into metal solidification. However, x-ray energies favor the examination of small volumes and low density metals. Here we use high energy proton radiography for the first time to image a large metal volume (>10,000 mm3) during melting and solidification. We also show complementary x-ray results from a small volume (<1 mm3), bridging four orders of magnitude. Real-time imaging will enable efficient process development and the control of structure evolution to make materials with intended properties; it will also permit the development of experimentally informed, predictive structure and process models.

A Quantitative Investigation of Cementite Dissolution Kinetics for Continuous Heating of Hypereutectoid Steel

Cementite dissolution kinetics in austenite was investigated in a hypereutectoid steel alloy during continuous heating. The quantitative change in cementite volume fraction as a function of thermal history was determined from dilation curves by using the martensite start temperature to calculate prior austenite carbon content. Two characteristics of the cementite dissolution kinetics were found: (1) the cementite dissolution rate increased with time regardless of heating rate due to the increased surface area of cementite particles, and (2) the rate of cementite dissolution was strongly affected by heating rate. An empirical equation combining the effects of cementite volume change and heating rate is proposed to describe cementite dissolution kinetics. A continuous heating transformation diagram for hypereutectoid steels was obtained and compared with the DICTRA simulations and metallographic analyses.

The effect of distribution of second phase on dynamic damage

For ductile metals, dynamic fracture occurs principally through void nucleation, growth, and coalescence at heterogeneities in the microstructure. Previous experimental research on high purity metals has shown that microstructural features, such as grain boundaries, inclusions, vacancies, and heterogeneities, can act as initial void nucleation sites. In addition, other research on two-phase materials has also highlighted the importance of the properties of a second phase itself in determining the dynamic response of the overall material. However, previous research has not investigated the effects of the distribution of a second phase on damage nucleation and evolution. To approach this problem in a systematic manner, two copper alloys with 1% lead materials, with the same Pb concentration but different Pb distributions, have been investigated. A new CuPb alloy was cast with a more homogeneous distribution of Pb as compared to a CuPb where the Pb congregated in large “stringer” type configurations. These m...

Kinetics of Lamellar Decomposition Reactions in U-Nb Alloys

Discontinuous precipitation (DP) and discontinuous coarsening (DC) reactions have been observed in numerous alloy systems [1]. DP has been observed in the U-Nb system [2, 3, 4, 5]. The U-Nb phase diagram (Fig. 1) exhibits a continuous γ-BCC solid solution at high temperatures and a two-phase mixture of a-orthorhombic and γ-BCC below the 647°C monotectoid isotherm. The DP reaction occurs during continuous cooling and isothermal aging over 300-647°C. No metallographic evidence of a DC reaction in U-Nb has been published, although this is suggested from x-ray observations of distinct changes in the Nb content of the γ phase upon prolonged holding after the DP reaction [2, 3, 6]. This study will provide direct evidence for a DC reaction. Discontinuous and other aging reactions [7] are undesirable in U-Nb alloys, since they degrade corrosion resistance [5], ductility [8], and the shape-memory effect [9]. Hence, an improved understanding of the kinetics of these discontinuous phase transformations in U-Nb alloys is of practical interest.

Microcantilever bend testing and finite element simulations of HIP-ed interface-free bulk Al and Al–Al HIP bonded interfaces

Abstract We report on the strength of Al–Al interfaces and the effects of chemical segregation and interfacial void formation on bond strength using microcantilever bend testing. Interfaces are synthesised via hot isostatic pressing. Microcantilevers of several nominal dimensions were fabricated via focused ion beam and deformed in a nanoindenter. We find increased cantilever strength as a function of decreasing sample size, with a linear dependence of the yield strength on the inverse square root of the length scale characteristic to the cantilever cross-section. The presence of pores and chemical segregation decreases the yield strength of the material by 17% and the accommodated strain energy by 10–15% for strain values in the 6–12% range.

Mössbauer Spectroscopy and Transmission Electron Microscopy Analysis of Transition Carbides in Quenched and Partitioned Steel

Quenching and partitioning (Q&P) is a novel steel heat treatment that produces microstructures of martensite and retained austenite (Fig.1) [1]. Q&P consists of quenching to a temperature (QT) between the martensite start and finish temperatures, partitioning at a temperature the same or greater than the QT, followed by quenching to room temperature (RT). The goal of the heat treatment is to partition carbon (C) from martensite to austenite, thereby stabilizing the austenite prior to the final quench. Competing reactions such as transition carbide formation can reduce the extent of C partitioning, resulting in less retained austenite and mechanical property variations. The small volume fractions, carbide thicknesses below ~50 nm, and numerous overlapping peaks makes X-ray diffraction characterization of transition carbides challenging. In contrast, Mössbauer spectroscopy (MS) with correlative transmission electron microscopy (TEM) is better suited for identifying and quantifying carbides. Most MS studies on transition carbides have focused on quenched and tempered microstructures in binary Fe-C steels with high C, extensive amounts of carbides, and MS spectra primarily comprised of resonance from a limited number of unique Fe sites [2]. Q&P steels with lower C and carbide fractions, alloying additions of Manganese (Mn), Silicon (Si), and other elements, and significant amounts of retained austenite in the microstructures have more complex MS spectra, requiring more precise analysis methods.

LANL Experience Rolling Zr-Clad LEU-10Mo Foils for AFIP-7

The cleaning, canning, rolling and final trimming of Low Enriched Uranium-10 wt. pct. Molybdenum (LEU-10Mo) foils for ATR (Advanced Test Reactor) fuel plates to be used in the AFIP-7 (ATR Full Size Plate In Center Flux Trap Position) experiments are summarized. Six Zr-clad foils were produced from two LEU-10Mo castings supplied to Los Alamos National Laboratory (LANL) by Y-12 National Security Complex. Details of cleaning and canning procedures are provided. Hot- and cold-rolling results are presented, including rolling schedules, images of foils in-process, metallography and local compositions of regions of interest, and details of final foil dimensions and process yield. This report was compiled from the slides for the presentation of the same name given by Duncan Hammon on May 12, 2011 at the AFIP-7 Lessons Learned meeting in Salt Lake City, UT, with Los Alamos National Laboratory document number LA-UR 11-02898.