Amy J. Clarke

Critical Assessment 7: Quenching and partitioning

Abstract Quenching and partitioning is a relatively new heat treatment concept to generate microstructures containing retained austenite stabilised by carbon partitioning from martensite. Research on quench and partitioning has been conducted by numerous groups, and this critical assessment provides some of the authors’ perspectives on progress and understanding in the field, with particular focus on the physical metallurgy and transformation mechanisms, process variations, mechanical behaviour, and industrial implementation. While much progress has been made, the field provides rich opportunity for further understanding and development.

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.

Three-dimensional Dendritic Needle Network model with application to Al-Cu directional solidification experiments

We present a three-dimensional (3D) extension of a previously proposed multi-scale Dendritic Needle Network (DNN) approach for the growth of complex dendritic microstructures. Using a new formulation of the DNN dynamics equations for dendritic paraboloid-branches of a given thickness, one can directly extend the DNN approach to 3D modeling. We validate this new formulation against known scaling laws and analytical solutions that describe the early transient and steady-state growth regimes, respectively. Finally, we compare the predictions of the model to in situ X-ray imaging of Al-Cu alloy solidification experiments. The comparison shows a very good quantitative agreement between 3D simulations and thin sample experiments. It also highlights the importance of full 3D modeling to accurately predict the primary dendrite arm spacing that is significantly over-estimated by 2D simulations.

From Solidification Processing to Microstructure to Mechanical Properties: A Multi-scale X-ray Study of an Al-Cu Alloy Sample

We follow an Al-12 at. pct Cu alloy sample from the liquid state to mechanical failure, using in situ X-ray radiography during directional solidification and tensile testing, as well as three-dimensional computed tomography of the microstructure before and after mechanical testing. The solidification processing stage is simulated with a multi-scale dendritic needle network model, and the micromechanical behavior of the solidified microstructure is simulated using voxelized tomography data and an elasto-viscoplastic fast Fourier transform model. This study demonstrates the feasibility of direct in situ monitoring of a metal alloy microstructure from the liquid processing stage up to its mechanical failure, supported by quantitative simulations of microstructure formation and its mechanical behavior.

Unraveling the Age Hardening Response in U-Nb Alloys

Complicating factors that have stymied understanding of uranium-niobium’s aging response are briefly reviewed, including (1) niobium inhomogeneity, (2) machining damage effects on tensile properties, (3) early-time transients of ductility increase, and (4) the variety of phase transformations. A simple Logistic-Arrhenius model was applied to predict yield and ultimate tensile strengths and tensile elongation of U-4Nb as a function of thermal age. Fits to each model yielded an apparent activation energy that was compared with phase transformation mechanisms.

X-ray microscopy for in situ characterization of 3D nanostructural evolution in the laboratory

X-ray microscopy (XRM) has emerged as a powerful technique that reveals 3D images and quantitative information of interior structures. XRM executed both in the laboratory and at the synchrotron have demonstrated critical analysis and materials characterization on meso-, micro-, and nanoscales, with spatial resolution down to 50 nm in laboratory systems. The non-destructive nature of X-rays has made the technique widely appealing, with potential for “4D” characterization, delivering 3D micro- and nanostructural information on the same sample as a function of sequential processing or experimental conditions. Understanding volumetric and nanostructural changes, such as solid deformation, pore evolution, and crack propagation are fundamental to understanding how materials form, deform, and perform. We will present recent instrumentation developments in laboratory based XRM including a novel in situ nanomechanical testing stage. These developments bridge the gap between existing in situ stages for micro scale XRM, and SEM/TEM techniques that offer nanometer resolution but are limited to analysis of surfaces or extremely thin samples whose behavior is strongly influenced by surface effects. Several applications will be presented including 3D-characterization and in situ mechanical testing of polymers, metal alloys, composites and biomaterials. They span multiple length scales from the micro- to the nanoscale and different mechanical testing modes such as compression, indentation and tension.

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.

Update on LANL DU U-10Mo Casting and on Characterization of Y-12 LEU Casting

This report presents an overview of LANL casting procedure and characterization of recent DU U-10 Mo castings and characterizes recent LEU U-10Mo casting from Y-12.

From Alloy Processing to Performance: An In Situ Experimental and Modeling Effort

Solidification is present in almost all materials. It is influenced by grain size and shape, chemical homogeneity, defect type and density, and mechanical properties. During micro-mechanical testing, the following occur: 1) Micro-CT (as processed) - Map Initial 3D Microstructure 2) Nano-Radiography (In situ under Tension) - Observe of Damage Initiation/Propagation 3) Micro-CT (Post Mortem) - Global Fracture Study 4) Nano-CT (Post Mortem) - High-Resolution Fracture Study.

Imaging the Rapid Solidification of Metallic Alloys in the TEM

The macroscopic properties of a metal solidified from a liquid melt are strongly dependent on the final microstructure, which in turn is the result of the solidification conditions. With the growing popularity of laser-based additive manufacturing (AM), there is an increasing need to understand the microstructures that result from rapid solidification processes. Rapidly solidified alloy microstructures are typically far from equilibrium and therefore traditional thermodynamic approaches used to predict structure and composition (i.e., phase diagrams) must be extended to describe these deviations from equilibrium and ensuing metastable states. This work highlights progress toward corroborating predictive (phase-field) modeling capabilities [1] with in situ experimental observations [2] in order to better understand the non-equilibrium structures produced during rapid solidification following laser melting.