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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Redesigned Nylon 6 Variants with Enhanced Recyclability, Ductility, and Transparency

Geminal (gem−) disubstitution in heterocyclic monomers is an effective strategy to enhance polymer chemical recyclability by lowering their ceiling temperatures. However, the effects of specific substitution patterns on the monomer's reactivity and the resulting polymer's properties are largely unexplored. Here we show that, by systematically installing gem-dimethyl groups onto ϵ-caprolactam (monomer of nylon 6) from the α to ϵ positions, both the redesigned lactam monomer's reactivity and the resulting gem-nylon 6’s properties are highly sensitive to the substitution position, with the monomers ranging from non-polymerizable to polymerizable and the gem-nylon properties ranging from inferior to far superior to the parent nylon 6. Remarkably, the nylon 6 with the gem-dimethyls substituted at the γ position is amorphous and optically transparent, with a higher T g (by 30 °C), yield stress (by 1.5 MPa), ductility (by 3×), and lower depolymerization temperature (by 60 °C) than conventional nylon 6.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Ductile Tearing Resistance Indexing of Automotive Grade DP 590 Steel Sheets: EWF Testing Using DENT Specimens

The essential work of fracture (EWF) method has been explored for indexing the ductile tearing resistance of DP 590 automotive grade dual-phase steel sheet both in longitudinal (L-T) and transverse (T-L) orientations. The simplest possible test and analysis procedures have been adopted. The EWF method is found to be eminently suitable for routine quality control and product development purposes for such materials. Areas for further research for improving the experimental strategy are highlighted. For the investigated steel sheet, the estimated tearing resistance is found to be distinctly higher for the L-T orientation compared to the T-L orientation; the reason thereof merits further investigation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The Selection of Heat Treatment Parameters to Obtain Austempered Ductile Iron with the Required Impact Strength

The effect of austempering parameters on impact energy of non-alloyed cast iron with nodular graphite was examined. The analysis involved the austenitizing temperature in the range of 830-950 °C for the time of 60 min and the austempering temperature in the range of 300-400 °C for the time of 8, 16, 32 and 64 min. The value of the impact energy KV was determined. It has been found that the austenitizing temperature and austempering conditions affect the impact energy of ADI. The conducted analytical studies helped to develop a method for the selection of heat treatment parameters to obtain the ausferritic ductile iron with appropriate impact energy KV. The applied machine learning methods included support vector machine, decision trees, artificial neural networks and the method of variational component analysis.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Evaluation of the Role of Metallic Matrix on Environmentally Assisted Embrittlement of Austempered Ductile Iron (ADI) Using High Si Steel

The objective of this work stems from the environmentally assisted embrittlement suffered by austempered ductile iron (ADI) when its surface is submerged in water. This phenomenon leads to a drop in elongation and ultimate tensile strength of about 75% and 10%, respectively, while the 0.2% offset strain stress remains unaffected. Heat-treating a high Si steel, an ausferritic matrix similar to that present in the ADI under study is obtained, yielding giving a very good combination of tensile strength and elongation. In this manner, it was possible to examine the influence of the metallic matrix such as that present in ADI, though without nodules and cell boundaries, also called last to freeze zones (LTF). Tensile test evaluations on ADI and steel samples under dry and wet conditions were carried out. Additionally, a very specific test, introducing surface cracks, was conducted with the purpose of expanding the current knowledge on this particular kind of fracture. Earlier investigations on ADI have suggested that the process of embrittlement starts in a surface crack generated at the early stages of plastic deformation in LTF zones, which results from the solidification process, where pores, inclusions and unreacted austenite are present. In this manner, water penetrates and weakens the material strength, making the crack grow very fast and leading to an instantaneous samples failure. The main results obtained in this work include the confirmation of the deleterious effect of the LTF zones in the embrittlement process and the fracture surface analysis showing characteristic features of this kind of failure.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Ti Modification on Microstructures and Properties of Carbidic Austempered Ductile Iron

In this paper, we investigated the effect of titanium modification on microstructures and properties of carbidic austempered ductile iron (CADI) containing 3.72 wt.% C, 2.77 wt.% Si, and 0.51 wt.% Mn. The results showed that with the addition of 0.1 wt.% Ti, TiC particles were formed, and distributed in carbides and matrix. Microstructural observation indicated that the maximum diameter of graphite nodules decreased from 58 to 40 μm, and the shape of carbides transformed from network to fragmentation, which was related to the nucleation of the eutectic structure (γ + (Fe, Cr){sub 3}C) where TiC particles act as the nucleus. After isothermal quenching at 300 °C, the hardness of Ti-bearing CADI was 3.5 HRC higher than ordinary CADI, and the impact toughness was increased by 14.3%. Additionally, due to the decrease in exfoliation, the wear loss of Ti-bearing CADI was 14.8% lower than ordinary CADI during the block-on-ring wear test at the surface load of 300 N.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Influence of Austempering Temperature and Time on the Microstructure and Mechanical Properties of Ductile Iron Weldment Using Developed Coated Electrode

Sound welded joints of ductile iron were first made by developing coated electrodes as well as weld procedure. The weldments were then austenitized at 900 °C for 2-h holding time followed by austempering at 300 and 350 °C for three different holding times (1.5, 2 and 2.5 h) at each austempering temperature. The influence of austempering temperature and time on the microstructural and mechanical properties was studied. Microstructural characterization, phase analysis, microhardness and mechanical properties were performed to understand the effect of austempering heat treatment on microstructure, austempering kinetics and mechanical properties of welded joints. In spite of significant variation in microstructures among all the three zones of DI weldment before austempering, the response of both weld metal and HAZ to austempering heat treatment similar to a base metal was noted. However, a significant variation in matrix structures (bainitic ferrite, retained austenite) as well as graphite nodules among the three zones was observed with changing the austempering temperature and holding time. Also, 100% joint efficiency of the welded joints was achieved at both 300 and 350 °C for 2-h holding time. Hardness and charpy impact toughness mostly depended on the volume fraction of retained austenite and carbon content. Based on the results of tensile and charpy impact testing, 350 °C and 2-h holding time could be considered as an optimum austempering condition for ADI weldment.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Nickel Content on Microstructural Evolution in Austempered Solution-Strengthened Ferritic Ductile Cast Iron

The effects of Ni content and austempering temperature on microstructural evolution during austempering in solution-strengthened ferritic ductile cast iron with Ni contents between 0 and 1.1 mass% were investigated. The microstructure observation for the samples austempered at temperatures between 375 and 325 °C using an infrared gold image furnace was carried out with optical microscopy, x-ray diffraction measurements, and electron back-scattered diffraction pattern (EBSP) analysis. In addition, the Vickers hardness of both the austempered and as-quenched samples was measured under a load of 0.9807 N. According to the microstructure observations, the matrix of the samples austempered at temperatures between 375 and 325 °C exhibited finer acicular ferrite microstructure with increasing Ni content. Optical microscopy and EBSP analysis revealed that the amount of high-carbon austenite phase for the austempered samples increased with increasing Ni content and austempering temperature. Although samples with higher Ni content were found to contain more high-carbon austenite phase in the matrix, the Vickers hardness of the samples tended to increase with increasing Ni content at the same austempering temperature. The reason for this may be the higher carbon content in the matrix of the samples with higher Ni contents.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Tailoring the Properties of Calcium Aluminate Macro-Defect-Free Cements: From Brittle to Ductile Behavior

Cementitious macro-defect-free (MDF) materials can be obtained processing a mixture of a water-soluble polymer, namely polyvinyl alcohol (PVA) and the calcium aluminate cement. Based on process techniques typical of plastics, the MDF products can be obtained in complex forms and, thanks to the reduced water to cement ratio, w/c, their strength is very high compared to standard cements. However, these materials show both the sensitivity typical of water-soluble polymers, presenting a low glass transition temperature (T{sub g}), and the brittle behavior typical of ceramic materials and therefore low toughness. The aim of this work is to modify the mechanical properties of MDF cements by tuning their unacceptable brittle behavior. Water solution of carboxylate styrene butadiene rubber is used as reactive additives, and different mixture formulations are tested changing the rubber-like content both with respect to the fixed water content and PVA amounts. It is shown that the modifications of the organic phase imply small or negligible effects on paste formation of different recipes as detected by the torque–time responses. From flexural and fracture toughness tests, a wide range of responses was observed, and depending on the particular recipe, the mechanical behavior can be tuned from a brittle to a ductile behavior.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Lattice distortion in a strong and ductile refractory high-entropy alloy

Here, the maximization of the mixing entropy with the optimal range of enthalpy in high-entropy alloys (HEAs) can promote the formation of a stable single solid-solution phase with the absence of competing intermetallic compounds. The resultant effects, such as lattice distortion, can contribute to excellent mechanical properties, which has motivated numerous efforts to develop and design single-phase HEAs. However, challenges still remain, particularly on quantifying the lattice distortion and relating it to materials properties. In this study, we have developed a NbTaTiV refractory HEA with a single body-centered-cubic (BCC) structure using an integrated experimental and theoretical approach. The theoretical efforts include thermodynamic modeling, i.e., CALculation of PHAse Diagram (CALPHAD). The microstructural evolutions have been investigated by systematic heat-treatment processes. The typical dendrite microstructure was observed, which is caused by the elemental segregation during the solidification in the as-cast condition. The structural inhomogeneity and chemical segregation were completely eliminated by the proper homogenization treatment at 1200 °C for 3 days. The homogeneous elemental distribution was quantitatively verified by the Atom Probe Tomography (APT) technique. Importantly, results indicate that this HEA exhibits the high yield strength and ductility at both room and high temperatures (up to 900 °C). Furthermore, the effects of the high mixing entropy on the mechanical properties are discussed and quantified in terms of lattice distortions and interatomic interactions of the NbTaTiV HEA via first-principles calculations. It is found that the local severe lattice distortions are induced, due to the atomic interactions and atomic-size mismatch in the homogenization-treated NbTaTiV refractory HEA.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and residual elastic strain at graphite nodules in ductile cast iron analyzed by synchrotron X-ray microdiffraction

The microstructure and residual elastic strain at graphite nodules (GNs) in ductile cast iron produced using either a fast or slow cooling rate have been characterized using synchrotron 3D X-ray Laue microdiffraction. The results demonstrate that thermal stress is introduced during cooling and that part of this stress is relaxed by plastic deformation of the polycrystalline ferrite matrix. It is observed that the plastic deformation is accommodated by the formation of dislocations and dislocation boundaries, which are organized in a cell structure. The dislocation density quantified based on the microstructure is most pronounced at the GN/matrix interface around small GNs in the fast cooled sample. Residual elastic strain is also present, which is mainly compressive with a maximum of 6.0–9.9 × 10 -4 near the GNs. Gradients of plastic deformation and elastic strain field around the GNs are observed. The results document for the first time that both the elastic strain field and the plastic strain field averaged over the grains around the GNs is approximately scaling with GN size and not affected by the cooling rate. The experimental data are compared with simulations by a finite element method, and agreement and disagreement are considered in detail.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Multimodal 3D characterization of voids in shock-loaded tantalum: Implications for ductile spallation mechanisms

Predicting the failure of crystalline materials at high strain rates requires knowledge of the underlying failure mechanisms and their dependence on microstructure. However, little experimental consensus exists on the underlying micromechanics of spallation. In this study, a 3D-EBSD characterization experiment is performed on tantalum prior to and after partial spallation by plate impact, which allowed for the statistical assessment of the microstructural neighborhoods surrounding incipient voids. In analyzing the resulting dataset containing 5884 grains and 467 voids, it is observed that the voids were roughly spherical and consistent in size throughout the spalled material. The voids mostly resided at quadruple points, at triple junctions, at grain boundaries, and within grains, in decreasing order of prevalence. Moreover, voids tended to form at grain boundaries with high degrees of plastic incompatibility, growing into the plastically soft grain but orienting primarily with or perpendicular to the loading direction. Here, the statistics from these analyses of 3D microstructural data support dynamic cavitation models for ductile spallation.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Multi-objective materials bayesian optimization with active learning of design constraints: Design of ductile refractory multi-principal-element alloys

We report Bayesian Optimization (BO) has emerged as a powerful framework to efficiently explore and exploit materials design spaces. To date, most BO approaches to materials design have focused on the materials discovery problem as if it were a single expensive-to-query ‘black box’ in which the target is to optimize a single objective (i.e., material property or performance metric). Also, such approaches tend to be constraint agnostic. Here, we present a novel multi-information BO framework capable of actively learning materials design as a multiple objectives and constraints problem. We demonstrate this framework by optimally exploring a Refractory Multi-Principal-Element Alloy (MPEA) space, here specifically, the system Mo-Nb-Ti-V-W. The MPEAs are explored to optimize two density-functional theory (DFT) derived ductility indicators (Pugh’s Ratio and Cauchy pressure) while learning design constraints relevant to the manufacturing of high-temperature gas-turbine components. Alloys in the BO Pareto-front are analyzed using DFT to gain an insight into fundamental atomic and electronic underpinning for their superior performance, as evaluated within this framework.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Machining-inducted residual strain and ductile-brittle transition in sapphire revealed by X-ray nanoprobe diffraction microscopy

Hard X-ray nanoprobe diffraction microscopy is employed to investigate machining-induced residual lattice strain in single-crystal sapphire following ultra-precision orthogonal plunge cutting. In this study, tilt-series and single-angle acquisition strategies are systematically compared to evaluate their capability for resolving lattice variations associated with the machining process. An analytical framework is developed to enable the efficient and reliable extraction of strains, which serve as strain-based indicators of changes in the dominant deformation mechanism. The measured high-dimensional strain exhibits spatial heterogeneity and systematic evolution along the cutting path, consistent with deformation-regime changes from ductile response to crack-prone behavior.

Liang, Rui↗
DOE OSTI2018

A computationally efficient ductile damage model accounting for nucleation and micro-inertia at high triaxialities

The computational formulation of a micro-mechanical material model for the dynamic failure of ductile metals is presented in this paper. The statistical nature of porosity initiation is accounted for by introducing an arbitrary probability density function which describes the pores nucleation pressures. Each micropore within the representative volume element is modeled as a thick spherical shell made of plastically incompressible material. The treatment of porosity by a distribution of thick-walled spheres also allows for the inclusion of micro-inertia effects under conditions of shock and dynamic loading. The second order ordinary differential equation governing the microscopic porosity evolution is solved with a robust implicit procedure. A new Chebyshev collocation method is employed to approximate the porosity distribution and remapping is used to optimize memory usage. The adaptive approximation of the porosity distribution leads to a reduction of computational time and memory usage of up to two orders of magnitude. Moreover, the proposed model affords consistent performance: changing the nucleation pressure probability density function and/or the applied strain rate does not reduce accuracy or computational efficiency of the material model. The numerical performance of the model and algorithms presented is tested against three problems for high density tantalum: single void, one-dimensional uniaxial strain, and two-dimensional plate impact. Here, the results using the integration and algorithmic advances suggest a significant improvement in computational efficiency and accuracy over previous treatments for dynamic loading conditions.

42 ENGINEERING↗
DOE OSTI2017

A dislocation-based crystal plasticity framework for dynamic ductile failure of single crystals

We developed a framework for dislocation-based viscoplasticity and dynamic ductile failure to model high strain rate deformation and damage in single crystals. The rate-dependence of the crystal plasticity formulation is based on the physics of relativistic dislocation kinetics suited for extremely high strain rates. The damage evolution is based on the dynamics of void growth, which are governed by both micro-inertia as well as dislocation kinetics and dislocation substructure evolution. Furthermore, an averaging scheme is proposed in order to approximate the evolution of the dislocation substructure in both the macroscale as well as its spatial distribution at the microscale. In addition, a concept of a single equivalent dislocation density that effectively captures the collective influence of dislocation density on all active slip systems is proposed here. Together, these concepts and approximations enable the use of semi-analytic solutions for void growth dynamics developed in [J. Wilkerson and K. Ramesh. A dynamic void growth model governed by dislocation kinetics. J. Mech. Phys. Solids, 70:262–280, 2014.], which greatly reduce the computational overhead that would otherwise be required. The resulting homogenized framework has been implemented into a commercially available finite element package, and a validation study against a suite of direct numerical simulations was carried out.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Strength-ductility synergy through microstructural and compositional heterogeneity in directed energy deposition additive manufacturing of face-centered cubic materials

Directed energy deposition (DED) is an additive manufacturing (AM) process based on welding technology and offers the advantages of large build volume, high deposition rate, and ability to fabricate multi-material parts. Epitaxial continuous columnar grain growth is a characteristic microstructural feature of DED processed alloys. In this study, a bamboo-like microstructure (periodic alternation of equiaxed and columnar structure) was produced by adopting an intermittent deposition strategy in 316L stainless steel and Inconel 625. The formation of a bamboo-like alternating microstructure was confirmed through electron backscattered diffraction (EBSD) analysis. Hardness mapping showed that the columnar to equiaxed transition (CET) occurred at the region right below the fusion line. A finite element (FE) model was used to investigate the relationship between the temperature gradient (G) and the solidification rate (R). The FE model showed a low G/R ratio at the region right below the interface promoting the CET. The grain size and material-dependent deformation behaviors are analyzed using digital image correlation (DIC). The lower deformation on the fine-grain regions observed in DIC analysis is attributed to a higher strain hardening rate, which is confirmed through dislocation density analysis on a tensile-interrupted specimen. The periodically alternating grain size coupled with the microstructural changes caused by intermittent deposition strategy result in a better strength-ductility synergy in both single-material and bimetallic specimens.

36 MATERIALS SCIENCE↗
DOE OSTI2013

In-situ measurement of phase transformation kinetics in austempered ductile iron

Austempered ductile iron (ADI) alloyed with 0.42% Mn and 0.72% Cu was heat treated in a mirror furnace and the phase transitions were studied in-situ by neutron diffraction. The heat treatment consisted of austenitisation at 920 °C and isothermal austempering at 400 °C, 350 °C and 300 °C, respectively. Due to the growth of ferrite platelets, the austenite content decreases rapidly at all temperatures within the first 15–20 min and reaches a stable plateau after 35 min (400 °C) to 80 min (300 °C). The carbon content of the residual austenite, which was monitored and characterised by the change of the lattice parameter, increases up to 1.6 wt.% caused by redistribution from the newly formed ferrite. While at higher austempering temperatures this takes place almost parallel to the phase transformation, at 300 °C the redistribution of carbon to austenite lags behind considerably. Furthermore the neutron data revealed an austenite peak asymmetry during austempering which is attributed to successive phase transformation. It results temporarily in two fractions of austenite, an initial low-carbon and an enriched high-carbon modification. - Highlights: • The heat treatment of ADI was studied in detail by in-situ neutron diffraction. • The phase fractions were monitored and evaluated quantitatively. • The austenite carbon content increased up to 1.6 wt.% during austempering. • Peak asymmetries indicate two austenite fractions during highest transformation rates.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Suppressing heating rate-dependent martensitic stabilization in ductile Cu-Al-Mn shape memory alloys by Ni addition: An experimental and first-principles study

Highlights: • Additional nearest neighbor bond between Ni and Al atoms was produced theoretically in the Cu-Al-Mn-Ni (Ni≤2 at.%) alloys. • Shape memory effect of Cu-Al-Mn alloys heated at 20 K/min was improved from 85% to 92% after the addition of 2 at.% Ni. • Addition of 2 at.% Ni can suppress heating rate-dependent martensitic stabilization in Cu-Al-Mn alloys heated at 0.01 K/min. - Abstract: The present paper aims to suppress the heating rate-dependent martensitic stabilization of ductile Cu-Al-Mn shape memory alloys through restricting the mobility and thus the behavior of vacancies. A combined study based on first-principles calculation and experimental results were presented to unveil the effect of Ni addition into ternary Cu-Al-Mn alloys on the bonding force between atoms. The results showed that additional nearest neighbor bond between Ni and Al atoms in quaternary Cu-Al-Mn-Ni alloys can be produced. The addition of Ni (≤2 at.%) into ternary Cu-Al-Mn alloys can not only efficiently increase the shape memory effect, but also suppress the occurrence of heating rate-dependent martensitic stabilization. The addition of 2 at.% Ni improved the shape memory effect of ternary Cu-Al-Mn alloys from 85% to 92% when heated at 20 k/min. The shape memory effect of ternary Cu-Al-Mn alloys dropped to 10% as the heating rate decreased to 1 K/min, but it remain at 92% even when the heating rate was sufficiently lowered to 0.01 K/min after the addition of 2 at.% Ni.

36 MATERIALS SCIENCE↗