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At least 1,495 records · Page 83
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Microstructure and Mechanical Properties of Laves Phase-strengthened Fe-Cr-Zr Alloys

Laves phase-reinforced alloys have shown some preliminary promising performance at room temperatures. This paper aims at evaluating mechanical properties of Laves phase-strengthened alloys at elevated temperatures. Three Fe-Cr-Zr alloys were designed to favor the formation of eutectic microstructures containing Laves and body-centered cubic phases with the aid of thermodynamic calculations. Microstructural characterization was carried out on the alloys in as-processed and aged states using optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. The effect of thermal aging and alloy composition on microstructure has been discussed based on microstructural characterization results. Mechanical properties have been evaluated by means of Vickers microhardness measurements, tensile testing at temperatures up to 973.15 K (700.15 °C), and creep testing at 873.15 K (600.15 °C) and 260 MPa. Alloys close to the eutectic composition show significantly superior strength and creep resistance compared to P92. Finally, however, their low tensile ductility may limit their applications at relatively low temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Comparative evaluation of cast aluminum alloys for automotive cylinder heads: Part II: Mechanical and thermal properties

The first part of this study documented the as-aged microstructure of five cast aluminum alloys namely, 206, 319, 356, A356, and A356+0.5Cu, that are used for manufacturing automotive cylinder heads (Roy et al. in Metall Mater Trans A, 2016). In the present part, we report the mechanical response of these alloys after they have been subjected to various levels of thermal exposure. In addition, the thermophysical properties of these alloys are also reported over a wide temperature range. The hardness variation due to extended thermal exposure is related to the evolution of the nano-scale strengthening precipitates for different alloy systems (Al-Cu, Al-Si-Cu, and Al-Si). The effect of strengthening precipitates (size and number density) on the mechanical response is most obvious in the as-aged condition, which is quantitatively demonstrated by implementing a strength model. Significant coarsening of precipitates from long-term heat treatment removes the strengthening efficiency of the nano-scale precipitates for all these alloys systems. Thermal conductivity of the alloys evolve in an inverse manner with precipitate coarsening compared to the strength, and the implications of the same for the durability of cylinder heads are noted.

36 MATERIALS SCIENCE↗
DOE OSTI2024

A General Model for the Ductility of Intermetallics Applied to Fe-Co Alloys

The mechanical properties, specifically ductility, of high performing soft magnets such as Fe-Co alloys are a limiting factor to their broader use in a number of systems. The understanding of the mechanical robustness in these materials is currently insufficient to be able to support the growing interest in applications such as magnetic shielding or electric motors. Fe-Co alloys provide the highest commercially available magnetic saturation and high magnetic permeability but have very poor ductility. The addition of vanadium to these alloys has allowed for significant commercialization and some ductility improvements, but the fundamental reasons for the observed improvements are not well understood. In most published work on mechanical properties in these alloys, the precise chemistry of the alloys investigated, often a critical aspect of intermetallics, is not reported or controlled and thermal history is unclear. This work creates a ductility model that is sensitive to changes in chemistry and can predict relative strain to failure as well as brittle fracture mode for intermetallics and is applied to Fe-Co alloys. Through the application of density functional theory (DFT), this model identifies defect stabilities, anti-phase boundary (APB) energies, the energy necessary to cross-slip, and cleavage energies and combines them through energetic competition to determine a relative failure strain. The model correctly predicts ductility improvements with the addition of vanadium as well as the transition from intergranular to transgranular cleavage, though more precise experiments are necessary to appropriately validate the various improvements observed.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Full-Strength Additive Manufacturing of Pure Aluminum Alloy 7075 Using Liquid Metal Jet Printing

Additive manufacturing (AM) of high-strength aluminum alloys can produce high-performance aerospace and biomedical parts with unique geometries and functions. However, many high-performance alloy systems such as AA 7075 pose challenges during AM due to thermal cycles that cause hot cracking, limiting AM to casting-friendly alloys. Stock AA 7075 alloy poses additional challenges due to the excessive growth of insoluble precipitates during casting and needs to be thermomechanically processed to obtain acceptable mechanical properties. Here, this work uses liquid metal jet printing to produce high-resolution pure AA 7075 parts at high strength (593 MPa) and ductility (10 pct), surpassing the specific strength of any type of steel. The parts are printed by depositing fine droplets on a high-temperature substrate that reduces cooling rates, preventing hot cracking. The results show that fully dense, crack-free parts can only be printed at substrate temperatures of 500 °C with a cooling rate threshold of < 2000 K/s to prevent cracking and < 500 K/s to eliminate porosity. The tensile strength, microhardness, and ductility of the parts matched or were beyond the wrought alloy specifications after heat treatment, possibly due to high solidification rates that produce finer insoluble precipitates. Our results indicate that many challenging high-performance alloy systems can now be 3D printed based on the critical thresholds for cooling rates using any AM method that can satisfy these conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2018

A Low-Cost Lightweight Entropic Alloy with High Strength

In this study, a series of lightweight Al–Mg system entropic alloys containing Zn, Cu, and Si were designed based on the order/disorder or entropy, and eutectic concepts. The alloys of Al{sub 58.5}Mg{sub 31.5}Zn{sub 4.5}Cu{sub 4.5}Si{sub 1}; Al{sub 63}Mg{sub 27}Zn{sub 4.5}Cu{sub 4.5}Si{sub 1}; Al{sub 66.7}Mg{sub 23.3}Zn{sub 4.5}Cu{sub 4.5}Si{sub 1}; Al{sub 80}Mg{sub 14}Zn{sub 2.7}Cu{sub 2.7}Si{sub 0.6}; Al{sub 85}Mg{sub 10.5}Zn{sub 2.025}Cu{sub 2.025}Si{sub 0.45}; and Al{sub 90}Mg{sub 7}Zn{sub 1.35}Cu{sub 1.35}Si{sub 0.3} were prepared by induction melting under a high-purity argon atmosphere and then casted into stainless steel molds. The microstructures which were tested in the as-cast state exhibited multiphases and contained apparent volume fractions of intermetallic compounds and solid solutions. Then, the compressive mechanical properties of the alloys were measured, and high fracture strengths of 577, 677, 590, 498, 814, and 794 MPa, respectively, were determined. Strong hardening phenomena were observed in the Al{sub 80}Mg{sub 14}Zn{sub 2.7}Cu{sub 2.7}Si{sub 0.6}, Al{sub 85}Mg{sub 10.5}Zn{sub 2.025}Cu{sub 2.025}Si{sub 0.45}, and Al{sub 90}Mg{sub 7}Zn{sub 1.35}Cu{sub 1.35}Si{sub 0.3} alloys at room temperature, with amazing plasticity percentages of 13.8, 24.8, and 32.7%, respectively. The property differences in the lightweight alloys were analyzed using the following parameters: the critical values of the enthalpy of mixing; atomic size differences; ratio of entropy to enthalpy; valence electron concentration (VEC); and Pauling electronegativity difference. Finally, three of the aforementioned parameters (atomic size difference, enthalpy of mixing, and Pauling electronegativity difference) were regarded in this study as the crucial rules for the lightweight multicomponent alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructural Evolution and Properties of 24CrNiMoY Alloy Steel Fabricated by Selective Laser Melting

24CrNiMoY alloy steel samples were fabricated by selective laser melting (SLM), and a phase transformation model was established to study the alloy steel microstructural evolution. Meanwhile, microhardness and tensile properties of 24CrNiMoY alloy steel prepared by different laser energy densities (E{sub v}) were investigated. Results indicate that the microstructural evolution of 24CrNiMoY alloy steel is consistent with the phase transformation model. The main microstructure changed from martensite to bainite with the increase in thermal cycle numbers. In addition, a suitable E{sub v} plays an important role in refining the bainite structure and improving the alloy steel properties. When the E{sub v} decreases from 210 to 140 J/mm{sup 3}, the bainite lath width reduces from 1.7 to 0.6 μm. Simultaneously, the relative density, tensile strength and microhardness of the fabricated samples increase first and decrease later. 24CrNiMoY alloy steel sample prepared by 160 J/mm{sup 3} has fine mechanical properties: The tensile strength is 850 MPa and microhardness is 360 HV{sub 0.2}.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Aqueous Corrosion Behavior of Cast CoCrFeMnNi Alloy

CoCrFeMnNi high-entropy alloy demonstrates good mechanical properties and can be further strengthened by thermomechanical processes. A sample alloy prepared by vacuum casting in a graphite mold was further investigated for its corrosion resistance. One-millimeter thin strips of cast alloy were used to investigate its corrosion behavior at room temperature. Linear potentiodynamic tests of the alloy in sodium chloride solution confirmed uniform corrosion property comparable to stainless steel AISI 304. However, a positive cyclic hysteresis loop indicated susceptibility to pitting corrosion. In sodium hydroxide solution, metastable pit formation was observed, while sulfuric acid solution passivated the alloy. In both cases, the alloy did not show any spontaneous pitting tendency.

36 MATERIALS SCIENCE↗
DOE OSTI2019

On the Microstructure and Solidification Behavior of N-Bearing Ti-46Al-8Ta (at.%) Intermetallic Alloys

Ti-46Al-8Ta (at.%) intermetallic alloys exhibit acceptable strength at low and high temperatures with good oxidation resistance. Different alloying elements are usually added to TiAl-based alloys to develop required properties. In this work, effects of N addition up to 2 at.% on microstructural features and solidification behavior of the Ti-46Al-8Ta alloy is investigated. The achieved results showed that alloying with N causes dendrite refinement and reduction of the retained metastable B2 phase in the as-cast microstructure. The dendrite morphology is also changed from fourfold to fivefold and then sixfold symmetry by increasing N content. The effect of N on interlamellar spacing, amount of constituent phases, and lattice parameters was not uniform. Increasing N content resulted in an increase in both small punch maximum load and hardness up to 1.05 at.%, whereas further N alloying, despite increasing of hardness, led to a drop in the maximum load.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Temperature-Dependent Configurational Entropy Calculations for Refractory High-Entropy Alloys

The cluster expansion formalism for alloys is used to construct surrogate models for three refractory high-entropy alloys (NbTiVZr, HfNbTaTiZr, and AlHfNbTaTiZr). These cluster expansion models are then used along with Monte Carlo methods and thermodynamic integration to calculate the configurational entropy of these refractory high-entropy alloys as a function of temperature. Many solid solution alloy design guidelines are based on the ideal entropy of mixing, which increases monotonically with N , the number of elements in the alloy. However, our results show that at low temperatures, the configurational entropy of these materials is largely independent of N , and the assumption described above only holds in the high-temperature limit. This suggests that alloy design guidelines based on the ideal entropy of mixing require further examination.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Time-Resolved In Situ Measurements During Rapid Alloy Solidification: Experimental Insight for Additive Manufacturing

In research and industrial environments, additive manufacturing (AM) of metals and alloys is becoming a pervasive technology, though significant challenges remain before widespread implementation of AM can be realized. In situ investigations of rapid alloy solidification with high spatial and temporal resolutions can provide unique experimental insight into microstructure evolution and kinetics that are relevant for AM processing. Hypoeutectic thin-film Al–Cu and Al–Si alloys were investigated using dynamic transmission electron microscopy to monitor pulsed-laser-induced rapid solidification across microsecond timescales. Solid–liquid interface velocities measured from time-resolved images revealed accelerating solidification fronts in both alloys. We observed microstructure evolution, solidification product, and presence of a morphological instability at the solid–liquid interface in the Al–4 at.%Cu alloy are related to the measured interface velocities and small differences in composition that affect the thermophysical properties of the alloys. These time-resolved in situ measurements can inform and validate predictive modeling efforts for AM.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Insights into Defect-Mediated Nucleation of the Equilibrium B2 Phase in Face-Centered Cubic High-Entropy Alloys

A fine scale second phase distribution is important for strengthening metallic materials. High precipitate density is often difficult to achieve due to sluggish precipitation kinetics and limited nucleation sites. Based on the CALPHAD predictions, transition element-based complex concentrated alloys (CCAs)/ High Entropy Alloys (HEAs), have a highly stable face-centered cubic (FCC) phase and intermetallic B2 phase. The precipitation of the strengthening B2 phase is often limited to low fractions in these alloys due to a high nucleation barrier in the FCC matrix. Here, by illustrating in three different alloys, we present a defect mediated nucleation of a high fraction equilibrium B2 phase. The degree of cold work is used to manipulate the phase fraction, morphology, and distribution of B2 precipitates. Our approach synergistically improves the strength and ductility of these alloys and is widely applicable to many other complex alloys.

Sharma, Abhishek↗
DOE OSTI2017

A combined APT and SANS investigation of α' phase precipitation in neutron-irradiated model FeCrAl alloys

Here, FeCrAl alloys are currently under consideration for accident-tolerant fuel cladding applications in light water reactors owing to their superior high-temperature oxidation and corrosion resistance compared to the Zr-based alloys currently employed. However, their performance could be limited by precipitation of a Cr-rich α' phase that tends to embrittle high-Cr ferritic Fe-based alloys. In this study, four FeCrAl model alloys with 10–18 at.% Cr and 5.8–9.3 at.% Al were neutron-irradiated to nominal damage doses up to 7.0 displacements per atom at a target temperature of 320 °C. Small angle neutron scattering techniques were coupled with atom probe tomography to assess the composition and morphology of the resulting α' precipitates. It was demonstrated that Al additions partially destabilize the α' phase, generally resulting in precipitates with lower Cr contents when compared with binary Fe-Cr systems. The precipitate morphology evolution with dose exhibited a transient coarsening regime akin to previously observed behavior in aged Fe-Cr alloys. Similar behavior to predictions of the LSW/UOKV models suggests that α' precipitation in irradiated FeCrAl is a diffusion-limited process with coarsening mechanisms similar to those in thermally aged high-Cr ferritic alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Design of a V–Ti–Ni alloy with superelastic nano-precipitates

Stress-induced martensitic transformations enable metastable alloys to exhibit enhanced strain hardening capacity, leading to improved formability and toughness. As is well-known from transformation-induced plasticity (TRIP) steels, however, the resulting martensite can limit ductility and fatigue life due to its intrinsic brittleness. In this work, we explore an alloy design strategy that utilizes stress-induced martensitic transformations but does not retain the martensite phase. This strategy is based on the introduction of superelastic nano-precipitates, which exhibit reverse transformation after initial stress-induced forward transformation. To this end, utilizing ab-initio simulations and thermodynamic calculations we designed and produced a V 45 Ti 30 Ni 25 (at%) alloy. In this alloy, TiNi is present as nano-precipitates uniformly distributed within a ductile V-rich base-centered cubic (bcc) β matrix, as well as being present as a larger matrix phase. We characterized the microstructure of the produced alloy using various scanning electron microscopy (SEM) and transmission electron microscopy (TEM) methods. Furthermore, the bulk mechanical properties of the alloy are demonstrated through tensile tests, and the reversible transformation in each of the TiNi morphologies were confirmed by in-situ TEM micro-pillar compression experiments, in-situ high-energy diffraction synchrotron cyclic tensile tests, indentation experiments, and differential scanning calorimetry experiments. The observed transformation pathways and variables impacting phase stability are critically discussed.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

On the origin of internal obstacles to dislocation glide in single-phase NiFe random alloys

Over the past decade, a new class of unconventional alloys, where several constitutive elements are presented in close to equal concentrations, is considered as promising structural materials in many applications. Properties and behavior of many concentrated solid solution alloys (CSSAs) under different conditions, including mechanical loading, differs from that of conventional alloys and has been the subject of intensive studies by different techniques. A key feature of these materials is that there is no clear distinction between solute and solvent species of atoms, which is the conventional approach to considering dislocation motion in alloys. To understand this regime, we have recently reported on the glide of a screw dislocation glide in a simple equiatomic NiFe alloy and reported the two-mode character. At low applied stress, the dislocation motion is similar to a glide through a field of obstacles. In this work, we report the results of a detailed study on the nature of these internal obstacles. We demonstrate that these are not conventional localized “obstacles” but is the result of significantly different energies of instantaneous dislocation configurations over the alloy bulk. The strong pinning positions are associated with composition changes in the partial dislocation cores, while observing no significant composition changes along the stacking faults. Local energies can vary significantly, as demonstrated using simulations of short-periodicity dislocation lines. Therefore, the critical stress to move a dislocation under such conditions is not a constant Peierls stress but represents a wide spectrum of barriers between different non-straight valleys that the dislocation line accommodates in the bulk at particular stress/strain conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effect of substitutional doping and disorder on the phase stability, magnetism, and half-metallicity of Heusler alloys

Spintronics is the fast growing field that will play a key role in optimizing power consumption, memory, and processing capabilities of nanoelectronic devices. Heusler alloys are potential candidates for application in spintronics due to their room temperature (RT) half-metallicity, high Curie temperature, low lattice mismatch with most substrates, and strong control on electronic density of states at Fermi level. In this work, we investigate the effect of substitutional doping and disorder on the half-metallicity, phase stability, and magnetism of Heusler alloys using density functional theory methods. Our study shows that electronic and magnetic properties of half/full-Heusler alloys can be tuned by changing electron-count through controlled variation of chemical compositions of alloying elements. We provide a detailed discussion on the effect of substitutional doping and disorder on the tunability of half-metallic nature of Co 2 MnX and NiMnX based Heusler alloys, where X represents group 13–16 and period 3–6 elements of the periodic table. Based on the idea of electron count and disorder, we predicted a possible existence of thermodynamically stable half-metallic multicomponent bismuthides, for example, (CuNi 3 )Mn 4 Bi 4 and (ZnNi 7 )Mn 8 Bi 8 , through substitution doping at Ni site by specific Cu and Zn composition in half-Heusler NiMnBi. We believe that the design guide based on electron-counts presented for half-metals will play a key role in electronic-structure engineering of novel Heusler alloys for spintronic application, which will accelerate the development and synthesis of novel materials.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Hydrogen and water interactions with CrMnFeCoNi alloy from density functional theory calculations

High entropy alloys (HEAs) are a promising class of materials with remarkable mechanical and catalytic properties. Among these, the quinary CrMnFeCoNi alloy (also called “Cantor alloy”) has attracted considerable attention given its thermodynamic stability and remarkable mechanical properties under different temperatures. Given that various degradation mechanisms involve multiple contaminants, such as hydrogen and water in hydrogen embrittlement and surface poisoning, respectively, understanding their interactions with the Cantor alloy is critical for its practical applications as structural, nuclear, or hydrogen storage material. In this work, we perform first-principles calculations based on Density Functional Theory (DFT) to investigate such interactions when considering various microstructures, including bulk materials and those containing certain defects, such as grain boundaries, stacking faults, and vacancies. We also employ Global Sensitivity Analysis to identify the importance of different factors in the stability of the impurities. We find that the accuracy of the H formation energy is significantly affected by spin polarization and chemical short-range order. The study also identifies a strong tendency for hydrogen interstitials to segregate to Σ5(210)/[001] symmetric tilt grain boundary, even when H concentrations are high, suggesting that a certain type of grain boundaries acts as H sinks within the alloy. Further, this result is reinforced by the low formation energy of vacancy-hydrogen complexes, which can contain multiple hydrogen atoms. Finally, the surface reactivity analysis reveals that the adsorption energy of oxygen and hydroxyl groups is highly sensitive to the specific metal atom involved in the binding, with a clear preference for chromium atoms, which could have implications for the alloy’s oxidation and corrosion behavior.

36 MATERIALS SCIENCE↗
DOE OSTI2019

High temperature oxidation of Ni alloys in CO 2 containing impurities

Next-generation power systems need structural alloys resistant to high-temperature oxidation in CO 2 containing combustion impurities. In this report we evaluate the oxidation performance of several commercial Ni alloys exposed to 95%CO 2 -4%H 2 O-1%O 2 , with/without 0.1% SO 2 , at 750°C and 1 atm. All alloys formed protective chromia layers and showed no signs of carburization or sulfidation, and minimal chromia volatilization. The microstructural evolution of the alloy surface, secondary oxidants, and minor alloy additions all played a part in promoting protective chromia formation/growth. The results show that Ni alloys are promising candidates for high-temperature portions of next-generation CO 2 -based power systems.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effect of Cu on the passivity of Ti–xCu (x = 0, 3 and 5 wt%) alloy in phosphate-buffered saline solution within the framework of PDM-II

The effect of copper content on the passivity of Ti–xCu alloys in a phosphate-buffered saline electrolyte (pH 7.4), in the steady-state condition, was examined using potentiostatic polarization, electrochemical impedance spectroscopy, and Mott-Schottky analysis. The study demonstrates that the oxide layer forms on the Ti–xCu alloys surface have n-type semiconducting character, and the steady-state thickness of the oxide layer is observed linearly and depends on the applied potential. The study observations are in line with the predictions of the Point Defect Model (PDM-II), which provides a physiochemically realistic description of the oxide layer formation on Ti–xCu alloys. The observations reveal that the Cu additions result in a decrease in the charge transfer resistance and capacitances associated with the hydroxide and barrier layer, an increase in the donor density, and reduce the electrochemical resistance of the Ti–xCu alloys. The study also exhibits that the decrease in the steady-state current density after the initial addition of 3 wt% of Cu is attributed to the progressive substitution of copper on the cation sublattice of the film resulting in enhanced electrostatic interaction between the immobilized copper $Cu^{x'}_{Ti}$, and the mobile cation interstitials $Cu^{+}_{i}$, which carries the excess current over and that conveyed by oxygen vacancies $V^{••}_o$ in the barrier layer. Moreover, it is also observed that over a hundred-year implant period, about 0.006 cm (0.06 mm) of Ti–3Cu alloy is predicted to lose due to steady-state corrosion in the PBS solution under the given set of conditions, it is further seen that, as the Cu amount increased from 3 wt% to 5 wt% in the Ti–xCu alloys, the steady-state corrosion rate decreases.

Electrochemical Impedance Spectroscopy↗