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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

The roles of kinematic constraint and diffusion in non-equilibrium solid state phase transformations of Ti-6Al-4V

A solid state phase transformation of Ti-6Al-4V was studied using high speed in situ x-ray diffraction measurements made during rapid cooling of a cold metal transfer arc weld bead deposited onto a water cooled substrate. Analysis of body centered cubic (BCC) and hexagonal close packed (HCP) lattices revealed an abrupt, nonlinear shift in the lattice parameters of both phases just after the HCP phase had nucleated. Postmortem transmission electron microscopy confirmed that V diffusion was mostly suppressed during cooling. Together, these results indicate that at this cooling rate of approximately 10 4 K/s, which is representative of cooling rates of many additive manufacturing and welding processes, kinematic coherency of the BCC–HCP interfaces gives rise to the anomalous lattice expansion and contraction behaviors of both phases during the initial nucleation and growth stages of (mostly) martensitic transformation from BCC to HCP; the role of diffusion in such lattice anomalies is shown to be minimal.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Numerical Simulation of Residual Stress for Laser Welding of Ti-6Al-4V Alloy Considering Solid-State Phase Transformation

In the present work, a thermal-metallurgical-mechanical model which considered the influence of phase transformation was used to predict the residual stress field of Ti-6Al-4V after laser welding. In this model, the Johnson–Mehl–Avrami equation was used to predict diffusional solid-state phase transformations: α phase to β phase during heating and β phase to α phase during cooling. The Koistinen–Marburger equation was used to predict diffusionless solid-state phase transformation of β phase to martensite during cooling. The properties of material were obtained using a mixing rule based on the phase fraction and the property of each pure phase. By applying this model, the effect of solid-state phase transformations of Ti-6Al-4V alloy on temperature field and stress field during laser welding was considered in the simulation. In addition, experiments were carried out for comparison, which significantly verified the accuracy of the applied model.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Unconditionally stable, second-order accurate schemes for solid state phase transformations driven by mechano-chemical spinodal decomposition

Here, we consider solid state phase transformations that are caused by free energy densities with domains of non-convexity in strain-composition space; we refer to the non-convex domains as mechano-chemical spinodals. The non-convexity with respect to composition and strain causes segregation into phases with different crystal structures. We work on an existing model that couples the classical Cahn-Hilliard model with Toupin’s theory of gradient elasticity at finite strains. Both systems are represented by fourth-order, nonlinear, partial differential equations. The goal of this work is to develop unconditionally stable, second-order accurate time-integration schemes, motivated by the need to carry out large scale computations of dynamically evolving microstructures in three dimensions. We also introduce reduced formulations naturally derived from these proposed schemes for faster computations that are still second-order accurate. Although our method is developed and analyzed here for a specific class of mechano-chemical problems, one can readily apply the same method to develop unconditionally stable, second-order accurate schemes for any problems for which free energy density functions are multivariate polynomials of solution components and component gradients. Apart from an analysis and construction of methods, we present a suite of numerical results that demonstrate the schemes in action.

36 MATERIALS SCIENCE↗
DOE OSTI2018

High Pressure Induced in Situ Solid-State Phase Transformation of Nonepitaxial Grown Metal@Semiconductor Nanocrystals

Considering the large lattice mismatch induced interface strain between nonepitaxial grown monocrystalline semiconductor shell and metal core, we studied the solid-state phase transformation of such nonepitaxial grown Au@CdS core/shell NCs under high pressure in this paper. Consistent with HRTEM characterizations, the high resolution Raman spectra and synchrotron angle-dispersive X-ray diffraction (ADXRD) spectra evolution were utilized to investigate the hydrostatic pressure (0–24 GPa) induced gradual phase transformation. Here, due to the strong lattice interactions between Au core and CdS shell, the different behavior and improved stability under high pressure appeared compared to single quantum dots (QDs).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Numerical and Experimental Analysis of Dissimilar Repair Welding Residual Stress in P91 Steel Considering Solid-State Phase Transformation

Repair welding is a popular method to repair the cracked zones of pipes in power plants. However, residual stresses induced by repair welding are inevitably generated and have a significant effect on the performance of repair-welded components during service. The purpose of this research is to investigate the effects of solid-state phase transformation (SSPT) on the repair welding residual stress distribution of multi-layer and multi-pass welding in P91 steel. Based on thermal elastic-plastic theory, a three-dimensional thermal–metallurgical–mechanical finite element model considering the SSPT is established, and shielded metal arc welding (SMAW) heat input is modelled as one double elliptical heat source. The effect of phase transformation on residual stress is investigated by considering effects with and without phase transformation in the database of materials. The results show that transverse compressive stress and a low longitudinal stress area are generated in the final pass due to martensite transformation. Compared with different zones along the thickness direction, the effect of SSPT is greater around the outer surface than around the internal fusion zone. The rationality of the developed finite element model is verified by the experimental method, and good agreement is obtained between those results from the numerical analysis and experimental methods.

36 MATERIALS SCIENCE↗
NASA NTRS1972

Low-temperature solid-state phase transformations in 2H silicon carbide

Single crystals of 2H SiC were observed to undergo phase transformations at temperatures as low as 400 C. Some 2H crystals transformed to a structure with one-dimensional disorder along the crystal c axis. Others transformed to a faulted cubic/6H structure. The transformation is time and temperature dependent and is greatly enhanced by dislocations. Observations indicate that the transformation takes place by means of a slip process perpendicular to the c axis. Cubic SiC crystals were observed to undergo a solid state transformation above 1400 C.

Will, H. A.↗
NASA NTRS1972

Low-temperature solid-state phase transformations in 2H silicon carbide.

Study of the phase transformations taking place in 2H SiC single crystals at temperatures as low as 400 C. Some crystals transformed to a structure with one-dimensional disorder along the crystal c axis. Others transformed to a faulted cubic/6H structure. The transformation is time and temperature dependent, and is greatly enhanced by dislocations. The transformation takes place by means of a slip process perpendicular to the c axis. Cubic SiC crystals were observed to undergo a solid-state transformation above 1400 C.

Powell, J. A.↗
DOE OSTI2024

Phase Selection During Solidification and Solid-State Phase Transformations in an Al-10Ce-8Mn (wt pct) Alloy

In multicomponent Al-Ce alloys, and especially after additive manufacturing (AM), complex and metastable solidification microstructures are frequently observed. Here, in this research, the relationship between solidification conditions and phase selection are explored for an Al-10Ce-8Mn (wt pct) alloy using a systematic study of laser melting conditions. Three solidification modes were observed: primary Al 10 Mn 2 Ce; primary Al 20 Mn 2 Ce; and eutectic FCC Al + Al 20 Mn 2 Ce. These solidification modes were correlated to specific liquid-solid interface velocities using a simple thermal model, showing the change in primary solidification phase for low (< 6.8 × 10 −4 m/s), moderate (between 8.2 × 10 −4 and 5.9 × 10 −2 m/s) and high solidification velocities (> 6.2 x 10 −2 m/s) for the above three solidification microstructures, respectively. These results were rationalized by using interface response function (IRF) theory to describe the solidification undercooling for the possible primary intermetallic phases. The implication of the local phase selection from differing solidification conditions is summarized by a comparison of hardness which demonstrates the potential variance of Vickers hardness from 101 to 242 (VHV) by changing the laser velocity from 1 to 83 mm/s. Interestingly, on heat treatment at 400°C, the decomposition pathways of the solidification microstructure and hardness were also found to be different, thereby opening multiple pathways for spatial microstructure and property control within AM components.

Sisco, Kevin↗
DOE OSTI2021

Low modulation index 3-phase solid state transformer

A solid-state transformer (SST) comprises a transformer core, a primary winding, a secondary winding, a primary-side switch bank, and a secondary-side switch bank. Each of the switch banks includes six 4-quadrant switches. The twelve 4-quadrant switches are toggled on and off over six clock cycles in a repetitive sequence with a period that is a function of a desired operating frequency of the transformer. The sequence is configured such that at any given time, 2 of 3 input and output phases are connected to the primary and secondary windings. The SST further includes L-C filter circuits that are configured to filter high-frequency components of current and voltage waveforms such that these components are not back-fed to the electrical mains or delivered to a load. The SST includes a primary-side filter circuit and a secondary-side filter circuit that can each include respective L-C filters for three input or output phases.

Koplow, Jeffrey P.↗
DOE OSTI2019

Integrated Simulation Framework for Additively Manufactured Ti-6Al-4V: Melt Pool Dynamics, Microstructure, Solid-State Phase Transformation, and Microelastic Response

In this work, to accelerate the establishment of fundamental understanding of the additive manufacturing (AM) process and its influence on microstructural evolution and related properties, we develop a multiphysics and multiscale modeling framework that integrates: (1) a high-fidelity powder-scale three-dimensional simulation of transient heat transfer and melt flow dynamics, (2) cellular automaton simulation of solidification grain structure and texture, (3) phase-field modeling of precipitation and dissolution of second-phase precipitate during repeated thermal cycles, and (4) microstructure-based micro- and mesoscopic elastic response calculation. Using Ti-6Al-4V as a model system, we demonstrate the application of the integrated framework to simulate complex microstructure evolution during a single-track laser powder bed fusion process and the associated mechanical response. Our modeling framework successfully captures the solidification β grain structure as a function of laser power and scanning speed, α precipitation upon subsequent cooling with different rates, and elastic response of the resulting (α + β) two-phase microstructure. The key features of solidification and second-phase precipitate microstructures, and their dependence on processing parameters, agree well with existing experimental observations. Finally, the established modeling framework is generally applicable to other metallic materials fabricated by AM.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Solid-state transformation of Fe-rich intermetallic phases in Al–5.0Cu–0.6Mn squeeze cast alloy with variable Fe contents during solution heat treatment

The Al–5.0 wt.% Cu–0.6 wt.% Mn alloys with a variable Fe content were prepared by squeeze casting. Optical microscopy (OM), Deep etching technique, scanning electron microscopy(SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to examine the solid-state transformation of Fe-rich intermetallics during the solution heat treatment. The results showed that the Chinese script-like α-Fe, Al{sub 6}(FeMn) and needle-like Al{sub 3}(FeMn) phases transform to a new Cu-rich β-Fe (Al{sub 7}Cu{sub 2}(FeMn)) phase during solution heat treatment. The possible reaction and overall transformation kinetics of the solid-state phase transformation for the Fe-rich intermetallics were investigated. - Graphical abstract: Display Omitted - Highlights: • The α-Fe, Al{sub 6}(FeMn) and Al{sub 3}(FeMn) phases change to the β-Fe phases. • Possible reactions of Fe phases during solution heat treatment are discussed. • The overall fractional transformation rate follows an Avrami curve.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Numerical estimation of phase transformations in solid state during Yb:YAG laser heating of steel sheets

This work concerns the numerical modeling of heat transfer and phase transformations in solid state occurring during the Yb:YAG laser beam heating process. The temperature field is obtained by the numerical solution into transient heat transfer equation with convective term. The laser beam heat source model is developed using the Kriging interpolation method with experimental measurements of Yb:YAG laser beam profile taken into account. Phase transformations are calculated on the basis of Johnson - Mehl - Avrami (JMA) and Koistinen - Marburger (KM) kinetics models as well as continuous heating transformation (CHT) and continuous cooling transformation (CCT) diagrams for S355 steel. On the basis of developed numerical algorithms 3D computer simulations are performed in order to predict temperature history and phase transformations in Yb:YAG laser heating process.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1967

Solid state phase detector replaces bulky transformer circuit

Miniature solid state phase detector using MOSFETs is used in a phase lock loop with a sun-bit detector in an integrated data-link circuit. This replaces bulky transformer circuits. It uses an inverter amplifier, a modulator switch, and a buffer amplifier.

Moberly, C. L.↗
DOE OSTI2017

Understanding the role of carbon atoms on microstructure and phase transformation of high Nb containing TiAl alloys

The microstructure and solidification behavior of high Nb containing TiAl alloys with the composition of Ti-46Al-8Nb-xC (x = 0.1, 0.7, 1.4, 2.5 at.%) prepared by arc-melting method have been investigated in this work. The results give evidence that the addition of carbon changes the solidification behavior from solidification via the β phase to the peritectic solidification. And carbon in solid solution enriches in the α{sub 2} phase and increases the microhardness. As the carbon content increases to 1.4 at.%, plate-shape morphology carbides Ti{sub 2}AlC (H phase) precipitate from the TiAl matrix which leads to the refinement microstructure. By aging at 1173 K for 24 h after quenching treatment, fine needle-like and granular shape Ti{sub 3}AlC (P phase) carbides are observed in the matrix of Ti-46Al-8Nb-2.5C alloy, which distribute along the lamellar structure or around the plate-shape Ti{sub 2}AlC. Transmission electron microscope observation shows that the Ti{sub 3}AlC carbides precipitate at dislocations. The phase transformation in-situ observations indicate that the Ti{sub 2}AlC carbides partly precipitate during the solid state phase transformation process. - Highlights: •Carbon changes the solidification behavior from β phase to peritectic solidification. •Dislocations in solution treated γ phase act as nucleation sites of Ti{sub 3}AlC precipitations. •Ti{sub 3}AlC precipitates as fine needle-like or granular shape in the solution treated matrix. •Ti{sub 2}AlC carbides precipitate during the solid state phase transformation process.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Widmannstätten laths in Ti48Al2Cr2Nb alloy by undercooled solidification

Widmannstätten laths in Ti48Al2Cr2Nb alloy were obtained by containerless electromagnetic levitation during undercooled solidification rather than heat treatments in most cases. The nucleation of γ{sub Widmannstätten} is confirmed by the thermal history of the second recalescence behaviour. High dislocation density, stacking faults and dislocation slip are observed by transmission electron microscopy. By quantitative analysis of the transformation energy, the driving force for γ{sub Widmannstätten} in the solid state phase transformation is calculated, compared with that of heat treatments. Meanwhile, the proper undercoolings (ΔT = 257–300 K), post-solidification cooling rates (11.1–29.1 K/s), the fine lamellar colony sizes (306–467 μm) and proper undercoolings in the solid state phase transformation are in favour of promoting the formation of Widmannstätten laths at moderate undercoolings. - Graphical abstract: Display Omitted - Highlights: • Widmannstätten laths were observed in undercooled TiAl alloy. • The influencing factors were discussed in undercooled TiAl alloy. • An analysis of the nucleation of γ{sub Widmannstätten} is carried out.

36 MATERIALS SCIENCE↗