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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Alloying effects on the microstructure and properties of laser additively manufactured tungsten materials

A large body of literature within the additive manufacturing (AM) community has focused on successfully creating stable tungsten (W) microstructures due to significant interest in their application for extreme environments. However, cracking and additional embrittling features at grain boundaries have resulted in poorly performing materials, stymying the application of AM as a manufacturing technique for W. Several alloying strategies, such as ceramic particles and ductile elements, have emerged with the promise to eliminate cracking while simultaneously enhancing stability against recrystallization. Here, in this work, we provide new insights regarding the defects and microstructural features that result from the introduction of ZrC for grain refinement and NiFe as a ductile reinforcement phase – in addition to the resulting thermophysical and mechanical properties. ZrC is shown to promote microstructural stability with increased hardness due to the formation of ZrO 2 dispersoids. Conversely, NiFe forms into micron-scale FCC phase regions within a BCC W matrix, producing enhanced toughness relative to pure AM W. A combination of these effects is realized in the WNiFe + ZrC system and demonstrates that complex chemical environments coupled with the tuning of AM microstructures provides an effective pathway for enabling laser AM W materials with enhanced stability and performance.

36 MATERIALS SCIENCE↗
NASA NTRS1987

Effect of LID (Registered) processing on the microstructure and mechanical properties of Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo titanium foil-gauge materials

A study was undertaken to determine the mechanical properties and microstructures resulting from Liquid Interface Diffusion (LID -Registered) processing of foil-gauge specimens of Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo coated with varying amounts of LID material. In addition, the effects of various elevated temperature exposures on the concentration profiles of the LID alloying elements were investigated, using specimens with a narrow strip of LID material applied to the surface. Room and elevated temperature tensile properties were determined for both coated and uncoated specimens. Optical microscopy was used to examine alloy microstructures, and scanning electron microscopy to examine fracture surface morphologies. The chemical concentration profiles of the strip-coated specimens were determined with an electron microprobe.

Balckburn, Linda B.↗
DOE OSTI2019

Effects of Heat Treatment Parameters on the Microstructure and Properties of Bainitic Steel

The results obtained in the present study demonstrate the effects of various types of heat treatment processes on the microstructure and hardness of new trip-assisted carbide-free bainitic steel. The steel was subjected to three variants of heat treatment processing with an isothermal bainitic transformation temperature in a range from 350 to 480 °C. Changes of temperature and time of isothermal holding caused changes in the values of retained austenite (RA) volume fraction and carbon content. Reduction in the isothermal holding temperature resulted in the increased concentration of carbon in austenite. Also, the investigations of the microstructure showed that size and morphology of the austenite evolved during heat treatment. The SEM observations revealed that the steel subjected to heat treatment is composed of the carbide-free bainite with ferrite plates and a high volume fraction of retained austenite in the form of thin layers or islands. With the lower isothermal holding temperature and the higher degree of bainitic transformation, the more the RA morphology changed from island to layer type. The application of the lowest isothermal temperature resulted in a significant refinement of the microstructure components: the bainitic ferrite plates and the RA layers. Also, the mechanical properties obtained from the tensile testing and hardness measurements were correlated to the microstructure of the investigated steel after different isothermal holdings.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and Properties of A356 Alloy Wheels Fabricated by Low-Pressure Die Casting with Local Squeeze

The low-pressure die-casting process (LPDC) is widely used in fabricating aluminum alloy automotive wheels. With good castability and mechanical properties, A356 alloys are important and common materials for LPDC wheels. However, in practical casting process, defects often appear, especially porosities in the wheel thick-walled parts (i.e., spoke) due to inadequate interdendritic feeding, which greatly decreases the mechanical properties. In this investigation, to deal with this problem, local squeeze (LS) was added in conventional LPDC to enhance the interdendritic feeding. The practical A356 alloy wheel castings were fabricated with different LS velocities and times. The quantitative characteristics of α-Al cells, Si particles, and porosities with and without the LS analyzed by optical microscope, scanning electron microscope, and 3D x-ray computed tomography were compared. Their effects on the tensile and impact properties were discussed. The results have shown that the LS can increase the cooling rate and feed shrinkage of spoke parts, hence resulting in finer grains and denser microstructure. In result, the properties of spoke parts were improved with the LS.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and Properties of Inconel 718 Fabricated by Directed Energy Deposition with In-Situ Ultrasonic Impact Peening

Many inherent issues, such as the detrimental residual stress, columnar grains with anisotropy, and weak mechanical properties, have severely impeded the adoption of metal additive manufacturing (AM) techniques including powder bed fusion and directed energy deposition (DED) processes. In this study, a hybrid AM process that consists of layer-wise laser metal deposition (i.e., a DED process) and in-situ ultrasonic impact peening (UIP) was applied to obtain Inconel 718 superalloy workpieces. Also, for further property enhancement, a post-heat treatment was applied to the deposited material obtained by the hybrid AM process. Scanning electron microscopy and transmission electron microscope were used to investigate the microstructure morphology and reveal the underlying strengthening mechanism. Electron backscatter diffraction was employed to quantitatively study the microstructure resulted from the hybrid AM process and the post-heat treatment. The profile of residual stress along the depth direction was obtained through X-ray diffraction. The results demonstrate that this hybrid AM process is capable of producing high-quality metal parts with significantly refined microstructure, and beneficial compressive residual stress along the depth into surface. Severe plastic strains are introduced by UIP, and the resulted mechanical twinning and dynamic recrystallization play an important role in refining microstructure. The material microstructure is further refined down to 100 µm, and the texture anisotropy is significantly diminished after solution treatment at 980 °C for 1 hour. Under the as-built condition, in-situ ultrasonic peening alters the residual stress component from a tensile state to an overall compressive state with a maximum value of − 190 MPa within the range of measurement depth.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Beam Current Effect on Microstructure and Properties of Electron-Beam-Melted Ti-6Al-4V Alloy

In this study, a noncommercial 3D printing machine was used to fabricate Ti-6Al-4V alloy by electron-beam melting (EBM). The influence of beam current on the microstructure, phase composition and mechanical properties of electron-beam-melted Ti-6Al-4V alloy was investigated. Numerical simulation is implemented to evaluate thermal fields during electron-beam melting of Ti-6Al-4V powder. The decrease in beam current from 3.5 to 2.5 mA leads to refinement of microstructure: The average width of α plates decreases from 10 down to 6 μm. The formation of finer microstructure is attributed to higher cooling rate at lower beam current confirmed by simulation. The phase composition of EBM Ti-6Al-4V indirectly depends on the beam current. High content of β phase (7%) was achieved at the beam current of 3 mA. The produced Ti-6Al-4V samples are characterized by high microhardness (470-520 HV).

36 MATERIALS SCIENCE↗
DOE OSTI2021

Crystal mechanics-based thermo-elastic constitutive modeling of orthorhombic uranium using generalized spherical harmonics and first-order bounding theories

In earlier works, a mathematical procedure for invertible microstructure-property linkages was developed using computationally efficient spectral methods for polycrystalline cubic and hexagonal metals. This paper formulates such invertible microstructure–property linkages for orthorhombic polycrystalline metals relying on the generalized spherical harmonics (GSH) spectral basis. The procedure is used to compute property closures of orthorhombic polycrystals. The closures represent the complete set of theoretically possible combinations of effective properties for a selected material. The procedure relies on the first-order bounding theories and considers orientation distribution functions (ODFs) as the main microstructural descriptor influencing homogenized properties. Numerous examples of these closures involving second-rank thermal expansion and fourth-rank elastic stiffness tensorial properties over a broad range of temperatures are presented for α-uranium (α-U). In doing so, certain key properties of these closures are exploited to facilitate their computation with drastically reduced computational effort. Along with the recently developed GSH-based interpolation procedure for ODFs from coarsely spaced experimental measurement grids to finely spaced finite element mesh resolution grids presented in Barrett et al., the developed computationally efficient ODF-effective property linkages are used to establish a crystal mechanics-based simulation framework coupled with the finite element method (FEM). The ODF dependent thermal expansion and elastic stiffness tensors are efficiently calculated at every integration point and used by the FEM to predict the overall distortion of a hemispherical part made of α-U during heating. In conclusion, it is shown that the developed framework can be used to simulate microstructurally heterogeneous components under thermo-mechanical loadings in a computationally efficient manner.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Effects of carbonitrides and carbides on microstructure and properties of castable nanostructured alloys

In this work, to develop advanced reduced-activation ferritic-martensitic (RAFM) steels for fusion reactor structural applications, both carbonitride- and carbide-strengthened castable nanostructured alloys (CNAs) were explored for higher densities of MX (M = Ti/Ta/V/etc. and X = C/N) nanoprecipitates. Additionally, systematic comparisons between the two types of CNAs indicated generally similar microstructures and comparable tensile properties and creep resistance. However, the carbide-CNAs did show some advantages over the carbonitride-CNAs in terms of the uniformly distributed higher density of MC nanoprecipitates, greater Charpy impact upper shelf energies, less deuterium retention and swelling, and potentially less transmutation-induced composition changes and consequently thermodynamically more stable carbides. The carbide-CNAs showed the best-balanced high performance in the examined properties, in contrast to the significantly varied performance of oxide-dispersion-strengthened alloys and the generally lower performance of current RAFM steels.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Formability, Microstructure and Mechanical Properties of Flow-Formed 17-4 PH Stainless Steel

The subject of the research that has been conducted in this paper was to analyze precipitation-hardened martensitic stainless steel 17-4 PH after flow forming with four different strains and subsequent standard heat treatment. Four cylinders were obtained with a 16, 30, 48 and 68% reduction in thickness, respectively. The samples were analyzed in terms of their mechanical properties and microstructural changes before and after the heat treatment. The results showed that a higher strain resulted in an overall higher strength (up to 1200 MPa UTS) and refinement of the structure, although at a cost of the elongation. High deformation influenced the precipitation process, and the ratio of the grain boundaries significantly increased. Nonetheless, comparing the obtained results with other similar research, it seems that the formation of nano-precipitates of Cu is the key-strengthening mechanism. Strain hardening contributes to an increase in the strength of the steel, although the effect decreases after heat treatment. The relatively small values of residual stress in the steel, especially after the heat treatment, confirmed these claims. Overall, flow forming allowed high deformations of the 17-4 PH steel to be obtained although it did not significantly change the mechanical properties of the material due to the dominant precipitation hardening mechanism.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Boron Content on Microstructure, Mechanical Properties, and Oxidation Resistance of Mo-Si-B Composites

Mo-Si-B composites with boron content from 0 to 8 wt.% were prepared by spark plasma sintering using Mo, Si, and B elemental powders. The effects of boron content on the phase composition, mechanical properties, and high-temperature oxidation resistance of the Mo-Si-B composites were investigated. The results show the evolution of the major phase constitution of the composites with increasing boron content is in the following sequence: (MoSi{sub 2}, Mo{sub 5}Si{sub 3}) → (MoSi{sub 2}, Mo{sub 5}Si{sub 3}, Mo{sub 5}SiB{sub 2}) → (MoSi{sub 2}, MoB) → (MoSi{sub 2}, MoB, MoB{sub 2}). As the boron content increases, both the hardness and flexural strength of the composites increases, but the fracture toughness of the composites gradually decreases. The strengthening mechanisms for the composites are second-phase strengthening and grain refinement strengthening. However, the formation of brittle phases, i.e., Mo{sub 5}SiB{sub 2}, MoB, and MoB{sub 2}, reduces the toughness of the composites. In addition, since a dense barrier layer of SiO{sub 2}-B{sub 2}O{sub 3} formed on the surface of the composites inhibits the inward diffusion of oxygen, B-doped Mo-Si-B exhibits better oxidation resistance compared with non-doped composites.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Chemical Composition on the Microstructure and Tribological Properties of Sn-Based Alloys

This article presents the test results of two bearing alloys on a tin matrix, differing in the content of alloy additions. Microscopic tests were performed using scanning electron microscopy, along with Brinell hardness tests and tests of tribological properties in the conditions of technical dry friction and wet friction using a TU-32 oil. Additionally, wear mechanisms were analyzed depending on the process conditions. The results indicate better tribological properties of the B82 alloy, with a higher content of antimony compared to the B89 alloy. This alloy contains more precipitates of hard phase β—SnSb, which both influence the level of the mechanical properties and determine the tribological properties. Additionally, it was proven that using the TU-32 oil as a lubricant practically eliminates adhesive wear, and the post-friction surfaces for both alloys are of a similar nature.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Build orientation, surface roughness, and scan path influence on the microstructure, mechanical properties, and flexural fatigue behavior of Ti–6Al–4V fabricated by electron beam melting

In the present work, microstructure and fatigue behavior of Electron Beam Melted (EBM) Ti–6Al–4V have been systematically studied. Fatigue behavior was investigated using four-point bend tests of rectangular bars with internal features fabricated with different build orientations, scan paths, and surface conditions. Microstructural variations were evaluated by optical microscopy and Vickers micro-hardness indentations, both of which were found to be consistent throughout the build and within ranges reported in the literature. The fatigue performance of the EBM material was comparable to wrought Ti–6Al–4V when the loading direction was parallel to the build direction, but severely limited when loaded perpendicular to the build direction or if surface roughness defects were present. Scanning electron microscopy and interrupted fatigue tests were utilized to characterize fracture surfaces, failure mechanisms, crack initiation, and propagation behavior. Crack initiation sites in the vertically-oriented specimens were identified to be melt-related defects formed by insufficient melting.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Quantification of solution annealing effects on microstructure and property in a laser powder bed fusion 316H stainless steel

Solution annealing (SA) is an effective way to mitigate microstructural heterogeneity and to optimize mechanical performance of alloys manufactured by laser powder bed fusion (LPBF). In this study, a comprehensive and quantitative understanding of the recovery and recrystallization processes in the SA temperature range of LPBF 316H stainless steel is provided using results from analytical electron microscopy and in-situ high-energy synchrotron x-ray scattering. The profound effect of dislocation structures and secondary phase particles on mechanical performance, particularly under tension and creep conditions, is rationalized using deformation models that incorporate microstructural inputs. This study, for the first time, quantifies the broad effect of nano oxide inclusions on dislocation recovery kinetics, on grain growth and recrystallization kinetics, and on tension strength and creep resistance. The fundamental differences between the LPBF and the conventional wrought materials are revealed. The findings address critical questions in post-build processing of AM materials and pave the way for their rapid qualification for high temperature applications.

In-situ X-ray diffraction↗
DOE OSTI2019

Design and Development of Galvannealed Dual-Phase Steel: Microstructure, Mechanical Properties and Weldability

It was attempted to produce galvannealed dual-phase (DP) steel through a commercial continuous galvanizing line (CGL). The main challenge was to form the DP microstructure even with the slow cooling rate after annealing in the CGL. A two-pronged approach was adopted for this, viz. design of the steel chemistry with calculations and optimization of the CGL parameters with simulation using Gleeble-3800 thermomechanical simulator. The steel thus produced in CGL revealed ferrite and 18 ± 2% martensite microstructure, with tensile strength exceeding 600 MPa and 24% total elongation along with good formability. Weldability study using resistance spot welding indicated the possibility of achieving sound joint with a 4-kA width in weld time–current lobe despite the high carbon equivalence.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Microstructure and properties of additively manufactured Al–Ce–Mg alloys

Additive manufacturing of aluminum alloys is largely dominated by a near-eutectic Al-Si compositions, which are highly weldable, but have mechanical properties that are not competitive with conventional wrought Al alloys. In addition, there is a need for new Al alloys with improved high temperature properties and thermal stability for applications in the automotive and aerospace fields. In this work, we considered laser powder bed fusion additive manufacturing of two alloys in the Al–Ce–Mg system, designed as near-eutectic (Al–11Ce–7Mg) and hyper-eutectic (Al–15Ce–9Mg) compositions with respect to the binary L → Al + Al 11 Ce eutectic reaction. The addition of magnesium is used to promote solid solution strengthening. A custom laser scan pattern was used to reduce the formation of keyhole porosity, which was caused by excessive vaporization due to the high vapor pressure of magnesium. The microstructure and tensile mechanical properties of the alloys were characterized in the as-fabricated condition and following hot isostatic pressing. The two alloys exhibit significant variations in solidification structure morphology. These variations in non-equilibrium solidification structure were rationalized using a combination of thermodynamic and thermal modeling. Both alloys showed higher yield strength than AM Al-10Si-Mg for temperatures up to 350 °C and better strength retention at elevated temperatures than additively manufactured Scalmaloy.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Comparative study of Sc and Er addition on microstructure, mechanical properties, and electrical conductivity of Al-0.2Zr-based alloy cables

Highlights: • The comprehensive properties of Al-0.2Zr-0.1Sc were better than those of Al-0.2Zr-0.1Er alloy cable. • The microstructures evolved from fibrous grains to equiaxed grains with increasing annealing time. • The Zener drag force increased with increasing ratio of volume fraction (F{sub V}) to average diameter F{sub V}/ d-bar . • The capability to inhibit recrystallization was as follows: Al{sub 3}(Zr, Sc) > Al{sub 3}(Zr, Er) > Al{sub 3}Zr. - Abstract: The effect of the addition of 0.1 wt% Sc and 0.1 wt% Er on the deformation microstructure, Vickers microhardness, and electrical conductivity of Al-0.2 wt% Zr-based alloy cables annealed at 400 °C for different times was investigated. The comprehensive properties of Al-0.2Zr-0.1Sc were better than those of Al-0.2Zr-0.1Er. The optimal conditions for the Al-0.2Zr-01Sc alloy cable were a microhardness of 575 MPa, an electrical conductivity of 34.5 MS/m for an annealing time of 36 h, and those for the Al-0.2Zr-0.1Er alloy cable were a microhardness of 550 MPa, an electrical conductivity of 33.6 MS/m for an annealing time of 8 h. During the annealing process, dispersed Al{sub 3}(Zr,Sc/Er) precipitates were generated, which substantially impeded recrystallization by Zener pining of grain boundaries. Based on the ratio of volume fraction (F{sub V}) to average diameter ( d-bar ) of the precipitates, their capability to inhibit recrystallization was as follows: Al{sub 3}(Zr, Sc) > Al{sub 3}(Zr, Er) > Al{sub 3}Zr. Moreover, the electrical conductivity of Al-0.2Zr-0.1Sc was better than that of Al-0.2Zr-0.1Er, probably because Sc could better inhibit recrystallization than Er, leading to fewer grain boundaries and hence lesser electron scattering.

36 MATERIALS SCIENCE↗