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At least 1,387 records · Page 77
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Characterization and Rationalization of Microstructural Evolution in GRCop-84 Processed by Laser-Powder Bed Fusion (L-PBF)

In this study, prismatic geometries of GRCop-84 [Cu-8Cr-4Nb (at. pct)] were built with laser-powder bed fusion (L-PBF) process. The samples were sectioned parallel or perpendicular to the build direction and characterized in the as-built and after post-processing with a hot-isostatically pressing (HIP) treatment. The microstructure and phase evolutions were evaluated with optical microscopy, scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), and high-temperature X-ray diffraction (HTXRD) up to 1223 K. The samples in the as-built conditions exhibited predominantly columnar epitaxial and misoriented Cu-FCC grains. The microstructure evolutions are discussed based on locations within the overall build geometry, the dynamics of small melt pool shape and sectioning effects. The above grain structure did not change significantly during post-process HIP treatment. The stability of this FCC grain structure is attributed to the formation of primary stable Cr 2 Nb (Laves phase) during L-PBF, even before the emergence of FCC grains from liquid. The stability of Cr 2 Nb in both as-built and HIPed samples were evaluated using high-temperature X-ray diffraction measurements and compared with that of gas-atomized powder. The significance of these results is discussed with reference to aerospace applications.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Electrosynthesis of Ti{sub 5}Si{sub 3}, Ti{sub 5}Si{sub 3}/TiC, and Ti{sub 5}Si{sub 3}/Ti{sub 3}SiC{sub 2} from Ti-Bearing Blast Furnace Slag in Molten CaCl{sub 2}

Ti{sub 5}Si{sub 3}, Ti{sub 5}Si{sub 3}/TiC, and Ti{sub 5}Si{sub 3}/Ti{sub 3}SiC{sub 2} have been electrochemically synthesized from the Ti-bearing blast furnace slag/TiO{sub 2} and/or C mixture precursors at a cell voltage of 3.8 V and 1223 K to 1273 K (950 °C to 1000 °C) in molten CaCl{sub 2}. The pressed porous mixture pellets were used as the cathode, and a solid oxide oxygen-ion-conducting membrane (SOM)-based anode was used as the anode. The phase composition and morphologies of the cathodic products were systematically characterized. The final products possess a porous nodular microstructure due to the interconnection of particles. The variations of impurity elements, i.e., Ca, Mg, and Al, have been analyzed, and the result shows that Ca and Mg can be almost completely removed; however, Al cannot be easily removed from the pellet due to the formation of Ti-Al alloys during the electroreduction process. The electroreduction process has also been investigated by the layer-depended phase composition analysis of the dipped/partially reduced pellets to understand the detailed reaction process. The results indicate that the electroreduction process of the Ti-bearing blast furnace slag/TiO{sub 2} and/or C mixture precursors can be typically divided into four periods, i.e., (i) the decomposition of initial Ca(Mg,Al)(Si,Al){sub 2}O{sub 6}, (ii) the reduction of Ti/Si-containing intermediate phases, (iii) the removal of impurity elements, and (iv) the formation of Ti{sub 5}Si{sub 3}, TiC, and Ti{sub 3}SiC{sub 2}. It is suggested that the SOM-based anode process has great potential to be used for the direct and facile preparation of Ti alloys and composites from cheap Ti-containing ores.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Wittichenite Cu3BiS3: Synthesis and Physical Properties

Polycrystalline Cu 3 BiS 3 was synthesized and densified using hot pressing in order to investigate the physical properties of this material. Both the thermal conductivity and the Seebeck coefficient of Cu 3 BiS 3 are reported for the first time in order to investigate the thermoelectric properties of this material. The ultralow thermal conductivity coupled with the relatively high Seebeck coefficient, 0.17 W/m-K and 540 μV/K at room temperature, respectively, suggest Cu 3 BiS 3 may show promise for thermoelectric applications.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Vacuum Pressureless Sintering of Ti-6Al-4V Alloy with Full Densification and Forged-Like Mechanical Properties

Ti-6Al-4V ingots with a nearly 100% density, fine and homogeneous basket-weave microstructure, and better comprehensive mechanical properties (UTS = 935 MPa, Y.S. = 865 MPa, El. = 15.8%), have been manufactured by vacuum pressureless sintering of blended elemental powders. Coarse TiH{sub 2} powder, Al powder (2, 20 μm), V powder, and Al-V master alloy powder were used as raw materials to produce different powder mixtures (D{sub 50} = 10 μm). Then, the compacts made by cold isostatic pressing were consolidated by different sintering curves. A detailed investigation of different as-sintered samples revealed that a higher density can be obtained by generating transient molten Al in the sintering process. Coarse Al powder and a rapid heating rate under the melting point of Al contribute to molten Al formation. The presence of temporary liquid phase changes the sintering mechanism, accelerating the sintering neck formation, improving sinterability of the powder mixtures. Density of 99.5% was achieved at 1150 °C, which is markedly lower than the sintering temperatures reported for conventional blended elemental powder metallurgy routes. In addition, low interstitial content, especially for oxygen (0.17 wt.%), is obtained by strict process control.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Porous Titanium Parts Fabricated by Sintering of TiH{sub 2} and Ti Powder Mixtures

A new simple powder metallurgy process by sintering TiH{sub 2} powders was used to manufacture porous Ti components. The effects of the processing parameters (pressure of cold isostatic pressing and sintering temperature) and the TiH{sub 2}/Ti ratio in the powder mixtures on the impurities, the linear shrinkage and the pore properties (including overall and open porosities) were comprehensively determined. The addition of TiH{sub 2} as a reactant has been found beneficial for the synthesis of porous Ti components. The formation mechanisms of pores were demonstrated based on the dehydrogenation process of TiH{sub 2} during sintering, resulting in highest reactivity due to the “in statu nascendi” generation of the metal. In addition, the hardness and corrosion resistance of all the sintered samples were evaluated, related to the overall and open porosities. As a result, an optimal composition of Ti-40 wt.% TiH{sub 2} was defined, as its maximum open porosity was about 23%.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of TiN Addition on 3Y-TZP Ceramics with Emphasis on Making EDM-Able Bodies

In this study, to produce electrically conductive ceramics, rapid hot press (RHP) sintering of 3 mol.% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) and 3Y-TZP/TiN composites with TiN amounts of 25, 35, and 45 vol.% was performed at 1300, 1350, and 1400 °C. Interestingly, the toughness and hardness were improved in the presence of TiN up to 35 vol.% and maximum fracture toughness and hardness of 5.40 ± 0.05 MPa m{sup 1/2} and 14.50 ± 0.06 GPa, respectively, were obtained. However, the bending strength was decreased which could be attributed to the rather weak interfaces of nitride and oxide phases. Regarding the zirconia matrix, the effect of grain size on fracture toughness of the samples has been studied using x-ray diffraction and field emission scanning electron microscope (FESEM) analysis. It was also found that electrical resistivity decreased to the value of 6.88 × 10{sup −6} Ω m at 45 vol.% of TiN. It seems the TiN grains form a network to impose conductivity on the ZrO{sub 2} body; however, below 35 vol.% TiN, due to lack of percolation effect, this conductivity could not be maintained according to FESEM studies. Finally, electrically conductive samples were successfully machined by electrical discharge machining (EDM).

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Mechanical Properties of W-ZrC Composites Synthesized by Reactive Melt Infiltration of Zr{sub 2}Cu into Porous Preforms from Partially Carburized W Powders

W-ZrC composites with different W contents from 48 to 73 vol.% have been synthesized by reactive melt infiltration of Zr{sub 2}Cu melt into porous preforms from partially carburized W powders at 1300 °C for 1 h in vacuum. The influences of carbon content and porosity in the preforms on microstructure and mechanical properties of W-ZrC composites are investigated. Cold isostatic pressing followed by pre-sintering process is used to produce porous preforms with suitable porosities of 53.6-47% under a pressure of 100 MPa to allow sufficient penetration of Zr{sub 2}Cu melt into the preforms. Small amounts of Cu-rich phases form in the synthesized W-ZrC composites after a complete reaction of y/2xZr{sub 2}Cu(l) + WC{sub y}(s) = y/xZrC{sub x}(s) + W(s) + y/2xCu(l). These Cu-rich phases are distributed not only at the phase boundaries of W matrix and ZrC grains, but also in the interior of ZrC{sub x} grains. With decreasing W content from 73 to 48 vol.% in the W-ZrC composites, the flexural strength and fracture toughness increase from 519 to 657 MPa and from 9.1 to 10.6 MPa m{sup 1/2}, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Composites Strengthened with Graphene Platelets and Formed in Semisolid State Based on α and α/β MgLiAl Alloys

MgLiAl base composites strengthened with graphene platelets were prepared by semisolid processing of ball-milled alloy chips with 2% of graphene platelets. Composites strengthened with graphene platelets show higher hardness and yield stress than the cast alloys, i.e., 160 MPa as compared to 90 MPa for as-cast alloy MgLi9Al1.5. Mechanical properties for MgLiAl-based composites were similar or higher than for composites based on conventional AZ91 or WE43 alloys. The strengthening however was not only due to the presence of graphene, but also phases resulting from the reaction between carbon and lithium, i.e., Li{sub 2}C{sub 2} carbide. Graphene platelets were located at globules boundaries resulting from semisolid processing for all investigated composites. Graphene platelets were in agglomerates forming continuous layers at grain boundaries in the composite based on the alloy MgLi4.5Al1.5. The shape of agglomerates was more complex and wavy in the composite based on MgLi9Al1.5 alloy most probably due to lithium–graphene reaction. Electron diffraction from the two-phase region α + β in MgLi9Al1.5 indicated that [001]α and [110]β directions are rotated about 4° from the ideal relationship [001] hex || [110] bcc phases. It showed higher lattice rotation than in earlier studies what is most probably caused by lattice slip and rotation during semisolid pressing causing substantial deformation particularly within the β phase. Raman spectroscopy studies confirmed the presence of graphene platelets within agglomerates and in addition the presence mainly of Li{sub 2}C{sub 2} carbides in composites based on MgLi4.5Al1.5 and Mg9Li1.5Al alloys. From the character of Raman spectra refinement of graphene platelets was found in comparison with their initial size. The graphene areas without carbides contain graphene nanoplatelets with lateral dimension close to initial graphene sample. Electron diffraction allowed to confirm the presence of Li{sub 2}C{sub 2} carbide at the surface of agglomerates found from Raman spectroscopy results.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Enhanced Hot Tensile Ductility of Mg-3Al-1Zn Alloy Thin-Walled Tubes Processed Via a Combined Severe Plastic Deformation

In the current study, combined parallel tubular channel angular pressing (PTCAP) and tube backward extrusion (TBE), as a recently developed severe plastic deformation (SPD) method, were applied at 300 °C on a commercial Mg-3Al-1Zn alloy tubes to achieve an ultrafine grained structure. Then, the microstructure, hardness, tensile properties, and fractography evaluations were done at room temperature on the SPD-processed samples. Also, to study the hot tensile ductility of the SPD-processed samples, tensile testing was performed at an elevated temperature of 400 °C, and then, the fractured surface of the tensile samples was studied. It was observed that a bimodal microstructure, with large gains surrounded by many tiny ones, was created in the sample processed by PTCAP followed by TBE. This microstructure led to reach higher hardness and higher strength at room temperature and also led to reach very high elongation to failure (~ 181%) at 400 °C. Also, the value of elongation to failure for this sample was ~ 14.1% at room temperature. The fractographic SEM images showed the occurrence of predominately ductile fracture in the samples pulled at 400 °C. This was mostly due to the nucleation of microvoids and their subsequent growth and coalescence with each other.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Sintering Temperature on Density and Mechanical Properties of Solid-State Sintered Silicon Carbide Ceramics and Evaluation of Failure Origin

Densified silicon carbide (SiC) is prepared through dry pressing of SiC powder followed by pressureless sintering in the presence of boron carbide and carbon as the additives. Sintering of SiC parts shows the increase in density with the increase in temperature (1950-2180 °C) by resulting in higher than 98% relative density (RD) at 2150 °C and above for 1 h. Sintered specimens are evaluated concerning the phase, microstructure and mechanical properties including hardness and flexural strength. The machined surface of SiC parts with about 98% RD is studied for the origin of failure. The estimated size of critical flaws (32-110 µm) based on flexural strength and fracture toughness indicates that machining defects are one of the primary reasons for failure in SiC ceramics. The order of magnitude of defects on machined surfaces of sintered SiC by SEM studies is found to be comparable with the estimated size of critical flaws. The failure behavior of SiC is discussed with the help of Weibull statistics with respect to the variation of four-point flexural strength.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Thermoelectric Properties of TiNiSn Half Heusler Alloy Obtained by Rapid Solidification and Sintering

Thermoelectric TiNiSn-based half Heusler compound reveals high Seebeck coefficient and electrical conductivity, leading to high power factor. As a consequence of the solidification path, TiNiSn single phase cannot be obtained directly from the liquid phase. Thus, a long annealing step is needed to homogenize the alloy. In this work, we present a new processing route for half Heusler compound formation, combining arc melting of pure elements, rapid solidification of the molten alloy and sintering. Rapid solidification of the molten alloy allows to obtain almost single TiNiSn phase, limiting the formation of the primary TiNi{sub 2}Sn phase as a consequence of the deep undercooling of the liquid. The rapidly solidified alloy was ground to powder and sintered by open die pressing. As-sintered samples show a density around 95% of the theoretical value. Thermal cycling of the sintered samples shows evolution of the phases, suggesting that after sintering the sample is not in equilibrium yet. After the second thermal cycle thermoelectric properties become reproducible, indicating the attainment of the equilibrium. In conclusion, the proposed processing route allows to obtain dense TiNiSn in bulk form avoiding the time-consuming annealing step, typically used to homogenize this alloy after solidification.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Achieving Grain Refinement and Related Mechanical Property Improvement of an Al-Zn-Mg-Cu Alloy Through Severe Plastic Deformation

Microstructural features and mechanical properties of an Al-Zn-Mg-Cu alloy processed by equal-channel angular pressing (ECAP) have been investigated. The equivalent plastic strain obtained from the simulation agrees well with that from the analytical calculation in a way that both increase with an increase in deformation, but the strain distribution is non-uniform, appearing as a strain gradient along the norm direction. Microstructure investigation shows that grain refinement is achieved in frequency of low-angle grain boundaries fabricated by ECAP. The pole density is reduced by ECAP, during which the Brass and Goss type textures are introduced. TEM results show chain-like recrystallized grains formed at the grain boundaries and broken second-phase particles come into being due to the heavy shear force. After ECAP, the hardness and strength increase significantly while the elongation is reduced. Besides, an extreme strengthening is confirmed because of the balance between dynamic hardening and softening. The microstructure evolution mechanism is revealed in terms of grain refinement, precipitate phase breakage and texture evolution.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Synthesis and Characterization of Aluminum Matrix Composites Reinforced with (Ni,Cu){sub 3}Al Intermetallic Particles via Mechanical Milling Technique

This study investigates the dispersion of (Ni,Cu){sub 3}Al intermetallic particles in an aluminum matrix via low-energy mechanical milling as an alternative to conventional stir casting. The precursor powders consisted in (Ni,Cu){sub 3}Al and reactive-grade Al particles with an average size of 15 and 3 microns, respectively. The concentrations used were 3, 5, 8 and 10 wt.% of the intermetallic powders. After 4 h of milling, the obtained powders were compacted in a unidirectional press with a pressure of 930 MPa and the obtained green compacts were sintered at 500 °C in a controlled atmosphere oven for 2 h. Result showed that the bulk composites displayed an increase in the Vickers microhardness as well as an increase in compressive strength and stiffness as the percentage of dispersed particles increased. The Al-8 wt.%(Ni,Cu){sub 3}Al sample achieved the best performance and is related to the findings in SEM micrographs that showed that higher reinforcement concentration increased the formation of microporosities that reduce the decohesion of the bulk sample and impact heavily on the mechanical performance of the material.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Synthesis and Properties of Porous Ti-20 wt.% HA Nanocomposites

In the present study, porous titanium-20 wt.% hydroxyapatite nanocomposites were produced by the combination of mechanical alloying and powder metallurgical process with the addition of titanium hydride particles as the blowing agent. The powder mixture of Ti, HA and spacer particles were milled, pressed and heat-treated to decompose the hydride particles and to sinter into highly porous Ti-HA materials. The resulting microstructures were characterized using x-ray diffraction and scanning electron microscope with energy-dispersive spectrometry. The porosity, compressive strength and corrosion resistance of the porous nanocomposites were investigated. The results show that the compressive strength of porous Ti-HA can be tailored between 210 and 497 MPa by changing the content of blowing agent between 25 and 75%. The increase in the weight ratio of titanium hydride (from 25 to 75%) causes almost a twofold increase in the porosity and decreases the corrosion resistance (by almost two orders of magnitude). The present study has demonstrated that the porous Ti-HA nanocomposites are promising scaffold biomaterials for bone tissue engineering by virtue of their appropriate mechanical and corrosion properties and highly porous structure.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Enhanced Mechanical, Tribological, and Thermal Properties of Fe{sub 3}Al Composites with Carbon Nanotubes

In this work, Fe{sub 3}Al-based composites reinforced with carbon nanotubes were fabricated by a hot-press sintering process under various sintering pressures. The effects of sintering pressure and additives (alumina and lanthanum) on both the physical and the mechanical properties were systematically investigated. Results showed that with the sintering pressure of 15 MPa and incorporation of carbon nanotubes (CNTs), the microhardness of Fe{sub 3}Al composite exhibited a peak value at 1038.1 HV, which was ~ 60% higher than those without the addition of carbon nanotubes (644.9 HV). Also, both the compressive yield strength and the thermal conductivity of Fe{sub 3}Al composite were also significantly improved. Results of wear test showed the Fe{sub 3}Al composite exhibited a low wearing rate (10{sup −6}-10{sup −5} order of magnitude) and a friction coefficient (0.53-0.67).

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure, Texture, and Mechanical Properties of AM60 Magnesium Alloy Processed by Extrusion and Multidirectional Forging

In the present paper, multidirectional forging (MDF), as a severe plastic deformation process, was carried out on an extruded AM60 magnesium alloy. The microstructure, texture, and mechanical properties of the MDFed samples were examined. Microstructural findings indicated that the grain size of the alloy decreased from 10.2 to 3.7 µm after six MDF passes, carried out at a constant temperature of 220 °C. Textural studies revealed that after six MDF passes, the fiber texture of the extruded material was transformed to another texture, in which the basal planes tend to rotate toward 45{sup o} to the transverse direction. Shear punch testing (SPT) was employed to assess the shear yield stress (SYS) and ultimate shear strength (USS) at room temperature. Both SYS and USS were enhanced after two MDF passes, but decreased in the next pressing stages, despite developing finer grains at higher levels of imposed strain. The observed drop in strength is ascribed to texture softening that counterbalances the grain refinement strengthening effects. The variation in average microhardness values after different passes of MDF showed the trend that was in concurrence with the rise and fall of the SPT results. Finally, the changes in mechanical properties were discussed, based on the microstructural and textural findings.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of ECAP Die Angles on Microstructure Mechanical Properties and Corrosion Behavior of AZ80 Mg Alloy

In this article, the effect of channel angles on material properties was investigated during equal channel angular pressing of AZ80 magnesium alloy using processing route R at 325 °C processing temperature. Channel angles of 90° and 110° and common corner angle 30° have been considered for this study. It has been revealed that the channel angle has a significant influence on deformation homogeneity, microhardness, ultimate tensile strength, ductility and corrosion behavior of Mg alloys. Investigation with reference to as-received AZ80 Mg alloy indicates 18.47% improvement in UTS and 76.07% enhancement in ductility after processing through 3P-90° and 2P-110° ECAP, respectively. Also, the corrosion rate reduces to 89.47% after processing the sample with 3P-110° ECAP die.

36 MATERIALS SCIENCE↗
DOE OSTI2019

A Study on Peripheral Grain Structure Evolution of an AA7050 Aluminum Alloy with a Laboratory-Scale Extrusion Setup

A laboratory-scale hot extrusion setup was designed to investigate recrystallization and grain growth behavior of an AA7050 alloy during extrusion and subsequent heat treatments. Compared with industrial extrusion, the laboratory-scale process enabled rapid water quenching of extrudate with less delay so that the dynamic grain structure development was captured. After extrusion, static microstructure evolution in the extrudates was studied using salt bath annealing for 5 and 15 s at 490 °C and solutionization treatment for 1 h at 490 °C. The salt bath annealing was a simulation of the delay of press quenching in typical industrial extrusion practices. In the as-quenched extrudates, the peripheral region mainly exhibited continuous dynamic recrystallization and geometric dynamic recrystallization, whereas in the core region discontinuous dynamic recrystallization dominated. A <100> and <111> double fiber texture was identified in extrudates, and recrystallization behavior was found to be orientation dependent. The <100> oriented grains contained more sub-grain boundaries and better-defined sub-grains and had a higher tendency to fragment via continuous recrystallization, while the <111> oriented grains produced less sub-grain boundaries and did not recrystallize. Subsequent heat treatments resulted in static recrystallization and abnormal growth of the continuously recrystallized grains. Additionally, the effects of extrusion temperature (440, 480 and 520 °C) and punch speed (0.7, 1.4 and 2.1 mm/s) on grain structure were discussed. A revised grain structure evolution mechanism based on the observation of 7050 extrusion was proposed.

36 MATERIALS SCIENCE↗