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At least 1,567 records · Page 87
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DOE OSTI2019

Enhanced Surface Properties and Microstructure Evolution of Cr12MoV Using Ultrasonic Surface Rolling Process Combined with Deep Cryogenic Treatment

In this study, the enhanced surface properties and microstructure evolution of quenched and tempered cold work die steel Cr12MoV, which were induced by ultrasonic surface rolling process combined with deep cryogenic treatment (UDCT), were systematically investigated. The results indicated that UDCT had an advantage over conventional ultrasonic surface rolling process (USRP) in improving surface mechanical properties, including smaller surface roughness and smoother machined surface with less cracks and defects; higher surface microindentation hardness with the value of ~ 890 HV (increased by ~ 6.2% compared with USRP); and lower friction coefficient. Such enhancements in surface properties are mainly attributed to the martensitic transformation of retained austenite, the dispersion strengthening of small secondary carbides and the homogenized carbides distribution during UDCT.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Influence of Rolling Temperature on the Mechanical Properties and Microstructure of Variable-Plane-Rolled Mg-3Al-1Zn Alloy

AZ31 magnesium alloy was produced via variable-plane rolling (VPR) under different temperatures. The effects of rolling temperature on the microstructure, texture and mechanical properties were investigated. A double-peak basal texture was formed during VPR treatment. Dynamic recrystallization (DRX) was observed during the VPR treatment, and it can reduce the texture intensity owing to the coalescence of sub-grains. Four types of twins were observed in the VPR treatment: {10-12} extension twins, {10-13}, {10-11} contraction twins and {10-11}-{10-12} double twins. The suitable temperature of AZ31 magnesium alloy for VPR treatment is determined as 623 K through experiments with tensile yield strength of 240 MPa, compressive yield strength of 162 MPa and elongation of 13%. Owing to the important impact of temperature on mechanical properties, the alloy was further VPRed with a decrease in temperature pass-by-pass. The high temperature of 623 K at the initial passes randomized the texture, which led to notably high ductility.

36 MATERIALS SCIENCE↗
DOE OSTI2019

The Effect of Brazing on Microstructure of Honeycomb Liner Material Hastelloy X

Labyrinth seals with abradable metallic honeycomb structures are common in gas turbines to ensure a tight sealing between rotor and stator. These seals have a big impact on the turbine efficiency. The honeycombs made from nickel-based superalloys are brazed onto backing segments in order to form liners. In this work, the microstructure of Hastelloy X honeycombs after brazing with a nickel-chromium-silicon braze filler alloy is investigated. It was found that the braze filler can wet the honeycomb foils over their entire height up to the contact surface with the rotor. After solidification, the braze filler alloy forms γ and silicides (NiSi and Ni{sub y}(Mo, Cr){sub x}Si). The intermetallic silicides give high hardness to the honeycomb liners. Between the braze filler alloy and the honeycomb foils, interdiffusion zones are observed. For the rub-in behavior of the honeycomb liners, not only the foil material but also the comportment of the composite made up of the foil material, the braze filler material and the interdiffusion zones has to be considered.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Microstructure and Properties of Ni-WC Composite Coatings on Their Solid Particle Erosion Behavior

Metal matrix composites (MMCs) of WC in Co or Ni matrices are widely used due to their high wear resistance. Previous studies showed that tailoring the mechanical properties through optimum WC content can significantly extend the life cycle of MMCs. In this study, composite coatings of Ni with various WC content ranging from 10 to 55 vol.% were cold sprayed using agglomerated WCNi powders and two different sizes of Ni powders, − 10 + 4 (d{sub 50} = 7 µm) referred to as Ni(7) and − 35 + 15 (d{sub 50} = 25 µm) to as Ni(25). Microstructural characterization, including the deformed structure of Ni splats and retention and distribution of WC, was performed by scanning electron microscopy (SEM). Composite coatings were subjected to solid particle erosion following ASTM G76-13 standard. At low WC content coatings, erosion occurred primarily by plastic deformation of Ni matrices and knocking out of WC particles. At high WC concentrations, network distribution of compacted WCNi particles in the matrices led to a significant drop in erosion rates of coatings, due to a change in erosion mechanism from severe cutting and plowing of Ni and WC dislodging to WCNi splats spalling. Composite coatings sprayed using finer size Ni powder were usually more erosion resistant.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and Mechanical Characterization of Incoloy 925 Welds in the As-Welded and Direct Aged Conditions

The joints of Incoloy 925 obtained from gas tungsten arc welding (GTAW) with pulsating current utilizing ERNiCrMo-3 and ERNiCrMo-10 were investigated under as-welded and direct aged conditions. The microstructure showed the microsegregation of Nb-Mo-, Ti-rich phases in the weld zone of ERNiCrMo-3 in the as-welded condition. Post-weld heat treatment (PWHT) involving direct aging was carried out on the weldments at 732 °C for 4 h followed by air cooling. On direct aging, the tensile strength of the base metal increases by 40% when compared to the as-received ones. The failures were experienced in the parent metal of Incoloy 925 upon tension testing in as-welded conditions. Although the tensile strength was improved considerably, the failures occurred at the fusion zones, for both the fillers in the direct aged conditions. The joint efficiencies of Incoloy 925 welds employing ERNiCrMo-3 and ERNiCrMo-10 were reported as 95 and 86%, respectively. The impact toughness of the weldments in both the as-welded and PWHT conditions was considerably greater compared to that of the candidate metal.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and Mechanical Properties of As-Cast γ-TiAl Alloys with Different Cooling Rates

γ-TiAl alloys composed of Ti-45Al-5Nb-1W-1B (at.%) solidified at three different cooling rates by vacuum electromagnetic induction melting under a controlled degree of vacuum of under 2 Pa. The influence of the cooling rate on the microstructure and mechanical properties of the γ-TiAl alloy samples was investigated. A relatively large temperature gradient at the beginning of solidification led to a thin fine-grained layer around the graphite mold, and undercooling had two main effects: (a) promoting nucleation in the grain-refining surface layer and (b) promoting the growth of columnar crystals beneath the surface. Moreover, all prepared ingots of this alloy could be clearly divided into Regions S, M and C from the surface to the core. The colony size of Region M was larger than those of the other two regions due to composition segregation and the growth of columnar grains. The existence of tungsten led to micro-inhomogeneity, and the mechanical properties of the alloy changed according to concentration fluctuations. The degrees of homogeneity were expressed as H{sub 0.1} and H{sub 0.2}. The alloy with the highest values, which possesses the best mechanical properties, solidified at a relatively low cooling rate. The deformation mechanism is also discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructural Analysis of Powder Metallurgy Tool Steels in the Context of Abrasive Wear Behavior: A New Computerized Approach to Stereology

The present work describes a new methodology designed to characterize the microstructures of tool steels containing carbide hard phases, with the focus set on their abrasive wear resistance. A series of algorithms were designed and implemented in MATLAB® to (i) recognize each of the features of interest, (ii) measure relevant quantities and (iii) characterize each of the phases and the alloy in function of attributes usually neglected in wear description applications: size distribution, shape and contiguity of the hard phases. The new framework incorporates new parameters to describe each one of these attributes, as observed in SEM micrographs. All three aforementioned stages contain novel contributions that can be potentially beneficial to the field of materials design in general and to the field of alloy design for severely abrasive environments in particular. Models of known geometry and micrographs of different powder metallurgy steels were analyzed, and the obtained results were compared with the obtained by the linear intercept method. The relation between the new parameters and the ones available in the scientific literature is also discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effects of Bi-Sb Addition and Solution Treatment on Microstructures and Mechanical Properties of Al-20 wt.% Mg{sub 2}Si Composites

In this study, the effects of compound modification with Bi and Sb and solution treatment on the microstructures evolution and the mechanical properties of Al-20 wt.% Mg{sub 2}Si composites were investigated. The specimens were observed by optical microscope, scanning electron microscope equipped with an energy-dispersive spectrometer and x-ray diffraction. The characterization results showed that 2.0 wt.% Sb and 0.2 wt.% Bi addition refined the primary Mg{sub 2}Si from 95 to 10 μm, accompanied by a change in morphology from coarse dendrites to dispersive fine polygons. After solution treatment at 540 °C for 15 h, the sharp polygonal primary Mg{sub 2}Si particles were blunted and the flake-like eutectic Mg{sub 2}Si phases were changed to small spherical particles, resulting in the increase in the Brinell hardness of the composites from HB 94.53 to 134.67. Mechanical tests revealed that both tensile strength and hardness of the composite were improved by compound modification with Bi and Sb, and the ultimate tensile strength (UTS) was increased from 216 MPa unmodified to 257 MPa with optimum modification of 2 wt.% Sb and 0.2 wt.% Bi. A propounded change in hardness from HB 94.53 to 130.67 was also investigated.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effects of Annealing on the Microstructures and Mechanical Properties of Cold-Rolled TB8 Alloy

The microstructure and mechanical properties of TB8 cold-rolled plate with a thickness reduction of 50% during annealing at different temperatures are investigated. The results show that annealing for 1 h below 750 °C does not induce the recrystallization transition, remaining the main texture components of {001} <110>, {113} <110> and {112} <110>. The ellipsoidal ω phase is formed when the alloy is annealed at 400 °C. α phase is formed when the alloy is annealed from 450 to 750 °C. The size and volume fraction of α phase increases first and then declines in control of driving force of phase transformation and diffusion. The synergistic effect of the precipitation of ω phase and α phase and the disappearance of dislocation makes the tensile strength of the plate decrease first, then increase, and finally decrease with the increase in annealing temperature. The change in plasticity is opposite to that of strength.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure Characteristics and Comparative Analysis of Constitutive Models for Flow Stress Prediction of Inconel 718 Alloy

An accurate constitutive model is essential for analyzing deformation behavior of material and reliable numerical simulations in metal forming processes. In this study, hot tensile tests of Inconel 718 alloy have been conducted over a wide range of temperatures (300-973 K at an interval of 100 K), strains (0.01-0.3 at an interval of 0.01) and quasi-static strain rates (0.0001, 0.001, 0.01 s{sup −1}). Flow stress behavior is significantly affected by test temperatures and strain rates. Microstructure characteristics of deformed test specimens have been examined using scanning electron microscope and electron backscatter diffraction (EBSD). The fractography study revealed that fracture is mix-mode type, i.e., ductile and brittle. Subsequently, EBSD analysis shown that dynamic recrystallization mechanism is more pronounced at a higher temperature. Furthermore, four constitutive models, namely modified Cowper–Symonds, modified Johnson Cook, modified Zerillie-Armstrong and integrated Johnson Cook–Zerillie-Armstrong (JC-ZA) models have been investigated for flow stress prediction. Capability of models has been evaluated based on the correlation coefficient (R), average absolute error (Δ) and its standard deviation (δ). Accurate prediction of flow stress behavior is found by integrated JC-ZA model with R = 0.9873, Δ = 2.44 and δ = 4.08%.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Chemical Composition on the Microstructure and Properties of the Ag-Ga-Ge Alloys

This study presents a comparison of thermodynamically calculated phase compositions of Ag-Ga-Ge alloys at fixed temperatures with phase analysis results of alloys annealed at respective temperatures. Three isothermal sections of the Ag-Ga-Ge system at 25, 100 and 300 °C have been extrapolated using optimized thermodynamic parameters from the literature. An experimental analysis of the microstructure was carried out using optical microscopy, scanning electron microscopy and energy-dispersive spectrometry. Results obtained by x-ray powder diffraction methods (identified phases and lattice parameters) were compared with the results of the predicted phase equilibria. Good overall agreement between experimental and calculated values was obtained. The Brinell hardness and electrical conductivity of selected alloys were measured and obtained results were analyzed with respect to their overall compositions and phase constituents.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and Corrosion Behavior of Cr Coating on M50 Steel Fabricated by Electrospark Deposition

Cr coatings are deposited on M50 steel by electrospark deposition with different parameters (100 V-60 μF, 100 V-90 μF, 100 V-120 μF, 150 V-60 μF, 150 V-90 μF and 150 V-120 μF). Influences of the depositing parameters on the microstructure and the corrosion behavior of the coatings were experimentally investigated. There is a linear trend of growth between the mass gain per area and the deposition cycles before reaching the maximum values. The voltage has more influence on the mass gain per area than the capacitance. The coatings display a splash-like pattern decorated with a few of micro-cracks. The micro-cracks are more sensitive to the voltage than the capacitance. The anti-corrosion ability of the coating is significantly better than the M50 steel. Local corrosion occurs in the coating when there are through-coating cracks. The coating deposited under 150 V-60 μF has the minimum defect density and the best anti-corrosion behavior.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Heat Input on Microstructure, Residual Stress, and Corrosion Resistance of UNS 32101 Lean Duplex Stainless Steel Weld Joints

In the present work, the influence of heat input on phase balance, residual stresses and corrosion resistance of UNS 32101 lean duplex stainless steel welds has been investigated. Three weldment types were obtained with different heat inputs by varying welding current, using gas tungsten arc welding process. The results revealed that increasing heat input caused a significant microstructural evolution with high austenite fraction in weld metals. The residual stresses were measured on the surface and through the sample’s thickness by x-ray diffraction technique based on Sin{sup 2}ψ method. The results showed an inversely proportional relationship between the heat input and the compressive residual stresses on the surface and through the thickness of the weld metal. Potentiodynamic polarization tests of the different welded joints evaluated in 3.5% NaCl solution exhibited high corrosion resistance of the weld metal produced with the low heat input.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effects of CeO{sub 2} on the Microstructure and Properties of Laser Cladding 316L Coating

Laser cladding was used to distribute a CeO{sub 2}-modified 316L powder coating onto a 316L stainless steel surface. The cladding layer phase composition was evaluated through x-ray diffraction. An analysis of the microstructure was conducted using an optical microscope and scanning electron microscope, while the micro-hardness was measured using a tester. The coating corrosion behavior in response to 3.5% NaCl solution was analyzed by polarization curves and electrochemical impedance spectroscopy. Finally, the overall performance of coatings with different CeO{sub 2} concentrations, 1, 2, and 3%, was compared. Results showed that adding CeO{sub 2} significantly refined the grains, reduced the coating porosity, and effectively limited the appearance of local corrosion. Also, additive CeO{sub 2} shifted the coating’s electrode potential in a positive direction, minimizing the current density of corrosion, creating a stable passivation film, and improving the coating’s corrosion resistance. The coating performance (hardness and corrosion behavior) was optimal when 2% CeO{sub 2} was employed as the powder mass.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Experimental Study on Microstructure and Hardness of Pure Titanium Subjected to Torsion Deformation at Different Temperatures

The studies of mechanical property, microstructure evolution and fracture analysis in pure titanium processed by torsion deformation at 298, 673 and 1073 K are conducted systematically. The variations of mechanical property of deformed pure titanium are shown through Vickers hardness evaluation. During torsion at 298 K, the grains are refined and elongated, but the α phase with different shapes precipitates for twisted samples at 673 and 1073 K. The fracture appearance indicates that the elongated dimples occur on fracture surface at 298 K. Besides, a large number of shear facets are arranged. However, typical intergranular fracture appearance with lots of blocks in polyhedral shape covers the fracture surface at 673 and 1073 K, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Nb Content on Microstructures and Mechanical Properties of Ti-xNb-2Fe Alloys

β-Type Ti-Nb-based alloys exhibit satisfactory biocompatibility and low Young’s modulus for biomedical applications. The microstructure and mechanical properties of a series of Ti-(14, 16, 18, 20, 22, 24)Nb-2Fe alloys fabricated by arc melting were investigated by XRD, optical microscopy, and tensile tests. Both ω and α″ phases existed in the Ti-14Nb-2Fe alloy, while just a single β phase existed in the other alloys. Twinning is an important deformation mechanism that causes work hardening and twinning-induced plasticity. It was found in the Ti-(14, 16, 18, 20)Nb-2Fe alloys and not in the Ti-22Nb-2Fe alloy. The Ti-14Nb-2Fe alloy exhibited the highest tensile strength and the highest Young’s modulus owing to the existence of the ω phase. The tensile strength decreased gradually from 830 MPa (highest) for the Ti-14Nb-2Fe alloy to 540 MPa (lowest) for the Ti-24Nb-2Fe alloy with an increase in the Nb content. The Young’s modulus decreased from 90 GPa for the Ti-14Nb-2Fe alloy to 63 GPa for the Ti-22Nb-2Fe alloy and then increased to 71 GPa for the Ti-24Nb-2Fe alloy. Elongation shows the same trend as the Young’s modulus. The Ti-22Nb-2Fe alloy, with a low Young’s modulus of 63 GPa, tensile strength of 570 MPa, and 15% elongation, was found suitable for biomedical applications. The Ti-20Nb-2Fe alloy also exhibits a high tensile strength, a Young’s modulus ratio of 9.24 × 10{sup −3}, and 18% elongation and is thus considered another valuable Ti alloy for biomedical applications.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Influence of FSW Repairing Process on the Microstructures and Mechanical Properties of Friction Stir-Welded 6082Al Alloy

When two 42-mm 6082Al plates were welded by friction stir welding (FSW), a discontinuously distributing groove defect is formed on the surface of the joint. Subsequently, the FSW repairing process was conducted at the defect location. The influence of the repairing process on the microstructures and mechanical properties of the joints were studied, and a description of the softening mechanism of the repaired joints is provided. The results show that TMAZ at the advance side is the weakest zone of the as-welded joints. The repairing process increases the heat input into the joints and thus promotes the coarsening and transformation of the precipitates in the joints, which further weakens the repaired joints. The average tensile strength of the joints repaired with a 24-mm pin is 205 MPa, which is 91% of that of the as-welded joint. The tensile strength generally increases when the size of the repaired pin decreases. The average tensile strength of the joints repaired with an 8-mm pin is 220 MPa, which is equivalent to the initial joints.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Heat Treatment on Microstructure Evolution and Mechanical Properties of Selective Laser Melted Inconel 718 Alloy

The microstructure characteristics of the selective laser melted Inconel 718 alloy under as-deposited, homogenization + solution + aging (HSA) and solution + aging (SA) conditions were studied. The anisotropy and heterogeneity of mechanical properties under different conditions were also investigated. Under the as-deposited condition, the morphology and size of the grains are heterogeneous. The dendrite structures which grow nearly perpendicular to the molten pool traces, accompanying interdendritic Laves phase, can be observed within the grains. The dendrite structure completely disappeared, and the Laves phases embedded in the interdendritic regions also dissolved into the matrix to precipitate γ′ and γ″ phases after heat treatments. Fully recrystallized grains were obtained under the HSA condition, while only incomplete recrystallized grains were obtained under the SA condition. However, the characteristics of γ′ and γ″ phases are very similar under the HSA and SA conditions. Significant improvement in strength after heat treatments was due to the dissolution of undesirable Laves phase and the precipitation of γ′ and γ″ phases. For all the three conditions, different tensile properties were observed depending on the orientation of the specimens. The scatter of mechanical properties is notable, and heat treatments increased the scatter of mechanical properties.

36 MATERIALS SCIENCE↗