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At least 1,351 records · Page 75
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Martensitic Transformation and Magnetic Properties of the CuAl, CuAlMn, and CuAlMnZn Alloys

In the present work, the effect of alloying elements on the phase transformation which are formed by cooling effect in CuAl, CuAlMn, and CuAlMnZn (wt%) alloys are investigated using scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM) characterization techniques. From SEM observations, the martensite formation with different morphologies is observed in these alloys. But the results of the scanning electron microscope investigations presented in this work have not formed the precipitates in the CuAl alloy with Mn or Zn additions. We found that magnetic saturation and coercivity values decreased with the addition of Mn and Zn elements to the CuAl alloy. The findings confirmed the variation in magnetic properties, which occurred in the result of adding different alloying elements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Metamagnetism, Multiple Magnetic Transitions and Wide Temperature Range Magnetocaloric Effect in Dy{sub 6.5}Co{sub 2}Si{sub 2.5} Alloy

We report the structure, magnetic, thermodynamic, transport, and magnetocaloric properties of Dy{sub 6.5}Co{sub 2}Si{sub 2.5} alloy. It shows that this alloy has been formed in three phases viz., Dy{sub 5}Si{sub 3}, Dy{sub 3}Co{sub 2.2}Si{sub 1.8}, and Dy{sub 3}Co with four successive magnetic transitions named as T{sub 1}, T{sub 2}, T{sub 3},T{sub 4}, at 43, 74.7, 90.5, and 132.4 K, respectively. The magnetic measurements display competing ferromagnetic and antiferromagnetic interactions in the alloy. At 2.5 K, the alloy exhibits hard ferromagnetic properties with a coercive field of 0.8 T. Metamagnetism is also evidenced in the isothermal magnetization measurements. The Arrot plot shows that this material undergoes first-order magnetic transitions below 47 K, and above which, magnetic transitions are of second order. The four successive magnetic transitions together with competing magnetic interactions give rise to a broadened magnetocaloric effect (MCE) peak with temperature width of ΔT = 83.8 and 93.2 K and consequently a large RC value of 474 and 739 J/kg, for a magnetic field change of 0–5 and 0–7 T, respectively. Additionally, it is found that the alloy exhibits negative magnetoresistance and attains a value of − 11 and − 7% at 2.5 and 50 K, respectively, in applied magnetic field of 9 T. Thus, the peculiar magnetic and magnetocaloric properties make the Dy{sub 6.5}Co{sub 2}Si{sub 2.5} alloy promising not only for low-temperature magnetic refrigeration but also for fundamental study perspectives.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Comparison of first principles and semi-empirical models of the structural and electronic properties of Ge{sub 1−x}Sn{sub x} alloys

We present and compare three distinct atomistic models—based on first principles and semi-empirical approaches—of the structural and electronic properties of Ge{sub 1−x}Sn{sub x} alloys. Density functional theory calculations incorporating Heyd–Scuseria–Ernzerhof (HSE), local density approximation (LDA) and modified Becke–Johnson (mBJ) exchange-correlation functionals are used to perform structural relaxation and electronic structure calculations for a series of Ge{sub 1−x}Sn{sub x} alloy supercells. Based on HSE calculations, a semi-empirical valence force field (VFF) potential and sp{sup 3}s{sup ∗} tight-binding (TB) Hamiltonian are parametrised. Comparing the HSE, LDA+mBJ and VFF+TB models, and using the HSE results as a benchmark, we demonstrate that: (1) LDA+mBJ calculations provide an accurate first principles description of the electronic structure at reduced computational cost, (2) the VFF potential is sufficiently accurate to circumvent the requirement to perform first principles structural relaxation, and (3) VFF+TB calculations provide a good quantitative description of the alloy electronic structure in the vicinity of the band edges. Our results also emphasise the importance of Sn-induced band mixing in determining the nature of the conduction band structure of Ge{sub 1−x}Sn{sub x} alloys. The theoretical models and benchmark calculations we present inform and enable predictive, computationally efficient and scalable atomistic calculations for disordered alloys and nanostructures. This provides a suitable platform to underpin further theoretical investigations of the properties of this emerging semiconductor alloy.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Corrosion Performance of Fe-Based Alloys in Simulated Oxy-Fuel Environment

The long-term corrosion of Fe-based alloys in simulated oxy-fuel environment at 1023 K (750 A degrees C) was studied. Detailed results are presented on weight change, scale thickness, internal penetration, microstructural characteristics of the corrosion products, and the cracking of scales for the alloys after exposure at 1023 K (750 A degrees C) for up to 3600 hours. An incubation period during which the corrosion rate was low was observed for the alloys. After the incubation period, the corrosion accelerated, and the corrosion process followed linear kinetics. Effects of alloy, CaO-containing ash, and gas composition on the corrosion rate were also studied. In addition, synchrotron nanobeam X-ray analysis was employed to determine the phase and chemical composition of the oxide layers on the alloy surface. Results from these studies are being used to address the long-term corrosion performance of Fe-based alloys in various coal-ash combustion environments and to develop methods to mitigate high-temperature ash corrosion.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Evaporation of Cu, Sn, and S from Fe-C-Cu-Sn-S Liquid Alloys in the Temperature Range from 1513 K to 1873 K (1240 °C to 1600 °C)

Evaporation of Cu or Sn from liquid iron alloys containing C and S was experimentally investigated. The initial C concentration, [pct C] 0 , in the liquid alloy was varied from zero to C saturation, and the evaporation temperature was varied from 1513 K to 1773 K (1240 °C to 1500 °C). Along with the report by one of the present authors, the evaporation mechanism of Cu and Sn from liquid Fe-C-S alloy is proposed, after a modification from the previous mechanism. It was proposed that Cu and Sn evaporate as Cu(g) and Sn(g) and also evaporate as CuS(g) and SnS(g), which are more volatile species. Therefore, availability of S in the alloy affects the overall evaporation rate of Cu and Sn. At the same time, C in the alloy also forms volatile carbosulfides CS(g) and CS 2 (g), thereby competing with Cu and Sn. Moreover, C increases the activity coefficients of Cu, Sn, and S. This increases the thermodynamic driving force for the formation of CuS(g) and SnS(g). Therefore, increasing [pct C] partly accelerates the evaporation rate of Cu and Sn by increasing the activity coefficient but partly decelerates the evaporation rate by lowering the available S content. S partly accelerates the evaporation rate by increasing the available S for the sulfide gas species but partly decelerates the evaporation rate due to the surface poisoning effect. Increasing the reaction temperature increases the overall evaporation rate. All these facts were taken into account in order to develop an evaporation rate model. This model was extended from the present authors’ previous one by taking into account (1) CS(g), S(g), and CS{ 2 (g) (therefore, the following species were considered as dominant evaporating species: Cu(g), CuS(g), Sn(g), SnS(g), S(g), CS(g), and CS 2 (g)); (2) the effect of C and temperature on the activity coefficients of Cu, Sn, and S; (3) the effect of C and temperature on the density of the liquid alloy; and (4) the effect of temperature on the S adsorption coefficient. This revised evaporation model was used in order to explain the experimental data, and it showed good agreement. In particular, it was found that the temperature showed a significant effect on the evaporation rate, and the effect of temperature and C content on the activity coefficients of Cu, Sn, and S also significantly affected the evaporation rate. The chemical reaction rate constant of the individual evaporation reaction (k i } R ) and residual rate constant (k i r ) could be obtained as a function of temperature. The activation energy of each evaporation reaction was derived and discussed. The evaporation rate model can be applied in order to predict the content of Cu and Sn remaining in liquid iron under various conditions of temperature and [pct C].

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of Copper Content and Mold Temperature on the Hot Tearing Susceptibility of Mg-7Zn-xCu-0.6Zr Alloys

The effects of copper (Cu) content and mold temperature on the hot tearing susceptibility (HTS) of Mg-7Zn-xCu-0.6Zr (x = 0, 1, 2) alloys were investigated experimentally using a T-type hot tearing mold. The crack volumes were measured by paraffin permeation method, and the solidification process was analyzed by thermal analysis and by using the Clyne and Davies model. The microstructure and morphology of the crack zone of Mg-7Zn-xCu-0.6Zr alloys were characterized by scanning electron microscopy, and the composition of alloys was analyzed by X-ray diffraction. The results showed the formation of a new phase of MgZnCu in the alloys with Cu addition. Moreover, addition of Cu also led to the refinement of microstructures of Mg-7Zn-xCu-0.6Zr alloys. The HTS of Mg-7Zn-xCu-0.6Zr alloys decreased with the increase in both the content of Cu and the mold temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Microstructure and Thermal Stability of A357 Alloy With and Without the Addition of Zr

The principal purpose of this research was to evaluate the effects of Zr on the microstructure and thermal stability of an A357 alloy that has been subjected to an aging treatment (T6) and thermal exposure (250 °C). The results show that the addition of Zr had a significant influence on the refinement of the grain size, which enhanced the hardness and tensile strength of the A357 alloy under the T6 condition. During thermal exposure at 250 °C, the rodlike metastable β’-Mg{sub 2}Si precipitates transformed into coarse equilibrium phase β-Mg{sub 2}Si precipitates, resulting in a significant drop in the hardness and tensile strength of the T6 heat-treated A357 alloy. However, after thermal exposure, coherent, finely dispersed Al{sub 3}Zr precipitates were found to be formed in the T6 heat-treated A357 alloy. The addition of 0.1% Zr played a critical role in improving the high-temperature strength. Consequently, the A357 alloy with the addition of Zr demonstrated better mechanical properties at room temperature and high temperature than the alloy without Zr, in terms of both microstructure and thermal stability.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Influence of MAO Treatment on the Galvanic Corrosion Between Aluminum Alloy and 316L Steel

To slow down the galvanic corrosion of aluminum alloy and 316L stainless steel in subsea water, a micro-arc oxidation (MAO) coating was prepared on the surface of the Al alloy, and no treatment was performed on the surface of the 316L. The surface morphology of MAO-coated Al alloy was evaluated using a scanning electron microscope (SEM) before and after corrosion. A micro-hardness tester was used to measure the micro-hardness. Corrosion behaviors were evaluated by open-circuit potential (OCP), potentiodynamic polarization (PDP) and electrode impedance spectroscopy (EIS) tests in a 3.5 g/L NaCl solution. The results of PDP testing show that the corrosion potential of the MAO-coated galvanic pair was more positive than that of the uncoated galvanic pair and that the corrosion current density was smaller than that of the uncoated galvanic pair. EIS results show that the impedance of the galvanic pair increased after MAO coating. SEM images show that the corrosion damage of the uncoated Al alloy was more severe than that of the MAO-coated one, and the post-corrosion images of the surface of the 316L connected with MAO-coated Al alloy were more compact than those of the 316L connected with uncoated Al alloy. A physical model was developed to discuss the influence of MAO treatment on the galvanic corrosion process and corrosion mechanism.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of Scandium Addition and T7 Heat Treatment on Thermal Stability and Corrosion Properties of A201 Alloy

This study elucidates how scandium addition and T7 heat treatment affects thermal stability and corrosion properties of A201 alloy. The as-cast and the T7 heat-treated specimens were thermally exposed at 300 °C for 120 h to investigate the evolution of the Sc-free and the Sc-containing alloys. The results indicated that adding 0.6 wt.% scandium to A201 alloy could produce Al{sub 3}Sc, W phase (Al{sub 8-5.4}Cu{sub 6.6-4}Sc) and grain refinement, significantly enhancing the hardness and tensile strength in as-cast condition. The as-cast A201 alloy containing Sc also showed the least loss of hardness and strength during thermal exposure at 300 °C. In addition, the presence of W phase weakened the peak hardness and tensile strength of the Sc-containing A201 alloy during T7 heat treatment. Adding Sc to A201 alloy increased corrosion resistance in T7 heat-treated condition, but slightly decreased corrosion resistance in as-cast condition.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Constitutive Model for Hot Deformation of the Cu-Zr-Ce Alloy

Hot compressive deformation behavior of the Cu-Zr-Ce alloy has been investigated according to the hot deformation tests in the 550-900 °C temperature range and 0.001-10 s{sup −1} strain rate range. Based on the true stress–true strain curves, the flow stress behavior of the Cu-Zr-Ce alloy was investigated. Microstructure evolution was observed by optical microscopy. Based on the experimental results, a constitutive equation, which reflects the relationships between the stress, strain, strain rate and temperature, has been established. Material constants n, α, Q and ln A were calculated as functions of strain. The equation predicting the flow stress combined with these materials constants has been proposed. The predicted stress is consistent with experimental stress, indicating that developed constitutive equation can adequately predict the flow stress of the Cu-Zr-Ce alloy. Dynamic recrystallization critical strain was determined using the work hardening rate method. According to the dynamic material model, the processing maps for the Cu-Zr and Cu-Zr-Ce alloy were obtained at 0.4 and 0.5 strain. Based on the processing maps and microstructure observations, the optimal processing parameters for the two alloys were determined, and it was found that the addition of Ce can promote the hot workability of the Cu-Zr alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Functional Properties of Porous Ti-48.0 at.% Ni Shape Memory Alloy Produced by Self-Propagating High-Temperature Synthesis

The functional behavior of the porous shape memory alloy produced by self-propagating high-temperature synthesis from the Ti-48.0 at.% Ni powder mixture was studied. It was found that a large unelastic strain recovered on unloading and it was not attributed to the pseudoelasticity effect. A decrease in deformation temperatures did not influence the value of strain that recovered on unloading, while the effective modulus decreased from 1.9 to 1.44 GPa. It was found that the porous Ti-48.0 at.% Ni alloy revealed the one-way shape memory effect, where the maximum recoverable strain was 5%. The porous Ti-48.0 at.% Ni alloy demonstrated the transformation plasticity and the shape memory effects on cooling and heating under a stress. An increase in stress did not influence the shape memory effect value, which was equal to 1%. It was shown that the functional properties of the porous alloy were determined by the TiNi phase consisted of the two volumes Ti{sub 49.3}Ni{sub 50.7} and Ti{sub 50}Ni{sub 50} where the martensitic transformation occurred at different temperatures. The results of the study showed that the existence of the Ti{sub 49.3}Ni{sub 50.7} volumes in the porous Ti-48.0 at.% Ni alloy improved the functional properties of the alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Study on the Microstructure, Mechanical Properties and Corrosion Behavior of Mg-Zn-Ca Alloy Wire for Biomaterial Application

Due to their excellent biocompatibility and biodegradability, magnesium alloy wires have attracted much attention for biomaterial applications including orthopedic K-wires and sutures in wound closure. In this study, Mg-3Zn-0.2Ca alloy wires were prepared by cold drawing combined with proper intermediate annealing process. Microstructures, texture, mechanical properties and corrosion behavior of Mg-3Zn-0.2Ca alloy wire in a simulated body fluid were investigated. The results showed that the secondary phase and average grain size of the Mg-3Zn-0.2Ca alloy were refined in comparison with the as-extruded alloy and a strong (0002)<10-10>//DD basal fiber texture system was formed after multi-pass cold drawing. After the annealing, most of the basal planes were tilted to the drawing direction (DD) by about 35°, presenting the characteristics of random texture, and the texture intensity decreased. The as-annealed wire shows good mechanical properties with the ultimate tensile strength (UTS), yield strength (YS) and elongation of 253 ± 8.5 MPa, 212 ± 11.3 MPa and 9.2 ± 0.9%, respectively. Electrochemical and hydrogen evolution measurements showed that the corrosion resistance of the Mg-3Zn-0.2Ca alloy wire was improved after the annealing. The immersion test indicated that the Mg-3Zn-0.2Ca wire exhibited uniform corrosion behavior during the initial period of immersion, but then exhibited local corrosion behavior.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Sm-Rich Phase on Corrosion Behavior of Hot-Extruded AZ31-1.5Sm Magnesium Alloy

The effects of Sm on the corrosion and microstructure behavior of hot-extruded AZ31 magnesium alloy were investigated by SEM, TEM, weight loss analysis, and electrochemical measurements. The results indicated that granular Al{sub 2}Sm phase ~ 4 μm in size in the hot-extruded AZ31 magnesium alloy modified with 1.5 wt.% Sm leads to significant grain refinement. The corrosion rate decreased from 15.98 × 10{sup −4} to 11.19 × 10{sup −4} g cm{sup −2} h{sup −1} in the transverse section and from 8.57 × 10{sup −4} to 6.20 × 10{sup −4} g cm{sup −2} h{sup −1} in the longitudinal section. Compared to the unmodified alloy, the corrosion potential of the Sm-modified alloy in the transverse and longitudinal sections increased by 98 and 62 mV, respectively, and the R{sub ct} value (charge transfer resistance) in the transverse and longitudinal sections of the modified alloy increased from 1764 and 1756 to 2928 and 2408 Ω cm{sup 2}, respectively. The results showed that the corrosion resistance of hot-extruded AZ31 magnesium alloy was significantly improved by Sm addition due to the grain refinement, the decreased dislocation density, and the suppression of micro-galvanic corrosion caused by Al-Sm-(Mn) intermetallic compounds.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Notch Sensitivity and Failure Behavior of TiAl and K418 Alloys

The notch sensitivity of tensile specimens of TiAl and K418 alloys has been investigated, and the notch strength has been quantitatively analyzed. The fracture surface of the specimens has also been observed and analyzed by scanning electron microscopy (JEOL-6700F). By comparing notch rod tensile specimens and special notch rod tensile specimens, it was found that the basic nature, fracture driving force, and fracture criteria of the brittle TiAl alloy and the ductile K418 alloy are different. The final fracture of the K418 alloy is controlled by strain and not by stress, and the specimens do not exhibit notch sensitivity. However, the final fracture of the TiAl alloy is controlled by stress, and the specimens exhibit small notch sensitivity. For the special notch rod tensile specimens, the K418 specimens do not exhibit notch sensitivity. However, for the TiAl alloy, when the notch depth reaches 10% of the specimen diameter, the tensile strength decreases, and when the notch depth reaches 20% of the specimen diameters, the tensile strength sharply decreases.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Improving Corrosion Resistance and Corrosive Wear Resistance of Aluminum Alloy Drill Pipe by Surface Nanocrystallization and Micro-arc Oxidation

To improve the corrosion and wear resistance performance of aluminum alloy drill pipes in geological and petroleum drilling applications, a strengthening coating was prepared on the surface of the 2618 aluminum alloy by combining treatment with ultrasonic cold forging technology (UCFT) and micro-arc oxidation (MAO). The effects of the strengthening coating on the corrosion and wear resistance of the aluminum alloy were evaluated by electrochemical and tribological tests. The results demonstrated that the corrosion and wear resistance of the aluminum alloy were improved by the UCFT and MAO technologies, especially the salt corrosion resistance. The corrosion morphology of the strengthening coating primarily included pitting, micro-cracks and NaCl attachment. In addition, the UCFT + MAO composite coating exhibited the best wear performance. The wear rate of the UCFT + MAO composite coating was lower than that of the aluminum alloy by 94%. The results provided support for this excellent surface treatment technology for the corrosion and wear resistance of aluminum alloy drill pipes, and they also present a feasible evaluation method for drill pipes.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Heat Treatment Conditions on Mechanical Properties and Precipitates in Sheet Metal Hot Stamping of 7075 Aluminum Alloy

Due to the low formability at room temperature, the application of 7XXX aluminum alloy sheet is limited. The hot forming-quenching process is an effective technology for the forming of some complex-shaped aluminum alloy parts. In this study, heat treatment was integrated with sheet metal stamping to improve formability and strength of 7075 aluminum alloy sheet. The effect of different heat treatment parameters on the strength of 7075 aluminum alloy sheet was investigated under stamping and heat treatment integrated process, based on which the optimal heat treatment combination was determined. Furthermore, the evolution of precipitates with the variation of aging temperature and time was studied by transmission electron microscope. In addition, quenching under water-cooled dies was conducted for 7075 aluminum alloy sheet after solution heat treatment, in which the hardness under die quenching was compared with that of underwater quenching and air quenching. Finally, the tensile test illustrates that, after die quenching and successive artificial aging, the tensile strength of 7075 aluminum alloy sheet was achieved at around 548 MPa. Shear fracture with long strip step-shaped pattern was observed in aging state.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Corrosion Resistance of Cu-Ni-Si Alloy Under High-Temperature, High-Pressure H{sub 2}S and Cl{sup −} Environments

In this study, Cu-Ni-Si alloy was subjected to corrosion resistance tests under simulated high-temperature, high-pressure H{sub 2}S and Cl{sup −} environments. The corrosion rates, electrochemical performance, surface morphologies, and chemical compositions of the Cu-Ni-Si alloy after corrosion were analyzed by the mass loss method, electrochemical impedance spectroscopy, scanning electron microscopy, energy-dispersive x-ray spectrometry, and x-ray diffraction. The results showed that, at the initial stage of corrosion, because of the loose corrosion product layer initially formed on the surface, a high corrosion rate was observed for the Cu-Ni-Si alloy under high-temperature, high-pressure environments. With the progress of corrosion, the corrosion rate gradually decreased and reached a steady state. Because at the later stage, a passive film was formed on the alloy surface, further inhibiting the corrosion of the alloy and improved its corrosion resistance. The Cu-Ni-Si alloy exhibited good corrosion resistance under high-temperature, high-pressure H{sub 2}S and Cl{sup −} environments.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Ni Addition to Sn0.7Cu Solder Alloy on Thermal Behavior, Microstructure, and Mechanical Properties

The addition of Ni on lead-free solder alloys Sn0.7Cu was tested in this study. A small amount of Ni was added to the solder alloys to evaluate the thermal behavior, microstructure, and mechanical properties of the composite solders after sintering and isothermal aging for 3 days and then compared with the results of the binary Sn0.7Cu solder. The results indicated that the ultimate tensile strength and yield tensile strength changed when adding 0.25 wt.% of Ni. The tested results of the differential scanning calorimeter showed that the addition of Ni (such as 0.5 and 1 wt.%) could obviously increase the solidification temperature of Sn0.7Cu alloys in the cooling process. When the Ni content was increased to 0.25 wt.% in the ternary condition, which includes the Sn0.7Cu-xNi (x = 0, 0.1, 0.25, 0.5, and 1 wt.%) solder alloys for the indentation creep test, the minimum creep rate reached the maximum; however, the trend was reversed as the Ni content was higher than this level. In addition, the microstructure of Sn-0.7Cu-xNi solder alloys was obviously different with the eutectic Sn-0.7Cu solder, such that the Ni gradually accumulated in the (Cu, Ni){sub 6}Sn{sub 5} IMCs within the Ni-containing solder alloys. Additionally, this process refined the microstructure of the Sn-0.7Cu solder. The fracture surface of the eutectic Sn-0.7Cu solder revealed ductile fracture modes; however, there was no mixed ductile–brittle fracture mode occurred when the Ni content was in the range of 0-1 wt.%.

36 MATERIALS SCIENCE↗