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Erdeniz, D.; Nasim, W.; Malik, J.; Yost, A.R.; Park, S.; De Luca, A.; Vo, N.Q.; Karaman, I.; Mansoor, B.; Seidman, D.N.; Dunand, D.C. |
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Effect of vanadium micro-alloying on the microstructural evolution and creep behavior of Al-Er-Sc-Zr-Si alloys |
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Journal Article |
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2017 |
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Acta Materialia |
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124 |
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501-512 |
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Aluminum alloys; Precipitation strengthening; High temperature creep; Atom-probe tomography; Microstructure |
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Abstract Al-Er-Sc-Zr-Si alloys, strengthened by L12-ordered, coherent Al3(Er,Sc,Zr) nanoscale precipitates, can be used for automotive and aerospace applications up to 400 °C. Vanadium, due to its small diffusivity in aluminum and its ability to form L12-ordered tri-aluminide precipitates, is a possible micro-alloying addition for further improving the service temperature of these alloys. Moreover, vanadium-containing Al3(Er,Sc,Zr,V) precipitates are anticipated to have a smaller lattice parameter mismatch with the matrix, thereby improving the alloy's coarsening resistance. In this study, the temporal evolution of microstructural and mechanical properties of an Al-0.005Er-0.02Sc-0.07Zr-0.06Si alloy micro-alloyed with V are investigated utilizing isochronal, isothermal and double-aging treatments and compared to the results obtained from an alloy that does not contain V, but otherwise has the same composition. Both isochronal and isothermal aging treatments reveal slower precipitation and coarsening kinetics for the V-containing alloy. A peak microhardness value of ~600 MPa is obtained after a double-aging treatment at 350 °C/16 h, followed by aging at 400 °C for 12 h. Transmission electron microscopy reveals a duplex-size precipitate microstructure, with the smaller precipitates having a mean radius <3 nm. Despite the expectation of a reduced creep resistance due to a lower precipitate/matrix lattice mismatch, both alloys have similar creep behavior at 400 °C, characterized by a threshold stress of 7.5 and 8 MPa under peak-aged and over-aged conditions, respectively. Thus, micro-additions of V to an Al-Er-Sc-Zr-Si alloy lead to enrichment of V in the Al3(Er,Sc,Zr,V) nano-precipitates, improving their coarsening resistance without deteriorating their ability to block dislocations under creep at 400 °C. |
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1359-6454 |
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NU @ karnesky @ |
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11515 |
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Lawrence, S.K.; Somerday, B.P.; Karnesky, R.A. |
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Elastic Property Dependence on Mobile and Trapped Hydrogen in Ni-201 |
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2017 |
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JOM |
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69 |
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1 |
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45-50 |
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Enhanced dislocation processes can accompany decohesion mechanisms during hydrogen degradation of ductile structural metals. However, hydrogen-deformation interactions and the role of defects in degradation processes remain poorly understood. In the current study, nanoindentation within specifically oriented grains in as-received, hydrogen-charged, aged, and hydrogen re-charged conditions revealed a "hysteresis" of indentation modulus, while the indentation hardness varied minimally. Thermal pre-charging with approximately 2000 appm hydrogen decreases the indentation modulus by 20%, aging leads to a slight recovery, but re-charging drives the modulus back down to values similar to those measured in the hydrogen-charged condition. This "hysteresis" indicates that dissolved interstitial hydrogen is not solely responsible for mechanical property alterations; hydrogen trapped at defects also contributes to elastic property variation. |
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1047-4838, 1543-1851 |
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NU @ karnesky @ lawrence_elastic_2017 |
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11516 |
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Karnesky, R.A.; Chao, P.; Buchenauer, D.A. |
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Hydrogen isotope permeation and trapping in additively manufactured steels |
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2017 |
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ASME Pressure Vessels and Piping Conference |
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6A |
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Additively manufactured (AM) austenitic stainless steels are intriguing candidates for the storage of gaseous hydrogen isotopes. Complex vessel geometries can be built more easily than by using conventional machining options. Parts built with AM steel tend to have excellent mechanical properties (with tensile strength, ductility, fatigue crack growth, and fracture toughness comparable to or exceeding that of wrought austenitic stainless steel). However, the solidification microstructures produced by AM processing differ substantially from the microstructures of wrought material. Some features may increase permeability, including both some amount of porosity and a greater amount of ferrite. Because the diffusivity of hydrogen in ferrite is greater than in austenite (six orders of magnitude at ambient temperature), care must be taken to retain the performance that is taken for granted due to the base alloy chemistry. Furthermore, AM parts tend to have greater dislocation densities and greater amounts of carbon, nitrogen, and oxygen. These features, along with the austenite/ferrite interfaces, may contribute to greater hydrogen trapping. We report the results of our studies of deuterium transport in various austenitic (304L, 316, and 316L) steels produced by AM (via either powder bed fusion or blown powder methods). The hydrogen permeability (an equilibrium property) changes negligibly (less than a factor of 2), regardless of chemistry and processing method, when tested between 150 and 500 °C. This is despite increases in ferrite content up to FN=2.7. However, AM materials exhibit greater hydrogen istotope trapping, as measured by permeation transients, thermal desorption spectra, and inert gas fusion measurement. The trapping energies are likely modest (<10 kJ/mol), but may indicate a larger population of trap sites than in conventional 300-series steels. |
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NU @ karnesky @ karnesky_hydrogen_2017 |
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11517 |
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Zhou, X.W.; Sills, R.B.; Ward, D.K.; Karnesky, R.A. |
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Atomistic calculations of dislocation core energy in aluminium |
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Journal Article |
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2017 |
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Physical Review B |
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95 |
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054112 |
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A robust molecular dynamics simulation method for calculating dislocation core energies has been developed. This method has unique advantages: it does not require artificial boundary conditions, is applicable for mixed dislocations, and can yield converged results regardless of the atomistic system size. Utilizing a high-fidelity bond order potential, we have applied this method in aluminium to calculate the dislocation core energy as a function of the angle \textbackslashBeta between the dislocation line and Burgers vector. These calculations show that, for the face-centred-cubic aluminium explored, the dislocation core energy follows the same functional dependence on \textbackslashBeta as the dislocation elastic energy: Ec = A.sin2\textbackslashBeta + B.cos2\textbackslashBeta, and this dependence is independent of temperature between 100 and 300 K. By further analysing the energetics of an extended dislocation core, we elucidate the relationship between the core energy and core radius of a perfect versus an extended dislocation. With our methodology, the dislocation core energy can be accurately accounted for in models of dislocation-mediated plasticity.e accurately accounted for in models of dislocation-mediated plasticity. |
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NU @ karnesky @ zhou_atomistic_2017 |
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11519 |
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Zhou, X.W.; Dingreville, R.; Karnesky, R.A. |
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Molecular Dynamics Studies of Irradiation Effects on Hydrogen Isotope Diffusion Through Nickel Crystals and Grain Boundaries |
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2017 |
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Physical Chemistry Chemical Physics |
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Experiments indicated that tritium permeation in 316 austenitic stainless steel is enhanced by a factor of ~2-5 after irradiation as compared to ex-reactor results. To understand this enhancement, we have performed extensive molecular dynamics simulations to study effects of both grain boundary structure (Σ3{111}, Σ5{100} and Σ11{311}) and the nature of point defects (vacancy, interstitial, and Frenkel pair) on hydrogen diffusivities in an exemplar fcc metal (nickel). By deriving diffusivities from mean square displacement, all possible atomic jump paths encountered during real diffusion are realistically sampled. By performing extremely long simulations, the statistical errors typically associated with this method are also significantly reduced. We found that within grains, interstitial defects increase diffusivity whereas vacancies have almost no effects. This mechanism might explain hydrogen permeation enhancements in irradiated materials with coarse grains. The largest increase in hydrogen diffusivity was found at a certain combination of grain boundary and point defect. This suggests that permeability of materials with finer grains can also be enhanced by irradiation depending on whether the grain boundary character is skewed. Our results shed new light on the enhancement of tritium permeation in 316 stainless steels during reactor operations. |
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NU @ karnesky @ |
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11520 |
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