U. Erb

Influence of grain boundary character distribution on sensitization and intergranular corrosion of alloy 600

The objective of this study was to assess, for the first time, the potential impact of ``Grain Boundary Design and Control`` on the bulk sensitization and intergranular corrosion resistance of a commercial corrosion resistant nickel-based austenitic alloy: Alloy 600 (UNS N06600). Increasing the special grain boundary ({Sigma} {le} 29) frequency in thermomechanically processed Alloy 600 from 37% to 71% has been shown to result in commensurate decreases in bulk intergranular corrosion susceptibility in both the solution annealed and sensitized condition; these findings being attributed to both the intrinsic corrosion resistance, and resistance to solute segregation and precipitation exhibited by structurally-ordered low {Sigma} grain boundaries. These results provide considerable promise for the practical application of grain boundary design and control considerations in the general field of corrosion prevention and control.

Electrodeposited nanocrystals: Synthesis, properties and industrial applications

Abstract Electrodeposition of nanocrystals is a technologically and economically viable production route to synthesize porosity free pure metals, alloys and metal matrix composites both in bulk form and as coatings.Certain properties of nanostructured electrodeposits such as hardness, wear resistance and electrical resistivity are strongly affected by grain size. On the other hand, properties such as thermal expansion, Youngs modulus and saturation magnetization show little grain size dependence. Potential applications of nanostructured electrodeposits range from corrosion and wear resistant coatings to soft magnetic materials for magnetic recording.

Effect of grain size on mechanical properties of nanocrystalline materials

Abstract The possibility of a dislocation mechanism in the deformation process of nanocrystalline materials is reviewed and analyzed. The present theoretical calculation, by taking the anisotropic characteristic of crystallographic symmetry and different choices of critical shear strength into account, results in a reasonable limit in grain size for applying dislocation pile-up theory to nanocrystalline materials. The deviation from the Hall—Petch relationship is rationalized in terms of a small number dislocation pile-up mechanism. A composite model is proposed to evaluate the strength of nanocrystalline materials. It is shown that this model can be used for interpreting the various cases observed in Hall—Petch studies. An analytical expression for assessing the creep rate of nanocrystalline materials by a diffusion mechanism, including triple line diffusion, is derived. It is predicted that the creep rate due to triple line diffusion will exhibit a stronger grain size dependence than that due to grain boundary diffusion.

Deviations from hall-petch behaviour in as-prepared nanocrystalline nickel

2,106,004 1/1938 Inglee .................................. 204/300 R 2,764,540 9/1956 Farin et al. .......................... 204/1297 3,103,235 9/1963 Stringham ................................. 138/97 3,125,464 3/1964 Harmes ................................... 18/105 3,287,248 11/1966 Braithwaite ............................. 204/260 3,618,639 11/1971 Daley et al. ............................ 137/28 3,673,073 6/1972 Tobey et al. ............................ 204/226 3,804,725 4/1974 Haynes .................................... 205/104 4,080,268 3/1978 Suzuki et al. ........................... 205/131 4,120,994 10/1978 Inoue ...................................... 427/239 4,200,674 4/1980 Inoue ...................................... 427/290 4,227,986 10/1980 Loqvist et al. .......................... 204/209 4,280,882 7/1981 Hovey ....................................... 205/50

Room temperature creep behavior of nanocrystalline nickel produced by an electrodeposition technique

Abstract Deformation processes of nanocrystalline (6–40 nm) nickel produced by an electrodeposition technique were studied. First, the results of unidirectional tensile tests were discussed with respect to the deviation from the Hall-Petch relationship. It was suggested that such a mechanical behavior exhibited by nanocrystalline materials could be described by a composite model proposed previously. Further experimental work on static and dynamic creep tests under the load control condition showed that nanocrystalline nickel electrodeposits exhibited a significant room temperature creep behavior. It appeared that grain boundary sliding and diffusive matter transport within the intercrystalline region played an important role in terms of deformation mechanisms of nanocrystalline materials. The contributions of dynamic creep to stress-strain behavior and, in turn, to the assessment of the Hall-Petch relationship for nanocrystalline materials are discussed.

Thermal stability of nanocrystalline Ni

Abstract Thermal stability of electroplated nanocrystalline Ni of 10 and 20 nm grain size was investigated by differential scanning calorimetry (DSC). The temperature dependence and heat release ΔH during grain growth have been determined by linear anisothermal measurements (linear heating at 10 K min −1 ). The corresponding change in microstructure has been monitored in the temperature range between 373 K and 693 K using transmission electron microscopy (TEM). The TEM and DSC studies identified three exothermic reactions: “nucleation” and abnormal grain growth (353–562 K), normal grain growth (562–593 K) and growth towards equilibrium (643–773 K). The grain growth behaviour, and the similar heat releases ΔH = 18 J g −1 , and 16 J g −1 measured for the 10 nm and 20 nm Ni nanocrystals respectively in the DSC experiments may be related to the observed sulphur segregation at grain boundaries and triple junctions.

Electrodeposition of nanocrystalline Ni-Fe alloys

Abstract Thick nanocrystalline Ni and Ni-Fe alloy deposits with grain sizes less than 30 nm and containing iron up to 28 wt.% were produced by electrodeposition at rates in excess of 100 μm per hour. It is shown that the grain size, macrotexture and microtexture of these materials is strongly dependent on the amount of iron co-deposited with nickel. With decreasing grain size, the hardness of the materials initially increases following the regular Hall-Petch relationship. However, starting at grain sizes of approximately 18 nm, a deviation from the regular Hall-Petch behavior is observed leading to softening at the smallest grain sizes.

Synthesis, structure and properties of electroplated nanocrystalline materials

Abstract Electroplating is an economically viable production route to synthesize porosity-free pure metals, alloys and composite materials both as coatings and in bulk form. Some properties of electroplated nanocrystals differ from those observed on nanostructured materials prepared by other techniques; these differences may be attributed to residual porosity. Triple junctions have been shown to strongly effect certain properties of nanostructured materials and should not be overlooked in future interpretation of properties of both nanocrystalline and conventional polycrystalline materials.

Isokinetic analysis of nanocrystalline nickel electrodeposits upon annealing

Abstract The grain growth kinetics of nanocrystalline nickel electrodeposits was studied by transmission electron microscopy and differential scanning calorimetry at different heating rates. It was found that, upon annealing, the nanocrystals in the nickel electrodeposits appeared to grow abnormally and released about 415.7 ± 3.5 J/mol of heat. The mechanism of the abnormal grain growth was attributed to the subgrain coalescence. The method for determination of the grain growth activation energy as well as all the other kinetic parameters in the Johnson-Mehl-Avrami equation was proposed based on an isokinetic analysis. This method is applicable to general types of transformation process governed by a single activation energy under the isokinetic condition. The activation energy for the grain growth of nanocrystalline nickel electrodeposits was found to be about 131.5 kJ/mol using this method. The difference between the present method and the Kissinger and Ozawa method was addressed in terms of their physical backgrounds.

Study of grain growth in electrodeposited nanocrystalline nickel-1.2 wt.% phosphorus alloy

The paper describes a transmission electron microscope study of the microstructural evolution of electrodeposited nanocrystalline Ni-1.2wt%P and pure nickel during heating experiments performed in situ. The grain structure of the Ni-1.2wt.%P alloy was found to be stable up to a temperature of 360 °C. The grain growth was concurrent with Ni3P precipitation, suggesting the important role of phosphorus in supersaturated solid solution in imparting the thermal stability to the microstructure. Normal grain growth was observed in Ni-1.2wt.%P at annealing temperatures up to 480 °C. Subsequent analysis revealed pinning of grain boundaries by Ni3P precipitates. The value of activation energy for grain growth in Ni-P alloy (2.25 eV), obtained from continuous scan rate DSC experiments, is consistent with the above observation. Electrodeposited nanocrystalline nickel, on the other hand, showed abnormal grain growth at temperatures as low as 260 °C. At 320 °C, the microstructure had largely transformed from the nanocrystalline to the microcrystalline state.