Fereshteh Ebrahimi

Synthesis and characterization of electrodeposited nanocrystalline nickel–iron alloys

Abstract Nickel–iron nanocrystalline alloys with different compositions and grain sizes were fabricated by electrodeposition. The iron content of the deposits was changed by varying the Ni/Fe ion ratio in the electrolyte. X-ray diffraction (XRD) analysis was applied for measuring the lattice parameter, grain size and the level of internal strain of the deposits. The strength of the alloys was evaluated by microhardness testing. The results of this study revealed that at a constant grain size of approximately 11 nm the hardness depends strongly on the iron content. The hardness showed a maximum around 20% Fe. The grain size of the alloys with 4–6% iron was very sensitive to the deposition conditions. The hardness of these alloys followed the Hall–Petch relationship. The alloys with equal or more than 35% Fe cracked upon deposition. The tendency to cracking was correlated with the level of internal stresses in the deposits.

Fracture anisotropy in silicon single crystal

In this study the effect of crystallographic orientation on fracture toughness and the fracture path of silicon single crystal was investigated. Vickers microhardness indentation was used to introduce cracks along various crystallographic orientations on the (110), (001), and (111) planes. The fracture toughness variation was found to follow the symmetry of the indentation axis with no distinct correlation with the elastic constants. The observations made suggest that the inclination angle of cleavage planes relative to the indent plane affects the fracture path and toughness significantly. Prestraining decreased the hardness and improved the toughness without a modification of fracture path.

Transition of deformation and fracture behaviors in nanostructured face-centered-cubic metals

Tensile stress–strain curves demonstrate that single-phase nanocrystalline face-centered-cubic (fcc) metals are intrinsically ductile and their failure begins with necking. However, the area reductions and the fracture behaviors were found to be dependent on the grain size. When plastic deformation is governed by dislocation activity, the nanocrystalline samples behave similar to the conventional coarse-grained materials. As the grain size is reduced to the regime where grain boundary sliding dominates, the material shows very high strain-hardening rate and the tensile samples fail by microcracking with no noticeable reduction in area.

The effect of current density on properties of electrodeposited nanocrystalline nickel

Nanocrystalline nickel electrodeposits were fabricated at 18, 25 and 50 mA cm−2 using a sulfamate-based electrolyte. The crystallite size of the deposits was evaluated by XRD technique and their mechanical properties were characterized by tensile testing. The results of this study confirmed that increasing the current density results in an increase in the grain size of nickel deposits. The strength of the deposits decreased consistently with increasing the crystallite size. However, the deposit fabricated at 50 mA cm−2, in comparison to nickel with conventional grain size (>1 μm), showed a relatively low strength and a surprisingly low tensile elongation. It is suggested that the enhanced evolution of hydrogen at high current densities is responsible for the formation of larger crystals and the unusual low tensile elongation.

An investigation of thermal stability and microhardness of electrodeposited nanocrystalline nickel-21% iron alloys

Abstract The thermal stability of an electrodeposited nickel-21% iron alloy with an average grain size of approximately 14 nm was investigated. Samples were annealed for 90 mins in the temperature range of 373–773 K. The results of this study showed that grain growth started after annealing at and above 400 K. Two regimes of grain growth were identified. It is suggested that the low temperature regime (below 575 K) is accommodated mainly by the grain boundary diffusion, while the high temperature regime is assisted by the lattice diffusion. The microhardness results also confirm the presence of these two regimes. The position of the (111) XRD peaks indicated a sudden decrease in the lattice parameter after annealing at 773 K, which is attributed to the loss of connectivity of the nanosize grains. The variation of hardness with grain size followed the Hall-Petch relationship.

Effect of stacking fault energy on plastic deformation of nanocrystalline face-centered cubic metals

The effect of stacking fault energy (SFE) on the tensile stress–strain behavior of nanocrystalline face-centered cubic (fcc) metals was investigated. The stacking fault energy of nickel was decreased by alloying with copper or iron. It was found that, as predicted by a recent simulation study, decreasing the SFE increases the strain hardening rate of the nanocrystalline fcc metals. The effect was more pronounced in the nickel–copper alloy, which had a smaller average grain size.

Brittle-to-ductile transition in polycrystalline NiAl

Abstract The brittle-to-ductile transition (BDT) of stoichiometric NiAl and Ni 49Al 1Ti alloys was investigated using tensile and fracture toughness testing methods. The BDT in these alloys was found to be associated with a sharp drop in strain hardening rate, which causes the general yielding and plastic instability (necking) to precede unstable fracture in notched and tensile specimens, respectively. The addition of Ti increases the strength, low temperature toughness, and BDT temperature of NiAl significantly. The occurrence of intergranular fracture in NiAl alloys has been correlated with the development of localized internal stresses near grain boundaries owing to a lack of sufficient slip systems. The results of this study suggest that the vacancy concentration in NiAl depends significantly on the stress state and the amount of deformation. This phenomenon may be responsible for the microvoid coalescence fracture mechanism observed in tensile specimens, and the enhancement of dislocation climb at moderate temperatures.

Ductile crack initiation in steels

The process of ductile crack initiation has been investigated for a ferritic, pearlitic and a bainitic structural steel. A three-dimensional analysis has revealed that the ductile crack initiation occurs by formation of disconnected microcracks along the crack front. While the distribution of inclusions and highly strained regions governs the microcracking in the ferritic-pearlitic steel, the geometrical inhomogeneities produced by fatigue precracking act as the sites for formation of microcracks in the bainitic steel. In the latter steel, the size of the carbides is two orders of magnitude smaller than the crack tip opening produced by fatigue precracking, and hence, ductile crack initiation can not be modeled as the coalescence of the first microvoid with the crack tip.

The role of point defects in the physical properties of nonstoichiometric ceria

Explicit, analytical expressions have been derived for the dependence of electrical conductivity, chemical expansion, and elastic modulus on point defect concentration and oxygen partial pressure using a consistent approach. In developing the model, expressions were first derived for the functional dependence of defect concentration on the oxygen partial pressure for fluorite oxides. Expressions for the chemical expansion, elastic modulus, and electrical conductivity as functions of defect concentration are then derived and verified with experimental data for ceria (CeO2−δ) with consistently good fits. The same values for the material constants were used in all of the fits, further validating our approach.

The effect of substrate on the microstructure and tensile properties of electrodeposited nanocrystalline nickel

Abstract Nanocrystalline nickel was fabricated on copper substrates using electrodeposition techniques. The microstructure and the crystallographic texture of the copper substrate were varied by cold rolling and subsequent annealing. This study revealed that the annealed copper substrates result in nickel deposits with finer grain sizes than the cold-rolled substrates do. This difference is suggested to be associated with the change in planar texture, which is expected to affect the nucleation rate of crystals on the substrate. The results of this study suggest that the size of crystals formed immediately on the substrate affects the size of the grains through the thickness of deposits. Tensile properties were found to depend on the average grain size, the grain size distribution, and the co-deposition of hydrogen.