Cold spray is a novel and promising technology to obtain surface coating. Notwithstanding the several technological advantages with respect to other processes, its diffusion is somewhat limited because of the limited knowledge on the mechanical properties of the cold sprayed materials and in particular, the fatigue behavior. Moreover, the existing data concerning fatigue behavior of coated specimens are controversial and different material system shows different behaviors. The aim of this study is to distinguish the involved parameters and their effect on fatigue behavior of cold sprayed systems. A critical discussion on four important parameters i.e. interface quality, material properties, deposition parameters and residual stress is given. The influential parameters are consolidated in one formula, which can predict the fatigue limit of cold spray system as a function of residual stress, coating hardness and stress gradient in the specimen.

The effect of strain rate on hydrogen embrittlement of low alloy 4340 steel was studied using double-notched tensile samples electrochemically charged in-situ with hydrogen in 1N H2SO4 + 5 mg/l As2O3 solution. The mechanical response of samples with prior austenitic grain sizes of 10 and 40 μm and martensitic hardness of 43-52 HRC were examined after hydrogen charging times of 0-20 min. Increasing the strain rate for hydrogen charged samples resulted in decreased failure strains and increased evidence of brittle fracture. Brittle fracture surfaces for the harder samples showed primarily intergranular fracture while softer samples exhibited predominantly quasi-cleavage.

A model is presented for computing the temperature increase associated with the formation of an adiabatic shear band. The hypothesis is that the heating is supplied by the difference in energy of a pile-up of n dislocations and the energy of n individual dislocations. The heating is assumed to occur within a volume determined by the grain size (i.e. slip band length) and an effective thermal length determined by the dislocation velocity. The model predicts increases in temperature with increasing shear modulus (G), increasing numbers of piled up dislocations (n), increasing Burgers vector (b), increased grain size (d), and increased dislocation velocity (vd). Increasing temperature is also predicted with decreasing heat capacity (c*) and thermal diffusivity (α) as would be expected. The model was applied to low carbon steel for which considerable data are available. Application to low carbon steel gives a temperature increase of about 1400K. The implied result that untempered martensite should be observed after adiabatic shear banding is in agreement with examples cited in the literature. Further investigation into the dynamics of pile-up release and the associated heat transfer mechanisms is discussed.

Uniform and multilayered nanocomposites are of growing interest due to their desirable mechanical properties and their performance under high stress, wear, and impact conditions. Composite structures offer an opportunity to combine the useful properties from multiple materials. Controlled variations in composition and microstructure within a composite material allow for tunable local variations in properties. The simplest version of such a variation is to periodically change the composite volume fraction to create a multilayered composite material. Such structures would have hard layers to maintain strength and softer layers to allow for greater plasticity and prevent brittle failure. We are able to manufacture uniform and layered composites of nickel matrices embedded with alumina nanoparticles using electrodeposition. In this method a rotating disk electrode (RDE) is used to directly control the rate of particle incorporation. Uniform composites are made by holding a constant RDE rotation rate while layered composites are manufactured by periodically varying the rotation rate during deposition. We have demonstrated this novel manufacturing process for large-scale samples, several square centimeters in area and hundreds of microns thick, while maintaining submicron microstructural resolution.

A way of estimating Stress Intensity Factors is proposed by extending available solutions (solid and crack configurations) to stress fields not considered in available formulations. The accuracy of the proposed estimation is considered with respect to fatigue life assessment and crack shape tracing. It is aimed as very fast initial estimation, in comparison with the use of Finite Elements, in those cases were a high stress gradient is observed: stress concentrations (holes, notches, grooves) or due to surface residual stresses produced by machining techniques or induced –on purpose- to improve fatigue life (for example, by shot-peening), where no SIF solutions are available.