Reza Mirshams

Tensile behavior and fracture in nickel and carbon doped nanocrystalline nickel

Abstract The potential engineering applications of nanocrystalline materials need more detailed study on deformation and fracture mechanisms at room and elevated temperatures under tensile loading. This paper reports results of a series of experiments carried out on nickel and carbon doped nanocrystalline nickel with different carbon concentrations from 500 to 1000 ppm at room temperature to 300°C. Grain growth was observed in nanocrystalline nickels as the testing temperature increases. A fast grain growth was noticed at 300°C. Pure nanocrystalline nickel experienced an abnormal grain growth at 500°C and its tensile properties reduced to a very low level. The addition of carbon exerted a potential effect to enhance the stability of the microstructure in nanocrystalline nickel at intermediate temperatures. However, carbon doped nickels exhibited lower tensile properties. Nanocrystalline nickels displayed a conventional Hall–Petch relationship. The results are discussed in relation to microstructural characteristics by using TEM and SEM.

Creep behavior of nanocrystalline nickel at 290 and 373 K

Abstract The uniaxial tensile creep behavior of porosity-free nanocrystalline nickel with 30 nm grains produced by an electrodeposition processing has been investigated under constant and step-load conditions at room temperature and 373 K in a load range 500–1050 MPa. The experimental results showed that significant creep deformation occurred even at room temperature at an initial applied stress of 600 MPa or higher. The creep resistance was very sensitive to test temperature. The grain size and microstructure of the as received and post-creep specimens have been characterized by conventional TEM techniques. An attempt has been made to explain the deformation behavior and creep mechanisms based on current findings.

R-Curve characterization of the fracture toughness of nanocrystalline nickel thin sheets

The fracture resistance curves of nanocrystalline nickel and carbon doped nanocrystalline nickel for different annealing temperatures have been generated and studied. The results indicate that crack growth resistance of pure nanocrystalline nickel is very sensitive to annealing temperatures. The crack growth resistance decreased with increasing annealing temperature for the nanocrystalline nickel. Carbon doping greatly reduces crack growth resistance of nanocrystalline nickel. However, the crack growth resistance of carbon-doped nanocrystalline shows improvement through annealing processing. A cluster model was used to explain the crack growth resistance behavior of nanocrystalline nickel.

Tensile strengthening in the nickel-base superalloy IN738LC

The tensile properties of superalloy IN738LC with different precipitate microstructures are evaluated at room temperature, 650 °C, 750 °C, and 85 °C at two different strain rates. The properties can be presented in two groups based on the comparable closeness of the values obtained—those of microstructures C and M, with coarse and medium size precipitates, and those of microstructures F and D, with fine and duplex size (medium + fine) precipitates. Preferred orientations, lattice parameters, and metallography are used to characterize the microstructure and tensile testing to determine the yield strength, tensile strength, and strain hardening coefficients. An anomalous increase in yield strength is observed, which occurs at temperatures about 100 °C higher with higher strain rate than with lower strain rate applied. The experimental results show that the yield strength is influenced by preferred orientations and precipitate size, while the tensile strength is effected by the size and morphology of precipitates.

Fracture behavior of superalloy IN738LC with various precipitate microstructures

Abstract Fracture studies on differently aged specimens of IN738LC, tensile tested to failure at different temperatures with two different strain rates, were carried out using a scanning electron microscope. Specimens with fine-size γ′ precipitates (F) and duplex-size γ′ precipitates (D) exhibited generally more cleavage type fracture than those with medium (M) and coarse (C) size γ′ precipitates. {100}-faceted cleavage-type fracture was dominant at room temperature, while dimple fracture was dominant at 650°C. A different type of cleavage fracture was observed in the specimens broken at 750°C, which is attributed to secondary strengthening and change in flow mechanism at this temperature under slow strain rate conditions. At 850°C, F and D showed wider faceted cleavage fracture, while M and C showed dimple-ductile fracture. Low tensile fracture toughness correlates with cleavage fracture, while specimens exhibiting dimple fracture have higher ductility and toughness. Presence of few (Ta, Ti)C-type carbide particles along the grain boundaries probably enables easy intergranular crack propagation. Dimples, however, are postulated to be initiated by decohesion at the interface between the γ′ precipitate and the matrix. Notably such decohesion is initiated only on coarsened particles, not on the fine ones.

Kinematics Analysis of the Chipping Process Using the Circular Diamond Saw Blade

One of the primary goals in the design of a diamond blade cutting system is to reduce the cutting force. By understanding the fundamentals of the kinematics of the sawing operation, these forces can be lowered and even optimized with respect to the machining parameters, In this work the material chipping geometries have been mathematically defined and derived through kinematic analysis. These geometries are bounded by four curves and depend on the parameters: depth of cut h, blade diameter D, transverse rate of the workpiece v T , peripheral speed of the saw blade v P , and grit spacing λ. From these chipping geometries, chip area and thickness relations have been obtained. A relation for the mean chip thickness to grit spacing ratio (t c /λ) has also been obtained as a function of the nondimensional machining parameter ratios, h/D and v T /v P . The effects of these parameters on t c were also investigated. It was found that increasing ω and D, reduces the chip thickness. Contrarily, increasing v T , λ, and h, increases the magnitude of the chip thickness, A review of older chipping models was performed, comparing well with the developed model. The results show an excellent agreement between the new model and the older ones, However, at moderately small to large h/D values the new model vields a more exact result. Thus, for h/D values greater than 0.08, it is recommended that the kinematic model be used to compute t c and other pertinent sawing parameters (i.e., grit force and grinding ratio) which are a function of t c .

Development of texture and texture gradient in Al-Cu-Li (2195) thick plate

Texture resulting from micromechanical processing plays an important role in the anisotropy of materials. The determination of texture components is a useful way to present texture data. The types of components present in the texture of a material can be related to other material aspects including predictions of yield loci using polycrystal methods. Al-Cu-Li 2195 thick plate was cold rolled to produce various reductions in thickness. Texture analysis was performed on the various rolled materials at different positions through the thickness of the plates. The texture components are consistent among the various rolled specimens at equivalent thickness positions. A texture gradient is observed to exist through the plate thickness that may indicate other microstructural information about the alloy. This texture gradient evidences the effects of increasing amounts of deformation on texture intensity, texture components observed, and changing modes of deformation.

Failure analysis of an elbow tube fitting

Abstract Safe operation of machinery depends strongly on the design of the system and the employment of proper components and materials. Unique to fluid powered systems are the proper selection of hydraulic lines, fittings, and control valves. This paper describes the cause of fracture in an elbow tube fittings due to deficiencies in the dimensional design and materials selection. The investigation included metallography, fractography, hardness testing and metrology. The results showed that the material was low carbon steel subjected to cold forming and normalized annealing processes during tube production. There were dimensional variations in the tube and the material strenght was low.

Fatigue Failure Analysis Case Studies

Fatigue fracture can occur in many components such as fasteners and tubular pole structures. In this paper, fatigue failure mechanisms have been described and the application of the principles for failure analysis for each case will be presented. Cyclic loading at stresses above the fatigue limit of the material can initiate cracks at the surface or at internal defects. Macroscopic and microscopic observations of fatigue crack initiation and approaches for characterization of fatigue failures have been described. Two case studies present application of laboratory analysis techniques to determine primary causes and modes of failures.

Mechanical Properties Measurement of Sand Grains by Nanoindentation

Nanoindentation was conducted on sand grains to measure the Young’s modulus, hardness, and fracture toughness of individual sand grains. An inverse problem solving approach was used to determine the stress-strain relationship of sand by allowing finite element simulated load-displacement relationship to agree with measurement data. A cube-corner indenter tip was used to induce cracks emanating from the corners of the indenter tip to measure the fracture toughness. X-Ray Diffraction (XRD) technique was used to determine the crystal structure of sand grains.