M.L. Weaver

Activation energy calculations for discontinuous yielding in Inconel 718SPF

Abstract The kinetics of yield point return and of dynamic strain aging (DSA) were investigated for a commercial heat of Inconel 718SPF. During uniaxial tensile tests, the alloy exhibited serrated flow on the load–elongation curves at intermediate temperatures. Types A, B and C serrations were observed depending upon the test temperature and strain rate. For type A and B serrations, apparent activation energies ranging from 97 to 133 kJ mol −1 were measured, which suggests that type A and B serrated flow results from the diffusion of an interstitial solute, most likely carbon. For type C serrations, apparent activation energies in the range ∼210 to ∼272 kJ mol −1 were measured, which suggests that type C serrations arise from the diffusion of a substitutional solute, most likely Cr. Yield points induced during strain aging under stress experiments increased with a t n relationship, where the exponent n ranged from 0.3 to 0.5 before reaching a plateau. This suggests that the yield points were a combined result of bulk lattice diffusion and dislocation pipe diffusion of solutes. The apparent activation energies, ranging from 42 to 50 kJ mol −1 , are consistent with prior observations, but cannot be explained by any classic models for static strain aging. It is suggested that the yield points result from the reorientation of carbon-vacancy pairs in the strain fields of dislocations.

Nanotribology studies of Cr, Cr2N and CrN thin films using constant and ramped load nanoscratch techniques

Abstract The nanotribological behavior of DC magnetron sputtered Cr, Cr 2 N and CrN thin films has been studied using nanoscratch techniques. Constant and ramped load (5 mN) scratches were made using a Nano Indenter II system on 500-nm-thick Cr, Cr 2 N and CrN thin films. Ramped-load scratch studies have been carried out on 10-nm-thick Cr, Cr 2 N and CrN. Wear tracks were imaged by atomic force microscopy. The dependence of the displacement, residual wear depth, percent elastic recovery, and friction coefficient on load is compared in constant load and ramped load tests. Under the same (maximum) load, constant load tests exhibit higher displacements, residual depths and friction coefficients, but lower percent elastic recoveries. The chromium nitride films (Cr 2 N and CrN) show less permanent damage, lower coefficients of friction and higher percent elastic recovery than pure chromium under the same (maximum) load. However, the adhesive strength of Cr 2 N needs to be improved.

Nanotribology and surface chemistry of reactively sputtered Ti-B-N hard coatings

Abstract The nanotribological performance of Ti–B–N protective coatings, 500 nm thick, have been studied in the range of 0–38.5 at.% N. A correlation was established amongst the chemical state, structure, mechanical properties, and nanowear resistance as a function of atomic percent nitrogen. The mechanical properties, elastic modulus and hardness, of the films were tested using a Hysitron Triboscope nanomechanical test instrument. The nanotribological performance of the films was evaluated using a Nanoindenter II with scratch capability. Single and reciprocating nanowear scratches, 10 μm in length, were performed at normal loads ranging from 50 to 750 μN. An atomic force microscope (AFM) was utilized to characterize the nanowear tracks with respect to depth and amount of plowing of material. The AFM images revealed that the reciprocating nanowear test caused grooving of the films with little to no material removal. Chemical and structural information was obtained by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction. Increasing N content correlated with increasing number of B–N bonds, structural disorder, and decreasing hardness, modulus, and wear resistance.

The effects of alloy purity on the mechanical behavior of soft oriented NiAl single crystals

Alloys based on the intermetallic compound NiAl are considered good candidates for high temperature structural applications due to their excellent oxidation resistance, low density and high thermal conductivity compared to nickel-base superalloys. In spite of these advantages, their developments into viable engineering materials has been limited by low ductility at ambient temperatures. However, recent efforts have resulted in room temperature tensile elongations of up to 6% for doped NiAl single crystals containing microalloying additions of Fe, Mo or Ga. The mechanism behind this increased ductility is unknown buy may be related to a gettering phenomenon since it is realized that some substitutional elements and interstitial impurities can cause significant embrittlement in bcc metals and B2 ordered compounds such as NiAl. Current observations are consistent with this viewpoint. Consequently, the purpose of this paper is to provide preliminary details concerning the effects of alloy purity on the mechanical properties of NiAl single crystals with differing impurity contents were studied. In addition, a NiAl alloy intentionally doped with Mo was investigated and a comparison of the mechanical properties of all three alloys is discussed in terms of relative compositions.

Apparent indentation size effect in a CVD aluminide coated Ni-base superalloy

Indentation hardness testing was used to assess the mechanical properties of a low activity CVD β-NiAl bond coat applied to a single crystal Ni-base superalloy substrate. Tests performed in the nano- and micro-hardness regimes using a Knoop indenter geometry revealed the presence of an indentation size effect (ISE) where the apparent hardness increased significantly with decreasing indentation test load. An energy balance analysis was applied to analyze the apparent ISE and to extract the load independent hardness from the experimental measurements. The energy balance model could not be successfully applied to the nanoindentation data. This is attributed in part to blunting of the nanoindenter tip, which resulted in changes in indenter contact area.

Biaxial modulus of fiber-textured cubic polycrystalline films with an arbitrary texture axis [hkl]

A simple method to estimate the effective biaxial modulus of cubic polycrystalline films with an arbitrary [hkl] fiber texture is presented. Analytic expressions of the Voigt [Lehrbuch der Kristallphysik (Teubner, Leipzig, 1910)] and the Reuss [Z. Angew. Math. Mech. 9, 49 (1929)] averages of the biaxial modulus are given in terms of the axis orientation [hkl] and single-crystal elastic constants. We then explain why the two averages of the fiber-textured films are identical to the corresponding appropriate biaxial modulus of the constituent grain. Finally, we show that for fiber axes equivalent to [100] or [111], the Voigt and Reuss averages coincide and are identical to the true effective biaxial modulus of the fiber-textured film. For other fiber axes, we further show that the Voigt-Reuss-Hill average of the biaxial modulus is an excellent theoretical estimation.

Kinetics of Static Strain Aging in Polycrystalline NiAl-based Alloys

The kinetics of yield point return have been studied in two NiAl-based alloys as a function of aging time at temperatures between 300 and 700 K. The results indicate that the upper yield stress increment, Δσu (i.e., stress difference between the upper yield point and the final flow stress achieved during prestraining), in conventional purity (CP-NiAl) and in high purity carbon-doped (NiAlC) material first increased with a t23 relationship before reaching a plateau. This behavior suggests that a Cottrell locking mechanism is the cause for yield points in NiAl. In addition, positive y-axis intercepts were observed in plots of Δσu versus t23 suggesting the operation of a Snoek mechanism. Analysis according to the Cottrell-Bilby model of atmosphere formation around dislocations yields an activation energy for yield point return in the range 70 to 76 kJ/mol which is comparable to the activation energy for diffusion of interstitial impurities in bcc metals. It is, thus, concluded that the kinetics of static strain aging in NiAl are controlled by the locking of dislocations by Cottrell atmospheres of carbon atoms around dislocations.

Observations of dynamic strain aging in polycrystalline NiAl

Abstract Dynamic strain aging has been investigated at temperatures between 77 and 1100 K in eight polycrystalline NiAl alloys. The 0.2% offset yield stress and work hardening rates for these alloys generally decreased with increasing temperature. However, local plateaus or maxima were observed in conventional purity and carbon doped alloys at intermediate temperatures (600–900 K). This anomalous behavior was not observed in low interstitial high-purity, nitrogen doped, or in titanium doped materials. Low or negative strain rate sensitivities (SRS) were also observed in all eight alloys in this intermediate temperature range. Coincident with the occurrence of negative SRS was the occurrence of serrated flow in conventional purity alloys containing high concentrations of Si in addition to C. These phenomena have been attributed to dynamic strain aging (DSA). Chemical analysis of the alloys used in this study suggests that the main species causing strain aging in polycrystalline NiAl is C but indicate that residual Si impurities can enhance the strain aging effect.

The effects of interstitial content, heat treatment, and prestrain on the tensile properties of NiAl

Abstract The tensile stress-strain response of polycrystalline NiAl was studied as a function of purity and pretest treatment (annealing and/or prestrain). After annealing at 1100 K for 7200 s (i.e. 2 h) followed by furnace cooling, high-purity and nitrogen-doped alloys exhibited continuous yielding, while conventional-purity or carbon-doped alloys exhibited a distinct yield point and Luders strain. Prestrain by hydrostatic pressurization removed the yield point, but it could be reintroduced by further annealing treatments. Yield points could be reintroduced more rapidly if the specimens were prestrained uniaxially rather than hydrostatically, owing to the arrangement of dislocations into cell structures during uniaxial deformation. The time dependence of the strain aging events followed a t 2 3 relationship. In total, these results suggest that the yield points observed in polycrystalline NiAl result from the pinning of mobile dislocations by interstitials, specifically carbon, i.e. classic static strain aging.

Microstructure and stress development in magnetron sputtered TiAlCr(N) films

Abstract TiAlCr(N) coatings were reactively magnetron-sputtered from a Ti-51Al-12Cr alloy target in this study by changing the nitrogen partial pressure over the range of 0–25% of the total pressure. The influence of nitrogen addition on the microstructure and stress development in the TiAlCr(N) films was investigated. With the increase of nitrogen partial pressure, the film microstructure undergoes a transition from amorphous-like metallic to crystalline nitride films. The crystalline phases are cubic Ti 1− x Al x N and Cr 1− x Al x N. The intrinsic stresses are compressive and become more so with nitrogen addition over most of the partial pressure range, while the stress–temperature curves during annealing vary significantly among the films. Nitrogen additions were found to increase the hardness of the films.