C.R. Brooks

Fatigue behavior of Zr52.5Al10Ti5Cu17.9Ni14.6 bulk metallic glass

Abstract In the present study, fatigue tests were conducted on a zirconium-based bulk metallic glass (BMG), BMG-11 (Zr–10Al–5Ti–17.9Cu–14.6Ni, atomic percent), in air and vacuum to elucidate the possible environmental effects. In air, the fatigue endurance limit and the fatigue ratio were found to be 907 MPa and 0.53, respectively. These values are better than many conventional high-strength crystalline alloys. Unexpectedly, the fatigue lifetimes in vacuum were found to be lower than in air. Additional testing indicated that dissociation of residual water vapor to atomic hydrogen in the vacuum via a hot-tungsten-filament ionization gauge, and subsequent hydrogen embrittlement of the BMG-11, could have been a factor causing the lower fatigue lifetimes observed in vacuum.

Temperature evolution during low-cycle fatigue of ULTIMET® alloy: experiment and modeling

Abstract The temperature variations of a cobalt-based ULTIMET alloy subjected to low-cycle fatigue were characterized by a high-speed, high-resolution infrared thermography. The change of temperature during fatigue, which was due to the thermal-elastic–plastic effect, was utilized to reveal the accumulation of fatigue damage. A constitutive model was developed for predicting the thermal and mechanical responses of ULTIMET alloy subjected to cyclic deformation. The model was constructed in light of internal state variables, which were developed to characterize the inelastic strain of the material during cyclic loading. The predicted stress–strain and temperature responses were found to be in good agreement with the experimental results.

The specific heat of aluminum from 330 to 890°K and contributions from the formation of vacancies and anharmonic effects☆

Abstract The specific heat of high purity aluminum has been measured from 330 to 890°K with an estimated accuracy of ± 0.7 per cent, using dynamic adiabatic calorimetry. Identical data were obtained for the sample annealed or quenched in ice water from 600°C. The experimental specific heat at constant pressure was converted to that at constant volume using experimental data of molar volume, volume expansivity and isothermal compressibility reported in the literature. A theoretical specific heat at constant volume was obtained by using the Debye specific heat, with a single Debye temperature, to which an electronic specific heat was added. The excess specific heat was determined by the difference between the specific heat converted from the experimental data and the calculated specific heat. The excess specific heat was about 10 per cent of the total specific heat at 930°K. The formation of equilibrium monovacancies accounts for only about one per cent of the excess specific heat at 930°K. The remaining excess specific heat is assumed to be due to anharmonic lattice effects. and the temperature dependence agrees approximately with the theoretical predictions for aluminum.

Physical contributions to the heat capacity of nickel

Abstract Heat capacity data for solid nickel have been re-evaluated and analyzed into physical contributions, 0–1726 K. Two new sets of measurements of C p (Ni), 333–1500 K, have been combined with literature data to produce an evaluated data set with uncertainty ⩽ ± 2%. These smoothed data have been analyzed into vibrational harmonic, electronic, magnetic and dilatational contributions with the aid of auxiliary measurements of expansion coefficient, compressibility, vibrational and electronic densities of states, elastic constants, and magnetic exchange integral and susceptibility obtained from the literature. The vibrational harmonic term is interpreted in terms of a θ D -vs- T curve in accord with predictions of the density-of-states distribution. The electronic contribution is smaller than predicted by free-electron theory due to a large electron-phonon effect. The electronic term for paramagnetic nickel is in good agreement with that predicted from band calculations. The magnetic contribution yields a magnetic entropy in accord with theoretical predictions, and a magnetic internal energy and critical-point behavior in agreement with the isotropic Heisenberg model. The experimental heat capacity can be accounted for without reference to vibrational anharmonic and vacancy contributions, in accord with recent calculations.

The specific heat of copper from 40 to 920°C∗

Abstract The specific heat of high-purity copper has been measured from 40 to 900°C using three adiabatic calorimeters. Two polycrystalline samples and one bicrystal sample were used. The data were obtained under widely varying operating conditions and the reproducibility was ±0.7 per cent. The average specific heat is about one per cent above most previous data up to 600°C, above which it shows an increasingly positive deviation from linearity. However, the specific heat of aluminum oxide has been measured with the same calorimeters and shows excellent agreement with the data obtained by the National Bureau of Standards. From these new results, in conjunction with previous data. are tabulated values of the specific heat of copper from 40 to 900°C which are believed to be the most accurate available.

High-frequency metal fatigue: the high-cycle fatigue behavior of ULTIMET ® alloy

Abstract ULTIMET® alloy is a relatively new commercial Co–26Cr–9Ni (wt.%) alloy, which exhibits good resistance to both wear and corrosion. A state-of-the-art high-frequency, 1000-Hz, material test system was used to study the high-cycle fatigue behavior of ULTIMET alloy up to 109 cycles. Fatigue experiments were conducted at high (1000 Hz) and conventional (20 Hz) frequencies in air at room temperature. The effects of the test frequency, the temperature increase during fatigue, and the change of crack initiation sites from the surface to subsurface on fatigue life are discussed. Although the fatigue life was comparable at test frequencies of 1000 and 20 Hz, the equilibrium temperature at 1000 Hz was considerably higher than that at 20 Hz. The fractographic study showed different morphologies of fracture surfaces at various frequencies. The high-cycle fatigue behavior of ULTIMET alloy at both high- and low-frequencies exhibited a typical two-stage fatigue-crack-growth process, i.e., (a) stage I fatigue-crack initiation in which the cracks formed on those planes most closely aligned with the maximum shear–stress direction in the grains of the fatigue specimen; and (b) stage II fatigue-crack growth in which the maximum principal tensile stress controlled crack propagation in the region of the crack tip.

Analysis of the excess specific heat of copper from 300 to 1200°K

Abstract Recent measurements of the specific heat of copper from 300 to 1200°K have been analyzed in order to estimate contributions from the formation of equilibrium lattice defects and anharmonic effects. This was accomplished by converting the experimental specific heat at constant pressure to that at constant volume using experimental data of specific volume, volume expansivity and isothermal compressibility reported in the literature. The excess specific heat was obtained by subtracting from the specific heat at constant volume a theoretical specific heat based on the Debye specific heat, using a single Debye temperature, corrected for electronic contributions. The excess specific heat increases more than linearly with temperature, becoming significant around 600°K and amounting to about 7 per cent of the specific heat at 1200°K. An analysis of the excess specific heat indicates that the formation of equilibrium monovacancies makes a contribution beginning around 800°K, accounting for about one third of the excess specific heat at 1200°K. It is concluded that the remaining excess specific heat is caused by anharmonic effects in the lattice.

The intergranular segregation of boron in Ni3Al: Equilibrium segregation and segregation kinetics

Abstract The grain boundary B content of high-purity Ni-24 at.% Al alloys containing 0.048, 0.144, 0.240 and 0.480 at.% B (100, 300, 500, 1000 ppm mass) has been determined for samples aged from 1323 to 873 K for sufficient times to attain equilibrium. The B content was derived from Auger electron spectra of the intergranular fracture facets. Many facets were exposed during fracture at ≈ 300 K, and additional facets were formed upon fracturing following hydrogen charging after heat treatment. For each alloy sample, about 25 facets were analyzed. The grain boundary B contents were in the range of 0.5–2.5 at.%. The grain boundary B content increased with decreasing temperature and with increasing bulk B content in the alloys. The energy of binding of a B atom to the grain boundary was calculated using McLeans segregation theory and assuming a unique binding energy for each alloy. The values were in the range of 0.15–0.45 eV/atom, and increased with increasing temperature and with decreasing bulk B content. These results have been rationalized in terms of a spectrum of binding energies for a given alloy. However, when the entropy of adsorption was taken into account, an enthalpy of adsorption of B to the grain boundary of 0.13 eV/atom was obtained, independent of temperatire and bulk B content. This is interpreted to mean that the spectrum of binding energies is quite restricted. The grain boundary B content of these alloys has also been measured as a function of annealing time at 773, 873, 973 and 1173 K. The diffusion coefficient of B in Ni 3 Al at 773 K is about 5 × 10 −21 m 2 /s, and the equilibrium grain boundary B content is attained at about 3000 s. The diffusion coefficient at 973 K is between 10 −16 and 10 −17 m 2 /s. The activation energy for diffusion of B in Ni 3 Al is between 200,000 and 300,000 J/mol.

Nondestructive evaluation of fatigue damage in ULTIMET® superalloy

Acoustic emission (AE) and positron spectroscopy were used to study fatigue damage in ULTIMET alloy. The linear-location method of AE was employed for identifying the positions of crack initiation for a cylindrical specimen subjected to fatigue. Positron lifetime spectroscopy, as a sensitive nondestructive technique, was utilized to reveal the fatigue damage. The results obtained by the AE system were generally in good agreement with those of the average positron lifetimes by positron spectroscopy. The detected crack initiation was further investigated by scanning electron microscopy, which was found to be consistent with the AE and positron spectroscopy results. The crack-initiation stage of ULTIMET alloy subjected to high-cycle fatigue was characterized.

The site location of Zr atoms dissolved in TiAl

The intermetallic compound TiAl is being considered for high temperature applications, but its adaption is hindered by its brittleness. There has been considerable research towards understanding this embrittlement, and especially towards improving the ductility by alloying while maintaining strength. In TiAl, the Ti and Al atoms lie on alternate (001) and (110) planes of a face-centered tetragonal lattice. (However, the structure is slightly distorted to tetragonal, with c/a = 1.02.) Thus, depending on the incident beam direction, the electron intensity will be concentrated at either the Ti or Al planes, and hence x-ray emission from one element will be enhanced relative to the other. This paper reports that by comparing the ratio of the characteristic x-ray intensity of a ternary element to that of Al and Ti, the concentration of the ternary element on the Al and Ti planes can be deduced. The method, which is used to locate the Zr atoms in the TiAl alloy studied, is ALCHEMI (Atom Locations by Channelling Enhanced Microanalysis). In this method, the characteristic x-ray intensity is compared for the electron diffraction conditions of S {gt} 0 and S {lt} 0.