O. Buck

Acoustic Harmonic Generation Due to Thermal Embrittlement of Inconel 718

This paper describes an attempt to characterize the deterioration of a structural materials mechanical properties by nonlinear acoustics. In this particular case, the damage was caused by “thermal embrittlement” during which the material, here the nickel-based alloy Inconel 718, loses a significant fraction of its fracture toughness. Harmonic generation was the experimental method used to characterize the microstructural changes in the material as a function of exposure time at elevated temperatures. Tests were performed on two heats of Inconel 718 with slightly different chemistries, with one heat showing particular sensitivity of the fracture toughness to the elevated temperature exposure with corresponding higher changes in the nonlinearity parameter. As a mechanical measure of the fracture toughness deterioration, a small specimen punch test was used in which the ductility of a thin slice of material is determined. A clear difference between the two heats was noted in the metallographic examination, which is reflected in the harmonic generation as well as the punch test data. An explanation for the changes of the harmonic generation during the embrittlement process is speculative at the present time.

Nonlinear Acoustics, a Technique to Determine Microstructural Changes in Materials

An important aspect of nondestructive evaluation is materials characterization, in particular, detection of changes in the microstructure, affecting the mechanical properties. The goal of this project is to correlate the mechanical properties of high strength alloys with nonlinear acoustical properties of the materials. Many high strength alloys are precipitation hardened, and therefore, their mechanical properties are dependent on microstructural changes during thermal aging. Since the precipitates are formed by diffusion a property sensitive to the precipitation changes is the electrical resistivity. Standard ultrasonic techniques, on the other hand, have been unreliable in microstructural characterization, since in these alloys linear acoustic properties (e.g. sound velocity, attenuation) change by no more than 1%. Literature indicates however that nonlinear acoustical properties change by roughly 50% [1]. From this evidence it appears a nonlinear acoustic technique would be an additional method to examine microstructural changes in precipitation hardened materials.

Acoustic harmonic generation at diffusion bonds

The distortion of a sinusoidal acoustic wave at unbonded interfaces has been determined in terms of the first and second harmonic amplitudes. The results demonstrate for the first time that the second harmonic can reach the theoretically predicted maximum value. As also predcted, the harmonic generation efficiency at unbonded interfaces first increases and then decreases with an externally applied compressive load. The technique has been applied to diffusion bonded specimens in an attempt to quantify their achieved strength. As already demonstrated earlier, the energy reflected from such diffusion bonds is also useful to characterize their strength. Indications are that a combination of reflected energy and harmonic generation data could be a powerful tool to quantify the strength of diffusion bonds, particularly those of nearly perfect strength. A strength determination of diffusion bonds by nondestructive evaluation is a necessity for the qualification of such bonds in critical applications.

Nondestructive characterization of the mechanical strength of diffusion bonds. I. Experimental results

The application of ultrasonic reflectivity for the characterization of a variety of copper diffusion bonds is described. The quality of each diffusion bond has been described by its ultimate tensile strength. Furthermore, fractography of the failed bonds provided information on the relative fraction of bonded areas as well as the geometry of the bonds. This paper provides the experimental results obtained as well as empirical correlations between the quantities determined.

Nondestructive characterization of the mechanical strength of diffusion bonds. II. Application of a quasi-static spring model

It has been shown that the acoustic response of imperfect interfaces may be described by a quasi-static spring model. In the present paper, experimental data on the geometry of the contacts between two diffusion-bonded blocks have been used to determine the “spring stiffness” of such interfaces which have been correlated with experimentally determined ultrasonic reflection coefficients. The correlation between the theoretical reflection coefficient and the “spring stiffness” was found to be in excellent agreement with experimentally-observed values, if the disbonds are of infinitesimally small thickness. For disbonds of finite thickness, the agreement is less satisfactory. Reasons for the discrepancy in the latter case are unknown at the present time.

Hydrogen-induced strengthening in V-Ti alloys

Abstract The results of this study show that hydrogen increases the yield strength and decreases the work hardening behavior in the low temperature region where embrittlement occurs in both hydride (vanadium and V-1%Ti) and non-hydride (V-5%Ti and V-10%Ti) forming V-Ti alloys. Solid solution softening as a result of additions of titanium is observed in non-hydrogenated V-1%Ti and V-5%Ti but it is not apparent in hydrogenated V-Ti alloys. Analysis of the strengthening in the hydrogenated V-Ti alloys shows that the predominant effect of hydrogen is to increase the thermally activated component of the yield stress. It appears that a mechanism based on stress-induced ordering of interstitial atoms around dislocations predicts strengthening behavior in reasonable agreement with the evperimental results.

Effects of hydrogen on some mechanical properties of vanadium-titanium alloys

The effect of hydrogen on the strength and ductility of V-Ti alloys was investigated from 78 to 300 K. Alloy softening which was observed at low temperatures for V-Ti alloys containing 5 at. pct of titanium or less was mitigated by the addition of hydrogen, and low temperature hardening took place. Hydrogen embrittlement, as measured by reduction of area, was observed in both the hydride forming V-1Ti alloy and some nonhydride forming V-Ti alloys. In alloys containing 10 at. pct of titanium the addition of hydrogen caused low temperature embrittlement, the range of which increased as the hydrogen concentration was increased. Failure of the severely embrittled alloys was found to be initiated transgranularly for the hydride forming V-1Ti alloy and intergranularly for alloys which did not form hydrides. A possible reason for this difference in crack initiation is discussed.

Effects of Crack Closure on Ultrasonic Transmission

Ultrasonic waves are attenuated as they propagate past the tip of a crack due to the reflection of the energy at the crack face and diffraction at the crack tip. Crack closure modifies the situation since partial transmission can occur at points along the crack face where asperities come in contact. This phenomenon is important in defining the ability to nondestructively detect closed cracks and in developing a more detailed understanding of the closure phenomenon itself. Modified compact tension specimens were used to investigate the effects of partial crack closure on focussed, through-transmission ultrasonic signals. Data obtained from fatigue cracks in 7075-T651 A1 provides evidence for a gradual transition from a fully closed crack condition at the crack tip to an essentially fully open condition at a distance of a few mm from the tip, with additional localized contact along the length of the crack. This interpretation of the data was aided by a two-dimensional, quasi-static model for ultrasonic interaction with a partially contacting interface. The model relates width and separation of asperity contacts to the frequency dependence of the ultrasonic reflection and transmission. These measurements were supplemented by tests in which water infiltrated into the crack opening. The frequency spectra of the ultrasonic transmitted signals for this case were used to estimate the average COD at various points along the crack length.

Interaction of Ultrasonic Waves with Simulated and Real Fatigue Cracks

It is now well accepted that the partial contact of fracture surfaces can have significant effects on the ultrasonic response of fatigue cracks. The authors and colleagues1–4 have developed an approximate model for this effect in which the array of contacts is replaced by an equivalent distributed spring with stiffness per unit area, K. A result of this model, the frequency dependent transmission and reflection coefficients, has been verified by comparison to exact solutions for special cases.5,6 Of particular note is the comparison to the transmission and reflection at a periodic array of strip contacts, as analyzed by Angel and Achenbach7, which is in good agreement with that of the spring model when the wavelength is large with respect to the contact spacing. Comparison to static elasticity solutions allows K to be determined for a variety of interesting interfacial topographies.5,6

Internal friction study of substitutional-Interstitial interaction in niobium-vanadium alloys

The effect of a small addition of a substitutional solute to both niobium and vanadium on the oxygen Snoek peak of each metal was investigated by an internal friction technique. A vanadium-oxygen interaction peak was identified in Nb-0.24 at. pct V and Nb-0.50 at. pct V alloys at a temperature which is appreciably higher than that for the oxygen Snoek peak in pure niobium. At oxygen concentrations greater than an oxygen/vanadium ratio of 1, the oxygen Snoek peak appears, thus providing strong evidence for oxygen trapping at vanadium in these alloys. A V-0.50 at. pct Nb alloy, on the other hand, shows no such interaction peak but rather only a somewhat broadened vanadium-oxygen Snoek peak. This is interpreted as indicating that oxygen atoms are not trapped by niobium in the vanadium matrix. The activation enthalpies and time constants for the oxygen relaxation for the peaks have been determined. The observed results are discussed in terms of the elastic and chemical interaction models for substitutional-interstitial solute interactions in a bcc lattice.