## 4 Results and Discussion
### Attenuation Behavior of Non-Ferrous Alloys
Tests N94 and N95 used an extruded plate of 2124 Al matrix with 14.1 wt% SiC whiskers (ARCO Silag Div., Green, SC, USA). Their attenuation spectra above 8 MHz showed quadratic dependence of the Datta-Kinra type, but attenuation was absent at lower frequencies. Attenuation was much higher in the extrusion direction than in the thickness direction. This difference reflects the alignment of whiskers (0.1 to 1 \(\upmu\)m in diameter and 0.4 to 4 \(\upmu\)m in length). Next, the MMC plate was reinforced by SiC particulates at 30 wt%. This plate was also finished by extrusion with the 2124 Al matrix (made at DWC Specialty Composites, Chatsworth, CA, USA). SiC particles had a platelet shape, with sizes ranging 5-10 \(\upmu\)m and aspect ratios ranging from 1 to 4 (see [87] for their mechanical and AE behavior). This MMC showed higher attenuation than most Al alloys, but the general trend was similar to the others, despite the presence of a large amount of SiC particles. The third MMC comprised 75% Be particles by weight and had the lowest density (2.06), except for Mg. This was an experimental MMC made at Lockheed Aircrafts (Burbank, CA, USA). The attenuation was linear in frequency and attenuation coefficients were moderate. This group showed low to moderate attenuation levels. The attenuation spectra of most non-ferrous metallic materials were in-line with the linear frequency spectra or Mason-McSkimin relation, indicating dislocation damping and Rayleigh scattering as the main mechanisms. The levels of attenuation were low in Al and Mg, but most others were comparable to typical ferrous alloys. However, available sample materials did not include certain alloy groups known for their high attenuation, such as cast brass and bronze samples. Two cold-worked plates in the in-plane orientations showed anisotropic behavior with quadratic frequency spectra, which was most likely a form of dislocation damping in these homogeneous materials. This behavior is predicted by the KGL theory [33; 34; 35; 36; 37], but the limited appearance of quadratic spectra makes this explanation less than convincing. The quadratic frequency spectra of Cu and brass are expected to vanish with annealing if they follow the trend found for low-C steel. In one test (test N35), this did not occur, indicating the need for further work. Other anisotropic attenuation behavior was observed, appearing to arise from mechanical processing steps, since the thickness direction typically showed higher attenuation. This behavior seems to be previously unreported. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Cu CW** & **Cu Annealed** & **Cu CW** & **Cu Annealed** \\ \hline Vickers Hardness & 99 & 54.1 & 94 & 77 \\ (values at 1 MHz & 35.4 & 66.9 & 33 & 43.5 \\ Test No. & N34 & N36 & N31 & N32 \\  & Ni 200 CW & Ni 200 CW & Ni 200 CW & Ni-211 annealed \\ Vickers Hardness & 234 & 122 & 115 & 80.7 \\ (values at 1 MHz & 16.1 & 45.4 & 60.4 & 123 \\ Test No. & N72 & N71 & N70 & N73 \\  & Beta-III 10\%CW & Beta-III ST & Beta-III ST & \\ Vickers Hardness & 339 & 278 & 277 & \\ (values at 1 MHz & 17.7 & 57.8 & 69.8 & \\ Test No. & N91 & N89 & N90 & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Vickers hardness and attenuation coefficient.