Figure 3. X-ray diffractometer scans taken from three orthogonal sides of a small hydrostatically pressed, pressureless-sintered beryllium cube. those of a pressureless-sintered mirror, which is shown in figure 5. This sample, although not perfectly random nor homogeneous, is far superior to hot-pressed samples. The actual thermal instability of this pressureless-sintered mirror was 0.41 x 10-3 X/°C. Thermal stability for performance of optical systems would be critical where the mirror would operate at temperatures other than that at which it was polished. In general, all the thermal instabilities in the mirrors tested were elastic in that they returned to their original surface figure when returned to the original test temperature. Beryllium mirrors that have large thermal instabilities, such as those made from the hot-pressed blanks, would of necessity have to be polished at a temperature near the operating temperature of the mirror. More critical to many applications is the long-term dimensional instability. Again, the only precise data available are for flat surfaces, but pressureless-sintered mirrors have been shown to be stable to values of X/20 per year. These values are approaching the longterm dimensional stability recorded for fused Introduction The Effects of Processing on the Robert E. Maringer 3. Relaxation of thermally-induced residual stresses resulting from the anisotropy of the thermal expansion coefficients of beryllium 4. A nonuniformity of the preferred orientation of beryllium, which results in nonuniformity of thermally-induced stresses. Beryllium has a number of distinctly advantageous properties that make it a promising optical mirror material. It has, above all, a high modulus (3.2 x 104 kg/mm2) and a low density (1.86 g/cc), which combine to give it the highest stiffness-to-density ratio of all the normally considered candidate materials. In addition, beryllium has a rela- Machining Stresses tively high thermal conductivity (1.7 watts/cm °K), which means that it will equilibrate readily after some change in temperature. Recognition of these properties has led to the use of beryllium in telescopic mirrors, gyros, accelerometers, instrument mounts, and other precision devices. The accumulated experience indicates that beryllium is not an ideally stable material. Recent research has resulted in a significant advance in the understanding of the reasons for this instability. It is the purpose of this discussion to point out some of the reasons for this instability and to indicate methods to avoid it. Mechanisms of Dimensional Instability At least four distinct mechanisms of dimensional instability (not including overstressing by the application of an excessive external load) have been identified in beryllium. These are: 1. Relaxation of residual stresses introduced during processing (especially machining) 2. Relaxation of residual stresses existing in an electroless nickel plate or relaxation of thermally-induced stresses between the plate and the beryllium substrate The most common source of residual stress in beryllium is machining damage. The machining operation itself causes considerable disruption of the surface layers, leaving behind a heavily cold-worked layer. If one examines this worked layer metallographically in cross-section, deformation twins are often observed, extending to a depth of perhaps 0.002 inch below the surface. These have some times been taken as evidence of surface damage or residual stress, and the beryllium surface has been etched to a depth of 0.002 inch or so to alleviate the situation. It has been shown (refs. 1 and 2), however, that the damage extends far deeper than this. Using the recommended tools, the recommended rake angle, and the recommended cutting speeds, it has been found that the residual stresses due to lathe machining will occasionally penetrate to depths greater than 0.010 inch, but penetration is more commonly of the order of 0.005 or 0.006 inch. Although the evidence is meager, it appears that the depth of penetration decreases with decreasing grain size and increasing material strength. These residual stresses introduced by machining have been reported (ref. 3) to be |