Introduction

It has been recognized for several years that beryllium should be extremely isotropic for use in optical systems. Aside from its anisotropy, beryllium is ideal for use in such instruments because it possesses low density, high modulus, and reasonably good thermal properties. A critical requirement for such precision parts is uniform expansion of the metal. Beryllium, however, crystallizes at low temperature with a hexagonal close-packed crystal structure that leads to anisotropy in many of its physical parameters. This presents a serious problem that can only be overcome by the use of randomly oriented polycrystalline bodies that are usually produced from beryllium powders.

Nearly all beryllium powder is prepared by some comminution process in which coarse beryllium particles are crushed to finer and finer sizes. Beryllium cleaves primarily on its basal plane, the powder taking on irregular platelet or acicular shapes in which the plane of the particle corresponds to the basal plane of the beryllium crystal. In packing such powders, preferred orientation occurs because the particles tend to align the plane of the platelets (basal plane) normal to the packing direction. During densification of the powders, such as hot-pressing, this tendency is further magnified because the basal plane is also the slip plane and additional particles will plastically deform into this same orientation under the pressing force. Using hydrostatic or isostatic pressing procedures and sintering techniques can greatly reduce the amount of anisotropic texture. Two such processes have been developed: pressureless sintering (ref. 1)

and hot-isostatic pressing (ref. 2). These techniques are capable of producing nontextured samples because the pressing forces are applied equally in all directions, not uniaxially as in conventional hot-pressing.

Anisotropy in polycrystalline materials can be estimated by a relatively simple diffractometer-scanning technique in which the degree of anisotropy is indicated by the relative intensities of selected diffraction peaks. Although such x-ray procedures are not as "foolproof" as pole figure analysis, diffractometer scans yield qualitative data that can be used to measure anisotropy.

Hot-Pressed Beryllium

Unlike most powder metallurgy products that are fabricated by cold-pressing followed by a high temperature sintering, nearly all commercially powdered beryllium is processed by vacuum hot-pressing. During vacuum hot-pressing, beryllium powders contained in a steel or graphite die are uniaxially compressed at pressures of the order of 1000 pounds per square inch for about 1 hour at 1000° to 1100° C. The beryllium billet produced is nearly theoretically dense, having a grain size only slightly larger than the initial powder particle size. Beryllium oxide is present in the same amount as in the initial powder and is usually 2 to 6 weight percent.

Mechanical deformation of polycrystalline beryllium at high temperatures causes the development of a texture (preferred orientation). The grains tend to become aligned with their c-axes parallel to the direction of the applied force. This worked texture is also observed to a lesser

degree in hot-pressed beryllium. For normal structural uses of beryllium, hot-pressed beryllium is considered to be isotropic; however, differences in ultimate tensile strength of 10 percent are often observed between longitudinal and transverse sections of hotpressed structural-grade beryllium billets (ref. 3). Differences in thermal expansion of nearly 5 percent have been measured for these two directions in hot-pressed billets; for optical application, such differences are intolerable.

The reason for strength and thermal expansion differences is the preferred orientation developed during densification of the powders. Figure 1 shows x-ray diffractometer scans of samples of a hot-pressed beryllium billet; the surfaces x-rayed were perpendicular and parallel to the hot-pressing direction. This figure shows the three most intense beryllium peaks. The (1011) peak should be the strongest in a random sample; the intensity of the (1010) peak should be 31 percent of the (1011) peak; and the (0002) peak should be 28 percent the intensity of the (1011) peak (ref. 4). As is evident in figure 1, neither of the scans yield peak intensities in the proper proportion. Because of the applied load during hot-pressing, a number of beryllium grains become aligned with their basal planes, (0002), normal to the pressing direction. X-ray examination in a direction 90 degrees to the pressing direction shows a lower number of basal planes than in a random sample. This behavior is observed in all hotpressed beryllium.

Isostatically Produced Beryllium

An alternative approach for producing randomly oriented beryllium is to use isostatic or hydrostatic pressing procedures in the densification operations. The pressurelesssintering technique produces dense (over 99 percent of theoretical), fine grain, randomly oriented beryllium (ref. 1). The microstructure of a pressureless-sintered beryllium specimen is seen in the polarized light micrograph of figure 2. A homogeneous, fine grain structure (average grain size of

about 10 to 15 μ) is evident. This sample contains about 2 weight percent beryllium oxide impurity, which appears as a small black precipitate in the micrograph. The oxide is generally present in the grain boundaries.

The degree of randomness of the pressureless-sintered beryllium samples is illustrated in figure 3. This figure shows three x-ray diffractometer scans taken from three orthogonal directions of a small specimen of a pressureless-sintered billet. The relative intensities of the three peaks taken in the three directions are nearly identical, and all are nearly in the proper ratios (ref. 4). X-ray data were also taken of sections cut at a 45-degree angle to the sections used for figure 3, and these data again are in the proper proportion. Finally, pole figures analysis on pressureless-sintered beryllium indicated that the samples examined were essentially random.

Optical Properties of Beryllium Mirrors

Mirrors have been made from the various classes of beryllium mentioned in this report, including several grades of commercially hotpressed, hot isostatically pressed, and pressureless-sintered beryllium. These mirrors were generally solid blanks, 4.2 inches in diameter by 0.50 inch thick. The samples were polished by Perkin-Elmer to obtain flat, high quality optical surfaces. The mirrors have been evaluated for short-term stability over thermal excursions between 25° and 80° C, called "thermal stability," and long-term flatness dimensional stability measured at constant temperature. The long-term dimensional stability is evaluated by two means (ref. 2). The first uses a precision surface interferometer that records flatness instabilities over a 2-week period at constant temperature; the data are then extrapolated to yield instabilities per year. A second dimensional stability measurement is simply to compare the flatness of the polished mirrors after shelf storage of 12 to 18 months. Longer times are not yet possible because control samples were produced less than 2 years ago.

The best hot-pressed beryllium showed thermal instabilities of 1.0 x 10-3/°C when thermally cycled between 25° and 80°C; X was 6328 angstroms for these measurements (ref. 5). Hydrostatically pressed and sintered beryllium, on the other hand, showed thermal

instabilities on on the order of 0.4 to 0.5 x 10-3 /°C (ref. 5). More recent measurements (ref. 6) indicate that the pressureless sintering will consistently produce beryllium samples 3 to 5 times more thermally stable than hot-pressing.