Influence of machining temperature on the surface damage, residual stress, and texture of hot-pressed beryllium
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The effect of lathe machining temperature ranging from room to 400~ on the depth of twinning, extent of microcracking, residual stress, and texture of hot-pressed, S200-type b e r y l lium was investigated. It was shown that twinning, microcracking, and residual s t r e s s were essentially eliminated when machining was performed above the ductile-to-brittle fracture transition range at 400~ This behavior has been related to the operation of multiple slip systems, which, in t e r m s of the yon Mises criterion, satisfies the condition for uninhibited deformation. Minimization of residual s t r e s s was attributed to r e c o v e r y resulting p r i m a r ily from dislocation annihilation and climb, the latter giving rise to the formation of welldefined low-angle boundaries and stable hexagonal networks adjacent to the machined s u r face.
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o c c u r r e n c e of a ductile-to-brittle fracture t r a n sition in beryllium with decreasing temperature is well known and has been adequately explained on the basis of available deformation modes and p r e f e r r e d orientation. ~-~ Because of the intrinsic restrictions to multiple slip and the strong propensity for basal plane cleavage below about 200~ conventional machining introduces high levels of surface residual stress, microcracking, and twinning which can lead to premature failure, e-a As a consequence, machined beryllium components normally are annealed a n d / o r chemically etched to a depth of 0.051 to 0.127 mm to remove the surface damage. Frequently, however, t h e d a m a g e extends beyond this depth such that etching alone will not ensure its complete removal. Furthermore, the necessity for maintaining accurate dimensional tolerances often p r e cludes the use of annealing and chemical etching t r e a t ments, with the result that part reliability under load is substantially reduced. The improved flow and fracture behavior of b e r y l lium which occurs with increasing temperature suggests that elevated temperature machining would p r o vide an effective approach for overcoming the surface damage problems encountered at room temperature. This concept is supported to some extent by Morrow e t e l . , 9 who in drilling tests on extruded ingot observed a decrease in twinning when the temperature was raised from room to 250~ Tensile elongation-temperature relationships established for h o t - p r e s s e d and extruded QMV beryllium, however, indicate that 400~ would constitute a p r e f e r r e d machining temperature. ~~ At this temperature, ductility reaches a maximum (first ductility peak), while twinning is almost completely suppressed. 1~ Higher t e m p e r a t u r e s are undesirable from the standpoint of oxidation and the o c c u r r e n c e of Be (Fe, A1) precipitation aging reactions. On the basis of this analysis, the effect of lathe machining temperature extending up to 400~ on the s u r E. H. RENNHACKis Research Specialist and Group Leader, Manufacturing Research, LockheedMissiles & Space Co., Inc., Sunnyvale, Calif. 94088. Manuscript su
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