Microstructural features of cracking in autogenous beryllium weldments
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I.
INTRODUCTION
BERYLLIUM components have been joined by a number of methods;[1] however, autogenous welding is not appropriate except in special situations.[2–6] The principal reason is that Be is prone to extensive fusion zone cracking in autogenous weldments. It has been established that the cause of cracking in autogenous Be welds is associated with the presence of low-melting-point impurities, principally Al.[7–13] Passmore[9] was the first to demonstrate that impurities such as Al, Fe, and Si segregate to the weld centerline. These elements were also found to be concentrated at fusion zone edges. In addition, he examined Be in five lots containing from 0.064 to 0.41 wt pct Al and found a correlation between Al content and cracking propensity. Further additions of Al, via filler wire, of up to 1 wt pct, increased the amount of cracking. In another study, Hauser and Monroe[5] found high levels of Al, Si, Ti, and Cr in second phases associated with fusion zone cracks. In a similar work by Fraikor et al.,[11] second-phase particles were found by transmission electron microscopy (TEM) of specimens removed from the fusion zone. These were identified as AlFeBe4 and, in areas where there was insufficient Al to form this phase, FeBe11.[14] In this case, the identified phases were not located at cracks, but at Be grain boundaries and within solidification bands. More in-depth investigation by scanning electron microprobe[12,15] revealed Al-rich precipitates within fusion zone cracks. The authors proposed that the cracking was a result of Al-rich hot-short regions which, combined with thermal stresses, produced intergranular cracks. The identity of the phase(s) associated with fusion zone fissures was not conclusively determined, although it was suggested that free Al was present. The purpose of this work is to characterize the fracture mechanisms of solidification cracks in autogenous Be welds and to identify the phase(s) associated with the fracture. It J.D. COTTON, formerly Team Leader, Metallography and Microscopy, with the Los Alamos National Laboratory, is now Principal Engineer, Metals Technology, with the Boeing Defense and Space Group, Metals Technology, Seattle WA 98124-2499. R.D. FIELD, Team Leader, Microstructural Characterization, is with the Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted August 21, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
has been found that a rather complicated solidification path, resulting in a multiple phase grain boundary structure, is associated with the cracking. This information is important, as it pertains to the phase equilibria which evolve in the fusion zone: Ultimately, this is the key to the compositional and/or processing controls necessary to both understand and minimize fusion zone cracking in autogenous Be welds. II.
EXPERIMENTAL PROCEDURE
Ingot-grade Be was utilized in the form of 6.35-mm-thick rolled coupons which had been machined to approximately 75 3 25 mm in shape. The chemical composition is shown in Table I, as determined b
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