A neutron diffraction and modeling study of uniaxial deformation in polycrystalline beryllium
- PDF / 303,776 Bytes
- 11 Pages / 606.24 x 786 pts Page_size
- 79 Downloads / 181 Views
20/6/03
9:41 PM
Page 1439
A Neutron Diffraction and Modeling Study of Uniaxial Deformation in Polycrystalline Beryllium D.W. BROWN, M.A.M. BOURKE, B. CLAUSEN, T.M. HOLDEN, C.N. TOMÉ, and R. VARMA The deformation of polycrystalline beryllium to strains of 0.8 pct in uniaxial tension and compression was studied by neutron diffraction and modeled using an elasto-plastic self-consistent (EPSC) model. The beryllium response is asymmetric with respect to tension and compression in both the macroscopic behavior, as displayed in the stress/strain curve, and the microscopic lattice response. The EPSC model qualitatively reproduces the lattice strain curves in tension and compression with the assumption of pyramidal slip being active, in addition to prism and basal slip and with the inclusion of thermal residual stresses developed during processing. Although it underpredicts the magnitude of the observed strains, it demonstrates that accounting for residual stresses of thermal origin is crucial for understanding the evolution of lattice strains during uniaxial loading.
I. INTRODUCTION
METALLIC beryllium has a number of potential engineering applications that could benefit from its strength, low density, and thermomechanical properties. Its elastic stiffness is comparable to steel, with roughly one-fourth of the density. The thermal conductivity of 147 W/m·K is comparable to aluminum (210 W/m·K), and its melting temperature is relatively high, 1551 K, making it attractive for some high-temperature applications. Unfortunately, at room temperature, beryllium has limited ductility that prevents its use in more wide-ranging applications. In the context of this study, beryllium is of fundamental scientific interest, because it has a hexagonal close-packed crystal structure and displays strong anisotropy of plastic deformation while at the same time relatively low elastic anisotropy. There have been several studies of the polycrystalline deformation of beryllium in uniaxial tension often aimed at enhancing its limited tensile ductility.[1–7] In tension, hotpressed beryllium typically exhibits a sharp yield point phenomenon, with upper and lower yield points between 325 and 275 MPa, respectively, with an elongation of 0.7 to 3 pct depending on the impurities, heat treatment, and grain size.[2,4,6,7] This yield phenomenon in beryllium has been attributed to dislocation pinning by iron-bearing precipitates.[2,3] However, Murr et al. saw little evidence of dislocation pinning and asserted that plastic deformation was dominated by localized slip at the grain boundaries.[1] Relatively few studies of uniaxial compressive deformation of polycrystalline beryllium exist,[8–11] and only those in References 10 and 11 concentrated on hot-pressed material. The upper/lower yield point phenomena reported in tension are not observed for hot-pressed beryllium in compression. No detailed microstructural studies of comD.W. BROWN, M.A.M. BOURKE, C.N. TOMÉ, and R. VARMA, Technical Staff Members, and T.M. HOLDEN, Visiting Professor, are with the Lo
Data Loading...
