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The ideal strength of iron in tension and shear

Description: The ideal strength of a material is the stress at which the lattice itself becomes unstable and, hence, sets a firm upper bound on the mechanical strength the material can have. The present paper includes an ab-initio calculation of the ideal shear strength of Fe. It is, to our knowledge, the first such computation for any ferromagnetic material. The paper also elaborates on our earlier calculation of the ideal tensile strength of Fe by studying the effects of strains which break the tetragonal symmetry. The strengths were calculated using the Projector Augmented Wave Method within the framework of density functional theory and the generalized gradient approximation. In <001> tension the ideal strength is determined by an elastic instability of the ferromagnetic phase along the ''Bain'' strain path from bcc to fcc. An <001> tensile strain also leads to instability with respect to transformation into a face centered orthorhombic structure, and to various magnetic instabilities. However, these are encountered at larger strains and, thus, do not affect the ideal strength. We also investigated the ideal shear strength of bcc iron in two prominent shear systems, <111>{l_brace}112{r_brace} and <111>{l_brace}110{r_brace}. In both shear systems the ideal strength is determined by the body centered tetragonal structure that defines a nearby saddle point on the energy surface. The ideal shear strengths are thus very similar, though they are not identical since the two shears follow slightly different strain paths from bcc to bct. We investigated the magnetic instabilities encountered during <111>{l_brace}112{r_brace} shear. These instabilities do not appear until the strain is significantly greater than the instability strain of the ferromagnetic crystal. Hence while Fe exhibits some novel effects due to magnetism, they do not affect the ideal strength, which is determined by the same elastic instabilities that determine the strengths of most other bcc metals.
Date: December 1, 2002
Creator: Clatterbuck, D.M.; Chrzan, D.C. & Morris, Jr., J.W.
Partner: UNT Libraries Government Documents Department

Elastic stability and the limit of strength

Description: The upper limit of strength (the ''theoretical strength'') has been an active subject of research and speculation for the better part of a century. The subject has recently become important, for two reasons. First, given recent advances in ab initio techniques and computing machines, the limits of strength can be calculated with considerable accuracy, making this one of the very few problems in mechanical behavior that can actually be solved. Second, given recent advances in materials engineering, the limits of strength are being approached in some systems, such as hardened or defect-free films, and their relevance is becoming recognized in others. The present paper discusses some interesting results from recent research on the limits of strength, with an intermixture of speculations based on those results. Topics include the inherent nature of {l_brace}100{r_brace} cleavage and ''pencil slip'' in bcc metals, the inherent ductility of fcc metals, the anomalous properties of Al, and the possibility of measuring ideal strength with nanoindentation.
Date: July 1, 2002
Creator: Morris Jr., J.W.; Krenn, C.R.; Roundy, D. & Cohen, Marvin L.
Partner: UNT Libraries Government Documents Department

Validation of Predicted Precipitate Compositions in Al-Si-Ge

Description: Aged alloys of Al-0.5Si-0.5Ge (at.%) contain diamond cubic (A4) precipitates in a dispersion that is much finer than is found in alloys with Si or Ge alone. To help understand this aging behavior, the present work was undertaken to determine alloy composition as a function of aging temperature. The composition was estimated theoretically using a CALPHAD approach, and measured experimentally with energy dispersive spectroscopy (EDS) in a high-resolution electron microscope. Theory and experiment are in reasonable agreement. As the aging temperature rises, the precipitates become enriched in Si, changing from 50 at. % in the low-temperature limit to about 80 at.% Si as temperature approaches 433 C, the high-temperature limit of the precipitate field.
Date: April 21, 2004
Creator: Dracup, B; Turchi, P A; Radmilovic, V; Dahmen, U & Morris, Jr., J W
Partner: UNT Libraries Government Documents Department

Detecting damage in steel with scanning SQUID microscopy

Description: A ''Holy Grail'' of NDE research is a non-destructive method for measuring fatigue damage prior to crack initiation. High-Tc scanning SQUID microscopy may be a useful tool. Because of the exceptional magnetic sensitivity of this technique, fatigue damage can be detected well before microcrack initiation, and in the absence of other obvious microstructure or property changes. Given the spatial resolution of the technique, undamaged material can be located and used to set internal standards.
Date: September 4, 2001
Creator: Lee, Tae-Kyu; Clatterbuck, David; Morris Jr., J.W.; Shaw, T.J.; R., McDermott & Clarke, John
Partner: UNT Libraries Government Documents Department

X-Ray Microdiffraction Characterization of DeformationHeterogeneities in BCC Crystals

Description: The deformation behavior of BCC metals is being investigated by x-ray microdiffraction measurements (mu XRD) for the purpose of characterizing the dislocation structure that results from uniaxial compression experiments. The high brilliance synchrotron source at the Advanced Light Source (Lawrence Berkeley National Lab) and the micron resolution of the focusing optics allow for the mapping of Laue diffraction patterns across a sample. These measurements are then analyzed in order to map the distribution of residual stresses in the crystal. An important finding is the observation of Laue spot ''streaking'', which indicates localized rotations in the lattice. These may represent an accumulation of same-sign dislocations. Theoretical modeling of the diffraction response for various slip systems is presented, and compared to experimental data. Preliminary results include orientation maps from a highly strained Ta bicrystal and a less highly strained Mo single crystal. The orientation maps of the bicrystal indicate a cell-like structure of dense dislocation walls. This deformation structure is consistent with previous OIM studies of the same crystal. The results suggest that mu XRD may be a particularly useful tool for microscale studies of deformation patterns in a multi-scale investigation of the mechanisms of deformation that ranges from macroscopic deformation tests to high resolution TEM studies of dislocation structures.
Date: November 22, 2004
Creator: Magid, K.R.; Lilleodden, E.T.; Tamura, N.; Florando, J.N.; Lassila, D.H.; LeBlanc, M.M. et al.
Partner: UNT Libraries Government Documents Department

Mapping mesoscale heterogeneity in the plastic deformation of a copper single crystal

Description: The work reported here is part of a 'multiscale characterization' study of heterogeneous deformation patterns in metals. A copper single crystal was oriented for single slip in the (111)[{bar 1}01] slip system and tested to {approx}10% strain in roughly uniaxial compression. The macroscopic strain field was monitored during the test by optical 'image correlation'. The strain field was measured on orthogonal surfaces, one of which (the x-face) was oriented perpendicular to [1{bar 2}1] and contained the [{bar 1}01] direction of the preferred slip system. The macroscopic strain developed in an inhomogeneous pattern of broad, crossed shear bands in the x-face. One, the primary band, lay parallel to (111). The second, the 'conjugate' band, was oriented perpendicular to (111) with an overall ({bar 1}01) habit that contains no common slip plane of the fcc crystal. The mesoscopic deformation pattern was explored with selected area diffraction, using a focused synchrotron radiation polychromatic beam with a resolution of 1-3 {micro}m. Areas within the primary, conjugate and mixed (primary + conjugate) strain regions of the x-face were identified and mapped for their orientation, excess defect density and shear stress. The mesoscopic defect structure was concentrated in broad, somewhat irregular primary bands that lay nominally parallel to (111) in an almost periodic distribution with a period of about 30 {micro}m. These primary bands were dominant even in the region of conjugate strain. There were also broad conjugate defect bands, almost precisely perpendicular to the primary bands, that tended to bridge primary bands and terminate at them. The residual shear stresses were large (ranging to well above 500 MPa) and strongly correlated with the primary shear bands; interband stresses were small. The maximum resolved shear stresses within the primary bands were oriented out of the plane of the bands, and, hence, could not recover the dislocation ...
Date: October 1, 2008
Creator: Magid, K. R.; Florando, J.N.; Lassila, D.H.; Leblanc, M.M.; Tamura, N. & Morris Jr, J. W.
Partner: UNT Libraries Government Documents Department

Development of a nanoindenter for in-situ transmission electron microscopy

Description: In-situ transmission electron microscopy is an established experimental technique that permits direct observation of the dynamics and mechanisms of dislocation motion and deformation behavior. In this paper, we detail the development of a novel specimen goniometer that allows real time observations of the mechanical response of materials to indentation loads. The technology of the scanning tunneling microscope is adopted to allow nanometer scale positioning of a sharp, conductive diamond tip onto the edge of an electron transparent sample. This allows application of loads to nanometer-scale material volumes couple with simultaneous imaging of the material response. The emphasis in this paper is experimental and descriptive, with particular attention given to sample geometry and other technical requirements. Examples of the deformation of aluminum and titanium carbide as well as the fracture of silicon will be presented.
Date: January 30, 2001
Creator: Stach, Eric A.; Freeman, Tony; Minor, Andrew M.; Owen, Doug K.; Cumings, John; Wall, Mark A. et al.
Partner: UNT Libraries Government Documents Department

Shape Change Studies of BCC Single Crystals Using a Non-Contact Image Correlation System

Description: A 3-D image correlation system has been used to study the deformation behavior of high purity molybdenum single crystals. This system, in conjunction with a recently developed experimental apparatus, provides the full field displacement and strain data needed to validate dislocation dynamics simulations. The accuracy of the image correlation system was verified by comparing the results with data taken from conventional strain gage rosettes. In addition, a stress analysis has been performed to examine the non-uniformities in stress. The results of the analysis show that after the sample has been strain 2%, the axial stress in the sample varies by {+-} 20%.
Date: August 31, 2004
Creator: Florando, J N; Lassila, D H; Leblanc, M M; Kay, G J; Perfect, S A; Arsenlis, A et al.
Partner: UNT Libraries Government Documents Department