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Papers by Thomas Broderick

PhDT, 2010

Composite alpha/beta stereographic projections and slip system misorientation diagrams were devel... more Composite alpha/beta stereographic projections and slip system misorientation diagrams were developed and used to establish a new understanding regarding anisotropic deformation behavior of alpha and beta phases aligned according to the Burgers orientation relationship. Application of these tools showed that a 2-fold, maximum common crystal symmetry existed between single variants of Burgers oriented alpha and beta phases, which placed specific

$Research paper thumbnail of Practical Considerations for High-Resolution Transmission Kikuchi Diffraction Mapping and Analysis in Titanium Alloys$

Microscopy and Microanalysis, Jul 1, 2016

Materials Characterization, 2021

Abstract Elastic strain and residual stress values in an engineering material were determined by ... more Abstract Elastic strain and residual stress values in an engineering material were determined by high-resolution EBSD (HR-EBSD) using reference patterns from annealed, recrystallized material. Specifically, rolled Ti-6Al-4 V sheet was analyzed using HR EBSD in both an as-annealed and a strained state, the latter established via a static two-point bend fixture. The commercial software CrossCourt4 (CC4) was used to evaluate the relative intragranular strain tensor components of the alpha grains mapped by HR-EBSD. The intragranular reference patterns identified in each grain by the software were then replaced with similarly oriented patterns from scans of the as-annealed, recrystallized material. The CC4 analysis was then repeated to provide calibrated strain and residual stress values. Elastic strains on the order of 10−3 were detected, with corresponding residual stress values on the order of 1 GPa. Subsequent analyses using high energy synchrotron x-ray diffraction showed good correlation with the elastic strains measured by HR-EBSD using the annealed reference patterns. Additionally, the maximum macroscopic stress analytically predicted for two-point bending was in agreement with the residual stress observed. Good repeatability of the methodology and insight into the state of individual grains were also observed. The findings provide encouraging evidence that accurate intragranular and long-range residual stress measurements of engineering significance can be made with HR-EBSD using a physical reference pattern library.

Scripta Metallurgica et Materialia, 1990

In~oduction A recent weldability study on the 02 dtanium aluminide, Ti-26 at% Al-11 at% Nb, has s... more In~oduction A recent weldability study on the 02 dtanium aluminide, Ti-26 at% Al-11 at% Nb, has shown a su'ong dependency of the weld microsuructure and mechanical properties on the cooling rate experienced during the weld thermal cycle (1). Initial welding trials found that moderate coofing rates associated with conventional gas tungsten-axe (GTA) welding (25-75°C/s) promote the formation of an extremely fine, acicular 02 rnicrostrucmre (HCP, DO m superiauice) which exhibits a low ductifity and high susceptibility to solid-state cracking during weld cooling. Subsequent weld simulation studies showed that improvements in GTA weld ducdfity and cracking resistance could be achieved by utilizing slower weld cooling rates (< 10°C/s) to produce a coarser o. 2 platelet + retained 13 microstructure (1). However, practically achieving such slow cooling rates is difficult as it requires heating the workpiece to relatively high temperatures during the welding operation (2). The application of pulsed Nd:YAG laser welding has been shown to offer an alternative for generating crack-free weldments in Ti-26 at% Al-1 1 at% Nb (2, 3). The exu'emely rapid cooling rates provided by this high energy density welding process promote the formation of a relatively ductile, ordered-13 microsu'ucture (BCC, B2 microsm.tcmm) within the fusion zone. Although this ~(B2) phase is highly metastable, n~',ent work (4) has shown the potential of postweld heat treatment for thermally stabilizing the microstructure and developing acceptable mechanical proper~es.

A new source of anisotropic deformation behavior for alpha and beta titanium aligned according to... more A new source of anisotropic deformation behavior for alpha and beta titanium aligned according to the Burgers orientation relationship has been understood from the perspective of 2-fold maximum common crystal symmetry. Overlays of stereographic projections viewed along {011} || (0001) and rotated so that <111> || <2-1-10>, establishing a Burgers orientation relationship, revealed the maximum common crystal symmetry between bcc-β and hcp-α phases in the composite α/β material was only 2-fold. This new observation had implications for necessary anisotropy of deformation behavior between and within a-basal, a-prism, a-pyramidal and c+a pyramidal slip systems. Consideration for requirements of 2-fold crystal symmetry, e.g. testing of slip systems at loading orientations within 180 of each other and relative orientation with respect to the {011} || (0001) mirror planes, showed that 3, 6, 6 and 12 unique responses were required when testing hcp-α slip systems at orientations o...

Metallurgical Transactions A, 1985

The effect of cooling conditions, giving estimated cooling rates in the range 104 °C per second t... more The effect of cooling conditions, giving estimated cooling rates in the range 104 °C per second to 107 °C per second, on the microstructure of Ti-6A1-4V has been evaluated. The microstructures of as-solidified particulates were martensitic, with the martensite lath length decreasing with beta grain size, L, which in turn decreased with increasing cooling rate. For material alpha + beta heat-treated or vacuum hot pressed, the alpha morphology was dependent on the prior cooling rate. For materials cooled at ~5 x 10 5 °C per second martensite transformed to lenticular alpha, while material cooled at >~5 x 10 5 °C per second developed an equiaxed alpha morphology. This change in morphology was explained in terms of high dislocation density or grain size refinement, both of which result from the high cooling rate. When the beta grain size (L) was plotted vs section thickness (z), and estimated cooling rate (T), power law relationships analogous to those reported for secondary dendrite arm spacing were found: L = 1.3-+ 0.4Z 0"89+0"06 (thin, chill-substrate quenched), L = 0.17 +-0.05z °-86±°-°j (thick, convection-cooled material), and L = 3.1 x 106 j" °93+°12 (all material), where L and z are in/xm and T is in K/s. The last relationship is in agreement with the 0.9 exponent predicted using a model developed for the effect of grain size on cooling rate assuming classical homogeneous nucleation and isotropic linear growth during solidification. The first two relationships were rationalized by assuming that the two materials cooled under near-Newtonian conditions.

Materials Science and Engineering: A, 1996

Samples of Ti-62222-0.23wt.%Si were heat treated and aged at temperatures ranging from 1150 °F to... more Samples of Ti-62222-0.23wt.%Si were heat treated and aged at temperatures ranging from 1150 °F to 1500 °F with the view of effecting selective precipitation of ~2 precipitates and silicides (i.e. Ti~Zr 5_xsi3). The effect of these intermetallic precipitates on the mechanical properties and fracture morphology was assessed via three separate microstructural conditions: Ti-62222S with % precipitates, Ti-62222S with % and silicide precipitates, and Ti-62222S with silicide precipitates. Both types of intermetallic precipitate appear to lower the fracture toughness, however % promotes intergranular fracture while silicides lead to transgranular failure and dimpling. The combined presence of the % and silicides leads to mixed mode failure. Further, since ~-2 is present in the ct phase and silicides precipitate out in the 13 phase, it appears that the effect of each of these intennetallics in Ti-62222S is additive rather than synergistic.

JOM, 2013

We document evidence of a unique phase transformation process occurring within the primary alpha ... more We document evidence of a unique phase transformation process occurring within the primary alpha regions of a bimodal microstructure during solidstate welding of Ti-6Al-4V. The transformation occurs when material is rapidly and locally heated above the b transus, but the time at temperature is insufficient for complete homogenization of the initially segregated alloying elements in the bimodal microstructure of the base metal. After cooling, the regions of prior primary alpha phase contain chemical, structural, and crystallographic changes. The constituent morphology, crystallography, and chemistry provided insight into the transformation mechanism.

Scripta Metallurgica, 1989