Mitsuhiro Murayama

Pre-precipitate clusters and precipitation processes in Al–Mg–Si alloys

In the continuing drive for automobile weight reduction, the 6000 series Al-Mg-Si alloys are considered the most promising candidates for age-hardenable bodysheet materials. The solute clusters and the metastable precipitates in aged Al-Mg-Si alloys have been characterized by a three-dimensional atom probe (3DAP) and transmission electron microscopy (TEM). After long-term natural aging, Mg-Si co-clusters have been detected in addition to separate Si and Mg atom clusters. The particle density of {beta}{double_prime} after 10 h artificial aging at 175 C varies depending on pre-aging conditions, i.e. pre-aging at 70 C increases the number density of the {beta}{double_prime} precipitates, whereas natural aging reduces it. This suggests that the spherical GP zones formed at 70 C serve as nucleation sites for the {beta}{double_prime} in the subsequent artificial aging, whereas co-clusters formed at room temperature do not. Atom probe analysis results have revealed that the Mg:Si ratios of the GP zones and the {beta}{double_prime} precipitates in the alloy with excess amount of Si are 1:1, whereas those in the Al-Mg{sub 2}Si quasi-binary alloy are 2:1. Based on these results, the characteristic two-step age-hardening behavior in Al-Mg-Si alloys is discussed.

Microstructure of two-phase Al-1.7 at% Cu alloy deformed by equal-channel angular pressing

Abstract The microstructural changes of an aged Al–1.7 at% Cu alloy associated with severe plastic deformation have been studied by transmission electron microscopy (TEM) and energy-filtered transmission electron microscopy (EF-TEM). θ′ precipitates are almost completely dissolved after eight passes of equal-channel angular (ECA) pressing, and nearly single-phase α with a fine grain size of approximately 500 nm is obtained. When a severely deformed sample is aged for 24 h at 100°C, precipitation of equiaxed θ phase is observed along the grain boundaries, whereas only GP zones are formed in the undeformed sample. The dissolution and precipitation processes in severely deformed Al–1.7 at% Cu alloy have been examined by TEM and energy-filter mapping.

Atom probe studies on the early stages of precipitation in Al-Mg-Si alloys

Abstract Pre-precipitation stages of Al–0.70Mg–0.33Si and Al–0.65Mg–0.70Si (at.%) alloys have been investigated by atom probe field ion microscopy (APFIM) and high resolution transmission electron microscopy (HREM). Atom probe results show that clusters of Mg atoms are present in the as-quenched state. After a prolonged aging at room temperature, clusters of Mg, Si and their co-clusters are detected, although no contrasts suggesting the presence of precipitates are observed in HREM images. In the specimens aged at 175°C for 30 min, small equiaxed Mg–Si precipitates are observed by TEM. APFIM results show that the ratio of Mg to Si atoms in the precipitates is close to 1, rather than 2, which is expected from the equilibrium concentration of Mg 2 Si.

ROLE OF VACANCY-SOLUTE COMPLEX IN THE INITIAL RAPID AGE HARDENING IN AN Al-Cu-Mg ALLOY

Abstract We have investigated the origin of the initial rapid hardening of an Al–1.3 at.% Mg–1.7 at.% Cu alloy by coincidence Doppler broadening of positron annihilation radiation and positron lifetime spectroscopy. Quenched-in vacancies are bound to Mg atoms rather than Cu atoms initially and the vacancy–Mg complexes easily migrate to vacancy sinks at 150°C. Vacancy–Mg–Cu complexes form during the initial 1 min aging at vacancy sinks, meanwhile vacancy density decreases rapidly. These results support that the dislocation–solute interaction is the origin of the initial rapid hardening.

Summary of the Fourth AIAA CFD Drag Prediction Workshop

Results of the Thrid AIAA Drag Prediction Workshop are summarized. The workshop is focused on the prediction of both absolute and differential drag levels for wing-body and wing-alone configuarations that are representative of transonic transport aircraft.

Grid Quality and Resolution Issues from the Drag Prediction Workshop Series

The drag prediction workshop series (DPW), held over the last six years, and sponsored by the AIAA Applied Aerodynamics Committee, has been extremely useful in providing an assessment of the state-of-the-art in computationally based aerodynamic drag prediction. An emerging consensus from the three workshop series has been the identification of spatial discretization errors as a dominant error source in absolute as well as incremental drag prediction. This paper provides an overview of the collective experience from the worksho series regarding the effect of grid-related issues on overall drag prediction accuracy. Examples based on workshop results are used to illustrate the effect of grid resolution and grid quality on drag prediction, and grid convergence behavior is examined in detail. For fully attached flows, various accurate and successful workshop results are demonstrated, while anomalous behavior is identified for a number of cases involving substantial regions of separated flow. Based on collective workshop experiences, recommendations for improvements in mesh generation technology which have the potential to impact the state-of-the-art of aerodynamic drag prediction are given.

Summary of Data from the Fifth Computational Fluid Dynamics Drag Prediction Workshop

Results from the Fifth AIAA Computational Fluid Dynamics Drag Prediction Workshop are presented. As with past workshops, numerical calculations are performed using industry-relevant geometry, methodology, and test cases. This workshop focused on force/moment predictions for the NASA Common Research Model wing-body configuration, including a grid refinement study and an optional buffet study. The grid refinement study used a common grid sequence derived from a multiblock topology structured grid. Six levels of refinement were created, resulting in grids ranging from 0.64×106 to 138×106 hexahedra, a much larger range than is typically seen. The grids were then transformed into structured overset and hexahedral, prismatic, tetrahedral, and hybrid unstructured formats all using the same basic cloud of points. This unique collection of grids was designed to isolate the effects of grid type and solution algorithm by using identical point distributions. This study showed reduced scatter and standard deviation fr...

Comparison Study of Drag Prediction by Structured and Unstructured Mesh Method

Comparison study of computations for the Third AIAA Computational Fluid Dynamics Drag Prediction Workshop is performed on the DLR-F6 wing-body configurations with and without the wing-body fairing using the structured grid solver UPACS and unstructured grid solver TAS code. Grid convergence study at a fixed C L using a family of three difference density grids and the results by a sweep are discussed. The self-made multiblock structured grids and mixed-element unstructured grids are employed. Another participants grid is also compared. Comparisons between the two codes are conducted using the same turbulence model. Moreover, the detailed comparisons are conducted on the grid topology at the comer of the wing-body junction, the turbulence models, and the thin-layer approximation in viscous terms using the multiblock structured grids. The reconstruction schemes to realize the second-order spatial accuracy are also compared on unstructured grids. By comparing the results, the sensitivity of drag prediction to these factors is discussed.

The combined effect of molybdenum and nitrogen on the fatigued microstructure of 316 type austenitic stainless steel

The mechanical properties of austenitic stainless steels, including the low cyclic fatigue behavior have been a subject of numerous studies. Since the role of Mo was not considered in most previous studies, the present study aimed at understanding the combined effect of Mo and Ni on the fatigue properties of 316 type austenitic stainless steels. For this purpose, the authors have investigated the fatigued microstructure and the distribution of nitrogen by a transmission electron microscope (TEM), a conventional atom probe (1DAP) and a three dimensional atom probe (3DAP).

Summary of the Fourth AIAA Computational Fluid Dynamics Drag Prediction Workshop

Results from the Fourth AIAA Drag Prediction Workshop are summarized. The workshop focused on the prediction of both absolute and differential drag levels for wing–body and wing–body/horizontal-tail configurations of the NASA Common Research Model, which is representative of transonic transport aircraft. Numerical calculations are performed using industry-relevant test cases that include lift-specific flight conditions, trimmed drag polars, downwash variations, drag rises, and Reynolds-number effects. Drag, lift, and pitching moment predictions from numerous Reynolds-averaged Navier–Stokes computational fluid dynamics methods are presented. Solutions are performed on structured, unstructured, and hybrid grid systems. The structured-grid sets include point-matched multiblock meshes and overset grid systems. The unstructured and hybrid grid sets comprise tetrahedral, pyramid, prismatic, and hexahedral elements. Effort is made to provide a high-quality and parametrically consistent family of grids for each g...