Richard J. Fields

Consolidation of nanoscale iron powders

Abstract The consolidation behavior of two types of nanoscale iron powders-vacuum condensed (nanograins in nanoparticles) and ball-milled (nanograins in microparticles), was studied. The consolidation of two microscale powders, atomized and ground, was also characterized for comparison. Consolidation techniques investigated were cold closed die-compaction, cold isostatic pressing (CIPing), and after CIPing, sintering or hot isostatic pressing (HIPing). The mechanical properties, density, and microstructure of the resulting compacts were found to depend on the original powder type and its consolidation history. Significant differences were found between the microscale and nanoscale powders. An additional reason, besides the dissimilarity in grain size, for the differences observed relates to the fact that the nanograin powders contained significant amounts of oxygen, which ultimately resulted in a distinctly two-phase bulk microstructure. The vacuum condensed powder achieved satisfactory green strength on CIPing, and high hardness (440 Hv) on low temperature sintering. While unnecessary for complete consolidation, HIPing at 500 °C was found to be beneficial and compacts of this powder thus treated were found to have a hardness of 520 Hv and high compressive yield strength (1800 MPa). For ball-milled powders, HIPing was found to be essential for achieving effective consolidation: ball-milled material, which remained friable after CIPing and sintering at 580 °C, achieved exceptionally high hardness (820 Hv) when HIPed at 580 °C and 175 MPa. The ductility was greatly improved when HIPed at temperatures between 700 °C and 850 °C, while preserving its relatively high strength. The behavior of these nanoscale powders can be understood by invoking the usual densification, particle bonding, and grain growth mechanisms. Optimization of these processes may result in unique mechanical properties of ball milled powders.

FRACTURE TESTING OF LARGE-SCALE THIN-SHEET ALUMINUM ALLOY.

Abstract : A series of fracture tests on large-scale, precracked, aluminum alloy panels were carried out to examine and characterize the process by which cracks propagate and link up in this material. Extended grips and test fixtures were specially designed to tension load the panel specimens in a l780-kN capacity universal testing machine. Single sheets of bare 2024-T3 aluminum alloy, approximately 4 m high, 2.3 m wide, and 1 mm thick were fabricated with simulated through cracks oriented horizontally at midheight. Using existing information, a test matrix was set up to explore regions of failure controlled by fracture mechanics, with additional tests near the boundary between plastic collapse and fracture. In addition, a variety of multiple site damage (MSD) configurations were included to distinguish between various proposed linkage mechanisms. All tests but one used antibuckling guides. Three specimens were fabricated with a single central crack, six others had multiple cracks on each side of the central crack, and one had a single crack but no antibuckling guides. The results of each fracture event were recorded on various media: film, video, computer, magnetic tape, and occasionally optical microscope. The video showed the crack tip with a load meter in the field of view, using motion picture film for one tip and super VHS video tape for the other. The computer recorded the output of the testing machine load cell, the stroke, and the twelve strain gages at 1.5-second intervals. A wideband FM magnetic tape recorder was used to record data from the same sources. The data were analyzed by two different procedures: (1) the plastic zone model based on the residual strength diagram and (2) the R-curve.

Ball milling induced bct phase formation in iron and iron alloys

A variety of attritor ball milled Fe and Fe alloy powders, processed in an argon or a nitrogen environment, were studied using X-ray diffraction techniques. Broadened XRD spectra were obtained due to severe plastic deformation during ball milling. The broadening was non-uniform, displaying a systematic deviation from the average, with the (310) reflection peak being the broadest, and the (222) reflection peak being the sharpest. The systematic variation in broadening is interpreted as a tetragonal distortion. The tendency towards a tetragonal distortion occurred in all ball-milled samples.

Uncertainty in Finite Element Modeling and Failure Analysis: A Metrology-Based Approach

In this paper, we first review the impact of the powerful finite element method (FEM) in structural engineering, and then address the shortcomings of FEM as a tool for riskbased decision making and incomplete-data-based failure analysis. To illustrate the main shortcoming of FEM, i.e., the computational results are point estimates based on “deterministic” models with equations containing mean values of material properties and prescribed loadings, we present the FEM solutions of two classical problems as reference benchmarks: (RB-101) The bending of a thin elastic cantilever beam due to a point load at its free end and (RB-301) the bending of a uniformly loaded square, thin, and elastic plate resting on a grillage consisting of 44 columns of ultimate strengths estimated from 5 tests. Using known solutions of those two classical problems in the literature, we first estimate the absolute errors of the results of four commercially available FEM codes (ABAQUS, ANSYS, LSDYNA, and MPAVE) by comparing the known with the FEM results of two specific parameters, namely, (a) the maximum displacement and (b) the peak stress in a coarse-meshed geometry. We then vary the mesh size and element type for each code to obtain grid convergence and to answer two questions on FEM and failure analysis in general: (Q-1) Given the results of two or more FEM solutions, how do we express uncertainty for each solution and the combined? (Q-2) Given a complex structure with a small number of tests on material properties, how do we simulate a failure scenario and predict time to collapse with confidence bounds? To answer the first question, we propose an easy-to-implement metrology-based approach, where each FEM simulation in a gridconvergence sequence is considered a “numerical experiment,” and a quantitative uncertainty is calculated for each sequence of grid convergence. To answer the second question, we propose a progressively weakening model based on a small number (e.g., 5) of tests on ultimate strength such that the failure of the weakest column of the grillage causes a load redistribution and collapse occurs only when the load redistribution leads to instability. This model satisfies the requirement of a metrology-based approach, where the time to failure is given a quantitative expression of uncertainty. We conclude that in today’s computing environment and with a precomputational “design of numerical experiments,” it is feasible to “quantify” uncertainty in FEM modeling and progressive failure analysis. DOI: 10.1115/1.2150843

Effect of nitrogen on the mechanical properties and microstructure of hot isostatically pressed nanograined Fe

The effect of nitrogen content in Fe powders on the mechanical properties and microstructure of cold isostatic pressed (CIPed) and sintered or hot isostatic pressed (HIPed) Fe powders which was produced by attrition ball milling was investigated. Microhardness and compression tests were used to determine the mechanical properties. Optical, scanning, and transmission electron microscopy were used to investigate the microstructural changes that occurred during consolidation. Sintering of the CIPed bodies at temperatures below 850°C caused no change in density, with only minor mechanical properties improvement, and the bodies remained friable. HIPing was found to be essential for effective consolidation of ball-milled powders. Increasing HIPing temperatures increased the density, the compression yield stress, the maximum compressive strength, and the hardness to a maximum value and then decreased. The maximum in density preceded the maximum in the compression yield stress, and its value depends on the nitrogen content in the powder. At low HIPing temperatures (<580°C) the nitrogen content has no influence on the mechanical properties. However, above 580°C, the nitrogen content reduces the mechanical properties as well as density, and increases the oxide precipitation. There are two microstructural effects of nitrogen on the microstructure: void formation in the Fe bodies HIPed above 580°C for the higher nitrogen concentrations, and severe grain boundary embrittlement above 965°C. Iron bodies processed in argon have smaller grain size than iron bodies processed in nitrogen throughout the entire consolidation temperature range. However, due to the smaller grain size, the optimum in mechanical properties (maximum strength with reasonable elongation) is reached only above 965°C. Processing in argon improves machinability.

HSST wide-plate test results and analysis

Abstract Fifteen wide-plate crack-arrest tests have been completed to date, ten utilizing specimens fabricated from A533B class 1 material (WP-1 and WP-CE series), and five fabricated from a low upper-shelf base material (WP-2 series). Each test utilized a single-edge notched specimen that was subjected to a linear thermal gradient along the plane of crack propagation. Test results exhibit an increase in crack-arrest toughness with temperature, with the rate of increase becoming greater as the temperature increases. When the wide-plate test results are combined with other large-specimen results the data show a consistent trend in which the K Ia data extends above the limit provided in ASME Section XI.

Wide plate crack arrest testing

Abstract Five wide-plate crack arrest tests have been performed between September 1984 and December 1985 on an ASTM A533B quenched and tempered steel. These tests were done in the 26 MN Universal Testing Machine located at the National Bureau of Standards. The specimens were fractured in a thermal gradient to make the crack initiate at a preexisting notch in a cold, brittle region, and arrest in a hot, tough region. To obtain a great deal of information from these tests, each specimen was thoroughly instrumented with thermocouples, strain gages, crack-mouth-opening displacement gages, timing wires, and/or acoustic emission transducers. Fast data response was obtained from these sensors during the run-arrest events, which generally lasted for less than 10 ms. Estimates of two types of important data were obtained: (1) crack velocity as a function of time, and (2) initiation and arrest toughness. Thus, a successful data collecting system was developed for acquiring the basic data required for the prediction of the behavior of nuclear pressure vessels subjected to pressurized thermal shock.

Experimental observations and analysis of creep cavitation in AISI type 304 stainless steel

Abstract Large numbers of creep cavities have been measured in AISI type 304 stainless steel using automatic image analysis techniques. Among other things, the data exhibit substantial amounts of scatter, and indicate that the distribution of creep cavities may not be spatially homogeneous, even in the presence of uniform stress and temperature fields. The cavity radii appear to be approximately distributed according to a conditional Weibull distribution. Indirect evidence is presented to support the view that the nucleation of cavities is heavily influenced by grain boundary sliding. Comparisons are drawn with the observed cavity radius distributions and the predictions of the analytical cavity growth model of Chen and Argon and a constrained cavity growth model due to Rice. It is found that the distribution predicted by the constrained growth model is considerably narrower than the experimental distribution, but that the model of Chen and Argon shows reasonable agreement with experiment.

X-ray scattering and imaging from plastically deformed metals

New ultra-small-angle X-ray scattering (USAXS) facilities at third generation synchrotron sources enjoy an additional one to three decades of X-ray brilliance over second generation instruments, and can now quantify microstructural features from 3 nm to 1.3 μm in size. These developments offer exciting possibilities for further exploration of dislocation and other deformation microstructures. To the portfolio of existing techniques we now add a promising experimental window, USAXS imaging, in which high angular resolution images are acquired at scattering vectors related to the observed microstructures. Early results from this ultra-sensitive technique indicate that the arrangements of creep cavities in mildly deformed polycrystalline copper can be observed on many length scales, and the results can be compared with the size distributions derived from a USAXS analysis. Many of the features observed in USAXS imaging are not seen using other existing experimental techniques.

A Strain Gage Analysis of Fracture in Wide Plate Tests of Reactor Grade Steel

A new method of fracture analysis is described. Strains recorded by gages in the immediate vicinity of a propagating crack are analyzed. From the analysis, crack tip position, propagation toughness, and crack velocity are determined. The analysis procedure is demonstrated using data from the dynamic fracturing of a large-scale, wide plate test. The results are then used to describe the propagation toughness-crack velocity-temperature relation for the 2.25 Cr-1 Mo steel used in the test.