Wendell B. Jones

Microstructural evolution of modified 9Cr-1Mo steel

The tempering and subsequent annealing of modified 9Cr-lMo steel have been investigated to determine the influence of trace amounts of V and Nb on the sequence of precipitation processes and to identify the basis for the enhanced high-temperature strength compared to the standard 9Cr-lMo composition. Air cooling (normalizing) from 1045 °C results in the precipitation of fine (Fe, Cr)3C particles within the martensite laths. Additional carbide precipitation and changes in the dislocation structure occur during the tempering of martensite at 700 °C and 760 °C after normalizing. The precipitation of M23C6 carbides occurs preferentially at lath interfaces and dislocations. The formation of Cr2C was detected during the first hour of tempering over the range of 650 °C to 760 °C but was replaced by V4C3 within 1 hour at 760 °C. During prolonged annealing at 550 °C to 650 °C, following tempering, the lath morphology remains relatively stable; partitioning of the laths into subgrains and some carbide coarsening are evident after 400 hours of annealing at 650 °C, but the lath morphology persists. The enhanced martensite lath stability is attributed primarily to the V4C3 precipitates distributed along the lath interfaces and is suggested as the basis for the improved performance of the modified 9Cr-lMo alloy under elevated temperature tensile and creep conditions.

Substructure and strengthening mechanisms in 2.25 Cr-1 Mo steel at elevated temperatures

The substructures of thermally aged, creep deformed and fatigued 2.25 Cr-1 Mo steel have been studied using optical and transmission electron microscopy. In agreement with earlier work, the substructure of the proeutectoid ferrite was found to be very stable when exposed to thermal aging or creep deformation. This stability is explained based on the tendency of molybdenum atoms to form pairs in the ferrite matrix. Nucleation and growth of additional carbide particles during creep testing was not observed. The results of these creep tests and those of Klueh have been interpreted on the basis of Mo pair stability and the affinity between molybdenum and carbon. Fatigue tests at 866 K, however, did produce a fine Mou2C precipitate which contributed to secondary cyclic hardening in tests lasting longer than 200 h. The alloy was found to undergo early cyclic hardening followed by abrupt softening within the first tens of cycles.

Mechanical Behavior of Alloy 800 at 838 K

Three commercial heats of Alloy 800 have been tested at 838 K in simple tension and in fatigue at constant plastic strain ranges from 0.2 pct. to 1.4 pct. Although all three heats met the ASTM specifications for Alloy 800, the heats exhibited significant differences in elevated temperature mechanical properties. These differences were functions not only of heat-to-heat chemistry variations, but also of the final annealing treatment given during their manufacture. The microstructures of several samples cycled to failure in the above tests were examined by transmission electron microscopy and compared with the microstructures of the as-received heats. It was found that significant additional precipitation occurred during the fatigue testing of two of the heats, and this was correlated with secondary hardening behavior found during the mechanical tests. The observed precipitation behavior could be explained using suggested isothermal transformation curves and solvus curves taken from the literature.

Study of deformation and fatigue of 316 stainless steel at room temperature by positron annihilation

Scoping studies have been initiated to evaluate the potential of positron annihilation as a volumetric nondestructive examination technique. The first experiments have shown that positron annihilation provides a sensitive monitor of plastic deformation caused by cold rolling and of fatigue cycling of 316 stainless steel at room temperature. Transmission electron microscopy examination of the fatigued samples has shown a direct relationship between dislocation density and the positron annihilation measurements. The effects of changing the strain amplitude of the fatigue cycling are also observed to be significant. The goal of these studies is to determine the positron annihilation response to high temperature creep and creep/fatigue damage encountered in advanced nuclear reactor components.

Enabling technology for human collaboration.

This report summarizes the results of a five-month LDRD late start project which explored the potential of enabling technology to improve the performance of small groups. The purpose was to investigate and develop new methods to assist groups working in high consequence, high stress, ambiguous and time critical situations, especially those for which it is impractical to adequately train or prepare. A testbed was constructed for exploratory analysis of a small group engaged in tasks with high cognitive and communication performance requirements. The system consisted of five computer stations, four with special devices equipped to collect physiologic, somatic, audio and video data. Test subjects were recruited and engaged in a cooperative video game. Each team member was provided with a sensor array for physiologic and somatic data collection while playing the video game. We explored the potential for real-time signal analysis to provide information that enables emergent and desirable group behavior and improved task performance. The data collected in this study included audio, video, game scores, physiological, somatic, keystroke, and mouse movement data. The use of self-organizing maps (SOMs) was explored to search for emergent trends in the physiological data as it correlated with the video, audio and game scores. This exploration resulted in the development of two approaches for analysis, to be used concurrently, an individual SOM and a group SOM. The individual SOM was trained using the unique data of each person, and was used to monitor the effectiveness and stress level of each member of the group. The group SOM was trained using the data of the entire group, and was used to monitor the group effectiveness and dynamics. Results suggested that both types of SOMs were required to adequately track evolutions and shifts in group effectiveness. Four subjects were used in the data collection and development of these tools. This report documents a proof of concept study, and its observations are preliminary. Its main purpose is to demonstrate the potential for the tools developed here to improve the effectiveness of groups, and to suggest possible hypotheses for future exploration.

Mechanical stability of ultrahigh strength steels

Abstract Four ultrahigh strength steels, 250 maraging, D6AC, HP9-4-20 and HY-180, have been tested at room temperature to determine both monotonic and cyclic stress-strain curves. The incremental step test was used to develop the cyclic stress-strain curves. Values for the work hardening exponent are presented for each steel in several heat treatment conditions. For the tempered or aged condition, all four steels had work hardening exponents less than 0.10 and showed cyclic softening. In no case did the 0.2% offset yield stress drop by more than 18% after cycling. The microstructures of these steels contain martensite with a high density of transformation dislocations and a distribution of fine carbide or intermetallic precipitates. The complex nature of these microstructures prevents the unambiguous identification of cyclic softening mechanisms. Monotonic and cyclic strength differntial effects were observed, with the cyclic effect generally being smaller than the monotonic effect.

Materials and mechanics issues of solder alloy applications

The key conclusion out of the Solder Mechanics Workshop is that, overall, the technical and scientific community does not know as much quantitatively about solder behavior as we had hoped. The good news, however, is that the microelectronics user community, which is largely served by solder activities, has come to terms with the current environment through active participation in the workshop. The air has been cleared. As a technical community, we have a better idea of what our priorities for continued research are. A much more detailed overview of the workshop will be available shortly in monograph form. Individual chapters will be written by authors present at the workshop and will emphasize background material (e.g., literature critiques) and detail the technical consensus on critical issues central to the workshop. The book will be available through TMS and will be the first volume in a new series of technical monographs sponsored by the societys Electronic, Magnetic and Photonic Materials Division.

Deformation Modeling and the Strain Transient Dip Test

Recent efforts in material deformation modeling reveal a trend toward unifying creep and plasticity with a single rate-dependent formulation. While such models can describe actual material deformation, most require a number of different experiments to generate model parameter information. Recently, however, a new model has been proposed in which most of the requisite constants may be found by examining creep transients brought about through abrupt changes in creep stress (strain transient dip test). The critical measurement in this test is the absence of a resolvable creep rate after a stress drop. As a consequence, the result is extraordinarily sensitive to strain resolution as well as machine mechanical response. This paper presents the design of a machine in which these spurious effects have been minimized and discusses the nature of the strain transient dip test using the example of aluminum. It is concluded that the strain transient dip test is not useful as the primary test for verifying any micromechanical model of deformation. Nevertheless, if a model can be developed which is verifiable by other experimentts, data from a dip test machine may be used to generate model parameters.

Comparison of tensile loading and stress relaxation as probes of mechanical state

Abstract Two of the commonly used experimental probes of mechanical state have been compared. Stress relaxation and work hardening rates were found not to agree in establishing sameness or differences in state and its evolution. Two prestressing and two recovery histories produced four different initial states according to work hardening measurements, while only two different initial states were present according to stress relaxation measurements. These findings reinforce previous conclusions that more than one internal variable is required to define mechanical state and, in addition, indicate that more than one experimental probe may be required to identify variables and their evolution state completely.

Microstructural evaluation and non-destructive examination of 2. 25 Cr--1 Mo steel. [886/sup 0/K]

Specimens of 2.25 Cr-1 Mo steel were damaged to various levels by creep and combined creep-fatigue cycling at 886/sup 0/K (1100/sup 0/F) with hold periods. Ultrasonic shear wave velocity measurements were performed and showed small changes with large amounts of creep or fatigue damage; these changes were deemed too small for practical application. Measurements of magnetic properties were unsuccessful in following damage. Optical and transmission electron microscopy (TEM) were performed on the samples. TEM observations showed that microstructural changes which occurred at 886/sup 0/K (1100/sup 0/F) did not occur homogeneously throughout the matrix; during creep, carbide spheroidization and coarsening began in widely separated regions, with an increasing fraction of the material undergoing the process with time. A precipitate was observed to form during fatigue which did not occur during simple aging or creep.