Doctoral Dissertations

Keywords and Phrases

Threshold stress

Abstract

"The high creep resistance of dispersion-strengthened alloys is a result of a threshold stress, which is determined by the attractive dislocation-particle interaction. The change of dislocation-particle interaction with temperature and the resultant threshold stress have been identified as research areas that need focused effort to improve the fundamental understanding of creep deformation of dispersion strengthened materials.

Al-6Mg-2Sc-1Zr, Al-6Mg-1Sc-1Zr-10 vol.% SiC and Al-5Y2O3-10SiC systems were chosen for this study. Conventional creep experiments were carried out at constant load to study the creep mechanism and threshold stress in creep. The temperature of creep experiment performed was in the range of 423 to 573 K. (0.4 to 0.6 Tm). Creep data obtained in all the system mentioned above ranges over 8 orders of magnitude of strain rate. The analysis of the creep data obtained in the investigation have provided information on the creep characteristics of the alloy including the shape of the creep curve, the temperature dependence of the creep rate, the stress dependence of the creep rate and the details of creep mechanism. Microstructures were evaluated using optical, scanning and transmission electron microscopy. A threshold type creep behavior was measured and explained by observed dislocation-particle interactions. When threshold stress is incorporated into the analysis, it is shown that the stress exponent is ~5 and activation energy is close to the value for lattice self diffusion of pure aluminum. These results suggest that dislocation climb in the aluminum alloy is the rate controlling the creep process in this composite. The composite showed increased creep resistance at higher temperature compared to the unreinforced alloy.

The dislocation-particle interaction was studied in composite by in-situ straining experiments in a transmission electron microscope. The experiments were conducted in order to find the deformation micromechanics involved at creep temperatures. In-situ TEM straining experiment showed that dislocations spend most of the time interacting with particle during the climb by-pass processes. Particle pinning strength was determined from the dislocation curvature and critical bow-out angle before the detachment at various temperatures and compared with the experimentally observed threshold stress values. The in-situ experiments support observations of attractive dislocation particle interaction obtained in last two decades by conventional TEM. At each temperature mean detachment stress values calculated from critical dislocation configuration lower than the experimental thresholds stress indicating a contribution from load transfer to reinforcement phase. A new positive climb model is proposed for the threshold stress in dispersion strengthen alloy. The model incorporates the concept of dislocation core relaxation at the particle-matrix interface which leads to attractive dislocation-particle interaction. The model predicts a temperature dependent threshold stress and the predicated creep behavior is in reasonable agreement with the behavior of an Al-Sc alloy"--Abstract, pages iv-v.

Advisor(s)

Mishra, Rajiv S.

Committee Member(s)

Van Aken, David C.
Newkirk, Joseph William
Miller, F. Scott, 1956-
Krishnamurthy, K.

Department(s)

Materials Science and Engineering

Degree Name

Ph. D. in Metallurgical Engineering

Sponsor(s)

National Science Foundation (U.S.)

Comments

Financial support provided by the National Science Foundation (NSF) of the United States under grant DMR-0100780

Publisher

University of Missouri--Rolla

Publication Date

Summer 2005

Journal article titles appearing in thesis/dissertation

  • Creep behavior and threshold stress of an extruded Al-6Mg-2Sc-1Zr alloy
  • Creep behavior of extruded Al-6Mg-1Sc-1Zr-10 vol. % SiC[subscript p] composite
  • Effect of stress state on primary creep behavior of PM Al-Mg-Sc-Zr-SiC[subscript p] composite
  • Investigation of creep threshold stresses using in-situ TEM straining experiment in an Al-5Y₂O₃-10SiC composite
  • Positive-climb dislocation-particle interaction model for creep threshold stress in dispersion strengthened aluminum alloy

Pagination

xvii, 137 pages

Note about bibliography

Includes bibliographical references.

Rights

© 2005 Sachin Deshmukh, All rights reserved.

Document Type

Dissertation - Restricted Access

File Type

text

Language

English

Subject Headings

Aluminum alloysAluminum alloys -- CreepDispersion strengthening

Thesis Number

T 8716

Print OCLC #

69678327

Electronic OCLC #

905650883

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