Doctoral Dissertations


Nilesh Kumar


"Friction stir processing (FSP) has been used in the present work to obtain ultrafine grained (UFG) microstructure in a twin-roll cast (TRC) Al-Mg-Sc alloy. An attempt has been made to establish a correlation between processing, microstructure, and properties (mechanical) in UFG regime for this alloy system.

The effect of initial microstructural conditions and processing parameters (tool rotation rate) were shown to have a profound impact on the development of UFG microstructure. FSP led to a microstructure consisting of grains with an average grain size as small as 390 nm, having high angle grain boundaries and a completely random texture. The thermal stability study of this alloy showed that UFG microstructure was maintained up to 723 K.

Mechanical properties were studied in terms of quasi-static tensile and cyclic fully reversible bending loading conditions. A transition from continuous to discontinuous yielding took place as grain size reached UFG regime. Negative strain rate sensitivity was observed in the present UFG alloy system. Activation volume was found to be typical of UFG alloys. In UFG regime, a criticality in grain size was noted below which materials ability to undergo uniform plastic deformation was significantly compromised. For cyclically deformed samples an improvement in fatigue life in low cycle fatigue regime was observed with distinctly different deformation behavior.

An effort was made to theoretically predict the yield strength of UFG alloy by using existing strengthening mechanisms and additivity rules. A linear additivity rule was found to be applicable for UFG alloy system. However, a negative deviation in Hall-Petch slope was noted during the modeling of grain boundary strengthening"--Abstract, page iv.


Mishra, Rajiv S.

Committee Member(s)

Sankaran, Krishnan K.
Kohser, Ronald A.
Newkirk, Joseph William
Dharani, Lokeswarappa R.


Materials Science and Engineering

Degree Name

Ph. D. in Materials Science and Engineering


Boeing Company
Missouri University of Science and Technology. Intelligent Systems Center

Research Center/Lab(s)

Intelligent Systems Center


Missouri University of Science and Technology

Publication Date

Fall 2011

Journal article titles appearing in thesis/dissertation

  • Microstructure and mechanical behavior of friction stir processed ultrafine grained Al-Mg-Sc alloy
  • Ultrafine grained Al-Mg-Sc alloy via friction stir processing
  • Thermal stability of friction stir processed ultrafine grained Al-Mg-Sc alloy
  • Quasi-static and cyclic mechanical properties and deformation behavior of ultrafine grained Al-Mg-Sc alloy
  • Critical grain size for change in deformation behavior in ultrafine grained Al-Mg-Sc alloy
  • Effect of friction stir processing on the microstructure and mechanical properties of equal channel angular pressed 5052Al alloy sheet
  • Additivity of strengthening mechanisms in ultrafine grained Al-Mg-Sc alloy


xix, 196 pages

Note about bibliography

Includes bibliographical references.


© 2011 Nilesh Kumar, All rights reserved.

Document Type

Dissertation - Restricted Access

File Type




Subject Headings

Aluminum alloys -- Microstructure
Aluminum-magnesium alloys
Friction stir welding

Thesis Number

T 9898

Print OCLC #


Electronic OCLC #


Link to Catalog Record

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