Doctoral Dissertations


Arun Mohan


"The ß-precipitate free AZ31, ß-precipitate rich AZ91 and a few Mg-Y-Zn alloys were studied to understand the dependence of Mg formability on microstructural condition and temperature. Friction stir processing (FSP) was used to refine the grain size and to obtain desirable distribution of precipitates in the matrix. Since Mg alloys exhibit challenges in achieving room temperature formability, superplastic deformation was utilized. This work focused on high strain rate superplasticity (HSRS, ≥10⁻² s⁻¹) which is more attractive for industrial applications as compared to the slower rates of conventional superplasticity.

Thermally stable fine grains and pinning particles at HSRS temperatures were essential for Mg-Al-Zn alloys. AZ31 alloys exhibited strain and strain rate induced abnormal grain growth (AGG) at the HSRS conditions, and therefore did not exhibit superplasticity. Ultrafine grained AZ91 alloy, on the other hand, showed exceptional elongation of 1251% at the HSRS conditions. Twin-roll cast and rolled coarse grained (29µ-140µ) Mg-Y-Zn sheets also exhibited HSRS, making the need for grain refinement processes like FSP unnecessary.

Analysis of the AZ91 HSRS data indicated that the deformation mechanism was grain boundary diffusion controlled grain boundary sliding. Analysis of the HSRS data of coarse Mg-Y-Zn alloys indicated the deformation mechanism to be lattice diffusion controlled solute drag creep (SDC). The promising results of the present work can be potentially used to design new HSRS Mg sheets as well as for optimum use of existing Mg alloys for HSRS applications"--Abstract, page iv.


Mishra, Rajiv S.

Committee Member(s)

Miller, F. Scott, 1956-
Kohser, Ronald A.
Newkirk, Joseph William
Chandrashekhara, K.


Materials Science and Engineering

Degree Name

Ph. D. in Materials Science and Engineering


United States Automotive Material Partnership


Missouri University of Science and Technology

Publication Date

Fall 2012

Journal article titles appearing in thesis/dissertation

  • Influence of specimen width on the tensile behavior of AZ31 alloys
  • Influence of microstructure and alloying addition on the mechanical proerties of hot rolled and friction stir processed Mg-Y-Zn alloys
  • Influence of strain and strain rate on microstructural evolution during superplasticitiy of Mg-Al-Zn sheet
  • High strain rate superplasticity in friction stir processed ultrafine grained Mg-Al-Zn alloys
  • A cost effective route for achieving high strain rate superplasticity in Mg-Y-Zn alloy sheets


xiv, 161 pages

Note about bibliography

Includes bibliographical references.


© 2012 Arun Mohan, All rights reserved.

Document Type

Dissertation - Open Access

File Type




Subject Headings

Magnesium alloys -- Microstructure
Magnesium alloys -- Effect of temperature on
Metals -- Formability
Friction stir welding

Thesis Number

T 10234

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