Doctoral Dissertations


Wei Yuan


"Friction stir processing (FSP) is emerging as an effective tool for microstructural modification. Mg-Al-Zn alloys with different aluminum contents were subjected to FSP to achieve fine grained and ultrafine grained (UFG) structure. The objective of this work was to study the influence of microstructural length scale on the governing deformation and strengthening mechanisms of FSP magnesium alloys.

FSP introduced strong basal fiber texture and processing parameter dependent grain/precipitate size and distribution. Grain refinement promoted non-basal dislocation activation at room temperature. The strength of the alloy increased as a consequence of grain refinement following classic Hall-Petch relationship to different extents due to texture effect. Remarkable difference in tensile behavior was observed when the dominant deformation mode changed from basal to non-basal slip for various grain sizes. Specimens which deformed mainly by basal slip exhibited low tensile strength, high degree of work hardening and fractured before developing a neck. However, specimens which deformed by non-basal slip showed high strength, low degree of work hardening and a large fraction of post-uniform strain as a result of increased strain rate sensitivity.

A high ductility of ~65% in FSP magnesium was attributed to extensive basal dislocation slip with the additional work hardening from activation of extension twin. Grain refinement reduced the uniform strain significantly, especially for UFG magnesium due to the low work hardening rate and high rate of dislocation recovery. High strength magnesium alloy with ~435 MPa in tensile strength and~23% in ductility was achieved. The promising combination of strength and ductility was attributed to the UFG structure with low dislocation density and fine ß-phase precipitates"--Abstract, page iv.


Mishra, Rajiv S.

Committee Member(s)

Miller, F. Scott, 1956-
Kohser, Ronald A.
Newkirk, Joseph William
Du, Xiaoping


Materials Science and Engineering

Degree Name

Ph. D. in Metallurgical Engineering


National Science Foundation (U.S.). Industry/University Cooperative Research Centers Program


Missouri University of Science and Technology

Publication Date


Journal article titles appearing in thesis/dissertation

  • Material flow and microstructural evolution during friction stir spot welding of AZ31 magnesium alloy
  • Effect of texture on the mechanical behavior of ultrafine grained magnesium alloy
  • Influence of grain size and texture on Hall-Petch relationship for a magnesium alloy
  • Grain size and texture effects on deformation behavior of AZ31 magnesium alloy
  • Achieving high strength and high ductility in friction stir processed cast magnesium alloy


xii, 136 pages

Note about bibliography

Includes bibliographical references.


© 2012 Wei Yuan, All rights reserved.

Document Type

Dissertation - Open Access

File Type




Subject Headings

Magnesium alloys -- Microstructure
Magnesium alloys -- Mechanical properties
Deformations (Mechanics)
Friction welding

Thesis Number

T 10239

Print OCLC #


Electronic OCLC #


Included in

Metallurgy Commons