Elastic and pseudoelastic behaviour of Mg-Al-Zn alloys

Sumitomo, Taro. (2003). Elastic and pseudoelastic behaviour of Mg-Al-Zn alloys PhD Thesis, School of Engineering, The University of Queensland.

       
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Author Sumitomo, Taro.
Thesis Title Elastic and pseudoelastic behaviour of Mg-Al-Zn alloys
School, Centre or Institute School of Engineering
Institution The University of Queensland
Publication date 2003
Thesis type PhD Thesis
Supervisor Dr. Carlos Caceres
Dr. Martin Veidt
Total pages 183
Language eng
Subjects L
291403 Alloy Materials
671001 Structural metal products
Formatted abstract

In the first part, this work investigates the effect of various factors which influence the elastic moduli of magnesium alloys. The effect of grain size, temper and solute content on elastic modulus values of pure Mg, a Mg-0.3Zr alloy, an AZ91 alloy and a number of Mg-Al and Mg-Zn binary alloys are determined using two ultrasonic methods. The effect of porosity and cross section thickness in diecast AZ91 is also assessed.

Coarse-grained pure Mg and fine-grained Mg-0.3Zr has similar elastic moduli. The modulus increases then decreases with increasing content of Al or Zn in solid solution, with a maximum value at an electron concentration of ca. 2.01. The modulus of aged sand-cast AZ91 specimens increases with the volume fraction of β-Al12Mg17 precipitates. The cross section thickness seems to have no influence on the modulus of die cast material, but decreases with the porosity content.

Equations describing the effect of these factors are combined to construct an elastic modulus chart to predict the elastic modulus for the alloy AZ91 of various casting methods and tempers

In the second part, large pseudoelastic strains which develop in alloy AZ91 after a small plastic strain resulting in non-linear unloading behaviour and strain hysteresis loops upon loading/unloading are investigated.

 Loops have been observed in sand-cast and ingot material in a variety of tempers, and in high-pressure die-cast material with different cross section thickness. The loops are observed after a nucleation strain of between 0.001 and 0.01% and grow to a maximum width after 1 ~ 2% plastic strain, becoming slightly narrower afterwards. When fially developed, the loops add a large (0.3~0.45%) pseudoelastic strain to the elastic strain, effectively decreasing the elastic modulus of the alloy by up to 70%. In sand-cast and ingot materials for a given temper, the effects tend to be more pronounced in compression while for a given loading direction the effect is slightly larger in die cast or aged sand-cast or ingot material. The phenomenon is discussed in terms of the partial reversion of {1012} twins upon unloading.

It is shown that the pseudoelastic component of the strain can be expressed as a function of the applied strain or the stress using Weibull functions for. For all the alloys, the strain and stress dependent Weibull functions can be combined to predict the loading and unloading paths of the cyclic loading flow curve with very good accuracy.

Keyword Magnesium alloys

Document type: Thesis
Collection: UQ Theses (RHD) - UQ staff and students only
 
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