By understanding microstructure and properties of thixo-molded AZ91D fabricated at different processing parameters

the fracture behavior was investigated.The results show that ambient microstructure of typical thixo-molded AZ91D alloy is mainly composed of un-melted α-Mg and solidified phase.The un-melted solid phase can be divided into four types

such as spherical solid particles

irregular solid particle

solid particle with liquid pool inside and solid particle with liquid alloy inside.Solidified phase includes primary α-Mg phase and eutectic phase

in which primary phase exhibits regular structure with fine grain

controlling by under-cooling rate of liquid alloy.Porosity rate is the main factor influencing the mechanical properties of the alloy with similar microstructure

while effects of microstructure on the mechanical properties of the alloy with similar porosity rate is dominant.The samples with a lower solid fraction and a finer primary solid phase exhibits acceptable properties.Crack in the sample during tensile testing is propagated along interface of un-melted solid phase and liquid phase or along primary solid phase and eutectic phase.With a higher solid fraction

crack is mainly propagated along interface of un-melted solid phase and liquid phase

resulting in the deterioration of mechanical properties

while with a lower solid fraction

crack is mainly propagated along interface of primary solid phase and eutectic phase

leading to the improvement of mechanical properties as a result of bridging effects though β phase.