This work investigates systematically the phase transformation and superelasticity in TiNbTaZr alloy prepared by friction stir processing (FSP). Multiple-pass FSP with a 100% accumulated overlap to three passes was implemented. The influence of the processing parameters on the microstructural evolution and superelasticity in stir zone, transition zone, and heat affected zone were investigated. After recrystallization, existing dislocations gradually accommodate themselves at sub-grain boundaries. Increase in the proportion of α″ martensitic phase is always accompanied by significant reduction in the fraction of ω phase precipitation. Metastable ω phase dissolving feature is evident and promoted by dislocation gliding, especially in the specimen processed at a higher rotation speed. In nanoindentation measurements, the closer the indent applied towards the transition zone, the more martensites nucleate, thereby gradually reducing both elastic modulus and hardness via re-orientation of martensites. This study provides new insight into the surface modification of beta titanium alloys through FSP method to achieve improved mechanical properties

Friction-stir processing (FSP) is used to prepare Ti-35Nb-2Ta-3Zr alloys via different processing routes. Dislocation walls and tangles, deformation-induced α″ martensite, and deformation-induced ω phase are observed. The dominant deformation mechanisms are altered from deformation-induced α″ martensitic transformation and dislocation walls to twinning upon increasing the FSP passes. A reverse deformation-induced ω to β transformation and de-twinning process are observed together with grain refinement to the nanoscale. Meanwhile, compressive distortions along [0001]ω direction are favorable for the transformation from ω to β

Using a Gleeble 1500 hot simulator, the effects of hot deformation parameters and accelerated cooling conditions on the microstructural characteristics of low carbon microalloyed steels were investigated by means of compression tests. It was found that the grain re® nement effect of single pass reduction in the recrystallisation or unrecrystallisation temperature ranges is weaker than that of two pass reduction in the recrystallisation and unrecrystallisation temperature ranges. However, four pass deformation in the recrystallisation and unrecrystallisation temperature ranges could result in rather ® ne grained microstructures and, when coupled with moderately high cooling rate, partially acicular ferrite microstructure could be obtained. With the increase of cooling rate, the microstructure becomes ® ner and the content of acicular ferrite increases. Under similar deformation and cooling conditions, the specimens with relatively high carbon content have more re® ned microstructures.MST/4689

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