Rapid Decomposition of Martensite in SLM-TC4 Titanium Alloy under Thermal Cycling Conditions
α′ martensite is the major phase in selective laser melted TC4 alloy,and the controlling of its decomposition is very important for the optimization of the formed components.Laser scanning was applied to the SLM-TC4 samples with full α′ martensite microstructure,the thermal history curves at different locations of the samples were derived with the aid of numerical simulations,and the phase transformation sequence of α′ martensite under thermal cycling conditions was analysed.Two transformation pathways are identified:α′→β→(α+β)and α′→(α+β),with different thermal histories.In the former pathway,the α phase no longer maintains the hierarchical structure of α′,which typically exhibits a mixed morphology of Widmanstätten colonies and basketweave structures.In the latter pathway,the α phase retains the crystallographic orientation and hierarchical structure of the martensite.It has also been shown that in the second transformation pathway,when the temperature of thermal cycling approaches βtr and the temperature difference between the peak and valley is significant,the decomposition of α′martensite and the globalization of the α phase are accelerated,with β precipitating at the twin boundaries,which can further contribute to the enhancement of the properties of the SLM TC4 components.
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