Mechanism of martensite decomposition during low temperature ageing process for TC21 alloy

Microstructure evolution and martensite decomposition mechanism of quenched TC21 alloy during long term ageing process at low temperatures were studied by TEM, XRD and microhardness analysis techniques. Result shows that the alloys exhibit good properties after long term isothermal treatment at 400-450°C due to α-phase particles uniformly distribute in the parent phase. Furthermore, TEM observation indicates that the nucleation of α-phase particle is related with the distribution of stacking faults in the quenched TC21 alloy. The transformation sequence of martensite decomposition for TC21 alloy is α→α+α _rich→α +β _metastable→α+β. Further increasing the ageing temperature and time, α particles will coarsen to lamellar structure and strength of the alloy will decrease. The low temperature ageing porcess mentioned in this paper creates a new way to improve mechanical properties of TC21 alloy.