Precipitation induced recrystallisation (PIX) in a Ti-Fe-Mo bcc-superalloy driven by lattice misfit

Abstract

Beta-Ti bcc-superalloys, comprising an A2 beta-Ti matrix reinforced by ordered intermetallic B2 beta-prime-TiFe precipitates, exhibit an unusual recrystallisation that occurs with no externally applied strain (i.e. no thermomechanical processing). Thermal ageing at 750 degrees Celsius for 72 h results in refinement of the grain size from 364 um to 30 um. This grain refinement is driven by discontinuous precipitation of beta-prime-TiFe lamellae with the beta-Ti matrix from grain/phase boundaries, which is associated with significant misorientation and increased dislocation density, attributed as precipitation induced recrystallisation (PIX).

Figures (9)

• Figure 1: Grain refinement in the Ti-Fe-Mo alloy aged without external strain: (a) Grain size and (b) hardness evolution as a function of the aging time; (c) SEM BSE micrograph and (d) EBSD IPF-x map after homogenization at 1170 $^{\circ}$C; as well as (e) SEM BSE micrograph and (f) EBSD IPF-x maps after aging at 750 $^{\circ}$C for 72h.
• Figure 2: Pseudo in-situ recrystallisation of TiFeMo. Microstructure evolution of a specific area by (a) BSE image and (b) EBSD IPF-x map after homogenisation at 1170 $^{\circ}$C and (c) BSE image and (d) EBSD IPF-x map after ageing for 72h at 750 $^{\circ}$C. (e) High magnification BSE of the area marked by red square in (c) showing new TiFe B2 precipitates. (f) XRD spectrum of the homogenised alloy and the aged one.
• Figure 3: Microstructure of typical lamellar colonies in TiFeMo aged at 750 $^{\circ}$C for an intermediate ageing time 24 hours. (a) BSE image showing the matrix-colonies microstructure. Matrix-colony boundaries are indicated by the yellow and orange dashed lines and the colony-colony boundary is the red dashed line. (b) A zoomed-in observation in a colony with (c) element mapping by SEM/EDS showing precipitates inside the colony. EBSD characterization in the same area as (a) showing the misorientation by (d) the IPF-x map, dislocation density by (e) GND map (GND density in dislocations m$^{-2}$) and internal strain by (f) $\epsilon_{11}$ strain map
• Figure 4: (a) STEM/HAADF image of TiMoFe aged at 750 $^{\circ}$C for 24 hours showing (c-j) subgrains in colonies and (k-m) the matrix lifted out from the area in Fig. \ref{['fig3']} with misorientation between them by (b) selected area diffraction (SAD) patterns. (c) STEM/EDS element mappings in (d) titanium, (e) iron and (f) molybdenum with (g,h,i) SADP of the structure of colonies and two precipitates (ppt) identified in (a). (j) STEM Bright-field images showing dislocations in a colony and interfacial dislocations between the colony and a B2 precipitate. (k) Dark-field (DF) image using a B2 superlattice reflection with (l) SAD patterns from the matrix along zone axis [011]. (m) STEM/EDS element mappings in (j) titanium (f) iron and (g) molybdenum.
• Figure 5: Schematic illustration depicting the precipitation-induced recrystallisation (PIX) and hardening following Hall-Petch relation in a A2+B2 TiFeMo alloy during pure heat treatment.