Microstructure evolution and deformation mechanisms during compression of a harmonic–structured Ti–24Nb–4Zr–8Sn alloy

Processing heterogeneous microstructures, especially the so-called harmonic structures consisting of soft core and hard shell regions, is an efficient way to achieve a strength-ductility trade-off in classical metallurgy. In this study, two harmonic samples with the same composition of Ti-24Nb-4Zr-8Sn but different microstructures were processed to exhibit different grain size heterogeneities between the core and the shell. Both samples were consolidated from a ball-milled powder using Spark Plasma Sintering (SPS) but applying two different sintering times, 1 and 60 min. The grain size heterogeneities were higher for the longer SPS sintering time due to the enhanced grain dimension in the core for 60 min consolidation time. The mechanical behavior of the two materials was studied via a monotonic quasi-static compression test. For both harmonic-structured Ti-24Nb-4Zr-8Sn alloys, a high compressive proof stress of about 1 GPa was detected. The strain-hardening rate was higher for the longer SPS time due to the higher grain size differences between the core and shell. A high dislocation density was detected in both materials after compression deformation (several tens of 1014 m-2). The dislocations tend to form cells and LAGBs during compression. The dislocation pile-ups at the core-shell interfaces caused a back stress of about 640 MPa after compression at 2-5% strains. The contributions of the different features of the microstructure (grain size, α phase precipitates, and oxygen concentration) to the proof stress were determined.