Qiaoyan Sun

The accurate prediction of alloying effects on the martensitic transition temperature (M_s) is still a big challenge. To investigate the composition-dependent lattice deformation strain and the M_s upon the β to α″ phase transition, we calculate the total energies and transformation strains for two selected Ti−Nb−Al and Ti−Nb−Ta ternaries employing a first-principles method. The adopted approach accurately estimates the alloying effect on lattice strain and the M_s by comparing it with the available measurements. The largest elongation and the largest compression due to the lattice strain occur along ±[011]_β and ±[100]_β, respectively. As compared to the overestimation of the M_s from existing empirical relationships, an improved M_s estimation can be realized using our proposed empirical relation by associating the measured M_s with the energy difference between the β and α″ phases. There is a satisfactory agreement between the predicted and measured M_s, implying that the proposed empirical relation could accurately describe the coupling alloying effect on M_s. Both Al and Ta strongly decrease the M_s, which is in line with the available observations. A correlation between the M_s and elastic modulus, C₄₄, is found, implying that elastic moduli may be regarded as a prefactor of composition-dependent M_s. This work sheds deep light on precisely and directly predicting the M_s of Ti-containing alloys from the first-principles method.

The mechanical behaviors and deformation mechanisms of solution treated and 450°C aged Ti-55531 alloy microsamples are investigated by the micro-compression and micro-bending. The results show that the dislocation slip is a concentrated movement in the solution treated microsamples which containing nanoscale ω phase. Under the tensile and compressive stress, there were serious strain bursts occurred in the plastic deformation of solution treated microsamples. But the dislocation slip is transmit at the phase interface in the 450°C aged microsamples which containing high density of α phase. The deformation behavior exhibits high strength and excellent stability in the micro-compression and micro-bending process. The different mechanical behaviors and deformation mechanisms of Ti-55531 alloy containing different second phase have significant guidance for tailoring the microstructure and mechanical properties.Key words: Titanium, Micro-compression, Micro-bending, Dislocation, Mechanical behavior

Excellent stability of plasticity and high strength are acquired in Ti55531 alloy micropillars via introducing a high density of deformable nanoscale α phase into a β matrix. The yield strength of the pillars is as high as 2.26 GPa irrespective of pillar sizes ranging from 6 to 0.3 μm, which is high enough to activate dislocation slip both in ductile α precipitates and the β matrix. The harmony slip model was proposed to interpret slip transmission between the nanoscale α phase and the divided β matrix, and both α and β accommodate their individual plasticity during compression. This results in an excellent combination of high strength and stable plasticity in Ti55531 alloy micron-to submicron pillars. The results highlight the novel strengthening and toughening mechanisms of nanostructured alloys and a specific type of microstructure that exhibits stable plasticity for nano/microdevices.