The Effect of Nanoscale Mesoscopic Structural States Associated with Lattice Curvature on the Mechanical Behavior of Ti–6Al–4V Alloy

The paper studies the effect of the temperature of helical rolling, which creates nanoscale mesoscopic structural states in lattice curvature zones, on the low-temperature impact toughness of Ti–6Al–4V alloy. The polymorphic transformation temperature Tс = 950°C is shown to play the decisive role in this effect. The impact toughness is very high at helical rolling temperatures above Tс, while in the low-temperature range up to T = –70°C it decreases monotonically. This is a very important aspect of the technology. Starting from the helical rolling temperature T = 950°C, the toughness sharply decreases at T = –70°C. At T = 900°C, it decreases sharply already at T = –20°C. At T = 850°C, the level of the entire toughness curve of the alloy is low. The high level of impact toughness at T > Tс is found to be due to a two-stage transformation of the bcc β phase into a mixture of (α + β) phases. The first stage involves nonequilibrium microscopic decomposition into the α and β phases. At the second stage, the nonequilibrium β phase decomposes into (α + β) subbands in accordance with the interstitial structural states associated with lattice curvature. In helical rolling at T < Tс, the formation of martensite phases in the close-packed lattice of the α phase of titanium reduces its impact toughness. The fatigue life of Ti–6Al–4V alloy helically rolled at T = 1000°C remains unchanged.