Effect of abc Pressing at 573 K on the Microstructure and Martensite Transformation Temperatures in Ti49.8Ni50.2 (at%)

Abstract

This paper presents experimental data on the microstructure and martensite transformation temperatures of Ti49.8Ni50.2 (at%) after abc pressing (multi-axial forging) to different true strains e from 1.84 to 9.55 at 573 K. The data show that increasing the true strain results in grain–subgrain refinement on different scales at a time. With e = 9.55 at 573 K, the average grain–subgrain size measured approximately 130 nm. Decreasing the abc pressing temperature from 723 to 573 K caused a decrease in all martensite transformation temperatures, a change in the lattice parameters, R phase formation, and angular shifts of diffraction peaks and their broadening. The largest change in the microstructure of Ti49.8Ni50.2 was provided by abc pressing to e = 1.84. Increasing the true strain to e = 9.55 resulted in a much smaller effect, suggesting that the alloy obtained a high density of structural defects even at e = 1.84. Two possible mechanisms of grain–subgrain refinement are discussed.

Crystal Structure Defects in Titanium Nickelide after Abc Pressing at Lowered Temperature

“…In addition, the composition of the microstructure shown in Figure 2a contains both grains-subgrains with a two-phase structure R + B19 and grains-subgrains in which either a martensitic R-phase or a martensitic B19 -phase are observed. The more detailed analysis of phase state and microstructure in Ti 49.8 Ni 50.2 alloy after abc pressing at 573 K are presented in [61].…”

Mechanical Properties of the Ti49.8Ni50.2 Alloy after Multi-Axial Forging at 573 K

“…As noted in the introduction, the evolution of the microstructure of the Ti 49.8 Ni 50.2 alloy samples was studied in [14] as a function of the abc-strain specified at 573 K. It was found that the refinement of the grain-subgrain microstructure occurs at two structuralscale levels. The first level (denoted in [14] as a macrolevel) is large grains of initial samples with high-angle grain boundaries. The average sizes of these large grains are denoted as D. The second level (denoted in [14] as the microlevel) is the level of the grain-subgrain structure inside large grains.…”

“…This caused an increase in the yield stress, σ y , and ultimate tensile strength, σ UTS , by 2.3 and 1.5 times, respectively. One of the well-known methods of severe plastic deformation, which makes it possible to obtain an ultrafine-grained structure in metals and alloys, is the forging method with a change in the deformation axis in three mutually orthogonal directions, which is called "multi-axial forging" or "abc-pressing" [9][10][11][12][13][14]. The abbreviation "abc-pressing" was used in further text.…”

“…Such a refinement of the grain structure had practically no effect on the ultimate tensile strength, but the yield strength increased by ≈2 times. In [14], the evolution of the microstructure of a nickel-titanium binary alloy with a nickel content of 50.2 at % after abc-pressing at a temperature of 573 K was studied. It was shown that with an increase in the total true strain during abc-pressing, the grain-subgrain structure is refined to an ultrafine-grained state: the grain-subgrain size was 130 nm at the maximum studied abc-strain e = 9.55.…”

Microstructure and Mechanical Properties of Titanium Alloys

“…Optimal mechanical performance of the alloy was shown to be achieved via β-annealing at 1005 • C for 70 min, followed by air cooling to room temperature and aging at 722 • C for 2 h with subsequent air cooling to room temperature. Kashin et al reported the results of the experimental study of the microstructure and martensite transformations of Ti 49.8 Ni 50.2 alloy after abc pressing at 573 K [7]. They showed that increasing the true strain resulted in grain-subgrain refinement on different scales and proposed possible mechanisms for this effect.…”