Microstructure and fracture toughness of Mn-stabilized cubic titanium trialuminide

Abstract

This thesis project is related to the fracture toughness aspects of the mechanical behaviour of the selected Mn-modified cubic L12 titanium trialuminides.

Fracture toughness was evaluated using two specimen types: Single- Edge- Precracked_ Beam (SEPB) and Chevron- Notched- Beam (CNB). The material tested was in cast, homogenized and HIP-ed condition. In the preliminary stage of the project due to lack of the ASTM Standard for fracture toughness testing of th4e chevron- notched specimens in bending the analyses of the CNB configuration were done to establish the optimal chevron notch dimensions.

Two types of alloys were investigated: a) boron-free and boron doped low-Mn (9at.% Mn), as well as (b) boron-free and boron-doped high-Mn (14.% Mn). 1000oC and was calculated from the maximum load. It has been found that toughness of coarse-grained "base" 9Mn-25Ti alloy exhibits a broad peak at the 200- 500-oC temperature range and then decreases with increasing temperature, reaching its room tempe4rature value at 1000oC. However, the work of fracture and the stress intensity factor calculated from it increases continuously with increasing temperature. Also the fracture mode dependence on temperature has been established.

To understand the effect of environment on the fracture toughness of coarse-grained "base", boron-free 9mN-25ti alloy, the tests were carried out in vacuum (~1.3x10-5 Pa), argon, oxygen, water and liquid nitrogen. It has been shown that fracture toughness at ambient temperature is not affected by the environments containing moisture (water vapor). It seems that at ambient temperature these materials are completely immune to the water- vapor hydrogen embrittlement and their use of brittleness is other than environment.

To explore the influence of the grain size on fracture toughness the fracture toughness tests were also performed on the dynamically recrystallized "base", boron-free 9Mn-25Ti material with the average grain size of 45 um. Further refinement of the grain size was obtained by ball-milling of powders in order to obtain a nanostructure material. These were subsequently consolidated by hot pressing with the objective of retaining the nanostructure to the largest extent possible. The estimated grain size of the powder compact was ~50- 200 nm. The indentation microcracking fracture toughness measurements were performed on the powder compacts. It has been found that fracture toughness is independent of the grain size in the range ~1300- 45 um and that for the finest grains (~50- 200 nm) it drops substantially and is equal to half of that for coarse-grained material.

A beneficial effect of boron doping, high-(Mn+Ti) concentration and combination of both, on the fracture toughness was observed at room and elevated temperatures. The addition of boron to a "base" 9at.% Mn- 25at.% Ti trialuminide improves the room temperature fracture toughness by 25- 50%. Addition of boron to a high (Mn+Ti) trialuminide improves the room temperature fracture toughness by 100% with respect to a "base" 9Mn- 25Ti alloy. Depending on the Mn+Ti concentrations and the level of boron doping, improvements of fracture toughness at 200- 600oC and 800- 1000oC ranges are also observed.