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Micropillar compression tests of Ti5Si3 single crystals were conducted at room temperature as a function of loading axis orientation and specimen size in order to investigate their room temperature plastic deformation behavior. Plastic flow by the operation of three deformation modes, {1$${\overline{1}}$$ 1 ¯ 00}[0001], {2$${\overline{1}}$$ 1 ¯ $${\overline{1}}$$ 1 ¯ 2} < 2$${\overline{1}}$$ 1 ¯ $${\overline{1}}$$ 1 ¯ $${\overline{3}}$$ 3 ¯  > and {1$${\overline{1}}$$ 1 ¯ 01} < 2$${\overline{1}}$$ 1 ¯ $${\overline{1}}$$ 1 ¯ $${\overline{3}}$$ 3 ¯  > slip were observed in [2$${\overline{2}}$$ 2 ¯ 05]-, [0001]- and [4$${\overline{3}}$$ 3 ¯ $${\overline{1}}$$ 1 ¯ 0]-oriented micropillar specimens deformed at room temperature, respectively. The CRSS values were evaluated to be very high above 2.7 GPa and were confirmed to increase up to about 6 GPa with the decrease in the specimen size. The fracture toughness values are evaluated to be 0.45 MPa m1/2 (notch plane // (0001)) and 0.73 MPa m1/2 (notch plane //(1$${\overline{1}}$$ 1 ¯ 00)) based on the results of micro-cantilever bend tests of chevron-notched specimens. The fracture toughness values are considerably lower than those for D8l-Mo5SiB2 and D8l-Nb5Si3 evaluated by the same method, indicating the inherent brittleness of binary Ti5Si3 compared to the other transition-metal silicides of the TM5Si3 type (TM: transition-metal).

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