Fabrication of (Ta,W)C surface gradient layer on Ta-10W alloy by in situ solid-phase diffusion

Abstract

A novel (Ta,W)C gradient layer was fabricated by in situ solid-phase diffusion on the surface of Ta-10W alloy at 1130 °C for 2, 4, 6, 8, and 10 h. The phase composition and phase distribution of the (Ta,W)C surface gradient layer and the morphologies, grain sizes and crystal structure of the (Ta,W)C particles were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The nanoindentation hardness, elastic modulus, fracture toughness, and adhesion strength of the (Ta,W)C surface gradient layer were tested by a nanoindenter, Vickers indentation tool, and scratch tester, respectively. The results showed that the phase composition of the (Ta,W)C surface gradient layer was mainly the (Ta,W)C phase and a small amount of the Ta-10W phase. The microstructure exhibited a gradient in the (Ta,W)C grain size that decreased gradually from approximately 920 nm to 100 nm, moving from the surface of the carburized layer to the Ta-10W substrate. Ingrowth of the (Ta,W)C gradient layer was controlled by the diffusion of carbon, and the thickness reached approximately 18 μm when annealed for 10 h. Formation of the (Ta,W)C layer could be attributed to a diffusion-type solid phase transition driven by the diffusion of carbon atoms into the Ta-10W substrate. In addition, moving from the surface of the (Ta,W)C layer to the Ta-10W substrate, the nanoindentation hardness and elastic modulus gradually increased from 25.1 to 28.82 GPa and 500.8 to 537.9 GPa, respectively. The layer exhibited a high surface fracture toughness (4.98 ± 0.31 MPa·m1/2) and excellent layer/substrate adhesion strength. The toughness mechanism of the (Ta,W)C gradient layer was mainly crack deflection, grain bridging and intergranular fracture. In situ solid-phase diffusion offers a new approach for preparing (Ta,W)C surface gradient layer that exhibit high hardness, high toughness and excellent layer/substrate adhesion strength.