||In this study, three nickel platings, A B and C, having high density of twin boundaries were prepared by electrodeposition technique. The effects of twin boundary (TB) on mechanical properties and thermal stability were studied.|
The three platings were deposited to 1.2 mm thick or more, and then cut to 1 mm thick for tensile test. Electron backscattering diffraction (EBSD) was used to analyze grain size and grain boundary properties. Transmission electron microscopy (TEM) was used to analyze the microstructure of the as-deposit samples and the annealed samples prior to deformation, and the deformation microstructure. The thermal stability of the platings were studied according to the hardness variation by using microhardness test after annealing.
EBSD results showed that the average grain size of plating A and B was 0.4 μm. About 50 % of the high angle grain boundaries (HAGBs) were twin boundaries. After the platings were annealed, the average grain size and the fraction of TB did not change. Plating C had larger grains of about 0.8 μm in average, and the fraction of TB was also 50%. The average grain size and the TB fraction kept at the same values after annealing.
Tensile test results showed that both the tensile strength and total elongation of plating A were higher than those of, plating B, though both platings have the same grain size. The tensile stress and total elongation of B plating were 800 MPa and 8.2%. The tensile stress of C plating was 905 MPa, slightly lower than that of plating A, but the average grain size of plating C was two times larger than that of plating A. However, the total elongation of plating B was only a half of that of plating C. After annealing, the tensile stress of all platings decreased 15 – 25%. The total elongation of plating A increased 50%, however, cohere as that of platings B and C did not change. The work hardening rate curve of the platings showed that plating A had the highest work hardening rate, followed by plating C, and plating B exhibit the lowest value.
TEM observation indicated that the dislocation density of plating B was the highest in the three platings, followed by plating C, and A group had the lowest value. A large number of tangled dislocations and dislocation cells in the grain interiors were observed in plating B which corresponded to the low work hardening rate of plating B at low strain. The dislocation density of A plating was the lowest, so its work hardening rate was the highest. After deformation, high density of dislocations were accumulated at the twin boundaries of plating A. High resolution micrograph showed that were many b ⃑ = 1/3<111> dislocations were formed at the twin boundaries, and b ⃑ = 1/6<211> partial dislocations were observed close to the twin boundaries. This showed that the twin boundaries of plating A could allow dislocations to slip across the boundaries to sustain funther plastic deformation.
For thermal stability, the hardnesses of both plating A and plating B decrease rapidly on annealing at 300 oC – 350 oC. However, the hardness of plating C remained at 70% of the as-deposite value after annealing at 500 oC. Accordingly, plating C exhibits the best thermal stability.