No abstract

5 mm thick brass plates were successfully friction stir welded at the tool rotation rates from 400 to 1000 rpm for a constant traverse speed of 100 mm/min. The nugget zone (NZ) consisted of the incompletely and completely-recrystallized regions. With increasing rotation rates, the fraction of the non-recrystallized grains decreased and the size of the recrystallized grains increased. The hardness values in the NZs were higher than those in the parent material (PM). Increasing the rotation rate did not exert a noticeable effect on the tensile and yield strengths of the welds, but increased the elongation. The tensile and yield strengths of the welds reached up to $99 and 80% of the PM, respectively. The fracture occurred in the heat affected zone that had the lowest hardness.

Alloying scattering of phonons is particularly effective in reducing the thermal conductivity. The alloying model considers the mass and strain fluctuations but the substitutional atoms are assumed to be positioned at the ideal lattice point, i.e., without lattice distortion. In the real case, the existence of the lattice distortion in the alloy is inevitable, and it should have an additional contribution to the phonon scattering. Such an effect, however, is usually ignored and can be partially ascribed to the difficulty of experimentally identifying and quantifying the lattice distortion. In this work, significant distortion of the crystal lattice is directly observed by the scanning transmission electron microscopy in YbCd 2 Sb 2 with multiple elements alloying. These results show that the plane distance of adjacent Ytterbium atoms along the direction b fluctuates in the range between 0.34 and 0.46 nm, a distortion from ≈−11.7% to ≈16.0%. The lattice distortion plays a remarkable role in phonon scattering and substantially reduces the lattice thermal conductivity to ≈0.45 Wm -1 K -1 at 700 K. As a result, a peak zT of ≈1.4 is achieved in (Yb 0.9 Mg 0.1 ) Cd 1.2 Mg 0.4 Zn 0.4 Sb 2 . These results indicate that tuning the lattice distortion can be a promising strategy for enhancing thermoelectric performance.