Guinier-Preston zone hardening during vacuum annealing of (Ti1-xAlx)0.92W0.08N (0.07 ≤ x ≤ 0.79) thin films

Abstract

We present a systematic study on the formation of Guinier-Preston (GP) zones and corresponding age hardening in metastable cubic (Ti_1-xAl_x)_0.92W_0.08N thin films at annealing temperatures relevant to cutting tool applications. The Al content, which determines the compound's phase stability, varies widely from x = 0.07 to 0.79. The density of GP zones forming after 2 h long annealing in vacuum at 950 °C is assessed by cross-sectional transmission electron microscopy. Concurrent to the spinodal decomposition of c-(Ti,Al,W)N into c-Ti(W)N and c-Al(W)N, tungsten forms atomic-plane-thick W disks on (111) planes of the cubic matrix. The density of GP zones is ∼ 6.3·10¹¹ – 1.2·10¹² cm⁻² in the 0.07 ≤ x ≤ 0.58 range and decreases to 1.8·10¹¹ cm⁻² with х = 0.65 concomitant with precipitation of w-AlN grains and bcc-W nanocrystallites. GP zone formation and age hardening is observed for all films with 0.07 ≤ x ≤ 0.58 with 2 to 14 % increase in hardness, starting from high levels between 23.5 and 32.9 GPa. Elevated hardness is, thus, retained over the entire temperature range: 520 to 950 °C. Finally, precipitation of bcc-W at х ≥ 0.65 during annealing above 950 °C is observed as a means to relax misfit strain between the c-Ti(W)N and w-Al(W)N components.