# Hot-Pressing of Ti-Al-N Multiphase Composite: Microstructure and Properties
## 3 Results and Discussion
### Corrosion Response Analysis  
Electrochemical studies, including electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV), confirmed the susceptibility of the sintered AlN-TiN(BN) alloy to pitting corrosion. EIS spectra, presented in Figure 11, reveal at least two overlapping capacitive loops, indicative of distinct electrochemical processes occurring on the examined substrate. Figure 11: Bode plot of the sintered AlN-TiN(BN) composite exposed to 1 M NaCl solution. Solid line—fit; dotted line—experimental data. Figure 10: Hardness distribution on the sample cross-section. The presence of a maximum phase angle over a wide frequency range (10\({}^{-2}\) Hz to 10\({}^{1}\) Hz) suggests the formation of a passive layer on the alloy surface. However, the relatively low phase angle values (\(-\theta\) = 70\({}^{\circ}\)) indicate reduced capacitive characteristics of the passive film (\(\eta\) = 0.93). Additionally, the low dielectric resistance of the layer (Table 1) results in a limited passive region which can be observed in potentiodynamic studies (Figure 12), spanning only \(\sim\)200 mV. The electrochemical parameters of the dielectric layer (\(\eta\) = 0.77, R = 375 \(\Omega\)) indicate its active dissolution in a 1 M NaCl solution. The potentiodynamic study, the results of which are shown in Figure 12, confirms the material's susceptibility to pitting corrosion. Figure 13 presents the surface topography (SEM image) after potentiodynamic testing in a 1 M NaCl solution, showing numerous deep pits distributed across the surface. Additionally, etched regions of the passive layer are visible between the pits. Analysis of the potentiodynamic curve (Figure 12) for the sintered AlN-TiN(BN) composite indicates the presence of a marginally stable passive state, with the passive layer persisting up to approximately 30 mV. Beyond this potential, anodic polarization initiates the formation of metastable pits, which continue up to \(\sim\)600 mV. The steady increase in current density within this range is attributed to the nitrides (TiN, AlN) in the alloy. These nitrides promote substrate oxidation during anodic polarization, leading to the formation of \begin{table}
\begin{tabular}{c c c c} \hline  & \multicolumn{2}{c}{**Environment**} & **Dielectric Layer** & **Double Layer** \\ \hline \multirow{3}{*}{Q\({}_{\text{CFE}}\)} & R (\(\Omega\)cm\({}^{2}\)) & 7 & 3.75 \(\times\) 10\({}^{2}\) & 9.85 \(\times\) 10\({}^{5}\) \\  & Y\({}_{0\text{ CFE}}\) (Fcm\({}^{-2}\) s\({}^{-1}\)) & 2.65 \(\times\) 10\({}^{-4}\) & 2.83 \(\times\) 10\({}^{-5}\) \\  & n & 0.77 & 0.93 \\ \hline \end{tabular}  
* R—resistance; Q\({}_{\text{CFE}}\)—capacity of the constant phase element; Y\({}_{\text{MCPE}}\)—admittance; n—coefficient of imperfections of the constant phase element (CFE), an empirical constant ranging from 0 to 1. It is worth noting that when n = 1, the CFE behaves as a pure capacitor, while when n = 0, the CFE behaves as a pure resistor.  
\end{table} 
Table 1: Characteristic electrochemical parameters of the investigated sintered alloy (EIS measurements). Figure 12: Potentiodynamic curves of sintered AlN-TiN(BN) composite exposed to 1 M NaCl at room temperature. mixed oxide-nitride phases (e.g., TiNO and AlNO) on the surface. A significant rise in current density beyond 2000 mV corresponds to the onset of pitting corrosion. Electrochemical parameters characterizing the material are summarized in Table 2.  
A comparison with published data reveals notable differences in the corrosion resistance of AlN-TiN(BN) composite materials fabricated through various techniques. For instance, Ti-AlN substrates produced via 3D printing methods [27] exhibit no pitting corrosion within the studied potential range (up to 1500 mV vs. SSCE). However, their corrosion resistance is compromised by a dendritic microstructure, multiphasic nature, and prevalent cracking.