Sintering Kinetics, Mechanical Properties, and Electrical Conductivity of Ti-67 at% Al Targets Fabricated via Spark Plasma Sintering

Abstract

Ti–Al alloys have widespread applications as targets in hard coatings by PVD (Physical Vapor Deposition). While the importance of target density is recognized, the densification mechanisms of Ti-67 at% Al targets, particularly during spark plasma sintering (SPS), remain poorly understood, hindering process optimization. This study aims to clarify these mechanisms by fabricating Ti-67 at% Al targets via SPS and examining their densification behavior through a detailed analysis of the creep model based on the stress exponent (n) and apparent activation energy (Q_d). The target’s relative density gradually increased in the temperature range of 370–530 °C, whereas the grain size remained relatively constant, indicating that the densification process dominated during this period. The results reveal that densification is primarily controlled by intergranular diffusion (n ≈ 2, Q_d = 97.29 kJ/mol) and dislocation climbing (n ≈ 3, Q_d = 158.74 kJ/mol). The target’s relative density reached 98.25% at 530 °C, with a corresponding grain size of 10.86 ± 1.08 μm. Additionally, as the temperature increased, the Vickers hardness of the target increased from 61.56 HV to 129.66 HV, and the electrical conductivity rose from 0.23 S/cm to 0.86 S/cm. This work provides a fundamental understanding of the densification kinetics in Ti-67 at% Al alloys during SPS, establishing a crucial guideline for fabricating high-performance PVD targets.

1. Introduction

Ti-67 at% Al alloy targets are mainly used for depositing TiAlN coatings on substrates by the PVD technique. To ensure the quality of the coatings, the targets are typically required to have characteristics such as high purity [1], high density, uniform microstructure, and a fine grain size [2]. Previous studies have explored the impact of target properties on coating performance. Li et al. [3] indicated that Ti–Al targets with fine grains could enhance the deposition rate and hardness of the coatings but had an adverse effect on the oxidation resistance and thermal stability of the coatings. Zhou et al. [4] pointed out that Ti–Al targets with an alloy phase structure exhibited disadvantages in terms of the deposition rate and coating surface roughness. Wu et al. [5] indicated that the Al_xCr_y composite phase had a negative effect on the bonding strength of the coating and the cutting performance of the coating tools. Additionally, the density of the target is crucial in determining its strength, electrical conductivity, and thermal conductivity [6]. High-density targets mean fewer internal defects, which helps improve film performance and extend the lifespan of the target. Huang et al. [7] pointed out that density was the primary factor affecting the mechanical properties of Mo-10Nb targets. While the importance of targets is widely recognized, a fundamental understanding of the sintering kinetics of Ti-67 at% Al targets under spark plasma sintering (SPS) conditions—particularly the underlying mechanisms governing densification and microstructural evolution—remains lacking. In-depth investigation of this process is crucial for optimizing processing parameters and ensuring quality control of the targets [8].

Traditional powder metallurgy techniques, like hot pressing (HP) and hot isostatic pressing (HIP), can achieve densification, but due to the long sintering time, they are prone to grain growth [9]. In recent years, spark plasma sintering (SPS) has been extensively applied to sintering various materials owing to its low sintering temperature, short holding time, and fast heating rate, which effectively restrict grain growth and produce materials characterized by high density and a finer microstructure [10,11,12]. Therefore, using SPS to sinter Ti-67 at% Al targets not only simplifies the sintering process but also suppresses grain growth, making it an ideal sintering method. Since the improvement of the creep model by Bernard-Granger [13], this method has been widely applied to analyze the densification of various ceramics and metals. Gendre et al. [14] studied the densification mechanism in SPS-sintered ZrC_0.94O_0.05 and pointed out that densification was mainly controlled by intergranular diffusion and dislocation climbing. Huang et al. [7] solidified Mo-10Nb targets with SPS and elucidated that particle rearrangement controlled densification under low stress and grain boundary sliding under high stress. Wang et al. [15] showed that the densification of FAST-sintered W-Si-C composites was influenced by a combination of particle rearrangement (n ≈ 1) and intergranular diffusion (n ≈ 2). Kang et al. [16] fabricated WC-CoCrFeNi/Co cemented carbides using spark plasma sintering (SPS) technology and systematically investigated the densification mechanism during sintering temperature elevation and its influence on mechanical properties on the basis of a creep deformation model. Therefore, it is feasible to use the creep model to analyze the densification mechanism of Ti-67 at% Al alloy targets during the SPS process.

In this study, Ti-67 at% Al targets were fabricated using the spark plasma sintering (SPS) technique within the temperature range of 370–530 °C and 30 MPa for 10 min. For the first time, a comprehensive application of the creep deformation model was employed to reveal the dominant densification mechanisms during the SPS process of Ti-67 at% Al targets. Simultaneously, the mechanical and electrical conductivity properties of the targets were characterized, and the influence of grain size and microstructural features on target performance was investigated.

2. Experimental Section

2.1. Materials and Processing

Commercial Ti and Al powders served as the raw materials to prepare the Ti-67 at% Al targets.

Table 1. Data of raw powders.

Powders	Suppliers	Average Particle Size (μm)	Purity (%)
Ti	Shanghai Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China)	59.71	99.5
Al	Hunan Jinhao Technology Co., Ltd. (Luxi, China)	37.68	99.85

provides the relevant properties, including the manufacturer, particle size, and purity. Figure 1a,b show the morphology, particle size distribution, and phase composition of the Al and Ti powders, respectively. The Al particles exhibit a spherical shape with an average size of 37.68 μm, while the Ti particles have an average diameter of 59.71 μm. Furthermore, no impurity peaks are observed in either the Ti or Al powders, indicating no significant impurities in the starting materials. High purity is crucial for ensuring the quality of the final Ti–Al target and its suitability for subsequent processing and applications.

Ti and Al powders were mixed at an atomic ratio of 33:67 in a planetary ball mill (Changsha Miqi Instrument Equipment Co., Ltd., Changsha, China). The corresponding ball milling parameters are provided in

Table 2. Ball milling conditions.

Jar composition	Polyethylene
Milling media	Zirconium oxide
Rotation speed	150 rpm
Ball-to-powder weight ratio	5:1
Milling time	5 h
Process control agent	Ethanol
Volume of the jar	105 mL
Balls number (by quantity ratio)	Large–medium–small = 1:1:2

, including the milling speed, ball-to-powder ratio, and milling time. Wet ball milling was used to reduce electrostatic attraction between the powder particles, preventing agglomeration and ensuring that the milled powder had good dispersion and uniformity. After ball milling, the powder was transferred to a vacuum oven and dried at 60 °C for 24 h. This process effectively removed residual solvents and moisture from the powder, thus ensuring its stability for subsequent processing. Figure 1c shows the relevant properties of the milled powder. The distribution of the milled powder was homogeneous, with an average particle size of 28.37 μm. The elemental analysis of the mixed powder conducted by X-ray fluorescence spectroscopy (XRF, ZSX Primus IV, Rigaku, Akishima, Japan) is shown in

Table 3. Elemental analysis of milled Ti-67 at% Al powders.

Ti-67 at% Al Powder		Measured Value	Unit
Composition	Al	66.85	at%
Impurity	Fe	8.258	ppm
	Si	4.104	ppm
	Cu	2.15	ppm
	Ga	0.03	ppm
	Zn	3.44	ppm
	Cl	0.007	ppm
	Ni	0.06	ppm
	Zr	1.75	ppm
	As	0.001	ppm
Gas element	O	920	ppm

, with a low content of impurity elements, meeting the requirements for subsequent sintering and synthesis.

The dried powder was loaded into a graphite mold with a 15 mm internal diameter, and a layer of graphite foil was inserted inside the mold to facilitate lubrication and sample removal after sintering. Before sintering, the powder was pre-compacted using a tablet press for 3 min. This pre-treatment process helped improve the initial density of the sample, providing a better foundation for the subsequent sintering process. The powder was then vacuum-sintered using SPS equipment. The sintering temperature was set between 370 °C and 530 °C, with an applied load of 30 MPa, and the sintering time was 10 min, ensuring that the material reached the desired sintering effect within the appropriate time. After sintering, the sample was gradually cooled to room temperature along with the furnace, resulting in the Ti-67 at% Al targets.

Densification curves can be used to analyze the densification of targets at various sintering temperatures. During the sintering process, the system automatically records the variation in temperature, pressure, and displacement with time. Therefore, the following formula can be used to determine the immediate relative density (D) [17] on the basis of the change in sample height

$$D=\left({\displaystyle \frac{L_f}{L_f+∆L_{max}-L}}\right)D_f$$

(1)

where L_max (mm) represents the punch’s greatest displacement, L_f (mm) represents the sample’s final height, D (%) represents the immediate relative density, L (mm) represents the punch’s instantaneous displacement, and D_f (%) represents the final relative density.

2.2. Sintering Model

The creep model proposed by Bernard-Granger and Guizard [13,14,18] can be used to elucidate the densification mechanisms involved in SPS and HP processes [19,20,21]. It has been extensively applied to various ceramic materials [18,22] and metals [23,24]. The kinetic equation is expressed as follows

$${\displaystyle \frac{1}{D}}{\displaystyle \frac{dD}{dt}}={\displaystyle \frac{A\varphi {\mu }_{eff}b}{kT}}{\left({\displaystyle \frac{b}{G}}\right)}^P{\left({\displaystyle \frac{{\sigma }_{eff}}{{\mu }_{eff}}}\right)}^n$$

(2)

where A is a fixed parameter, k (10⁻²³ J/K) is the Boltzmann constant, t (s) refers to the time, G (μm) is the grain size, $\mathsf{\varphi }$ (m²/s) denotes the diffusion coefficient, T (K) is the absolute temperature, b is the Burgers vector, σ_eff (MPa) represents the instantaneous effective stress, and μ_eff (MPa) represents the instantaneous shear modulus. The grain size exponent and effective stress exponent of the sintered sample are denoted by p and n, respectively.

According to Ashby [25] and Lam et al. [26], the equations for σ_eff and μ_eff are as follows

$${\sigma }_{eff}={\displaystyle \frac{1-D_0}{D^2\left(D-D_0\right)}}{\sigma }_{mac}$$

(3)

$${\mu }_{eff}={\displaystyle \frac{E_{th}}{2\left(1+{\nu }_{eff}\right)}}{\displaystyle \frac{D-D_0}{1-D_0}}$$

(4)

where σ_mac is the pressure applied to the powder during sintering (30 MPa), D₀ denotes the relative density of the green material (about 50%), and Eth and ν_eff are the Young’s modulus and effective Poisson’s ratio of the theoretical dense material.

Here, $\mathsf{\varphi }$ is temperature-dependent and follows the Arrhenius relation [6]:

$$\mathsf{\varphi }={\varphi }_0\mathrm{e}\mathrm{x}\mathrm{p}(-{\displaystyle \frac{Q_d}{RT}})$$

(5)

Considering Equations (3)–(5), Equation (2) can be changed to Equation (6)

$${\displaystyle \frac{1}{{\mu }_{eff}}}{\displaystyle \frac{1}{D}}{\displaystyle \frac{dD}{dt}}=K{\displaystyle \frac{e^{-\frac{Q_d}{RT}}}{T}}{\left({\displaystyle \frac{b}{G}}\right)}^P{\left({\displaystyle \frac{{\sigma }_{eff}}{{\mu }_{eff}}}\right)}^n$$

(6)

where Q_d (kJ/mol) is the apparent activation energy controlling the densification mechanism, R represents the gas constant (8.314 J/(mol·K)), K denotes a constant, and ${\varphi }_0$ represents the frequency factor.

2.3. Characterization

According to Archimedes’ principle, the polished samples were immersed in deionized water for 12 h, and each sample was tested five times. Characterization of the phase composition of powders and sintered samples using X-ray diffraction (XRD, Smart Lab, Rigaku, Japan). The measurement angles (2θ) were from 30° to 70°, with a scan step size of 0.01°. The phase identification was carried out using Crystallographica Search-Match software (Version 2.1.1.0, Oxford Cryosystems). A scanning electron microscope (JSM-IT500, JSM-7100F, JEOL Ltd., Tokyo, Japan) was used to observe the microstructure of the raw powder and the samples and to analyze the changes during the sintering process. Additionally, to obtain the grain size distribution of the target, electron backscatter diffraction (EBSD, Oxford Instruments, Abingdon, UK) technology was employed. Grain size was analyzed using Channel 5 software (Version 15.5.1 build-15018445), considering at least 300 grains to ensure the concentration and representativeness of the data. The microstructure and interfaces of the samples were investigated using transmission electron microscopy (JEM-F200, Japan). A Vickers hardness tester (430SVD, Wolpert, USA) was used to measure the target’s hardness under a 0.1 kgf load for 15 s. The electrical conductivity was measured using a four-point probe system (RTS-9, Guangzhou Four-Probe Technology Co., Ltd., Guangzhou, China). All figures were plotted using OriginPro (Version 2025b; OriginLab Corporation, Northampton, MA, USA) under an academic site license provided by our institution.

3. Results and Discussion

3.1. Microstructural Evolution of Ti-67 at% Al Targets

Figure 2 demonstrates the XRD results of the Ti-67 at% Al target at different sintering temperatures. Only diffraction peaks related to Ti and Al appeared in the samples sintered within the 370 °C to 530 °C range. Notably, diffraction peaks of Ti_xAl_y (TiAl₃/Ti₂Al₅) were observed when the sintering temperature reached 550 °C. This indicates that the sample underwent an alloying reaction at high temperature, leading to the formation of intermetallic compounds due to the mutual diffusion of Ti and Al. Intermetallic compounds are usually brittle, and susceptible to cracking and fracture under impact or external forces, which reduces the machinability and mechanical strength of the target. Therefore, 530 °C is identified as the optimal sintering temperature for the preparation of Ti-67 at% Al targets, as it ensures the material’s integrity while avoiding the negative effects of alloying.

Figure 3 illustrates the microstructural evolution of the target sintered at temperatures ranging from 370 to 530 °C. As the sintering temperature increases, subtle structural changes occur in the material. At 370 °C, significant pores are observed between powder particles, indicating insufficient interparticle contact and low densification. When the temperature rises above 470 °C, the porosity decreases markedly, and grayish precipitated regions become visible at the Ti/Al phase interfaces. Although new phases emerge, the overall microstructure of the target remains predominantly composed of Ti and Al phases.

According to the diffusion kinetics of the Ti–Al system, the lower melting point and higher diffusion coefficient of Al lead to its preferential diffusion into Ti particles during sintering, forming a solid solution of Al in Ti (α-Ti). Simultaneously, interfacial reactions occur at the Ti/Al boundaries, resulting in the formation of intermetallic compounds. TiAl₃ typically nucleates and grows readily at these interfaces, eventually enveloping the Ti particles [27]. In contrast, the Ti₂Al₅ phase is a metastable phase in the Ti–Al alloy system, which likely coexists with stable phases such as TiAl₃ (containing ~67–75 at% Al) or is in the process of transforming into them [28]. According to the XRD and EDS results, it is inferred that the gray precipitated phases consist of TiAl₃ and Ti₂Al₅.

To illustrate the relationship between sintering temperature and the average grain size of the sample, Figure 4 shows the EBSD pattern of the target and its associated grain size distribution. It can be seen from the figure that the target consists of equiaxed grains, indicating a uniform and isotropic microstructure. The average grain sizes of the samples sintered at 400 °C and 470 °C are 9.58 ± 0.34 μm and 10.03 ± 0.31 μm, respectively. This indicates that the grains slightly grow with increasing sintering temperature but remain relatively constant overall, which may be attributed to the enhanced atomic diffusion at higher temperatures.

To further investigate the microstructure of the target, TEM characterization was performed on the samples sintered at 500 °C. Figure 5a shows the distribution of Ti and Al phases and their interface in high-resolution (HRTEM) images. It can be observed that there are no microcracks, impurities, or pores at the interface, indicating good interfacial bonding. In the IFFT image in Figure 5b, a small number of dislocations are observed on the (111) crystal plane of Al, with a measured interplanar spacing of 0.230 nm. Figure 5c shows the selected area electron diffraction (SAED) pattern of the Al grains, which reveals a polycrystalline structure for Al, with diffraction rings corresponding to the (111), (200), (220), and (311) crystal planes of face-centered cubic (bcc) Al. Figure 5d-e present the IFFT image and the selected area’s electron diffraction pattern of Ti, with a measured interplanar spacing of 0.221 nm, which closely matches the (101) crystal spacing of hexagonal close-packed (hcp) Ti.

3.2. Comprehensive Characterization of Ti-67 at% Al Targets

To comprehensively evaluate the overall performance of the synthesized alloys, we systematically characterized their relative density, grain size, mechanical properties, and electrical properties. All key performance data are summarized in

Table 4. Comprehensive characterization of Ti-67 at% Al targets.

Temperature (°C)	Relative Density (%)	Grain Size (μm)	Hardness (HV)	Electrical Conductivity (S/cm)
370	86.14 ± 0.43	9.34 ± 0.93	61.56 ± 11.69	0.23 ± 0.02
400	87.32 ± 0.44	9.58 ± 0.34	69.3 ± 20.69	0.40 ± 0.03
450	91.78 ± 0.46	9.86 ± 0.78	74.87 ± 20.33	0.59 ± 0.04
470	92.8 ± 0.46	10.03 ± 0.37	80.37 ± 22.01	0.70 ± 0.04
500	95.72 ± 0.42	10.44 ± 0.56	97.2 ± 11.86	0.76 ± 0.03
530	98.25 ± 0.43	10.86 ± 1.08	129.66 ± 7.50	0.86 ± 0.04

.

Table 4 shows the variation curves of the relative density and grain size of the target with temperature. The target’s relative density increases from 86.14% to 98.25%, while the grain size increases from 9.34 μm to 10.86 μm as the sintering temperature rises. Although the target’s relative density gradually increased, the grain size remains largely unchanged. This is primarily attributed to the rapid heating rate and short dwell time during the SPS process. Due to the high heating rate, the material reaches the desired sintering temperature quickly, which promotes bonding and densification between the particles. However, the short residence time at high temperature does not allow sufficient time for grain growth, resulting in a minor increase in grain size. These results support the conclusion that the target’s densification process is dominant at sintering temperatures of 370–530 °C.

During the cathodic arc deposition process, the target is subjected to the impact of energetic particles, which can cause surface damage or sputtering. Therefore, the target needs to have good mechanical properties to withstand repeated arc actions without cracking. Table 4 shows the relationship between the Vickers hardness and porosity of the target and the sintering temperature. Porosity decreases and Vickers hardness increases gradually as the sintering temperature increases. At 530 °C, the hardness reaches its maximum value of 129.66 HV. Higher hardness helps prevent cracking during use, thus extending the target’s lifespan. Porosity and grain size typically influence the hardness of the sample, and can be expressed by the following equations [29]

$$H_V=H_{0}exp\left(-b\rho \right)$$

(7)

$$H_V=H_0+k_H{d}^{-1/2}$$

(8)

where H₀, b, and k_H are constants; d is the grain diameter; and ρ is the porosity. Since the grain size remains constant during the sintering process, it is believed that the hardness of the target is mainly influenced by the porosity as the temperature increases.

The electrical conductivity of the target is crucial for the stability of the deposition process and the final quality of the film, so all samples were tested for electrical conductivity. Table 4 presents the conductivity curves of the target at various sintering temperatures. As depicted in the figure, the conductivity of the target increases gradually with sintering temperature, which indicates that the sintering temperature significantly affects the electrical conductivity of the target. Specifically at 530 °C, the conductivity reaches a maximum value of 0.86 S/cm. This can be attributed to the improvement in the microstructure of the target in the process of sintering [30]. With increasing sintering temperature, the target forms a denser microstructure, which helps reduce pores and defects, thus enhancing the conductivity. The denser structure lowers the resistance to current flow, effectively improving the conductivity of the target.

3.3. Densification Mechanism of Ti-67 at% Al Targets

Figure 6 shows the variation curves of temperature, density, and displacement rate with time for the sample at 530 °C. It can be observed that the sintering process of the Ti-67 at% Al target prepared by SPS is divided into three stages. In Figure 6a, the first stage of densification occurs below 350 °C. Within this temperature range, bonding begins to form between the particles, establishing initial contact points. However, the shape of the particles remains essentially unchanged, and the density shows very little variation. The main function of this stage is to promote the initial bonding between particles, but due to the relatively low temperature, the plastic deformation of the particles is not significant, resulting in a slow densification rate. The second stage of densification occurs between 350 °C and 530 °C, where the sample is rapidly densified, a large number of pores are removed, and the displacement rate reaches a maximum. This process largely determines the properties of the final sample, especially in terms of porosity and the overall structural stability of the material. In the third stage, since most of the pores have been eliminated, densification mainly relies on minor adjustments between the particles and further surface bonding. The densification process in this stage is relatively slow, and the displacement rate of the punch hardly changes anymore, indicating that the shape of the sample is stabilizing and the sintering process is gradually entering its final phase. Overall, the sintering process of the Ti-67 at% Al target exhibits different physical phenomena at various temperature intervals. Particularly in the second stage, the rapid densification of the material is essential in controlling the final density and pore structure.

Figure 7 presents the densification curves for samples sintered at various temperatures. As shown in the figure, the initial relative densities of the samples sintered at 370, 400, 450, 470, 500, and 530 °C are 78.94%, 79.62%, 81.59%, 83.94%, 86.71%, and 87.56%, respectively. These data indicate that as the sintering temperature increases, the relative density of the samples gradually rises, reflecting enhanced bonding and densification effects between the material particles at higher temperatures. At the same time, the change in relative density with holding time follows a similar trend: it increases rapidly in the initial stage and then stabilizes. This behavior is consistent with previous studies [19,20,31].

In order to reduce the error, the Young’s modulus and Poisson’s ratio of the target at different densities were measured by the ultrasonic method. We used Equation (9) [15] to express the relationship between Young’s modulus (E) and the shear modulus (μ). The Young’s modulus (E) and Poisson’s ratio (ν) of the material can be calculated by using the ultrasonic speed of sound equation (Equations (10) and (11)) [32]. The specific equations are given below

$$\mu ={\displaystyle \frac{E}{2\left(1+\nu \right)}}$$

(9)

$$\mathrm{E}=\mathsf{\rho }{V}_T^2{\displaystyle \frac{3V_L^2-4V_T^2}{V_L^2-V_T^2}}$$

(10)

$$\mathsf{\nu }={\displaystyle \frac{V_L^2-2V_T^2}{2\left(V_L^2-V_T^2\right)}}$$

(11)

where ρ denotes the relative density, and V_T and V_L denote the transverse and longitudinal wave velocities of the target, respectively, which are used to calculate the shear modulus of the material. The shear modulus is associated with sample’s relative density. The linear fitting curve for the variation of shear modulus with relative density for Ti-67 at% Al targets is shown in Figure 8.

To further investigate the densification mechanism of the Ti-67 at% Al target, it is necessary to determine the values of p, n, and Q_d. In Table 4, the variation in the grain size of the sample is minimal. Therefore, p can be considered to be a constant. Assuming that Q_d is also a constant, Equation (6) can be rewritten as [6,13,15]

$${\displaystyle \frac{1}{{\mu }_{eff}}}{\displaystyle \frac{1}{D}}{\displaystyle \frac{dD}{dt}}=K_0{\displaystyle \frac{e^{-\frac{Q_d}{RT}}}{T}}{\left({\displaystyle \frac{{\sigma }_{eff}}{{\mu }_{eff}}}\right)}^n$$

(12)

To obtain n, rewrite Equation (12) as Equation (13): [6,7,15]

$$\ln \left({\displaystyle \frac{1}{{\mu }_{eff}}}{\displaystyle \frac{1}{D}}{\displaystyle \frac{dD}{dt}}\right)=n·\ln \left({\displaystyle \frac{{\sigma }_{eff}}{{\mu }_{eff}}}\right)+K_1$$

(13)

where K₀ and K₁ are constants, and n is the slope of the curve plotted via Equation (13).

The effective stress index n at various temperatures is depicted in Figure 9. The curves are divided into two sections: the upper part represents the early stage of the holding stage, and the lower part represents the late stage of the holding stage. Two distinct effective stress indices are observed at each temperature, reflecting various stress levels that act on the powder. In the early stage of holding at 370, 400, 450, 470, 500, and 530 °C, the n values are 2.1, 1.4, 1.5, 2.0, 1.9, and 2.1 (low effective stress indices), while in the late stage of holding, the n values are 2.7, 3.2, 3.3, 3.5, 2.9, and 3.5 (high effective stress indices), respectively. Different effective stress exponents typically represent different densification mechanisms [8,9]. Specifically, when n < 1, particle rearrangement is the main mechanism for sintering; when 1 < n < 2, intergranular diffusion is considered to be the key factor of densification; and when n ≥ 3, the creep behavior of the material is likely to be controlled by dislocation climbing. Thus, in the early stage (n ≈ 2), the primary densification mechanism is intergranular diffusion, while in the late stage (n ≈ 3), the densification mechanism is dominated by creep controlled by dislocation climbing.

Weertman proposed a steady-state creep model based on the dislocation climbing mechanism, with the corresponding creep rate equation given by [33,34,35,36]

$${\displaystyle \frac{1}{{\mu }_{eff}}}{\displaystyle \frac{1}{D}}{\displaystyle \frac{dD}{dt}}=K_3{\displaystyle \frac{e^{-\frac{Q_d}{RT}}}{T}}{\left({\displaystyle \frac{{\sigma }_{eff}}{{\mu }_{eff}}}\right)}^{4.5}$$

(14)

$${\displaystyle \frac{1}{{\mu }_{eff}}}{\displaystyle \frac{1}{D}}{\displaystyle \frac{dD}{dt}}=K_4{\displaystyle \frac{e^{-\frac{Q_d}{RT}}}{T}}{\left({\displaystyle \frac{{\sigma }_{eff}}{{\mu }_{eff}}}\right)}^3$$

(15)

where K₃ and K₄ represent two separate values of constants. In this study, n = 2.7, 3.2, 3.3, 3.5, and 2.9, which are near to 3, indicating that the densification mechanism during the later stages of sintering at 370–530 °C is dislocation climbing, and the dislocation density is related to the effective stress [37].

Figure 10 illustrates the connection between the densification rate and the relative density of Ti-67 at% Al targets at different temperatures. For n = 2 and n = 3, the values of ${\displaystyle \frac{1}{D}} {\displaystyle \frac{dD}{dt}}$ were chosen as 0.0025 s⁻¹ and 0.0010 s⁻¹, respectively, indicating a significant difference in densification rates at different effective stress exponents. When n = 2, the corresponding relative densities at temperatures of 370, 400, 450, 470, 500, and 530 °C are 80.49%, 81.9%, 84.98%, 86.76%, 88.59%, and 90.49%, respectively. For n = 3, the relative densities are 81.77%, 82.76%, 86.84%, 88.68%, 89.97%, and 92.92%, respectively. These data were used to calculate the σ_eff and μ_eff values.

When n is constant, Equation (6) can be rewritten as [6,7,15]

$$\ln \left({\displaystyle \frac{T}{{\mu }_{eff}}}{\left({\displaystyle \frac{{\mu }_{eff}}{{\sigma }_{eff}}}\right)}^n{\displaystyle \frac{1}{D}}{\displaystyle \frac{dD}{dt}}\right)=-{\displaystyle \frac{Q_d}{RT}}+K_2$$

(16)

Figure 11 depicts the relationship between $\ln \left[{\displaystyle \frac{\mathrm{T}}{{\mu }_{eff}}}{\left({\displaystyle \frac{{\mu }_{eff}}{{\sigma }_{eff}}}\right)}^{\mathrm{n}}{\displaystyle \frac{1}{\mathrm{D}}}{\displaystyle \frac{\mathrm{d}\mathrm{D}}{\mathrm{d}\mathrm{t}}}\right]$ and ${\displaystyle \frac{1}{T}}$ when n = 2 and n = 3. As can be seen from the figure, the apparent activation energies Q_d are 97.29 and 158.737 kJ/mol for n = 2 and n = 3, respectively. These calculations can provide an in-depth analysis of the combined effects of temperature, holding time, and other factors on the densification of the target during SPS, which can provide data support for the subsequent optimization of the sintering process and can help to improve the stability of the performance of the target and its service life.

3.4. Performance Comparison and Discussion

Table 5. Comparison of alloy properties with the literature for Ti–Al alloys.

Alloy (at%)	Powder	Method	T/°C	Relative Density/%	Microstructure	Hardness/HV	Grain Size/μm
Ti-67Al	Ti, Al	SPS	530	98.25	Ti, Al	129.66	10.86 ± 1.08
Ti-48Al [28]	Ti, Al	SPS	600	99.5	Ti, γ, α2	<100	/
Ti-48Al [38]	pre-alloyed Ti-48Al	SPS	1200	98	Γ, α2	<100	/
Ti-50Al [39]	Ti, Al	HIP	1250	99.72	γ	339.7	78.7
Commercial Ti-67Al	/	HIP	/	99.35	Ti, Al	135.4	12.68 ± 2.38

systematically compares the performance differences between the Ti-67Al target fabricated in this study (SPS, 530 °C) and previously reported Ti–Al materials as well as commercial targets. The experimental results demonstrate that a high relative density of 98.25% was achieved at a significantly lower sintering temperature, fully highlighting the low-temperature rapid densification advantage of SPS technology. More importantly, the low-temperature sintering process effectively suppressed interdiffusion and alloying reactions between the elements, thereby preserving the Ti/Al dual-phase microstructure. This characteristic sharply contrasts with the fully reacted γ-TiAl or α₂-Ti₃Al phases commonly observed in high-temperature sintered or hot isostatically pressed (HIP) samples [38,39]. Studies indicate that this dual-phase structure may be more conducive to enhancing arc stability and significantly reducing droplet sputtering during arc ion plating [40].

Although the hardness (129.66 HV) of our material is slightly lower than that of some fully alloyed and coarse-grained counterparts (e.g., HIP Ti-50Al with 339.7 HV), it is very close to the value of the commercial target (135.4 HV). This indicates that our SPS-treated material offers a favorable combination of density and mechanical properties, matching the performance of commercially available targets. Furthermore, the relatively low processing temperature effectively inhibited grain growth, resulting in a uniform fine-grained structure (10.86 ± 1.08 μm) that outperforms the commercial target (12.68 ± 2.38 μm) in terms of microstructural refinement. The refined and homogeneous microstructure is expected to enhance the erosion uniformity during sputtering and improve the quality of deposited films.

Our work indicates that spark plasma sintering (SPS) is an efficient and advanced powder metallurgy technology for preparing high-performance, fine-grained Ti–Al alloys and is particularly suitable for application scenarios with high requirements for microstructural control and production efficiency. Although it has limitations in the preparation of large-sized components, its advantages are very prominent in the field of small and medium-sized components with high strength and high conductivity.

4. Conclusions

Ti-67 at% Al targets were prepared by SPS at temperatures between 370 °C and 530 °C. The densification mechanism, mechanical properties, and electrical conductivity of Ti-67 at% Al targets were investigated in this work. Based on the results, the following conclusions can be made.

• A high relative density of 98.25% was obtained at 530 °C. Densification dominated in the sintering range of 370–530 °C, while grain size remained relatively constant.
• A creep model was applied to analyze the densification mechanism in detail. Intergranular diffusion dominated during the early stage of holding from 370 to 530 °C, while dislocation climbing dominated in the later stage of holding at 370 to 530 °C. Furthermore, the activation energies calculated at n = 2 and n = 3 were 97.29 and 158.737 kJ/mol, respectively.
• As the temperature increased, the Vickers hardness of the target rose from 61.56 to 129.66 HV, while its electrical conductivity also increased from 0.23 to 0.86 S/cm. This improvement was primarily due to the increase in the target’s relative density.

Therefore, selecting an appropriate sintering temperature not only optimizes the microstructure of the target but also enhances its mechanical and electrical properties, thereby ensuring the stability and uniformity of the coating deposition. These conclusions provide a deeper insight into densification mechanism of SPS and provide practical references for further improving the sintering process and optimizing the target’s properties.