2017 Volume 58 Issue 8 Pages 1190-1196
The aim of this work is to find the optimal sintering parameters of Cr-31.2 mass% Ti alloys and simultaneously investigate the effects of the TiCr2 Laves phases. In this study, Cr-31.2 mass% Ti alloys are fabricated by vacuum hot-press sintering. Different compositions of submicron titanium (760 nm) powders are added to micron chromium (4.66 μm) powders. The experiments utilize various hot-press sintering temperatures (1250, 1300, 1350 and 1400℃) with the pressure maintained at 20 MPa for 1 h, respectively. The experimental results show the optimal parameters of the hot-press sintered Cr-31.2 mass% Ti alloys to be 1400℃ at 20 MPa for 1 h. The relative density reaches 99.96% and the hardness and TRS (transverse rupture strength) reach 81.93 HRA and 475.73 MPa, respectively. In addition, the increase in temperature of the hot-press sintering is effectively decreased the Cr3Ti3O and slightly enhanced TiCr2 phases, which will lead to the better mechanical properties. Significantly, the optimal hot-press sintered Cr-31.2 mass% Ti alloys possess a dense microstructure, excellent mechanical property and good electrical conductivity value (1.51 × 104 S·cm−1).
Cr-Ti alloys have been the object of great research interest due to their promising high temperature strength, excellent wear resistance, good oxidation resistance, as well as electrical properties and hydrogen solubility.1) Recently, Cr-based Laves phases have also been investigated for high temperature structural applications due to their high melting point, high strength and reasonably good oxidation resistance at elevated temperatures. The relative investigation of alloy design, defect structure and deformation mechanism was carried out for Laves phases to improve the poor ductility at low temperature, which is a common disadvantage of Laves phases.2) On the other hand, Cr-Ti alloys with body-centered-cubic (BCC) structures have been widely studied due to their excellent hydrogen storage capacity. TiCr2, as an intermetallic compound, can absorb hydrogen under high pressure at room temperature.3–5) Moreover, it also has been reported that the Cr-Ti under layer is used to enhance the texturing and maintaining higher coercivity of magnetic thin films.6,7) Therefore, Cr-Ti based alloys have been rapidly developed and applied.
In addition, titanium alloys are important engineering materials that are well known for their excellent specific strength and corrosion resistance.8) Titanium and its alloys can be produced by conventional processes, like casting, primary working and machining, and by powder metallurgy (P/M) where powders are consolidated and sintered to produce near-net-shape products. The conventional pressing and sintering method allows to produce P/M products with properties similar to wrought materials. But they typically remain the residual porosity. In order to overcome or, substantially reduce this disadvantage, alternative processes, where the sintering step is carried out simultaneously with the application of high temperature and pressure, were developed. Within these methods, known as hot consolidation processes, four of them are normally employed in the industry: hot-pressing, hot isostatic pressing (HIP), hot powder extrusion and hot powder forging.9) Hot-pressing, the simplest of the hot consolidation techniques, consists of loading a loose powder into a graphite mold that is then placed between two punches and heated in an enclosed furnace. Moreover, the compaction and sintering take place simultaneously. Through the years, hot-press sintering technique is the most commonly used method for preparing ceramics and alloys.10,11)
Cr-Ti alloys are generally made by means of melting or P/M methods. However, ingredient segregation, heterogeneity and coarse grains defects in Cr-Ti alloys are frequently caused by the melting processes. P/M is a good method for fabrication of high melting material (ex Cr and Ti) with better mechanical properties. It offers two different types of materials with the objective of achieving higher strength, hardness and wear resistance etc.12,13) As mentioned previously, P/M technology is the conventional process for the production of Cr-Ti alloys. The hot-press sintering is another special P/M technology, which directly pressed and sintering the material through a graphite mold to transmit the pressure to the powders. Hot-press sintering can obtain the dense material at relatively lower sintering temperature.14–16) Consequently, it is a novel technique to produce the Cr-31.2 mass% Ti alloys. In this work, Cr-31.2 mass% Ti alloys were fabricated by means of the vacuum hot-press sintering process of P/M technology. The intermetallic compound of the TiCr2 Laves phase was also studied.
In this research, Cr-31.2 mass% Ti alloys were fabricated via the vacuum hot-press sintering process of P/M technology; 99.95% reduced micron-sized chromium and submicron-sized titanium powders were mixed and pressed to produce the Cr-31.2 mass% Ti alloys. A Microtrac X 100 laser was used to analyze the particle size of the micron and submicron-size powders. The morphology of the chromium and titanium powders is shown in Fig. 1. The shape of the reduced chromium powders was irregular; there was no smooth undulating surface, as shown in Fig. 1(a). The mean particle size was about 4.66 ± 0.32 μm. In addition, the morphology of the reduced submicron-size titanium powders showed more irregular shapes, as shown in Fig. 1(b). The mean particle size was 760 ± 41 nm. Owing to the relative refinement and ductility of titanium powders, the fine particles of the titanium powders cohered to the hard particles of the chromium powders, as shown in Fig. 1(c). No obvious cold welding morphology of the Cr-31.2 mass% Ti powders was produced by 1 h of ball milling.
SEM photographs of the surface morphology: (a) chromium, (b) titanium powders, and (c) Cr-31.2 mass% Ti alloy powders after 1 h ball mixing, respectively.
In order to investigate the effects of sintering temperatures and TiCr2 Laves phases on the mechanical properties, microstructures and electrical behaviors of the Cr-31.2 mass% Ti alloys. A series of vacuum hot-press sintering (1250, 1300, 1350 and 1400℃ maintained at 20 MPa for 1 h, respectively) treatments was performed. Meanwhile, the degree of vacuum was maintained at 1.33 × 10−1 Pa. And the specimen size of vacuum hot-press sintering compaction was 40 × 40 × 5 mm3. Generally speaking, the vacuum hot-press sintering process directly pressed and sintered the material through graphite mold to transmit the pressure onto the powders. As a result, it can obtain the more dense material at relatively lower sintering temperature. To evaluate the sintered characterization of the Cr-31.2 mass% Ti alloys via vacuum hot-pressed sintering (Yu Tai Vacuum Co., Ltd. HPS-1053) processes, the porosity, hardness, transverse rupture strength (TRS) tests, electrical tests and microstructure inspections were performed. Microstructural features of the specimens were examined by X-ray diffraction (XRD, Rigaku D/Max-2200) and scanning electron microscopy (SEM, Hitachi-S4700). Porosity tests followed the ASTM B311-08 and C830 standards, and the closed porosity measurement was carried out through the analysis software of Image Pro image.
The hardness of the specimens was measured by Rockwell indenter (HRA, Indentec 8150LK) with loading of 588.4 N, which complied with the CNS 2114 Z8003 standard methods. Meanwhile, Rbm was the transverse rupture strength (TRS), which determined as the fracture stress in the surface zone. F was maximum fracture load, L was 30 mm, k was chamfer correction factor (normally 1.00–1.02), b and h were 5 mm in the equation Rbm = 3FLk/2bh2, respectively. The specimen dimensions of the TRS test were 5 × 5 × 40 mm3. Moreover, it needs to slightly grind the surface of the specimen and tests at least three pieces. A four-point probe (LRS4-TG2) was used to measure sheet resistance. In addition, electrical conductivity (σ) was calculated according to the following formula:12)
| \[ \rho = {\rm R}/t = 1/\sigma \] |
Where the ρ is electrical resistivity, R is the resistance, t is the thickness of the test sheet, and σ is the electrical conductivity (Scm−1), respectively.
Figure 2 shows the XRD patterns of the Cr-31.2 mass% Ti alloys after the hot-press sintering process at various temperatures. Due to the crystal structure dependence on the XRD diffraction intensity, it was reasonable to surmise that all Cr-31.2 mass% Ti alloys possessed a good crystalline property after hot-press sintering under the different temperatures. The major diffractions appeared in the Cr (110), (200) and (211) planes, respectively. Increasing the temperature of the hot-press sintering slightly increased the intensity of the Cr diffractions (110). The TiCr2 and Cr3Ti3O peaks were also generated after the sintering process. It can be speculated that the degree of vacuum (1.33 × 10−1 Pa) was still not enough for the sintered titanium alloys. Particularly, as titanium possesses a high activity and is easily reactive with oxygen elements, once the titanium oxide was formed, it immediately reacted with the chromium, thereby producing the Cr3Ti3O phases. As a result, only a small amount of the titanium and chromium formed TiCr2 phases and the intensity of the TiCr2 intermetallic peaks was very weak. Previous literature has noted that the formation of TiCr2 intermetallic peaks is a diffusion process and needs more time at an elevated temperature.4) When the sintering temperature was increased to 1350 and 1400℃, the intensity of the TiCr2 phases increased slightly, as shown in Fig. 2. Besides, the previous study have indicated that the complex compound Cr3Ti3O phase possessed the like spinel structure, which showed the brittle properties. Moreover, these Cr3Ti3O phases contributed to reduction of wear of the Ti-Cr alloy materials.17) As can be seen in Fig. 2, increasing the temperature of the hot-press sintering slightly decreased the intensity of Cr3Ti3O phase. Conversely, the intensity of the TiCr2 enhanced slightly. It is reasonable to suggest that increase the temperature of the hot-press sintering is effectively decreased the Cr3Ti3O phase, which will lead to the better mechanical properties.
XRD patterns of Cr-31.2 mass% Ti alloys by hot-press sintering at different temperatures.
In this work, the high proportion of chromium (68.8 mass% Cr) easily resulted in the residual Cr phases after the sintering process. Significantly, the major diffraction of the XRD patterns was the Cr (110) plane. When the hot-press sintering temperature was increased, the chromium appeared to be toward the low-angle offset phenomenon. It was reasonable to surmise that the increased temperature effectively enhanced the solid-solution reaction between the chromium and titanium elements. Therefore, the solid-solution strengthening mechanism enhanced the mechanical and electrical properties of the Cr-31.2 mass% Ti alloys. This result will be confirmed in the following discussion.
A comparison was made of the sintered density, apparent porosity and closed porosity of the Cr-31.2 mass% Ti alloys after hot-press sintering at different temperatures, as shown in Fig. 3(a). Increasing the hot-press sintering temperature gradually enhanced the sintered density, while the apparent porosity and closed porosity of the Cr-31.2 mass% Ti alloys obviously decreased, as also shown in Fig. 3(a). The highest sintered density (6.21 g·cm−3) appeared after 1400℃ sintering for 1 h. The lowest apparent and closed porosities were 0.015% and 0.02%, respectively. The experimental results also indicated a slight increase in the relative density of the Cr-31.2 mass% Ti alloys, as shown in Fig. 3(b). Significantly, the Cr-31.2 mass% Ti alloys possessed the highest relative density (99.96%) and sintered characteristics after hot-press sintering at 1400℃ and 20 MPa for 1 h.
Comparison of the properties of Cr-31.2 mass% Ti alloys by hot-press sintering at different temperatures: (a) sintered density, apparent and closed porosities, and (b) relative density.
It is difficult to achieve a high density (> 95%) of the sintered alloys with solid-phase sintering (SPS) under an atmospheric pressure. The SPS is defined as follows: the shaped green body is heated to a temperature that is typically 0.5–0.9 of the melting point. No liquid is present and atomic diffusion in the solid state produces joining of the particles and reduction of the porosity.18) The characteristics of hot-press sintering are that the mixed powder is synchronously heated and pressed into the shape of a graphite mold. When hot-press sintering enters the sintering stage, the assistance in high compressive stress leads to effective atomic diffusion, plastic deformation and compaction results. In this experiment, increasing the hot-press sintering temperature (1250 → 1400℃) effectively provided the driving force of the sintering mechanism and helped the atomic diffusion due to the external pressure (20 MPa). In addition, the smaller-sized particles of the titanium powders (760 nm) effectively filled the pores of the larger-sized particles of chromium powder (4.66 μm) and increased the contact area between the particles during the high-temperature sintering process. The Cr-31.2 mass% Ti alloys can be sintered at a lower temperature (less than the eutectic temperature (1412℃)19), while still achieving good sintering results.
Figure 4 shows the SEM morphology observations of the Cr-31.2 mass% Ti alloys after the hot-press sintering at different temperatures. The existence of residual black pores (as the arrow indicates) was obvious after hot-press sintering at 1250℃ for 1 h, as shown in Fig. 4(a). However, the pores decreased as the hot-press sintering temperature increased (1300℃), as shown in Fig. 4(b). When the sintering temperature was raised to 1350 and 1400℃, due to the SPS effects, few internal pores of the Cr-31.2 mass% Ti alloy hot-press sintered specimens remained in the sample. Moreover, the refined particles of the chromium tended to coarsen slightly due to the solid diffusion of the high temperature (1400℃) and high pressure (20 MPa), as shown in Fig. 4(d). As regards the SEM observations, the results showed that the internal pores of the Cr-31.2 mass% Ti alloys slowly decreased as the sintering temperature increased under the same hot-press pressure (20 MPa). Figure 4(a) illustrates the many larger and dispersed pores in the specimen (closed porosity 0.61%). When the sintering temperature was increased, the larger pores apparently became smaller, as shown in Figs. 4(b), 4(c) and 4(d) (closed porosity of 0.10, 0.04 and 0.02%, respectively). Due to the compaction effects of the hot-press pressure, the original pores of the Cr-31.2 mass% Ti alloys gradually decreased and led to the lower amount of pores. The result, when further compared with Fig. 3, revealed that the relative density increased to 99.96% and the apparent porosity rapidly decreased to 0.015% after hot-press sintering at 1400℃ and 20 MPa for 1 h. According to the above discussion and results, the high density (> 95%) of the Cr-31.2 mass% Ti alloys for SPS was obtained.
SEM morphology observations of Cr-31.2 mass% Ti specimens by hot-press sintering at different temperatures: (a) 1250℃, (b) 1300℃, (c) 1350℃, and (d) 1400℃.
In the present research, the mean grain size of the Cr-31.2 mass% Ti alloy hot-press sintered specimens was measured using the linear intercept method.12) With increases to the hot-press sintering temperature (1250 → 1300 → 1350 → 1400℃), the mean grain size gradually increased (4.29 → 4.85 → 5.04 → 5.53 μm), as shown in Fig. 5. The grain coarsening phenomenon appeared in the Cr-31.2 mass% Ti alloy hot-press sintered specimens, but the magnitude of the grain growth was not obvious, with an increase of about 1.24 μm (4.29 → 5.53 μm). On the other hand, our previous studies12,14) indicated that rapid grain growth during the early stage of sintering has been found in many nano and submicron material systems. Here, extremely rapid grain growth was readily generated during the high-temperature and high-pressure sintering. At the same time, the Cr-31.2 mass% Ti powders seemed to lose their submicron-scale characteristics after hot-press sintering. This finding was in agreement with our previous studies.14,20)
Comparison of the mean grain size of Cr-31.2 mass% Ti alloys by hot-press sintering at different temperatures.
Additionally, low- and high-magnification SEM observations of the Cr-31.2 mass% Ti alloys after 1400℃ sintering for 1 h are shown in Fig. 6. It was dramatic to observe that similar lamellar pearlite structures appeared in the microstructures (points 1 and 6). The lamellar pearlite structures were the solid-solution reaction of the chromium and titanium phases. The composition ratio of chromium and titanium was about 85:15, as shown in Table 1. In addition, the bulge-like granular structures were also mainly the solid-solution reaction of the chromium and titanium phases, as shown in Figs. 6(a) and 6(b). They contained lamellar of TiCr2 (points 1 and 6) and non-lamellar of Cr-rich structures (points 2 and 7). The lamellar structures possess a higher interface energy, which results in the dislocation slip being more difficult. Therefore, this kind of strengthening mechanism could affect and improve the mechanical properties of the Cr-31.2 mass% Ti alloys. Further discussion of this will be made in subsequent sections. It was also found that some of the smaller titanium particles were not fully reacted, as shown in points 4 and 9 (Ti-rich). Notably, titanium oxide (TiO2) particles existed in the structures, as shown in points 5 and 10. It is reasonable to suggest that the titanium oxide was generated first and then reacted with the chromium. As a result, Cr3Ti3O was formed (as shown in points 3 and 8). These findings and results confirmed our previous discussion (Fig. 2).
SEM surface morphology observations of 1400℃ hot-press sintered Cr-31.2 mass% Ti specimens: (a) low magnification, and (b) high magnification.
| Position | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
|---|---|---|---|---|---|---|---|---|---|---|
| Cr (mass%) | 84.4 | 82.5 | 44.2 | 3.3 | 3.3 | 86.2 | 82.9 | 45.4 | 9.5 | 3.6 |
| Ti (mass%) | 15.6 | 17.5 | 49.6 | 96.7 | 79.9 | 13.8 | 17.1 | 48.9 | 90.5 | 81.0 |
| O (mass%) | - | - | 6.2 | - | 16.8 | - | - | 5.7 | - | 15.4 |
Figure 7(a) shows the TRS and hardness tests of the Cr-31.2 mass% Ti alloys by hot-press sintering at different temperatures. The TRS values showed an increasing trend as the hot-press sintering temperature increased. The highest TRS value (475.73 MPa) appeared in the Cr-31.2 mass% Ti alloys after hot-press sintering at 1400℃ and 20 MPa for 1 h. Although the grain sizes showed a slight coarsening phenomenon in the 1400℃-sintered specimens, the small variation in grain size did not seem to affect the TRS value. In addition, as the relative density increased, the higher density provided stronger binding to prevent the generation of rupture. In the present research, increasing the temperature (1250 → 1400℃) of the hot-press sintering slightly improved the relative density (99.35 → 99.96%) of the Cr-31.2 mass% Ti alloys. Therefore, there was still some other factor affecting the TRS values. The result was further compared with the SEM observation, as shown in Fig. 4. The lamellar pearlite structures tended to increase as the sintering temperature increased. As discussed previously, the similar lamellar pearlite structures can prevent dislocation movement, which then affects the TRS tests. As to the effect of complex compound Cr3Ti3O phase, the SEM images shown Cr3Ti3O phase did not a significant variation as the hot-press sintering temperature increased. Though the brittle oxide was the main constituent of the Cr3Ti3O phase within the matrix of Cr-Ti alloy powders oxidized in the air,17) no evidence of it was found within the matrix generated a significant effect on the TRS. Thus, the Cr3Ti3O phase affects the evolution of TRS could be ignored.
Comparison of the mechanical and electrical tests of Cr-31.2 mass% Ti alloys by hot-press sintering at different temperatures: (a) TRS and hardness tests, and (b) conductivity test.
As seen in Fig. 7(a), the hardness values of the Cr-31.2 mass% Ti alloys were almost similar for the different sintering temperatures. Previous literature revealed a positive relationship between the hardness and the relative density.12) The literature also noted that decreasing the porosities of the sintered materials effectively enhanced the plastic deformation resistance and hardness. The relative density reached almost 99.9% and the apparent porosity decreased to 0.02% after hot-press sintering at 1300 and 1400℃ for 1 h. Although the grain size slightly increased as the temperature of the hot-press sintering increased, the increased value was limited (4.85 → 5.53 μm). The load increase was good for hardness, as shown by the slight improvement in the relative density, but it was also disadvantageous to the hardness, as shown by the slight coarsening phenomenon in the grain size. Both of them were present and canceled each other's effect, and the similar hardness values appeared at different temperatures. However, the hardness of the 1350℃-sintered specimens reached the highest value (82.27 HRA), with only a slight variation at 1400℃ sintering (81.93 HRA).
The main densification mechanism of the hot-pressed sintered Cr-31.2 mass% Ti alloys was the initial diffusion creep of high temperature and plastic deformation during the SPS step. Hot-press sintering offers several advantages over conventional methods, primarily the very low temperature required for densification.14,21,22) When the hot-pressed sintering entered the sintering stage (at sufficiently high temperature), the assistance in high compressive stress led to effective plastic deformation and compaction results, but the coarsening phenomenon easily resulted in the lower hardness of the 1400℃-sintered specimens. Thus, it is reasonable to speculate that the minor difference in the hardness resulted from the relative density, SPS and grain size effects.
Figure 8 shows the fractographic observations of the Cr-31.2 mass% Ti alloys by hot-press sintering at different temperatures. The fractographs showed that an obvious brittleness existed in the Cr-31.2 mass% Ti alloys after the TRS test. As see in Fig. 8(a), residual pores clearly existed in the fracture surface, as indicated by the arrows. These pores produced stress concentration points and resulted in the decline in fracture strength. As the sintering temperature was increased (1300 → 1350 → 1400℃), the residual pores dramatically decreased, as shown in Figs. 8(b)–8(d), which led to the increase in fracture strength. In addition, the fracture mechanism was evidenced by the brittle chromium generating the cleavage and transgranular fractures. When the deformation of a specimen exceeds its elastic limit, it cracks under the concentrated stress. Here, this caused local chromium particles near the Cr-TiCr2-Cr3Ti3O interfaces to spread and expand rapidly, which was detrimental to the fracture resistance of the interface.
Fractographs observation of Cr-31.2 mass% Ti specimens by hot-press sintering at different temperatures: (a) 1250℃, (b) 1300℃, (c) 1350℃, and (d) 1400℃.
Generally speaking, the TiCr2 phase with the Laves structure forms in binary Ti-Cr alloys when the Cr content is higher than 15%. The microstructure and properties of these alloys are very sensitive to their Cr content and preparation method.1) When the ratio of chromium was up to 68.8 mass%, the X-ray analysis revealed that major Cr and minor TiCr2 phases were produced after hot-press pressing at the different temperatures (as shown in Fig. 2). Owing to the binding strength of the intermetallic compound of TiCr2, the Laves phase was quite weak and the TiCr2 Laves phase easily peeled away from the rupture surface during the TRS test, resulting in some small pores, as shown in Fig. 8(d). In addition, only a small amount of the titanium and chromium formed TiCr2 phases, while the intensity of the TiCr2 intermetallic peaks was very weak. Thus, the TiCr2 Laves phase was not a main fracture mechanism affecting the TRS values. It is reasonable to speculate that the main fracture mechanism of the Cr-31.2 mass% Ti alloys was generated by the brittleness of the chromium, which showed obvious cleavage and transgranular fractures. Consequently, the stepped cleavage planes and relatively flat fracture planes were clearly evident, as indicated by the arrows in Fig. 8(d).
Figure 7(b) shows the electrical and the sintered density of the Cr-31.2 mass% Ti alloys after hot-press sintering at different temperatures. The electrical performance and the sintered density of the sintered Cr-31.2 mass% Ti alloys exhibited an opposite tendency. When the hot-press sintering temperature was increased, the sintered density rapidly increased, but the electrical conductivity declined slightly. The highest electrical conductivity (1.55 × 104 S·cm−1) appeared in the 1250℃-sintered specimen. However, the difference to the other sintering temperatures and compounds like spinel Cr3Ti3O were not significant. The average distance which an electron travels before a collision is called the mean free path. A decrease in the mean free path means ‘less moving’, and results in an increase in conductivity. Our previous study has proven the relationship of the relative density of Cr-based alloys to the electrical properties.12,14) Increasing the sintered density dramatically improved the electrical properties of the Cr-based alloys; nevertheless, the electrical conductivity of the sintered Cr-31.2 mass% Ti alloys declined slightly with increases in the sintering temperatures. It is reasonable to suggest that the grain coarsening and lamellar pearlite structures hindered the movement of free electrons; hence, the electrical conductivity of the 1400℃-sintered specimen had a slightly lower value (1.51 × 104 S·cm−1). Accordingly, the hot-press sintering parameters of 1400℃ at 20 MPa for 1 h for the Cr-31.2 mass% Ti alloys resulted in the optimal mechanical properties and suitable electrical conductivity.
The experimental results showed the optimal parameters of the hot-press sintered Cr-31.2 mass% Ti alloys to be 1400℃ sintering at 20 MPa for 1 h. When the relative density reached 99.96%, the apparent porosity was 0.015% and the hardness and TRS reached 81.93 HRA and 475.73 MPa, respectively. Moreover, the electrical conductivity was 1.51 × 104 S·cm−1. The 1400℃-sintered Cr-31.2 mass% Ti alloys also possessed the optimal SPS results (closed porosity of 0.02%). The main densification mechanism of the hot-pressed sintered Cr-31.2 mass% Ti alloys was the initial diffusion creep of high temperature and plastic deformation during the SPS step. The sintered Cr-31.2 mass% Ti alloys had better crystalline properties after the hot-press sintering at different temperatures. As the sintering temperature was increased, the intensity of the TiCr2 phases increased slightly. In this work, titanium oxide was generated first and then reacted with the chromium. As a result, Cr3Ti3O was formed, which resulted in the decrease in TiCr2 phases.
This research is supported by the ASSAB STEELS TAIWAN CO., LTD. The authors would like to express their appreciation for Dr. Harvard Chen, Michael Liao and Mr. Meng-Yu Liu.
