2017 Volume 58 Issue 8 Pages 1235-1237
A new process, based on an infiltration and reaction method, is proposed to fabricate metal matrix composites reinforced with Al3Ni, intermetallic compounds. This study investigates the optimum conditions for refining intermetallic compound dispersion inside the metal matrix at the different reaction times (60 s, 300 s and 600 s) at molten alloy temperature of 973 K and applied pressure of 0.1 MPa; porous nickel with a specific surface areas of > 5800 m2/m3 was used. The counts, area fraction, and aspect ratio of the intermetallic compounds inside matrix are investigated by different of reaction times. In additions, Vickers hardness, three-point bending strength and thermal conductivity of intermetallic compounds reinforced matrix composites are investigated.
In order to fabricate metal matrix composites (MMCs) with high performance properties by the infiltration casting technique, the squeeze casting process1) is generally used for the infiltration by high pressure because of the poor wettability between the molten metal and preform. When wettability is improved by control of reinforcement material and metal matrix reaction, it is possible to decrease the applied pressure. Therefore, the MMC can be fabricated by the gravity casting2) and low pressure infiltration casting3,4). These process make it possible to fabricate large scale, complex shaped composites and simple production process. The objective of this paper is to investigate the effects the reaction time on the intermetallic compounds shape and size inside the matrix. And mechanical property and thermal property of intermetallic-compound-reinforced composite.
A366 alloy in ASTM, was used as a matrix. And preform was used porous nickel (Toyama Sumitomo Electric Co., Ltd.) for application to infiltration process of molten metal. Porous nickels, which has over 5800 m2/m3 specific surface areas was used. Infiltration and reaction method was used to fabricate the composites. The temperature of molten Al alloy and applied pressure were 973 K and 0.1 MPa. This fabrication condition by different specific surface area of porous nickel has been shown by previous studies5) to be most suitable for generation of the intermetallic compound. Reaction time was controlled from 60 to 600 s. The size of composite is ϕ 30 mm × height 10 mm. The microstructure of the composites was observed using an optical Microscope (OM) and a scanning electron microscope (SEM). Elemental mapping analyses was performed using an electron probe micro analyzer (EPMA, JXA8900RL; Japan). The counts, area fraction and aspect ratio (ratio of length and width) inside matrix were measured from picture analysis using the image pro-plus software. The area analysis of each composite was conducted over an area 480 mm2. Hardness was evaluated by Vickers hardness was measured 10 times under load of 49 N respectively. Flexural strength measurements were carried out using three-point bending tests. The specimen configuration and the testing condition were as specified in ASTM C 1341. The shape of the specimen was length 26 mm × width 3 mm × thickness 1 mm. The support span was 16 mm. The cross-head speed was 0.05 mm/min. Thermal conductivities of intermetallic-compound-reinforced composites were evaluated using a laser flash method thermal constants measuring system (TC-700, ULVAC-RICO Inc., Japan) at the room temperature in air. The specimen size for thermal conductivity is ø 10 mm × thickness 1.0 mm, and the specimen were in the form of a thin circular disc.
Figure 1 shows the microstructure of the intermetallic-compound-reinforced composite by three difference reaction time (a; 60 s, b; 300 s, c; 600 s). The analysis of microstructure observed in the middle part of the sample cross-section. Observations of the microstructure for the composites fabricated with reaction time of 60 s show many of intermetallic compound particles was delamination from the porous nickel surface. Porous nickel is thought to undergo an expansion reaction with the molten Al. The stress induced by the expansion probably works together with the shear force to refine the Al3Ni. It was reported the previous work3). However, intermetallic compound particles remain as porous nickel body without delamination from the porous nickel surface, were also observed. Porous nickel was confirmed to change the Al3Ni, intermetallic compound by EMPA analysis. And unreacted nickel was observed in matrix too (Fig. 2). Figure 1 (b) shows that most of the fine Al3Ni intermetallic compounds were homogeneously dispersed inside matrix. When the reaction time was 300 s, more fine intermetallic compounds were distributed as compared with other materials in Fig. 1(a) and (c). When the reaction time was 600 s, almost all of the intermetallic compounds was distributed inside the matrix. However, many intermetallic compound, with a needle-like Al3Ni shape, were observed as compared to a reaction time of 300 s.
Microstructures of intermetallic-compound-reinforced composites; reaction time 60 s (a), 300 s (b) and 600 s (c).
BSE image of Al3Nip/AC8A alloy composites of reaction time, 60 s and elemental mapping analyses by EPMA.
Figure 3 shows the results of aspect ratio (ratio of length and width) of intermetallic compound inside the matrix for different reaction time of 300 s and 600 s. For a reaction time of 60 s, intermetallic compounds were not dispersed inside the matrix. Therefore, the aspect ratio could not be evaluated. When the reaction time 300 s, the intermetallic compound which has the aspect ratio of 1～2 is 95.7%. On the other hand, when the reaction time 600 s, the intermetallic compound which has the aspect ratio of 1～2 is 86.5%. When the reaction time is longer, needle-like intermetallic compounds with an aspect ratio of 3 or more mostly exist. The results of reaction time 600 s was reported the counts and average size of intermetallic compounds the previous study6). Counts and average area of the intermetallic compounds analysis of each composite were determined for an area of 480 mm2. Counts of the intermetallic compound inside the matrix under reaction time, 300 s and 600 s were 2181 count and 747 counts. The counts of intermetallic compounds shows an increasing trend with decreasing reaction time. Average area of the intermetallic compound inside the matrix under reaction time, 300 s and 600 s were 868.4 μm2 and 2385.4 μm2 count. The average area of Al3Ni shows a decreasing trend with decreasing reaction time. The results indicate that reaction time of 300 s is necessary for small sized intermetallic compounds.
Aspect ratio (ratio of length and width) of intermetallic compound inside the matrix for different reaction time of 300 s and 600 s.
Vickers hardness and Three-point bending test was carried out using a composite materials with a relative density of 98%. Table 1 shows the results of vickers hardness of each composites. Vickers hardness of composite with reaction time of 300 s is 198.8 Hv, higher than that observed for other composites with a reaction time of 60 s and 600 s. The flexural strength of composite with a reaction time of 300 s is 239.5 MPa, higher than that observed for reaction time of 60 s and 600 s; these are because of the higher counts and fine intermetallic compounds inside the matrix. Figure 4 shows the different fracture surface of the intermetallic compound inside the matrix. In the case with no dispersion of the intermetallic compounds (Fig. 4(a)), non-reaction porous nickel was observed. It was observed non reaction nickel porous. Crack were propagated along the frame of porous nickel and ductile fracture was observed inside the matrix. In contrast, for the intermetallic-compound-reinforced composite with mostly fine microstructure, Fig. 4 (b), fracture along the frame of porous nickel was not observed. Brittle fracture inside the matrix was observed by small sized intermetallic compounds. However, large sized brittle fracture inside the matrix was observed also. When the reaction time of 600 s, more large sized brittle fracture was observed inside the matrix as compared to a reaction time of 300 s. This is the cause for the decrease in strength.
| Fabrication conditions of composites | Vickers hardness [Hv] |
Three point bending strength σ[MPa] |
Thermal conductivity TC [W/mK] |
|---|---|---|---|
| Reaction time, t [s] | |||
| 60 | 126.4 | 203.8 | 82.5 |
| 300 | 198.8 | 239.5 | 112.5 |
| 600 | 159.1 | 198.6 | 110.1 |
Fracture surface of intermetallic-compound-reinforced composites; reaction time 60 s (a), 300 s (b) and 600 s (c).
One side of thin circular disc (ø 10 mm × thickness 1.0 mm) is irradiated by laser beam and the temperature is detected using thermocouple at the other side. The thermal conductivity is calculated as follows:
| \[\lambda = \alpha \cdot \rho \cdot {\rm Cp}\] | (1) |
| \[\alpha = 1.37 \cdot {\rm L}^2/(\pi^2 \cdot {\rm t}_{1/2})\] | (2) |
Fine Al3Ni intermetallic compounds were homogeneously dispersed inside the matrix for when the reaction time were 300 s, 600 s (applied pressure of 0.4 MPa, temperature of molten alloy, 973 K. The counts of the intermetallic compounds, Al3Ni, showed an increasing trend with decreasing reaction time (300 s) for the same area analysis. The average area of the Al3Ni intermetallic compound showed a decreasing trend with decreasing reaction time. The results indicate that a long reaction time is needed for large sized intermetallic compounds. The vickers hardness and flexural strength of the composite obtained at a reaction time of 300 s are 198.8 Hv and 239.5 MPa, higher than that obtained under the other fabrication conditions. The thermal conductivity of the intermetallic-compound-reinforced composite is 112.53 W/mK, which is higher than the thermal conductivity observed for the composites fabricated at a reaction time of 60 s and 600 s. However, the thermal conductivity of the composite was almost similar to that of the matrix, A366 alloy.
This work was supported by JSPS KAKENHI Grant Number15K05678.
