Z.Y. Ma

Microstructural and mechanical characteristics of in situ metal matrix composites

During the past decade, considerable research effort has been directed towards the development of in situ metal matrix composites (MMCs), in which the reinforcements are formed in situ by exothermal reactions between elements or between elements and compounds. Using this approach, MMCs with a wide range of matrix materials (including aluminum, titanium, copper, nickel and iron), and second-phase particles (including borides, carbides, nitrides, oxides and their mixtures) have been produced. Because of the formation of ultrafine and stable ceramic reinforcements, the in situ MMCs are found to exhibit excellent mechanical properties. In this review article, current development on the fabrication, microstructure and mechanical properties of the composites reinforced with in situ ceramic phases will be addressed. Particular attention is paid to the mechanisms responsible for the formation of in situ reinforcements, and for creep failure of the aluminum-based matrix composites.

In-situ Ti-TiB metal-matrix composite prepared by a reactive pressing process

During the past decade, considerable research efforts have been directed towards developing metal matrix composites (MMCs) by means of the reactive processing technique. In this study, four reactive systems, namely Ti-B, Ti-TiB{sub 2}, Ti-B{sub 4}C and Ti-BN, are selected to fabricate in-situ titanium matrix composites by means of the reactive pressing technique. The microstructure and compressive mechanical property of these composites are examined. The objective of this work is to assess which system can produce in-situ titanium matrix composites with a higher mechanical strength.

High-temperature creep behaviour of powder-metallurgy aluminium composites reinforced with SiC particles of various sizes

Abstract Tensile creep tests have been performed at 573–673 K on pure aluminium and aluminium-based composites reinforced with SiC particles of different sizes (3.5, 10 and 20 μm) fabricated by powder-metallurgy (PM) techniques. Both the pure aluminium and the composites exhibited high values of the apparent stress exponent (14.3–26.1) and creep activation energy (253–261 kJ/mol). The results showed that the creep rate of composites reinforced with small particles (3.5 μm) was two–three orders of magnitude lower than that of pure aluminium. However, the creep resistance of composites reinforced with large SiC particles (10 and 20 μm) was essentially identical to that of pure aluminium. Various creep models were attempted to rationalize the creep data of pure aluminium and particulate composites.

High temperature creep behavior of in-situ TiB2 particulate reinforced copper-based composite

Abstract The tensile creep behavior of in-situ TiB 2 particulate reinforced copper-based (TiB 2 /Cu) composite and unreinforced monolithic Cu fabricated by the powder metallurgy process was investigated at 773–873 K. The creep rate of monolithic Cu exhibited an exponential dependence on the applied stress, whereas the power-law relationship prevailed for the experimental data of the TiB 2 /Cu composite. The composite exhibited a stress exponent of 25.5, 20.6 and 22.1 at 773, 823 and 873 K, respectively. Furthermore, the unreinforced Cu and in-situ TiB 2 /Cu composite exhibited a creep activation energy of 187 and 444 kJ/mol, respectively. It is indicated, by incorporating a threshold stress into the analysis, that for the in-situ TiB 2 /Cu composite, the true stress exponent was equal to 4.8 and the true activation energy was close to that for self-diffusion of copper, indicating that the creep of the composite is controlled by the lattice diffusion.

High-temperature creep behavior of SiC particulate reinforced AlFeVSi alloy composite

Abstract A 15vol.%SiC particulate reinforced Al–8.5Fe–1.3V–1.7Si (8009Al) composite and unreinforced monolithic alloy were subjected to creep tests at 723–823 K. High and variable apparent stress exponent and apparent activation energy for creep were observed for both the composite and monolithic alloy. The reinforced and unreinforced 8009Al alloys exhibited an almost identical creep resistance in the stress and temperature ranges investigated, indicating that the SiC particulates exert no strengthening effect on the 8009Al alloy at elevated temperatures. By incorporating a threshold stress into the power law creep equation, the creep data of both reinforced and unreinforced 8009Al alloys were analyzed. The threshold stress obtained by the extrapolation technique exhibits a strong temperature dependence. The calculated values of the apparent stress exponent and apparent activation energy were in good agreement with those obtained from the experimentally determined e T, σ relationships.

The performance of aluminium-matrix composites with nanometric particulate Si–N–C reinforcement

Aluminium-matrix composites reinforced with nanomatric Si-N-C particles (Si-N-C/Al) were fabricated by a powder-metallurgy process. X-ray diffraction, metallography, tensile, dynamic compression and high-temperature creep tests were used to characterize the microstructure and mechanical properties of these composites. The results showed that the tensile strength of the 1 vol. % Si-N-C/Al composite is equivalent to that of the 15 vol. % SiCp(3.5 mu m)/Al composite. However, the tensile properties of the Si-N-C/Al composite deteriorated considerably with increasing particle content owing to the clustering of particles. The creep resistance of the 1 vol. % Si-N-C/Al composite was at least two orders of magnitude higher than that of the 15 vol. % SiCp(3.5 mu m)/Al composite. Furthermore, the 1 vol. % Si-N-C/Al composite exhibited an apparent stress exponent ranging from 15.7 to 23.0 and an apparent activation energy of 248 kJ/mol. The creep data of the composite was rationalized by using the substructure-invariant model with a stress exponent of 8 together with a threshold stress. The dynamic compression tests indicated that the strength and strain hardening rate of the 1 vol. % Si-N-C/Al composite increase significantly with increasing the strain rate from 10(-3) s(-1) to 10(3) s(-1)

The dynamic mechanical response of Al2O3 and TiB2 particulate reinforced aluminum matrix composites produced by in-situ reaction

Abstract The dynamic mechanical responses of three aluminum matrix composites reinforced with in-situ Al2O3 and TiB2 particulates were investigated in compression at strain rates ranging from 1.7×10−3 s−1 to 1.5×103 s−1. The results showed that these composites exhibit a high strain-rate sensitivity. The strength and initial strain hardening rate of the composites tended to increase with increasing strain rate. And increasing the particulate content led to an increase in the strain-rate sensitivity of composites. Furthermore, composites containing Al3Ti plates exhibited the highest strain-rate sensitivity.

The high-temperature creep behaviour of aluminium-matrix composites reinforced with SiC, Al2O3 and TiB2 particles

Abstract Tensile creep tests have been carried out on particulate composites with matrices of aluminium and Al/3.2 wt% Cu alloy at 573 and 623 K. Particulate SiC was incorporated in these alloys which also contained particles of Al2O3 and TiB2 grown in situ during processing. The results showed that the pure aluminium-based composites exhibited an apparent stress exponent of 8.9 and 9.5, respectively, at 573 and 623 K, and an apparent activation energy of 177 kJ mol−1. The composites based on the Alue5f8Cu alloy exhibited an apparent stress exponent of 11.9 and 13.2 at 573 and 623 K, respectively, and an apparent activation energy of 323 kJ mol−1. The creep resistance of the Alue5f8Cu alloy composite was about two orders of magnitude higher than that of the pure aluminium composite. The creep data for the two composites were rationalised by using a substructure-invariant model with a stress exponent of 8 together with a threshold stress. The threshold stresses obtained from the extrapolation technique tended to decrease with increasing temperatures for these two composites.

The high-temperature creep behaviour of 2124 aluminium alloys with and without particulate and SiC-whisker reinforcement

Abstract Tensile creep behaviour of SiC-whisker and particle reinforced 2124Al-matrix composites and unreinforced 2124Al alloy was investigated at 573-673 K. The results showed that the unreinforced 2124Al alloy exhibited apparent stress exponents of 4.7–8.5 and an apparent activation of 288 kJ/mol, while the particulate and whisker-reinforced composites exhibited apparent exponents of 9.4–17.1 and 11.5–18.2, and apparent activation energies of 482 and 533 kJ/mol, respectively. Moreover, a critical strain rate was observed in both reinforced and unreinforced 2124Al alloys, below which the creep resistance of the composites was higher than that of the unreinforced 2124Al alloy. Above this value, the 2124Al alloy was more creep resistant than the composites. An increase in the critical strain rate with increasing temperature was observed. Several established models were used to rationalize the creep data for the composites. The creep data for both particulate and whisker-reinforced composites at 623 and 673 K can be satisfactorily interpreted by the substructure-invariance model with a stress exponent of 8, whereas those at 573 K cannot be reasonably rationalized by a stress exponent of 5 or 8.

Creep behavior of in-situ Al2O3 and TiB2 particulates mixture-reinforced aluminum composites

Abstract Tensile creep tests were carried out at 573, 623 and 673 K on in-situ formed Al2O3 and TiB2 particulates mixture-reinforced aluminum (Al–Al2O3.TiB2) composites fabricated from TiO2–Al–B and TiO2–Al–B2O3 systems, respectively. The composite fabricated from TiO2–Al–B system exhibited an apparent stress exponent of 15.8–17.4 and an apparent activation energy of 242 kJ mol−1 at 56 MPa whereas the composite fabricated from TiO2–Al–B2O3 system had an apparent stress exponent of 16.5–17.9 and an apparent activation energy of 305 kJ mol−1 at 55 MPa. Two composites exhibited essentially identical creep resistance for all three testing temperatures, though the composite fabricated from TiO2–Al–B2O3 system had lower in-situ particulate content due to the formation of strip-like Al3Ti. It was demonstrated, by incorporating a threshold stress into the analysis, that for these two in-situ composites, the true stress exponent was equal 8 and the true activation energy was close to the value for lattice self-diffusion of aluminum. The results suggested that the creep of two in-situ composites was controlled by the lattice diffusion with a constant structure.