Surface energy contributions to the work of infiltration in metal matrix composite processing

The work needed to mechanically drive molten metal into a porous solid preform when producing a composite material by infiltration can significantly exceed the energy change required for thermodynamically reversible infiltration. We measure, by quantitative metallographic analysis of partially infiltrated, particle- or fiber-based non-metallic preforms, the evolution with saturation of the three interfaces present during the process. Results show that irreversible energy losses in the infiltration of alumina preforms by molten copper, aluminium or aluminium-tin alloy cannot be ascribed to the creation of liquid meniscus surface area at intermediate metal saturation. This result agrees with similar observations in soil science and gives experimental confirmation of predictions from a recent simulation of capillarity-dominated metal infiltration [Acta Mater., vol. 210, 2021, 116831].

Processing and Mechanical Properties of Alumina Reinforced Copper-Aluminum Composites

Carmen Krüger

Copper-based composites reinforced with alumina particles are produced by two different pro- cesses, namely gas pressure driven infiltration of a porous alumina preform and a two-step in-situ process developed in the course of this work. The resulting composites are characterized for their microstructures and mechanical properties. It is shown that during gas-pressure infiltration of angular alumina particles with liquid copper- aluminum alloys at 1300◦C, rounding of the alumina particles occurs. Rounding is quantified by measurements of local curvature, and shown not to be found after infiltration of the same particles with pure copper at 1150◦C. Transport pathways necessary for rounding are examined by analysis of the thermodynamics and kinetics of the copper-aluminum-alumina system, drawing from the literature on wetting of alumina by copper and its alloys. Analysis leads to two different possible explanations for the phenomenon: (i) the presence of aluminum in the melt increases the equilibrium oxygen concentration and thus accelerates volume diffusion or (ii) segregation of aluminum in the liquid alloy to the alloy-alumina interface causes accelerated interfacial diffusion. The pressure infiltration of four different copper-aluminum alloys with varying titanium content (Cu8wt%Alxwt%Ti with x=0, 0.2, 1, 2) into preforms of two types of alumina particle leads to sound, pore-free composites reinforced with 56-59 vol% alumina. Depending on the titanium content of the matrix alloy, a new phase appears at the interface between the alumina particles and the metallic matrix; this phase was identified as the mixed oxide Ti3(Cu,Al)3O. A very thin (5-10 nm) titanium-rich layer between alumina and the ternary oxide is responsible for a mechanically strong interface between the copper-alloy matrix and alumina. Peak mechanical properties in these composites were obtained by infiltration of equiaxed Al2O3 particles with a Cu8wt%0.2wt%Ti alloy. Their measured properties displayed a Young’s modulus near 235 GPa, a density of 5.5 g/cm3, a tensile strength and elongation of 600 MPa and 1% respectively, together with a compressive strength near 1100 MPa reached at 9% strain. Copper matrix alumina composites are also produced via a process combining in-situ formation of alumina and deformation processing. Elemental powders are uniaxially compacted and subjected to a two-step heat treatment that leads to the formation of alumina films in a copper-aluminum solid solution matrix. During deformation processing, the composite is consolidated and the alumina films are broken up into particles of average diameter 1-2 μm. Further hot working in the form of upsetting and extrusion further refines the alumina particles and improves their distribution, which improves the composite mechanical properties. Cu7wt%Al matrix composites reinforced with 26 vol% alumina particles were obtained by this process. In optimum processing conditions of this work, these composites have a tensile strength of 846±44 MPa and a tensile ductility of 2.2±0.8%, with likely room for significant further improvement of these properties after more extensive deformation processing.

EPFL2012

High-temperature wettability and thermal conductivity of aluminium nitride reinforced metal matrix composites

Masahiro Kida

Aluminum nitride (AlN) has been applied to various applications, including electronic substrates and packaging, semiconductor production components, wide band gap semiconductor, light-emitting diodes (LED) and acoustic wave devices; at the same time, it is also attractive as a reinforcement in Metal Matrix Composites (MMCs) for thermal management applications with tailored thermal properties. In this thesis, two main subjects are investigated in terms of both the composite processing and its thermal property, namely (i) the high-temperature wettability between AlN and liquid metals and (ii) the thermal conductivity behavior of AlN reinforced metal matrix composites and the associated question of the interface thermal conductance at the AlN/metal interface. High-temperature wettability of AlN particle preforms with pure Al, Cu, Sn and Pb, was investigated by means of drainage curve measurements during pressure infiltration. The measured drainage curves show good agreement with the semi-empirical model in soil science proposed by Brooks and Corey. The total work of immersion, Wi, can be directly calculated by integration of the drainage curves, in turn giving the apparent contact angle during infiltration at high temperature. For Cu, Sn and Pb, the apparent contact angles deduced by integration of experimental drainage curves are in the range 120-140° and agree well with corresponding sessile drop data from the literature. Data for AlN and α-Al2O3 are furthermore similar for both methods of measurement, which results from the presence of a native oxide layer on the AlN particles, which is not eliminated from the particle surface in these systems. For the wettability between AlN/Pb and α-Al2O3/Pb systems, a slight difference is observed and this behavior is attributed to the transformation of the native oxide layer with temperature and the presence of dissolved oxygen in the melt. On the contrary, the wettability of AlN/Al system gives higher value than the sessile drop data measured in vacuum, corresponding rather to the value measured under He-H2 gas at 1373K. This indicates that in the present study there is no elimination of the native oxide layer on the AlN by the formation of Al2O gas, which normally occurs in high vacuum. Moreover, the results of the present technique gradually deviate to the sessile drop data as the initial contact angle gets close or below 90° especially in the α-Al2O3/Al system, suggesting that there is a minimum value, around 105-110°, below which the present infiltration-based contact-angle measurement technique may lose validity. Thermal conductivity behavior of AlN reinforced metal matrix composites produced by pressure infiltration technique has been characterized at both room and elevated temperature in two different microstructural systems, namely (i) particle reinforced system (PRCs) and (ii) partially sintered particle system, i.e., an interpenetrating composite in which the AlN is also a continuous phase (IPCs). The phase contrast of thermal conductivity between AlN (Κd) and matrix metals (Κm) is varied within a range of 0.4

EPFL2008

High-temperature wettability and thermal conductivity of aluminium nitride reinforced metal matrix composites

Masahiro Kida

EPFL2008

Processing and Mechanical Properties of Alumina Reinforced Copper-Aluminum Composites

Carmen Krüger

EPFL2012