A New Class of Engineering Materials: Particle-Stabilized Metallic Emulsions and Monotectic Alloys

Abstract

Al-matrix particulate composites are melted and mixed with immiscible metals to form their small droplets in liquid aluminum. It is shown that, in the Al-Si/SiC/Bi system, the Bi droplets are stabilized by the SiC particles in the liquid Al matrix. Upon solidification, homogeneous distribution of solidified Bi droplets is obtained in the Al matrix at the bottom part of the ingot. Thus, a new class of engineering materials (particle-stabilized monotectic alloys) is obtained.Monotectic alloys are produced by solidifying liquid metallic emulsions, i.e., small droplets of one liquid metal dispersed in the matrix of another immiscible liquid metal. [1] However, once the droplets are nucleated, they immediately approach each other and coalesce under the influence of gravity or the interfacial gradient force, making the production of monotectic alloys with homogeneously distributed second phase almost impossible, especially for thick castings with considerable cooling times. [2][3][4][5][6] Here, we show for the first time that solid particles with appropriate wettability can be used to stabilize liquid metallic emulsions, opening a principally new route to produce thick wall monotectic alloys with homogeneously distributed second-phase droplets (particles).There has been common knowledge in colloid chemistry for more than a century [7] regarding how to stabilize water-or oil-based liquid foams [8][9][10][11][12][13][14][15][16][17] and emulsions [13,[18][19][20][21][22] by solid particles. On the other hand, solid particle-stabilized metallic foams [23][24][25][26][27][28] were originally discovered quite empirically by metallurgists and independently from the aforedescribed common knowledge of colloid chemists. However, it has become clear that the law of stabilization of water foams and metallic foams by particles is a common law. [10,20] Due to this gap in ''common knowledge'' of the colloid chemists' and metallurgists' communities, it is not surprising that the metallic analogue of particlestabilized oil/water emulsions does not exist today ( Figure 1). The goal of this article is to show that particle-stabilized liquid metallic emulsions (PSLMEs) can be produced, at least in a laboratory scale. In addition to the scientific curiosity, this might have a serious impact on further materials development. Solidifying the PSLMEs, a new route opens to produce monotectic alloys with homogeneously distributed second-phase particles in virtually any size, which is impossible to do when the monotectic alloys are not stabilized by particles.Let us summarize in Figure 2 the experience gathered by colloid chemists for the last century. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] Figure 2(c) is a theoretical emulsion stability diagram [20] for particle-stabilized emulsions. One can see that the contact angle (H) and the liquid-phase ratio (/ B ) are the two independent parameters that determine whether, in the given three-phase (liquid A + liquid B + solid C) combination of materials, any emulsion can be stabil...

Monotectic Al/Cd alloys with homogeneously dispersed Cd-droplets stabilized by strontium aluminide precipitates

“…The parameters of emulsification/mixing are given in Table 1 for the 3 parallel experiments. Other parameters of the experiments are described elsewhere [1].…”

“…A new class of engineering materials -particles stabilized metallic emulsions and monotectic alloys e has been recently developed by the authors [1]. Monotectic alloys are produced by solidifying liquid metallic emulsions, i.e.…”

“…However, once the droplets are nucleated, they immediately approach each other and coalesce under the influence of gravity and/or the interfacial gradient force [3], making the production of monotectic alloys with homogeneously distributed second phase almost impossible, especially for thick castings with considerable cooling times [4e8]. In our previous paper SiC particles of the Al/SiC composite mixed with liquid Bi was found to stabilize liquid Bi droplets in an Al-matrix [1]. The principle and technology are analogous to those developed for water-in-oil or oilin-water emulsions in colloid chemistry [9e17].…”

On the Negative Surface Tension of Solutions and on Spontaneous Emulsification

“…Emulsions are one of the important classes of multiphase materials. − Usually, they are considered thermodynamically unstable because of their high inner interfacial area and can be stabilized only kinetically, for example, using particles. − However, this general view is challenged by numerous observations on spontaneous emulsification. − These observations hold a promise for thermodynamically stable emulsions.…”

Fabrication of SiC-Particles-Shielded Al Spheres upon Recycling Al/SiC Composites

“…[14] The A359 alloy was used as an aluminum alloy (for its composition, refer to Table I). Stoichiometric SiC powder of F600 type was applied with the powder size of 10 to 20 lm.…”