Intermetallic alloy formation reaction

Intermetallic compound formation may be observed as the result from the diffusion across an interface between the two solids. The transient formation of a liquid phase may aid the synthesis and densification processes. A further aid to the reaction speed and completeness may come from the non-negligible volatility of the component(s). An important factor influencing the feasibility of the reactions between mixed powders is represented by the heat of formation of the desired alloy the reaction will be easier if it is more exothermic. Heat must generally be supplied to start the reaction but then an exothermic reaction can become self-sustaining. Such reactions are also known as combustion synthesis, reactive synthesis, self-propagating high-temperature synthesis. 

These intermetallic compounds react with Hj at RT, provided that the pressure is high enough , but there is often an induction period ranging from seconds to days depending on previous treatment of the alloy and time of exposure to air . Freshly prepared samples not exposed to air usually react in seconds because of the catalytic action of Ni and Co on Fe. The intermetallic compound is oxidized at its surface to form the rare-earth oxide (e.g., LajOj) and free metallic Ni (or Co or Fe), which acts as a catalyst to dissociate H. Impurity gases, such as CO, and HjO, decrease the rates of hydride formation and can poison the alloy for reaction with Hj. 

The formation of IMCs in Pb-free solder systems is in many respects reminiscent of those observed in Pb-Sn, with several intriguing added complications. In Pb-free solders such as Sn-Ag-Cu, the Cu and Ag additives form IMCs with Sn. These elements also diffuse rapidly in Sn, thus a high flux of these constituents is available for otherwise favorable reactions, such as the formation of ternary IMCs at metallization interfaces. The evolution of the microstructure of Pb-free solder joints during reflow, cooling, and subsequent annealing has been shown to be a complex subject. As far as intermetallic compound formation and growth at metallization interfaces are concerned, Sn-Ag-Cu solder alloys were found to behave similar to eutectic Sn-Pb solder on Cu substrates. In contrast, distinct differences were observed on Ni surfaces between the behavior of Sn-Ag-Cu and Sn-Pb solder. 

Chemical reaction This involves the formation of distinct compounds by reaction between the solid metal and the fused metal or salt. If such compounds form an adherent, continuous layer at the interface they tend to inhibit continuation of the reaction. If, however, they are non-adherent or soluble in the molten phase, no protection will be offered. In some instances, the compounds form in the matrix of the alloy, for example as grain-boundary intermetallic compound, and result in harmful liquid metal embrittlement (LME) although no corrosion loss can be observed. 

Plutonium-noble metal compounds have both technological and theoretical importance. Modeling of nuclear fuel interactions with refractory containers and extension of alloy bonding theories to include actinides require accurate thermodynamic properties of these materials. Plutonium was shown to react with noble metals such as platinum, rhodium, iridium, ruthenium, and osmium to form highly stable intermetallics. Vapor pressures of phases in these systems were measured by the Knudsen effusion technique. Use of mass spectrometer-target collection apparatus to perform thermodynamic studies is discussed. The prominent sublimation reactions for these phases below 2000 K was shown to involve formation of elemental plutonium vapor. Thermodynamic properties determined in this study were correlated with corresponding values obtained from theoretical predictions and from previous measurements on analogous intermetallics. 

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