Brass, phase equilibrium

A square-wave modulation of 1% was then applied to the gas phase equilibrium volume, Vg. Such a small perturbation can generally satisfy the assumption of linearity. The modulation was affected by applying a current to each of the two electromagnets (Eq. 3) in turn, which moves the disc (Eq. 4) between the electromagnets rapidly (< 10 ms) and periodically. The brass bellows (Eq. 5) attached to the disc, which is part of the sorption gas... 

The potentials of film-free a-brass and /3-brass in solutions comparable to those existing inside the alloy at the advancing front of attack were found to be —0-38V and —0-56V (v. S.H.E.), respectively. It was also established, taking into account the activities of copper ions in equilibrium with the sparingly soluble corrosion product CU2CI2, that whereas Cu ions can be reduced to copper at —0 -16 V the reduction of Cu ions is possible only at potentials more negative than —0-41 V. Thus whereas the /3-phase of an a/3-brass can reduce both Cu and Cu ions, the a-brass can reduce only the Cu ion. 

The Cu-Zn system (see Figure 2.7) displays a number of intermediate solid solutions that arise due to limited solubility between the two elements. For example, at low wt% Zn, which incidently is the composition of alloys known as brass, the relatively pure copper a phase is able to accommodate small amounts of Zn as an impurity in the crystal structure. This is known as a terminal solid phase, and the solubility limit where intermediate solid solutions (such as a + /S) begin to occur is called the solvus line. Some of the three-phase transformations that are found in this diagram include a peritectic (5 - - L -> e) and a eutectoid (5 -> y - - e). Remember that these three-phase transformations are defined for equilibrium coohng processes, not heating or nonequihbrium conditions. 

P + Pi brasses of similar composition (Cu—38 wt.% Zn and Cu—41 wt.% Zn, respectively), it has been concluded that this is not simply due to an increased Zn concentration in the p phase, but is likely to reflect the structural differences of hoth phases [67]. Other factors that stimulate the dezincification of both O - and a p brasses are the presence of chlorides, high concentrations of CuCl2 ions, stagnant environments, differential aeration cells and elevated temperatures. In chloride solutions, such conditions are typically encountered at a later stage of corrosion when mass transport restrictions by a deposit, corrosion product, or crevice have heen established and create a local environment, in which the copper in the brass is nearly in equilibrium with CU2O, CuCl, and accumulated CuCl2 anions [68,69]. Experimentally, the necessity... 

Measure the amount of water, e.g. 1 litre, with a graduated cylinder and pour into the zinc or brass vessel. Seal the vessel with a rubber bung and then shake it for 1 minute. This produces the vapour-space equilibrium between the liquid and gas phases. Carefully discharge any gauge pressure and then fit the dispersion cylinder and measuring head in position in the vessel. 

There are some phase transitions that do not fall either into the first-order or second-order category. These include paramagnetic-to-ferromagnetic transitions in some magnetic materials and a type of transition that occurs in certain solid metal alloys that is called an order-disorder transition. Beta brass, which is a nearly equimolar mixture of copper and zinc, has a low-temperature equilibrium state in which every copper atom in the crystal lattice is located at the center of a cubic unit cell, surrounded by eight zinc atoms at the corners of the cell. At 742 K, an order-disorder transition occurs from the ordered low-temperature state to a disordered high-temperature state in which the atoms are randomly mixed in a single crystal lattice. The phase transition between normal liquid helium and liquid helium II was once said to be a second-order transition. Later experiments indicated that the heat capacity of liquid helium appears to approach infinity at the transition, so that the transition is not second... 

It is necessary to establish a foundation of definitions and basic concepts relating to alloys, phases, and equilibrium before delving into the interpretation and utilization of phase diagrams. The term component is frequently used in this discussion components are pure metals and/or compounds of which an alloy is composed. For example, in a copper-zinc brass, the components are Cu and Zn. Solute and solvent,... 

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