Alloys precipitation mechanism

Bum3] Bumm, H., Mueller, H.G., Copper Precipitation Mechanism in the Recrystallization of Iron-Nickel-Copper Alloys (in German), Metallwirtschaft, 17(24), 644-648 (1938) (Experimental, Magn. Prop., Electr. Prop., 16)... 

In the dissolution and precipitation mechanism, proposed by various authors [12-14] working with brasses, both the LN and MN components dissolve, but the MN component is deposited. Several difficulties arose with this mechanism outside of the brass system in that ions of the MN component were not detected in the solution during dealloying, but instead alloys of intermediate compositions were found on the dealloyed surface. 

In addition to RIS, radiation can cause precipitation of second phases in alloys. Typically there are two types of precipitation mechanisms (1) radiation-enhanced mechanism in which the precipitation is thermodynamically favorable and the high concentration of point defects under irradialion allows supersaturated solutes to achieve equilibrium through precipitation at significandy faster rates than under thermal conditions and (2) radiadon-induced mechanism in which the coupling between... 

Physical metallurgy is a rather wide field of applications of Mossbauer spectroscopy and it is possible to enumerate only the main topics phase analysis, order-disorder alloys, surfaces, alloying, interstitial alloys, steel, ferromagnetic alloys, precipitation, diffusion, oxidation, lattice defects etc. Alloys are well represented by the iron-carbon system, the mechanism of martensite transformation, high-manganese and iron-aluminium alloys, iron-silicon and Fe-Ni-X alloys. 

In addition, the heat-treatability of an alloy system is mentioned frequently. Heat-treatable designates an alloy whose mechanical strength is improved by precipitation hardening (Section 11.9) or a martensitic transformation (normally the former), both of which involve specific heat-treating procedures. 

Once the precipitates grow beyond a critical size they lose coherency and then, in order for deformation to continue, dislocations must avoid the particles by a process known as Orowan bowing(23). This mechanism appHes also to alloys strengthened by inert dispersoids. In this case a dislocation bends between adjacent particles until the loop becomes unstable, at which point it is released for further plastic deformation, leaving a portion behind, looped around the particles. The smaller the interparticle spacing, the greater the strengthening. 

Nickel—Copper. In the soHd state, nickel and copper form a continuous soHd solution. The nickel-rich, nickel—copper alloys are characterized by a good compromise of strength and ductihty and are resistant to corrosion and stress corrosion ia many environments, ia particular water and seawater, nonoxidizing acids, neutral and alkaline salts, and alkaUes. These alloys are weldable and are characterized by elevated and high temperature mechanical properties for certain appHcations. The copper content ia these alloys also easure improved thermal coaductivity for heat exchange. MONEL alloy 400 is a typical nickel-rich, nickel—copper alloy ia which the nickel content is ca 66 wt %. MONEL alloy K-500 is essentially alloy 400 with small additions of aluminum and titanium. Aging of alloy K-500 results in very fine y -precipitates and increased strength (see also Copper alloys). 

Stacking faults thereby providing barriers to sHp. If carbides are allowed to precipitate to the point of becoming continuous along the grain boundaries, they often initiate fracture (see Fracture mechanics). A thorough discussion of the mechanical properties of cobalt alloys is given in References 29 and 30 (see also Refractories). 

Alpha—beta aluminum alloys respond to heat treatment with a general improvement of mechanical properties. Heat treatment is accompHshed by heating to 815—870°C, quenching in water, and reannealing at 370—535°C, depending on the size and section of the casting. Different combinations of strength, hardness, and ductility can be obtained. Some nickel in aluminum bronze is in soHd solution with the matrix and helps refine the precipitate, and a smaller amount is in the K-intermetaUic compound. 

Griess has observed crevice corrosion of titanium in hot concentrated solutions of Cl , SOj I ions, and considers that the formation of acid within the crevice is the major factor in the mechanism. He points out that at room temperature Ti(OH)3 precipitates at pH 3, and Ti(OH)4 at pH 0-7, and that at elevated temperatures and at the high concentrations of Cl ions that prevail within a crevice the activity of hydrogen ions could be even greater than that indicated by the equilibrium pH values at ambient temperatures. Alloys that remain passive in acid solutions of the same pH as that developed within a crevice should be more immune to crevice attack than pure titanium, and this appears to be the case with alloys containing 0-2% Pd, 2% Mo or 2[Pg.169]

---