Grain refining

Alloying the metals with other components offers other strengthening mechanisms. Adding a second metal as substitutional atoms in the lattice is known as solid solution hardening. The size difference between the solvent (host) atoms and the solute (impurity) atoms strains the lattice and makes it difficult for dislocations to move. Adding smaller atoms that can go into the interstices produces a similar hardening in the lattice. This is the role that carbon plays in the strengthening of steel. 

Second phase particles can be made to precipitate from a supersaturated solute. By carefully controlling the time and temperature, these precipitates can ripen by solid-state diffusion until they reach the optimiun size in which their lattice nearly matches the host lattice, but their mismatch puts enough strain in the lattice to block the motion of dislocations. This process is called precipitation hardening or age hardening because the size of the precipitate is controlled by the time the alloy is held at a temperature the precipitate can grow by solid-state diffusion. The methods for forming such precipitates will be discussed in Chapter 14. 

Adding inert second phase particles results in dispersion hardening. Submicron oxide or carbide particles act as pinning sites that block the motion of dislocations. Intermetallic phases can be made to form, which not only block the motion of dislocations, but also help to stabilize grain boimdaries. The hardening effect of a dispersed phase is not as effective as a coherent precipitated phase, but dispersion hardening can be used in systems that do not 

---

Ytterbium metal has possible use in improving the grain refinement, strength, and other mechanical properties of stainless steel. One isotope is reported to have been used as a radiation source substitute for a portable X-ray machine where electricity is unavailable. Few other uses have been found. 

Fig. 3. Grain size as a function of antimony content A, without grain refiners B, with addition of selenium (4).

Fibrous stmctures represent a grain refinement of columnar stmcture. Stress-reHeving additives, eg, saccharin or coumarin, promote such refinement, as do high deposition rates. These may be considered intermediate in properties between columnar and fine-grained stmctures. 

Selenium acts as a grain refiner in lead antimony alloys (114,115). The addition of 0.02% Se to a 2.5% antimonial lead alloy yields a sound casting having a fine-grain stmcture. Battery grids produced from this alloy permit the manufacture of low maintenance and maintenance-free lead—acid batteries with an insignificant loss of electrolyte and good performance stability. 

Fig. 31. Electron micrographs that compare crystal size of (top) a grain-refined microcrystalline coating and (bottom) a conventional zinc phosphate conversion coating [54].

Hydrogenation, deformation and dehydrogenation result in a strong grain refinement. 

Of the elements commonly found in lead alloys, zinc and bismuth aggravate corrosion in most circumstances, while additions of copper, tellurium, antimony, nickel, silver, tin, arsenic and calcium may reduce corrosion resistance only slightly, or even improve it depending on the service conditions. Alloying elements that are of increasing importance are calcium especially in maintenance-free battery alloys and selenium, or sulphur combined with copper as nucleants in low antimony battery alloys. Other elements of interest are indium in anodesaluminium in batteries and selenium in chemical lead as a grain refiner ". 

Alloys containing zirconium as a grain-refining agent have the iron content automatically reduced to about 0 004% by settling out of impurities during the alloying procedure. 

Both titanium and boron can be added as grain refiners to ensure small grain size and hence high surface area grain boundaries. This reduces the risk of preferential attack at grain boundaries and promotes more uniform dissolution. 

T orr. Reaction (1) is a simplified version of a reaction that involves the formation of subfluorides such as ReF4. Grain refinement is obtained by the addition of a small amount of H2O in the gas stream.The hydrogen reduction of the chloride is not normally used since it produces gas phase precipitation which is almost impossible to avoid. 

The main grain refining methods for magnesium alloys can be classified into ... 

Du and co-workers [48, 66] studied the effect of carbon on the grain refinement of Mg-3A1 alloy. High grain refining efficiency was obtained when these alloys were refined by carbon. A further increase in efficiency was obtained by the combination of 0.2 wt% C and less than 0.2 wt% of a solute element (Ca, Sr) [48, 66], Addition of a higher Ca amount would increase the brittleness of the alloy [60, 66]. Similar results were demonstrated when 0.2 wt% Sr was added in the alloys instead of Ca [48]. 

---