Grain structure

Tin additions to lead-calcium alloys change dramatically the method of precipitation and age-hardening from discontinuous precipitation of PbsCa to a mixed discontinuous and continuous precipitation of PbsCa and (PbSn)3Ca and, finally, to a continuous precipitation of SnsCa. Such precipitation reactions have been described for alloys that contain low tin contents [45,48,68-70]. The reactions are not influenced or modifled by impurities in the lead alloys [71,72]. A ternary phase diagram has been proposed [41], which sets the areas of stability of PbsCa, SusCa, and mixed (PbSn)3Ca precipitates, and this is shown in Fig. 2.5. The phase diagram has been conflrmed [73]. 

At low tin contents, the calcium initially precipitates rapidly as PbsCa in the same manner as in binary lead-calcium alloys. These alloys have a fine grain structure and quickly reach high hardness as discontinuous precipitation predominates. At higher tin contents, the mode of precipitation changes to a mixed discontinuous precipitation of PbsCa followed by a continuous precipitation reaction of Pb cSn Ca. These reactions lead to overageing of the precipitates and a decrease in mechanical properties. 

Below a calcium content of 0.08 wt.% and above a Sn Ca ratio ( r value) of 9 1 (the value corresponding to stoichiometric SnsCa), the reaction changes to a continuous precipitation. In the continuous precipitation mode, the original cast grain structure is maintained. 

2 Mechanical properties of cast lead—calcium tin alloys 

The mechanical properties and ageing response of cast lead alcium-tin alloys have been described by several authors [44,74,75]. The effect of calcium at 0.5 and 

---

Signals from the notch tip can only be identified for the herringbone grain structure and not for the perpendicular one. 

S olid Propellant Mging, Mechanical Pehavior and Grain Structural Integrity, CPIA Pubhcation LS 77-27, CPIA, Johns Hopkins University, Laurel, Md., 1977. 

Fig. 4. Grain structure of lead—2 wt % antimony alloy battery grid at a magnification of 50x (a) no nucleants (b) containing 0.025 wt % selenium as a grain...

Figure 3.4 Grain growth in a UO2 fuel rod during high-temperature service, showing the three zones of the original grain structure (I), the equi-axed central zone (II), and the mainly single crystal zone (III) surrounding the central void...

Why is it undesirable to have a columnar grain structure in castings Why is a fine equiaxed grain structure the most desirable option What factors determine the extent to which the grain structure is columnar or equiaxed ... 

The anodes are generally not of pure metals but of alloys. Certain alloying elements serve to give a fine-grained structure, leading to a relatively uniform metal loss from the surface. Others serve to reduce the self-corrosion and raise the current yield. Finally, alloying elements can prevent or reduce the tendency to surface film formation or passivation. Such activating additions are necessary with aluminum. 

Sulfide Stress Cracking) on steels over Rockwell C 22. (4) static stresses. other equipment handling sour gas, oil and/or water wherein H2S and H2O (liquid phase) are present up to about 150 F, where sulfide stress cracking slows down perceptibly. stainless steels with Rockwell hardness over C 22. (4) into crystal structure, exact mechanism uncertain. Sulfur expedites absorption of atomic H into grain structure. (4) if feasible use inhibitors and/or resistant coatings where feasible time or heating up will permit H to diffuse out but will not relieve any areas when H2 has concentrated. 

The SEM can also be used to provide crystallographic information. Surfaces that to exhibit grain structure (fracture surfaces, etched, or decorated surfaces) can obviously be characterized as to grain size and shape. Electrons also can be channeled through a crystal lattice and when channeling occurs, fewer backscattered electrons can exit the surface. The channeling patterns so generated can be used to determine lattice parameters and strain. 

One of the original applications of AES, and still one of the most important, is the analysis of grain boundaries in metals and ceramics. Very small amounts of impurity or dopant elements in the bulk material can migrate under appropriate temperature conditions to the boundaries of the grain structure and accumulate there. In that way the concentration of minor elements at the grain boundaries can become... 

Exfoliation corrosion is especially prevalent in aluminum alloys. The grain structure of the metal determines whether exfoliation corrosion will occur. In this form of corrosion, degradation propagates below the surface of the metal. Corrosion products in layers below the metal surface cause flaking of the metal. 

To further characterize the event it is first necessary to identify critical features of the initial configuration that will strongly influence the process. For powder compacts, the most obvious features are the morphological characteristics of the powders, their microstructures, and the porosity of the compact. For solid density samples, the grain structure, grain boundaries, defect level, impurities, and inclusions are critical features. 

B. D. Knowlton, J. J. Clement, C. V. Thompson. Simulation of the effects of grain structure and grain growth on electromigration and the reliability of interconnects. J Appl Phys 81 6012, 1997. 

Fig. 8 Schematic representation of grain structure in the presence of grain-boundary liquid phases.

K. Higashi, "Deformation Mechanisms of Positive Exponent Superplasticity in Advanced Aluminum Alloys with Nano or Near-Nano Scale Grained Structures," in Materials Science Forum Vols. 170-172, pp. 131-140, T.G. Langdon ed., Trans Tech Publications, Switzerland, (1994). 

Phase composition and grain structure of Ti-6Al-2Zr-l.5V-lMo-0.35H alloy after heat treatment and after deformation were studied in comparison with the basic alloy. After hydrogenation and cooling to room temperature the structure of the specimens was a -martensite with 20-25% of the / -phase. 

Thus, hydrogenation and straining of the alloy at 680-780°C followed by final outgassing results in a very fine grain structure which is favorable for the mechanical properties of the final product. 

O.N. Senkov, I.O. Bashkin, S.S. Khasanov, and E.G. Ponyatovsky, Grain structure of titanium alloy VT20 after hydrogen treatment and deformation at moderate temperatures, Fiz. Met. Metallovedeniye, 76 128 (1993). 

E.G. Ponyatovsky, O.N. Senkov, and I.O. Bashkin, Str s-strain properties of the Ti-6A1-2Zr-1.5V-Mo alloy with the various grain structures and hydrogen contents, Phys. Met. Metall. 72 194 (1991). 

Figure 4-431. Ringworm corrosion—at the inside of the tubing where grain structure near edge of upset portion makes metal susceptible to rapid corrosion. (From Ref. [219].)...

Fig. 1.10 Grain structure of a wrought high-strength precipitation-hardening aluminium alloy showing potential crack growth paths...

Shot peening is a beneficial surface treatment since it puts the surface into a state of compression and generally obscures the grain structure. Subsequent painting of the peened surface is often useful. If pitting occurs then cracking can be expected in susceptible material when the attack penetrates the depth of the compressed surface layer. 

The mechanical properties of Watts deposits from normal, purified solutions depend upon the solution formulation, pH, current density and solution temperature. These parameters are deliberately varied in industrial practice in order to select at will particular values of deposit hardness, strength, ductility and internal stress. Solution pH has little effect on deposit properties over the range pH 1 0-5-0, but with further increase to pH 5 -5, hardness, strength and internal stress increase sharply and ductility falls. With the pH held at 3-0, the production of soft, ductile deposits with minimum internal stress is favoured by solution temperatures of 50-60°C and a current density of 3-8 A/dm in a solution with 25% of the nickel ions provided by nickel chloride. Such deposits have a coarse-grained structure, whereas the harder and stronger deposits produced under other conditions have a finer grain size. A comprehensive study of the relationships between plating variables and deposit properties was made by the American Electroplaters Society and the results for Watts and other solutions reported... 

Fig. 20.34 Light micrograph showing the highly elongated grain structure of a commercial wrought high-strength precipitation-hardening Al-Zn-Mg-Cu alloy (x 40, section perpendicular to the long transverse direction)...

---