Grain growth in metals of close-packed hexagonal structure

Abstract

The process of grain growth has been investigated for four close-packed hexagonal metals magnesium, zinc, cadmium and thallium. The tests were carried out with thin sheets of metal, at least 10 grains in thickness, which were cross-rolled and allowed to recrystallize completely before being raised to the temperature at which the measurements of grain dia­meter were made, the surface being etched before each grain count. It has been established that maintenance at a fixed temperature leads ultimately to a stable average grain diameter D e characteristic of the temperature, supposing the initial value of diameter to have been less than D e . The relation between D e and temperature T is D ½ e = C(T – T 0 ), where C and T 0 are characteristic of the metal. The way in which grain size increases with time falls into three stages. If the initial grain diameter D i is very small compared to D e , very rapid growth takes place: in an extreme case at high temperature a threefold increase of diameter in 1 s was established. If D i is not smaller than 0.65 D e the observed growth takes place, in general over a period of days, in close accordance with the law D = D e (1 + pe -qt ) -1 . This we call the normal stage of growth, to which most attention has been devoted. There is an intermediate stage which characterizes most of the measurements hitherto made by others. For the normal stage q = ae -∊/kT , giving an activation energy ∊ which is 2.25 times the latent heat of fusion. The statistical distribution of grain size is given by z/z m = exp[ – π 2 {( x/x m ) ½ – 1} 2 ], where z is the number of grains of diameter x, z m being the maximum value of z at x m . If the temperature is raised, the number of grains decreasing in diameter is 5.25 times the number increasing in diameter; 5.25 is about the average number of corners of a grain as seen in the plane section. Measurements have been made on the effect of external stress on grain growth. In the discussion of the results the D e ½ law is attributed to the existence of strips of good fit on the boundary between random crystal grains, and an attempt is made to account for grain growth in terms of the disappearance of small grains as a result of surface tension forces and of the rotation of the boundaries of large grains to a position of stability. It is suggested that if a metal could be brought to an amorphous state at T 0 it would be stable in that state at that temperature, in which connexion quenching condensation receives reference.

The effect of different second-phase particle regimes on grain growth in two-phase aggregates: insights from in situ rock analogue experiments

“…2), indicate that grain growth is not continuous on the grain-scale for the whole run-time of each experiment. As shown in Andrade and Aboav (1966), and in Fig. 9 in Covey-Crump and Rutter (1989), our experiments reveal different stages of grain growth.…”

“…But with proceeding time, 3-D grain growth becomes delimited by the glass plates between which the samples are confined, changing into a 2-D grain growth with growth vectors parallel to the plane of view. As a consequence, only three out of the original four stages of Andrade and Aboav (1966) can be discriminated in our experiments: during the first part of the 2-D growth, grain size increases with proceeding time (stage II), followed by a transitional period of reduced grain growth (stage III), ending with a stabilized grain size that does not change anymore with proceeding time (stage IV). Characteristically, pure norcamphor aggregates (diamond symbols) show larger grain sizes than the experiments containing glass beads (circles, see Fig.…”

“…Due to the small initial grain size (\50 lm) and sample thicknesses generally around 140 lm, three-dimensional overlap prevents an accurate grain size analysis during the first increments of the experiments. Hence, stage I of ''rapid grain growth'' of Andrade and Aboav (1966) cannot be analyzed and is therefore missing in our datasets. But with proceeding time, 3-D grain growth becomes delimited by the glass plates between which the samples are confined, changing into a 2-D grain growth with growth vectors parallel to the plane of view.…”

“…Therefore, type, amount, dispersion, and physical properties of second phases determine grain growth and the physical properties of a rock. Initially, research on grain growth behaviour and grain scale processes was experimentally conducted in the field of material sciences on monomineralic metals (e.g., Andrade and Aboav 1966;Simpson et al 1971) as well as on two-phase metallic systems and alloys (Smith 1948;Dai et al 1999;Ferry et al 2005). Meanwhile, experimental approaches are expanded by numerical modelling (Jessell et al 2001(Jessell et al , 2003Solomatov et al 2002;Moelans et al 2005;El-Khozondar et al 2006;Zheng et al 2006;Becker et al 2008).…”

“…Based on the aforementioned experiments, four stages of grain-size evolution can be distinguished (Andrade and Aboav 1966;Simpson, et al 1971;Olgaard and Evans 1988): a first stage (I) comprises densification of the aggregate and the processes of initial recrystallization, in which specifically the combination of internally stored strain and surface energy of a grain represent the driving forces during fast grain growth (see Fig. 1 in Olgaard and Evans 1988).…”