Instantaneous Rates of Grain Growth

Abstract

Isotope Cl» a(mgc/sec.) 205.2 ±4.6 170.6 ±3.9 6(mgc/sec.) 55.2 ±1.2 43.1 ±1.0 Q(10-2« cm*) (DFZZ) -7.95 ±0.2 -6.20 ±0.2 0(10-28 C m2) (Townes) -6.7 -5.1 Specimen No. 1 2 3 4 Heat treatment 125 min. at 400°C 125 min. at 400°C and 120 min. at 450°C 5 min. at 450°C 125 min. at 450°C D mm 0.235 0.340 0.230 0.344(3) (d/a) 36 = 0.269±0.006; (4) by observing the trajectory of the atoms giving rise to the line (2, 0)-*(1, -1) we conclude that the magnetic moment of Ci 35 is positive, confirming the observation of Kusch and Millman ; 6 (5) assuming their 6 value of g 35 /2 37 = 1.203±0.005 to give a u /a 37 , we obtain the a and b values of Table I.From atomic h.f.s. measurements of this sort, it is possible to obtain accurate values of the nuclear quadrupole moment Q, without resorting to fine structure data and estimates of an effective nuclear charge Z», since both the magnetic dipole and electric quadrupole interactions have the same r dependence on the electronic wave functions. From the formulae for a and b given by Casimir 7 neglecting relativity corrections, we obtain, in an obvious notation: Q=-{b/a) gl^y e*)( m /M p ){L+\){2L+2>)/J(J+\\and we get the Q values of Table I. The Q values given by Townes 4 are included for comparison.In view of the fact that Q values can be determined quite accurately, we believe that the significance of these values lies not in the fact that they disagree with the estimates of Townes, but rather that, when combined with microwave spectroscopy measurements of eQ d * v /dz 2 , they provide a method of actually measuring d 2 V/dz 2 at the position of the halogen nucleus in a wide variety of molecules.Values for a and b of considerably greater accuracy can be obtained when the transitions Av^ -3a-f-b are found. We plan to apply this method to other halogens, including radioactive ones, and to measure the interaction constants to higher precision than the present apparatus can achieve. We believe that ultimately this method is suitable for observing these atomic interactions to a few cycle/sec, for atoms in concentration of less than 10~8. P REVIOUS work 1 -2 showed that the isothermal increase of the average grain size (D) with the annealing time (t) in high purity aluminum follows the relationwhere A' and n are parameters depending on the temperature. The exponent n increases from 0.056 at 350°C to 0.32 at 600°C. From (1) the instantaneous rate of growth isSince »<1, dD/dt decreases with continued annealing. The absolute value of the negative exponent n -1 and, therefore, the rate of decay of dD/dt is increasing with decreasing temperature.More recent experiments demonstrated that the rate of grain growth in high purity aluminum after 33 percent reduction of area by rolling depends only on the instantaneous grain size and on the temperature, but that it is independent of the particular prior heat treatment used to produce the instantaneous grain size considered. A typical experiment was the following. Four specimens 0.020 in. thick, from ingot No. 18 (see reference 2), were annealed at...

Correlation between microstructure evolution and cavity stringer formation in the superplastic Zn-22 wt% Al alloy

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A new insight into grain growth kinetics in terms of polygonized sub-structure development

“…A new insight into grain' growth kinetics in terms of polygonized sub-structure development • In the last few decades the phenomenon of grain growth has received close attention from both theoreticians and practising metallurgists (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18) in view of its importance in understanding grain boundary behaviour and in modifying: the properties of polycrystalline materials. Although from the very beginning it was recognized that impurities were a major factor in controlling the rate of M. Fiset is a graduate student of Laval University, S. Bercovici, formerly research assistant at Laval University, is now at the Centre de Recherches Pechiney-Voreppe.…”