Dendrite coarsening and microsegregation in Al-Cu alloys
- PDF / 734,643 Bytes
- 9 Pages / 594 x 774 pts Page_size
- 45 Downloads / 250 Views
MANYstudies
have been published on the quantitative prediction of dendrite arm spacings and microsegregation both of which affect significantly the mechanical properties of cast structures.~ The dendrite structure that forms at the early stages of solidification is very unstable and becomes coarser with time. It is very important to understand the effect of coarsening on dendrite arm spacing, hence on solute distribution since this spacing affects the severity of microsegregation, 2-6 as well as the solution and homogenization treatments. ~ The effect of dendrite coarsening on microsegregation has not yet been quantitatively established. In calculations it is taken into account only by correcting the dendrite arm spacing by a specific factor depending on the alloy s y s t e m s ) : In A1-Cu alloys dendrite arm spacing, d, is related 3 to the local solidification time (i.e. the time from the beginning to the completion of solidification at a certain point), t:, as follows:
d= 7.5xtf
39
[1]
In a theoretical study, Brody and Flemings 2 have indicated that dendrite arm spacing changes the classical nonequilibrium solidification equation (Scheil Equation) within the volume element as follows:
f,
C *= kCo(1 - 1 + o/tk )(k-l)
[2]
where, C* is the solute concentration in the solid at the solid-liquid interface during solidification, k is the equilibrium partition ratio, f , is the fraction solid, Co is the alloy composition (average solute concentration in the alloy), or' = D,t:/1}, D s is the diffusion coefficient of solute in the solid, and 1: is half the dendrite arm spacing measured at the end o f solidification. This equation assumes that the diffusion in the liquid is complete, the thickening rate of dendrite arm is linear, and the diffusion in the solid is so limited that METIN BASARAN, formerly Associate Professor, Department of Mechanical Engineering, Technical University of lstanbul, Turkey, is now Manager of Process Research, Revere Research Inc., Edison, NJ 08837. Manuscript submitted June 12, 1980. METALLURGICAL TRANSACTIONS A
the concentration at the interface does not change. The diffusion in the solid affects microsegregation depending on the parameter r/' = t:/l}. Assuming a plate-like dendrite morphology, calculations 2,3 made through solution of Ficks second law with the finite difference explicit method and using Eq. [2] have indicated that a corrected dendrite arm spacing, de, should be taken as d c = g 9 d so that experimental results 7,8 and calculated results are in agreement. In AI-Cu alloys the correction factor, g, is found to be 0.32. A different correction factor was found in ironbase alloys? All correction factors were less than 1. It is necessary for the calculation to know the prevailing dendrite morphology because of its effect on microsegregation. In many alloys microsegregation is a function of distance from the chill and alloy composition, 2-4'9 parameters which affect the dendrite morphology. 1,3,4,1~ The alloys which do not contain much eutectic in their cast microstructure g
Data Loading...
