Quantification of Microsegregation in Cast Al-Si-Cu Alloys

Abstract

The random sampling approach offers an elegant yet accurate way of validating microsegregation models. However, both instrumental errors and interference from secondary phases complicate the treatment of randomly sampled microprobe data. This study demonstrates that the normal procedure of sorting the data for each element independently can lead to inaccurate estimation of segregation profiles within multicomponent, multiphase, aluminum alloys. A recently proposed alloy-independent approach is shown to more reliably isolate these interferences, allowing more accurate validation of microsegregation models. Application of this approach to examine solidification segregation of a 319-type alloy demonstrated that, for these slowly cooled castings, neither Sr or TiB 2 additions significantly affected coring of Cu within the primary a-Al dendrites. Comparison against predictions of CALPHAD-type Gulliver-Scheil models was less satisfactory. Consideration of back-diffusion and morphology effects through a one-dimensional (1-D) numerical model do not improve the agreement. Possible reasons for the lack of agreement are hypothesized.

Directional Solidification of Mg-Al Alloys and Microsegregation Study of Mg Alloys AZ31 and AM50: Part I. Methodology

“…It is emphasized that the WIRS is especially suitable if alloys with many components are analyzed, such as superalloys that may have up to 10 components; in that case, the final solute profiles may be significantly influenced by operator subjectivity in the choice of a leading element. Ganesan et al [1] also applied the WIRS recently to the 319 aluminum alloy, which contains copper and silicon as the two major alloying elements. Even though sorting according to the descending aluminum content appeared reasonable, an important limitation was highlighted; it is the uncertainty in the measurement of the leading element that is transferred into noise in other components.…”

“…In addition, the heat-treatment efficiency and final mechanical properties, such as yield strength or toughness, are strongly influenced by the extent of the as-cast segregation. [1] Various microsolidification models are available for elucidating the effects of microsegregation [2] and crystal growth morphology. [3] The models range from a simple Scheil model with the most restrictive assumptions up to the highly sophisticated approaches, such as the promising phase-field method.…”

“…However complex the model may be, it is obvious that the reliability of any microsolidification model needs to be validated against dedicated experimental measurements. Perhaps one of the most stringent tests for any microsegregation model is a comparison of the measured and model-predicted solute profile, [1] even if some doubt has been raised in this regard. [6] The term ''solute profile'' is used for the solute concentration (content) vs solid phase fraction during solidification.…”

“…Numerous studies involving the X-ray mapping of directionally solidified samples and data processing are found in the literature; most deal with steel, [6,8] aluminum alloys, [1,9,10] or superalloys, [7,11] but a few are found that deal with magnesium alloys. Zhang et al [12] recently combined directional solidification (DS), EPMA, and the Scheil-solidification model to study the morphology and microsegregation of a binary Mg-4 wt pct Al alloy.…”

Experimental and Numerical Modeling of Segregation in Metallic Alloys

“…[18] One can cite characterization by electron probe microanalyses on samples solidified in wellcontrolled conditions, e.g., using directional solidification and quenching. [19][20][21][22] Because the fraction of the phases can also be measured as a function of the temperature or local composition, it is then possible to conduct a comparison with predictions. Not only could the combined interpretation of such experimental and modeling analyses explain the effect of solid diffusion, but it was also used to identify the effect of the nucleation undercooling of a second phase.…”