A study of microsegregation in Al-Cu using a novel single-pan scanning calorimeter

Abstract

A single-pan scanning calorimeter has been developed that eliminates the smearing of latent heat that occurs in a conventional two-pan heat-flux differential scanning calorimeter (DSC). In the new calorimeter, accurate enthalpy/temperature data was obtained in pure Al without smearing, and excellent sensitivity to new phases was obtained in a multicomponent Al alloy (LM25). The calorimeter has been used to investigate microsegregation in an Al-4.45 wt pct Cu alloy. The enthalpy/temperature data fell between that calculated, assuming no mixing in the solid (Scheil) and complete mixing in the solid (equilibrium solidification). The amount of segregation agreed well with that calculated using a diffusion-based model of microsegregation. The difficulty of getting the fraction solid from the enthalpy data is discussed, and it is concluded that it is not possible to do so without using a microsegregation model. In addition, it is concluded that it is wrong to assume that the enthalpy of an alloy can be given by a specific heat term and a constant latent heat term that depend on fraction liquid as is assumed in most casting models.

A Quest for a New Hot Tearing Criterion

“…Current research efforts are aimed, in particular, at the quantitative description of structure evolution and its correlation to cracking. [16,46,56,[59][60][61] Recently, several mechanical and nonmechanical hot tearing criteria were evaluated by implementing them into a thermo-mechanical model of DC casting. [52] The criteria show different results in predicting the hot tearing susceptibility, as shown in Table II.…”

Corrigendum to “Application of single pan thermal analysis to Cu–Sn peritectic alloys” [Acta Mater 56 (2008) 1519–1528]

“…f s ðT Þ, directly from enthalpy plots. Dong et al rather used microsegregation models coupled to a thermodynamic database to get f s ðT Þ indirectly from experimental enthalpy measurements [6,10].…”

Microsegregation in Al-Cu alloys

“…[1,2] It was pointed out, in the articles, that it is not possible to measure directly the fraction solid as a function of temperature and, thus, the amount of microsegregation using calorimetry, unless it is assumed that the latent heat is constant and the heat evolved does not depend on the composition changes that occur during solidification. However, it was possible to compare the heat evolved using a model of microsegregation and a thermophysical software program like MTDATA.…”

“…[3] When this comparison was made, the experimental results compared well with the predictions, except at the highest Cu concentrations. [1] Despite the reasonable agreement, it was recognized that calorimetry was a relatively insensitive method for investigating the progress of microsegregation, because there was only a small change in the amount of heat evolved for the two extremes of equilibrium freezing and there was no diffusion in the solid. For this reason, additional experiments were carried out, investigating the amount of microsegregation using the more traditional method of measuring composition as a function of sample fraction.…”