Solidification of Peritectic Alloys

Mathew Muether

University of Missouri - Columbia

Physics and Mathematics

Advising Professor: David Johnson

Introduction

The morphology of peritectic alloys upon solidification is not well understood as a wide variety of two-phase microstructures can result in a seemingly unpredictable fashion. For example, as growth conditions are varied for some alloys, dendritic, banded, or even "eutectic-looking" microstructures can result. What is needed is a set of rules to predict the growth morphologies given the growth conditions, the nature of the phase diagram, and the crystal structure and orientation of the solid phase.

Objectives

  • Processing Al-Cu alloy near peritectic region (45% Cu by Wt)
  • Perform directional solidification experiments at a variety of growth conditions to produce a range of microstructures for analysis
  • Understand relationship of phase diagram and thermodynamic properties to final microstructure and composition

Experimental Approach

  • Prepare Cu-Al alloy (45% Cu by weight) by melting shot in arc melter
  • Re-melt alloy and draw into tube for use in image furnace
  • Directionally solidify the samples at a variety of rates (0.0002 cm/s, .00125cm.s, 0.002cm/s) in image furnace
  • Prepare a sample by quenching in water for examining liquid phase reactions
  • Mount samples (including as-cast) cross-sections, polish, and etch (NaOH) for optical microscopy
  • Examine samples under optical microscope and record the spacing of major features
  • Perform X-ray diffraction on sample to determine composition and lattice parameters
  • Use the AlCu phase diagram for interpretation of phases present and any coupled growth that may occur

Research Findings

The growth of fine coupled "eutectic-like" AlCu and Al2Cu is observed in the slowly directionally solidified sample and the quenched sample as is not common in peritectic alloys. This may be due to the Gibbs-Thompson effect that can convert an equilibrium peritectic reaction into a meta-stable eutectic reaction.


Fig. 1. AlCu alloy As-cast. Exhibits coarse dendritic properties.


Fig. 2. Slowly DS sample. Fine coupled growth ("eutectic like").