Improving Precipitation Hardened Al-Sc-Zr High Temperature Aluminum Alloys With New Element X

By Jeffrey D. Lin   |   Faculty Advisors: David C. Dunand & David N. Seidman |   Materials Science & Engineering   |   September 14, 2015   |   NURJ Online 2014-15

Abstract

A new Al-Sc-Zr-X alloy is conventionally casted and heat-treated isochronally and isothermally in order to study the effects of element X on the nucleation and growth kinetics, morphology, and distribution of precipitates in the alloy. Vickers hardness and electrical conductivity values are used to trace the precipitate evolution as a function of aging temperature and time, while local electrode atom probe tomography (LEAP) is used to determine precipitate concentration, number density, volume fraction, and size. These data are compared with those from past studies of the Al-Sc-Zr system, and suggest that element X may affect the nucleation rate of the precipitates as well as the growth of the Zr-rich shell of the precipitate. Element X is determined to be a useful component for designing Al-Sc-Zr based light high temperature superalloys.

Introduction

A new class of precipitation-strengthened aluminum superalloys with part per million additions of scandium and zirconium has been developed as a promising lightweight material for use in high temperature aerospace and automotive structural components1–4. Traditional precipitation-strengthened alloys are unable to be used at elevated temperatures since the second-phase particles that provide resistance against plastic deformation grow too big at temperatures beyond 200ºC to be effective5. Both scandium and zirconium form nanoscale Al3X (X = Sc or Zr) precipitates with the L12 structure, which is coherent with the face-centered cubic crystal structure of aluminum6–9. Scandium has the highest strengthening capability of aluminum on a per atom basis due to precipitation hardening and grain size refinement, as well as the ability to form precipitates that are thermally stable and coarsening resistant up to 300ºC10–13. Due to its sluggish diffusivity in aluminum even at 400ºC, zirconium allows the formation of precipitates stable at elevated temperatures8,9,14,15.

Combining both Sc and Zr in aluminum yields an alloy with superior heat-resistant and high strength due to the synergy of the two elements1,16,17. Precipitates with a core-shell morphology form in Al-Sc-Zr alloys during heat treatment (artificial aging), where the core is Sc-rich and the shell is Zr-rich due to the higher diffusivity of Sc than Zr in aluminum16–18. The Zr-rich shell provides a lower lattice mismatch between the precipitates and the matrix as well as diffusion barrier against Sc atoms, resulting in coarsening resistant precipitates stable at 400ºC up to 2 months1,16–18. Recently, additional elements such as erbium and silicon have been added to Al-Sc-Zr alloys and studied for effects on precipitate morphology, nucleation and growth rate, coarsening resistance, and strengthening at ambient and elevated temperatures18–21. While Er improves the alloy’s coarsening resistance at 400ºC by several months due to formation of Er-rich inner core, Si accelerates the nucleation and growth of the precipitates and improves the peak strength achieved during thermal aging due to favorable binding between Si atoms and vacancies18–21. This article focuses on the study of another element added to the Al-Sc-Zr alloy selected based on first principle calculations of binding energies between solute atoms and vacancies in aluminum performed by Wolverton et al22. Due to the proprietary nature of this work, the identity of this new element will remain undisclosed and will henceforth be referred to as element X.

Materials and Methods

Material preparation and heat treatment

Table 1. Chemical analysis results from DCP-OES of Al-Sc-Zr and Al-Sc-Zr-X alloys.

A 200 grams cylindrical ingot were conventionally cast for an alloy with nominal composition of Al-0.06%Sc-0.06%Zr-0.02%X (atomic percent) by melting pieces of 4N (99.99% purity) Al (Alcoa Inc), Al-Sc and Al-Zr master alloys (Alcoa Inc), and pure element X in an alumina crucible at 800ºC.The melt was stirred rigorously and poured into a graphite crucible placed on top of an ice-cooled copper plate in order to prevent the formation of internal cavities during solidification. Direct current plasma optical emission spectrometry (DCP-OES) was performed on samples of the cast by ATI Speciality Alloys and Components (Albany, OR), and the results are shown in Table 1. 

The cast was homogenized in an air furnace heated to 640ºC for 3 days and water-quenched so that the cast is a supersaturated solid solution. Samples of the cast are heat-treated sequentially from 100ºC to 575ºC with 25ºC h-1 steps, which is schematically shown in Figure 1. 300ºC and 400ºC are the temperatures selected to isothermally heat-treat the samples for times ranging from 0.5 hours to 2 months. Double isothermal aging at 300ºC for 4h and subsequently at 400ºC for times ranging from 0.5 h to 1 week was performed in order to develop a heat treatment sequence for maximum alloy strength. Vickers hardness and electrical conductivity measurements using Struers Duramin 5 hardness tester and Foerster SigmaTest 2.069 respectively are collected from heat-treated samples in order to trace the evolution of the precipitates as a function of aging temperature and time. 

Figure 1. Temperature evolution for isochronal aging.

Microstructural imaging

Specimens for three-dimensional local-electrode atom-probe (LEAP) tomography were prepared by cutting blanks with the Struers Accutom 5 precision saw to dimensions of ~0.45 x 0.45 x 10 mm3. The blanks were cut from samples isochronally aged to 450ºC with 25ºC h-1 steps and isothermally at 300ºC for 4h. These were electropolished at room temperature by thinning the blanks at 25-20V DC using a solution of 10% perchloric acid in acetic acid, followed by necking and final tip formation at 15-12V DC with a solution of 2% perchloric acid in butoxyethanol. Pulsed-laser APT with was performed using a LEAP 4000X Si-X tomograph (Cameca, Madison, WI) at a specimen temperature of 30K, UV laser energy of 25 pJ pulse-1, and pulse repetition rate of 500 kHz. The data was analyzed using IVAS reconstruction software (Cameca), and measurement errors were calculated using counting statistics and standard error propagation techniques.

A surface of the Al-Sc-Zr-X alloy specimen after homogenization at 640ºC is polished to a ~0.05 μm finish and etched with Keller reagent to reveal the grain size and dendritic structure of the alloy. The images were taken using the Nikon Eclipse MA200 optical microscope with bright and dark field mode, and grain sizes were measured using ASTM E112-13 line intercept methods.