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A Study of Interrupted Aging in Al-Cu-Mg Alloys

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A Study of Interrupted Aging in Al-Cu-Mg Alloys A STUDY OF INTERRUPTED AGING IN AL-CU-MG ALLOYS by Joseph Ming-Ju Tsai A thesis submitted to the Faculty and Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Metallurgy and Materials Engineering). Golden, Colorado Date: Signed: Joseph Ming-Ju Tsai Signed: Michael J. Kaufman Thesis Advisor Golden, Colorado Date: Signed: Professor Michael J. Kaufman, Department Head Department of Metallurgical and Materials Engineering ii ABSTRACT Recently, a novel interrupted aging treatment, T6I6, was reported to improve the strength and toughness of heat treatable Al alloys. It consisted of the following three stages, namely, 1) artificially underaging at a typical temperature (150-200 ◦C) and quenching to room temperature, 2) reheating to a lower aging temperature (60-100 ◦C) for times that result in hardening, and 3) reheating to the initial aging temperature again and holding until peak hardness is observed. The improvements were reported to be due to precipitate refinement and secondary precipitation during the low temperature age. However, the improvements in properties have not been consistently reproduced in the litera- ture, and an explanation for the discrepancies remains unclear. Therefore, the focus of this study was to investigate the interrupted aging processes and the effects of each aging stage, as well as the effect of cold work, on precipitate development in commercial 2024 Al and a high-purity variant of the commercial 2014 alloy, Al-4.2Cu-0.4Mg (wt%). The aging responses, precipitation behavior, and microstructural changes were examined using Vickers microhardness, electrical resistivity, and electron microscopy. Small angle X-ray scattering (SAXS) was also used to characterize the precip- itate size and volume fraction during the initial underaging and subsequent stages of interrupted aging. It was found that, although secondary hardening was observed during the low temperature aging, apparent softening upon re-heating was observed and the peak hardness showed little im- provement. In addition, in 2024 alloys which were cold worked after SHT or after underaging, no further strengthening was observed during secondary aging at 65 ◦C. Likewise, subsequent aging to peak hardness showed no improvements. In order to understand these results, an attempt was made to examine the mechanism of hardening during the underaging and secondary hardening treatments using SAXS. Formation of 1 nm diameter solute clusters up to ∼3 vol% was found to contribute primarily to the initial rapid hardening and secondary strengthening during interrupted aging. The subsequent softening was attributed to the reversion of these 1 nm clusters. Further analyses of the observed cluster hardening involved estimating the modulus and/or surface/chemical strengthening. The shear modulus of the clusters was estimated to be 2.9-3.3 GPa higher than that of 2024, and additional surface energy from cluster shearing was estimated to be 0.16-0.63 J/m2. iii TABLE OF CONTENTS ABSTRACT ::::::::::::::::::::::::::::::::::::::::::::: iii LIST OF FIGURES ::::::::::::::::::::::::::::::::::::::::: vii LIST OF TABLES :::::::::::::::::::::::::::::::::::::::::: xii ACKNOWLEDGEMENTS ::::::::::::::::::::::::::::::::::::: xiii CHAPTER 1 INTRODUCTION ::::::::::::::::::::::::::::::::: 1 CHAPTER 2 LITERATURE REVIEW ::::::::::::::::::::::::::::: 3 2.1 History of Age Hardening in Aluminum Alloys . .3 2.2 Solution Heat Treatment SHT and Thermodynamic Basis of Precipitation . .3 2.3 Classical Nucleation Theory . .6 2.4 Solute Clustering . .8 2.5 Precipitate Strengthening Mechanisms . 11 2.5.1 Strengthening through Penetrable Particles . 11 2.5.2 Strengthening through Particle Bypassing: Orowan Looping . 14 2.6 Artificial Aging and Precipitate Hardening in Al-Cu and Al-Cu-Mg Alloys . 15 2.6.1 Precipitation and Hardening during Aging of Al-Cu Binary Alloys . 15 2.6.2 Precipitation and Hardening during Aging of Al-Cu-Mg Alloys . 18 2.6.3 Initial Rapid Hardening in Al-Cu-Mg Alloys . 22 2.6.4 Proposed Mechanisms for Initial Rapid Hardening in Al-Cu-Mg Alloys . 24 2.6.5 Clusters, GPB zones, S', S Phases and Precipitation Sequence in Al-Cu-Mg Alloys Revisited . 25 2.7 Novel Heat Treatment: Interrupted Aging . 27 2.7.1 Secondary Hardening and Precipitation . 28 2.7.2 Interrupted Aging . 28 2.7.3 Evolution of Clusters and GP zones during Low Temperature Dwell . 32 2.8 Basic Principles of Small-Angle X-ray Scattering, SAXS . 36 2.8.1 Small Angle Scattering . 36 2.8.2 Interpretations of SAXS Intensity . 37 iv 2.8.3 Volume Fraction Calculation Using SAXS Intensity . 38 2.9 Scope of Thesis . 39 CHAPTER 3 EXPERIMENTAL PROCEDURE :::::::::::::::::::::::: 42 3.1 Sample Procurement and Preparation . 42 3.2 Introducing Cold Work . 42 3.3 Heat Treatment . 42 3.4 Hardness Measurements . 45 3.5 Electrical Resistivity Measurements . 45 3.6 TEM Analysis . 46 3.7 Small-Angle X-ray Scattering, SAXS . 47 CHAPTER 4 RESULTS ::::::::::::::::::::::::::::::::::::: 48 4.1 Single-Stage Aging Behavior of 2024 Alloys . 48 4.1.1 Hardness and Resistivity Changes of 2024 Alloys during T6 and T8 Aging . 48 4.1.2 TEM Microstructures of 2024-T6 Alloys at Initial Plateau and Peak Hardening 50 4.1.3 TEM Microstructures of 2024-T8 Alloys at Initial Plateau and Peak Hardening 53 4.2 Interrupted Aging of 2024 Alloys . 57 4.2.1 Interrupted Aging (T6I6) of 2024 Alloys . 59 4.2.2 TEM Analyses of 2024 After T6I4 Treatment . 62 4.2.3 Interrupted Aging Treatments T8I6 and T6I8 . 62 4.2.4 Stretching Behavior of 2024 Alloys in As Solution Heat Treated and Under- aged Condition . 67 4.3 Examination of Interrupted Aging in Al-4.2Cu-0.4Mg Alloy . 67 4.3.1 Single-Stage Aging of Al-4.2Cu-0.4Mg . 67 4.3.2 Interrupted Aging of Al-4.2Cu-0.4Mg . 68 4.3.3 TEM Analyses of Al-4.2Cu-0.4Mg . 72 4.4 Summary and Comment on Interrupted Aging of Al-Cu-Mg Alloys . 72 4.5 Data Interpretation of Small Angle X-Ray Scattering (SAXS) . 74 4.5.1 Obtaining Experimental SAXS Intensity and Converting to Absolute Units . 74 4.5.2 Fitting of Small-Angle X-Ray Scattering Data . 74 4.6 SAXS Analyses on the Evolution of Solute Clusters during Initial Rapid Hardening and Interrupted Aging . 79 v 4.6.1 Evolution of Cluster Formation During Initial Hardening of Naturally Aged 2024 Alloys . 83 4.6.2 SAXS Analyses for 2024 Alloys Aged to Plateau Hardness . 85 4.6.3 SAXS Analyses of 2024 Alloys After T6I4 and T6I4-Reheated Treatments . 88 4.6.4 Relationship Between Electrical Resistivity and 1 nm Clusters . 89 4.6.5 SAXS Analysis of Al-4.2Cu-0.4Mg Alloys . 92 4.7 Summary . 93 CHAPTER 5 DISCUSSION ::::::::::::::::::::::::::::::::::: 94 5.1 The Effect of Cold Work on Interrupted Aging . 94 5.2 The Relationship Between Resistivity and Volume Fractions of Clusters . 96 5.3 Reasoning Behind the Assumptions of Cluster Analysis Using SAXS . 96 5.4 Mechanism(s) of Cluster Hardening . 98 5.5 Predicted Strengthening of the T6I4 Material and the Subsequent Re-aging Curves . 99 5.6 Volume Fractions of Clusters and Interparticle Spacing in 2024 Alloys at Plateau Hardening, After Secondary Aging, and Re-aging. 101 5.7 Estimating the Shear Modulus Difference and Excess Surface Energy by Shearing . 102 5.8 Predicted Volume Fractions of 1 nm Clusters for Al-4.2Cu-0.4Mg Alloys Using Re- sistivity and Their Predicted Strengthening . 103 5.9 Aging Temperatures and Volume Fractions at Early Aging Plateau . 104 5.10 Possible SAXS Interference Effect in Naturally Aged 2024 Alloy . 105 5.11 Conclusions on Interrupted Aging of Al-Cu-Mg Alloys . 107 CHAPTER 6 SUMMARY :::::::::::::::::::::::::::::::::::: 109 6.1 Single Stage and Interrupted Aging of 2024 Alloys . 109 6.2 Single Stage T6 and Interrupted Aging T6I6 of Al-4.2Cu-0.4Mg Alloys . 110 6.3 Resistivity, Strengthening Mechanisms for Cluster Hardening, and Conclusion . 110 CHAPTER 7 FUTURE WORK ::::::::::::::::::::::::::::::::: 112 REFERENCES CITED ::::::::::::::::::::::::::::::::::::::: 113 vi LIST OF FIGURES 2.1 The first age hardening curve published by Alfred Wilm on Duralumin. .4 2.2 Schematic illustration of solution heat treatment and aging temperatures. .5 2.3 Schematic illustration of the change of Gibbs free energy (overall driving force) for de- composition/precipitation from SSSS at Tb in blue and Tc in red (Tb < Tc) as illustrated in Figure 2.2. .6 2.4 Schematic illustration of Gibbs free energies for phase with miscibility gap at Tb and Tc (Tb < Tc) shown overall driving force for decomposition of the SSSS. .7 2.5 TEM micrograph of the morphology of precipitates formed from spinodal decomposition in a Cu-33.5 wt% Ni-15 wt% Fe alloy after aging at 775 ◦C for 15 minutes. After Newbury et al...........................................8 2.6 Plot of ∆G with respect to r from Equation 2.1 and ∆Gd = 0 for homogeneous nu- cleation. ∆G∗ is the free energy barrier, or activation energy, for nucleation. ∆G = −V (∆GV − ∆Gs) + Aγ.....................................9 2.7 Schematic illustration of total change of Helmholtz free energy vs number of clustering solute atoms in a two-phase region. After Nie et al...................... 10 2.8 Increase in CRSS vs the precipitate particle radius r. Adapted from Martin. 12 2.9 Schematic illustration of particle shearing by a moving dislocation. Reproduced after Martin. 13 2.10 Schematic illustration of Orowan looping (a) a dislocation bowing around impenetrable particles; (b) continuing motion of dislocation with loops of dislocations left around the precipitate particles. 15 2.11 Al-rich corner of the Al-Cu phase diagram with metastable solvi of GP, θ00 and θ0, and the equilibrium θ in bold. 16 2.12 Schematic free energy diagram for Al-Cu binary alloys. 17 2.13 Schematic illustration of (a) GP zone and (b) θ00.
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