AN ABSTRACT OF THE THESIS OF Moon H. Lee for the degree of Doctor of Philosophy in Mechanical Engineering presented on August 14, 1984 Title: Fatigue Life, Fatigue Crack Propagation and Fracture Toughness Study of 7075 Aluminum Alloy Subjected to Thermomechanical Processing Redacted for Privacy Abstract approved: Dr. Murli Saletore Due to its high strength/weight ratio, 7075 aluminum alloy has been widely used as an aerospace material. However, it has rela- tively low fatigue strength and low fracture toughness in the T6 condition. Thermomechanical processing, including pre-cyclic load- ing and stretching at high and ambient temperatures, has been in- vestigated with the aim of improving these properties. Fatigue life tests show that all four thermomechanical pro- cesses investigated increase the fatigue life at low stress levels, but cause a reduction at high stress levels. The effect of the precipitate free zone (PFZ) width on the fatigue crack propagation rate and fracture toughness has also been investigated. The fatigue crack propagation rate of the alloy with a wide PFZ was found to be approximately the same as that of the al- loy with a narrow PFZ under similar conditions of cyclic loading. The alloy with the wide PFZ demonstrated a greater value of fracture toughness than that shown by the alloy with the narrow PFZ. These experimental results have been analyzed taking recently pro- posed theoretical models into account. Transmission electron microscopy showed that a large number of dislocations were generated within the soft PFZ areas as a result of the cyclic pre-loading. This is believed to contribute to the increase in fatigue life at low applied stress levels. Scanning electron microscopy has shown an intergranular frac- ture mode for the alloy with the narrow PFZ and a dimple rupture mode for the alloy with the wide PFZ. Fatigue Life, Fatigue Crack Propagation and Fracture Toughness Study of 7075 Aluminum Alloy Subjected to Thermomechanical Processing by Moon H. Lee A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Completed August 14, 1984 Commencement June 1985 APPROVED: Redacted for Privacy Assistant Professor of Mechanical Engineering in charge of major Redacted for Privacy Head of Department pf Mechnical Engineering Redacted for Privacy Dean of Grad a School dr Date thesis is presented August 14, 1984 Typed by Donna Lee Norvell-Race for Moon H. Lee ACKNOWLEDGMENTS This work would have been impossible without: my parents' absolute love toward me, my family members' profound belief in me, consistent support by my wife, Kyung-Sook, continuous financial aid from department head, Dr. J. W. Welty, enlightening guidance from my advisor, Professor M. Saletore, sincere encouragements from my committee members, Professor P. Burke, Professor T. Kennedy, Professor R. Thresher and Professor E. Fichter. Also, I would like to give my deep gratitude to J. Bauffer, A. Brickman, L. Sundberg, C. Svenson, A. Soedner and P. Romans for their technical assistance and advice. TABLE OF CONTENTS I. INTRODUCTION 1 II. LITERATURE SURVEY 4 11.1 7075 Aluminum Alloy 4 11.2 Precipitate Free Zone 5 11.2.1 The Origin of the Precipitate Free Zone. 5 . . . 11.2.2 The Effect of the PFZ on Mechanical Properties. 7 11.3 Thermomechanical Processing 7 11.4 Fatigue Life Studies 9 11.4.1 S-N Curve 9 11.4.2 Sites for Crack Nucleation 11 11.4.3 Fatigue Crack Nucleation Models 11 11.4.4 Lynch's Model for Crack Nucleation 13 11.4.5 Microstructural Factors which Affect Crack Nucleation in Aluminum Alloys with PFZ 14 . . . . 11.4.5.1 Alloying elements 14 11.4.5.2 Grain-size reduction 15 11.4.5.3 Degree of aging 16 11.4.5.4 Thermomechanical processing 16 11.4.5.5 Cyclic pre-loading as TMP 17 11.5 Fatigue Crack Propagation Studies 18 11.5.1 General Description of Fatigue Crack Propagation 18 11.5.2 Effect of Microstructure 19 11.5.2.1 The effect of degree of aging on fatigue crack propagation 20 11.5.2.2 Effect of grain-size on fatigue crack propagation 20 11.5.2.3 Effect of pre-deformation on fatigue crack propagation 21 11.5.2.4 Effect of stacking fault energy on fatigue crack propagation 22 11.5.2.5 The effect of PFZ on fatigue crack propagation 23 11.5.3 Fatigue Crack Propagation Models 24 11.6 Plane Strain Fracture Toughness Studies. 24 . . The Effect of Microstructure on Fracture Toughness 24 11.6.2 The Effect of PFZ on Fracture Toughness. 26 11.6.3 The Effect of Grain-size and Shape 27 III. EXPERIMENTAL PROCEDURES 28 III.1 Material 28 111.2 Thermomechanical Processing 28 111.2.1 Cyclic Pre-loading at High Temperature: FHT. 31 . 111.2.2 Cyclic Pre-loading at Room Temperature: FRT 32 111.2.3 Cyclic Pre-loading Combined with T6 Tempering: FT6 32 111.2.4 Stretching at High Temperature: SH5 32 111.3 Heat Treatment for Precipitate Free Zone 32 . . . 111.4 Fatigue Life Tests 32 111.4.1 Testing System 32 111.4.2 Specimen Design 33 111.4.3 Test Procedures 33 111.5 Fatigue Crack Propagation Test: FCP Test. 34 . 111.5.1 Equipment 34 111.5.2 Specimen Design 34 111.5.3 Test Procedures 34 111.5.4 (da/dN)Calculationemid Related Computer Program 35 111.6 Plane Strain Fracture Toughness Test 36 111.6.1 Equipment 36 111.6.2 Specimen Design 36 111.6.3 Test Procedures 36 111.6.4 K Calculation and the Computer Program 37 IC . . 111.7 Tensile Test 38 111.7.1 Equipment 38 111.7.2 Specimen Design 38 111.7.3 Test Procedures 38 111.8 Transmission Electron Microscopy 38 111.9 Scanning Electron Microscopy 39 111.10 Metallography. 40 IV. RESULTS AND DISCUSSION 41 IV.1 Thermomechanical Processing (TMP) 41 IV.1.1 Concepts of Thermomechanical Processing. 41 . . . IV.1.2 Cyclic Pre-loading at High Temperature (FHT) 42 . IV.1.3 Cyclic Pre-loading at Room Temperature (FRT) 43 . IV.1.4 Cyclic Pre-loading Combined with T6 Tempering (FT6) 43 IV.1.5 Stretching at High Temperature (SH5) 45 IV.1.6 Thermal Treatment for Precipitate Free Zone Formation (PFZ#1 and PFZ#2) 46 IV.2 Material and Tensile Tests 48 IV.2.1 As-received 7075-T651 Alloy 48 IV.2.2 Tensile Test of Solution-treated Alloy 49 . . . IV.2.3 Tensile Test of T6-aged Alloy 49 IV.2.4 Tensile Test of FHT Alloy 49 IV.2.5 Tensile Test of FRT Alloy 50 IV.2.6 Tensile Test of FT6 Alloy 51 IV.2.7 Tensile Test of SH5 Alloy 51 IV.2.8 Tensile Tests of PFZ#1 and PFZ#2 Alloys. 51 . IV.3 Fatigue Life Studies 52 IV.3.1 Fatigue Life Study of FHT Alloy 55 IV.3.2 Fatigue Life Study of FT6 Alloy 56 IV.3.3 Fatigue Life Study of SH5 Alloy 57 IV.3.4 Fatigue Life Study of PFZ#1 and PFZ#2 Alloy 57 IV.4 Fatigue Crack Propagation Studies 57 IV.4.1 Fatigue Crack Propagation of Plastically De- formed and Non-deformed.Alloys 57 IV.4.2 Fatigue Crack Propagation Study of Alloys with Different PFZ Widths 60 IV.5 Plane Strain Fracture Toughness Studies. 61 . . IV.5.1 Plane Strain Fracture Toughness of 7075-T651 and 7075-T6 Alloys 61 IV.5.2 Plane Strain Fracture Toughness of PFZ#1 and PFZ#2 Alloys 63 IV.6 Scanning Electron Microscopy 65 IV.6.1 SEM Study of Fatigue Crack Propagation . 65 . . . IV.6.2 SEM Study of Fracture Surfaces 66 IV.7 Transmission Electron Microscopy 68 IV.7.1 TEM Study of Thermomechanically Processed 7075 Aluminum Alloys 68 IV.7.2 TEM Study of PFZ#1 and PFZ#2 Alloys 69 IV.7.3 TEM Study after Fatigue Life Testing 69 . . V. CONCLUSIONS 70 VI. SUGGESTIONS FOR FURTHER RESEARCH 73 REFERENCES 75 BIBLIOGRAPHY 84 APPENDICES 1 The shape of flat-end smooth fatigue specimen . . . 92 2 The dimensions of flat-end smooth fatigue specimen. 93 3 The shape of special grip for fatigue crack propa- gation test 94 4 The shape of compact tension specimen for fatigue crack propagation test 94 5 The dimensions of clevis and pin for fatigue crack propagation test 95 6 The dimensions of the adapter for MTS 96 7 The dimensions of compact tension specimen for fatigue crack propagation test 97 8 The shape of compact tension specimen for KIC test 98 9 The shape of special grip for KIC test 98 10 The dimensions of compact tension specimen for K test 99 IC 11 The dimensions of clevis and pin for Km test . . 100 12 Fractograph of compact tension specimens showing fatigue pre-crack 101 13 The dimensions of standard tensile test specimen according to ASTM E-8 102 14 The dimensions of special adapter for tensile test of flat-end smooth fatigue specimen 103 15 Y.S. and U.T.S. of 7075 aluminum alloys subjected to various thermomechanical processings 104 16 Microstructure of 7075-PFZ#1 (100X) 105 17 Microstructure of 7075-PFZ#2 (100X) 105 18 Load versus displacement of 7075-T6 aluminum alloy 106 19 Load versus displacement of 7075-FHT before T6 tempering 107 20 Load versus displacement of 7075-FHT after T6 tempering 108 21 Load versus displacement of 7075-FRT before T6 tempering 109 22 Load versus displacement of 7075-FRT after T6 tempering 110 23 Load versus displacement of 7075-FT6 111 24 Load versus displacement of 7075-SH5 112 25 Load versus displacement of 7075-PFZ#1 113 26 Load versus displacement of 7075-PFZ#2 114 27 Stress amplitude versus fatigue life of 7075-T6 of this investigation compared with literature values 115 28 Stress amplitude versus fatigue life of 7075 aluminum alloy cyclically loaded compared with S-N curve of 7075-T6 116 29 Stress amplitude versus fatigue life of 7075 aluminum alloy stretched 5% plastically at 150°C compared with S-N curve of 7075-T6 117 30 Stress amplitude versus fatigue life of 7075- PFZ#1 aluminum alloy with PFZ of 800A compared with S-N curve of 7075-T6 118 31 Stress amplitude versus fatigue life of 7075- PFZ#2 with PFZ of 2000A compared with S-N curve of 7075-T6 119