Composition Gradient and Particle Deformed Zone: An Emerging Correlation
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TRODUCTION
AEROSPACE grade AA 7075, a high-strength aluminum alloy, is produced by direct chill (DC) casting.[1–4] However, the cast alloy typically has 2nd phase and micro-porosity.[1–7] These, and even the thermal residual stresses,[8–11] may differ significantly with the casting condition and location. And they may lead to significant degradation in mechanical properties, especially ductility. More specifically, porosity and microcracks at the matrix-2nd phase interface may provide an easy path for crack propagation[6,12] and are often attributed to ingot hot cracking.[8,13] Mitigation typically involves homogenization, or dissolution of the 2nd phase, and extensive hot working to eliminate micro-porosity.[1,5] Interestingly, even the
ADITYA PRAKASH and INDRADEV SAMAJDAR are with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai 400076, India, Contact mail: [email protected] TAWQEER NASIR TAK and P.J. GURUPRASAD are with the Department of Aerospace Engineering, Indian Institute of Technology Bombay, Mumbai 400076, India. ARIJIT LODH is with the IITB-Monash Research Academy, Indian Institute of Technology Bombay, Mumbai 400076, India. NIRAJ NAYAN is with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay and also with the Vikram Sarabhai Space Centre, Trivandrum 695 022, India. S.V.S. NARAYANA MURTY is with the Vikram Sarabhai Space Centre. Manuscript submitted September 23, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
as-cast specimens show significant improvement in ductility at elevated temperature.[8,14] It is thus apparent that the dislocation–precipitate interaction plays a critical role in the ductility of the 7XXX series. Dislocation–precipitate interaction has a reservoir of rich scientific literature.[15–23] Even in the early 1950s, it was recognized that ‘dislocations from activated FrankRead sources to form closed loops around the particles.’[15] There are excellent articles on strengthening through ordered[24] particles and precipitates with coherency[25] field. More relevant to this manuscript, Ashby[26] had shown that a strain incompatibility exists between the matrix and non-shearable second phase. This is accommodated by the generation of geometrically necessary dislocations (GNDs). For the so-called weak interfaces, the strain incompatibility may also lead to the formation of micro-voids (and pre-mature failure) at the interface. A series of classical research articles, particularly from Humphreys and co-workers,[27–29] clearly established that large non-shearable 2nd phase precipitates would produce particle deformed zone (PDZ) around them. The existence of PDZ especially becomes relevant with considerations of particle stimulated nucleation during subsequent recrystallization,[27] and the fact that PDZ may trigger extensive strain localizations.[28] Though it is apparent that the formation of the PDZ would depend on the deformation condition, and the existence of 2nd phase plus PD
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