Effects of cold work on precipitation in Al-Cu-Mg-(Ag) and Al-Cu-Li-(Mg-Ag) alloys
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INTRODUCTION
W H E R E A S cold working (e.g., 5 pct uniform elongation) of aluminum alloys during the interval between quenching from the solution-treatment temperature and artificial aging (T8 temper) generally leads to an acceleration of aging response, t~,2J the effects of such a treatment on the subsequent precipitation hardening depend upon the particular alloy system. Three types of behavior can be identified as follows: (1) Enhanced hardening is observed in alloys based on the A1-Cu, A1-Cu-Mg, and A1-Cu-Li systems in which each of the primary intermediate strengthening precipit a t e s - 0' (AI2Cu), S' or S (AlzCuMg), and T~ (A1RCuLi), respectively--nucleates with difficulty. In these alloys, the dislocations introduced during plastic deformation provide numerous sites at which heterogeneous nucleation may occur, with the result that precipitates are uniformly dispersed. (2) Response to hardening may be changed little. This applies to A1-Zn-Mg and A1-Mg-Si alloys for which nucleation of the respective intermediate precipitates r/' (MgZn2) and/3' (Mg2Si) already occurs easily at the sites of Guinier-Preston (GP) zones. (3) Hardening may be reduced. This situation occurs in special circumstances, such as with A1-Cu alloys containing trace concentrations of cadmium, indium, or tin.t3~ These elements normally cause enhanced hardening in artificially aged A1-Cu alloys (T6 temper) by stimulating nucleation of a fine dispersion of the phase 0'.t4] However, the effects of these trace elements may be nullified if the alloys are given a T8 temper, possibly because they represent relatively large atoms that are trapped at dislocations generated by the cold-working operation.I3] S.P. RINGER, formerly Research Fellow, Department of Materials Engineering, Monash University, is now Research Associate, Institute for Materials Research, Tohoku University, Sendai 980-77, Japan. B.C. MUDDLE, Reader in Materials Engineering, and I.J. POLMEAR, Emeritus Professor, are with the Department of Materials Engineering, Monash University, Clayton, Victoria, 3168, Australia. Manuscript submitted August 1, 1994. METALLURGICAL AND MATERIALSTRANSACTIONS A
Recent interest has been shown in the effects of small additions of silver (0.1 to 0.2 at. pct) and larger additions of lithium (typically 5 at. pct), individually or in combination, to A1-Cu-Mg alloys having high Cu:Mg ratios. With quaternary A1-Cu-Mg-Ag alloys such as A1-4 pct Cu-0.3 pct Mg-0.4 pct Ag (compositions in wt pct), the presence of silver changes the aging process s o that thin plates of a finely dispersed phase (designated 11) form on the {111}~ planes, ts,6,7] The l~ phase has been described as monoclinic, t8'91 hexagonal, t~~ orthorhombic, t~'121 and most recently, tetragonal, t131 It has a composition similar to that of 0' (AIaCu), with Mg and Ag segregated to the coherent interphase boundaries with the a matrix, m,~4,~5,161 Recent observations of Mg content within f~ plates t~61are not supported by atom probe studies of similar precipitates, t~41 The addition of th
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