Calorimetric Studies of Al-Cu Alloys: Quench Sensitivity and Sample Preparation
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y machining the 32 mm dia. discs or by punching the 1 mm thick sheet; both operations were performed after aging. These procedures allowed us to study separately the effects of the two variables considered in this work, namely, sample preparation and quenching conditions. Aging at 100 ~ was carried out in a silicone oil bath, and an air furnace was used to homogenize at 525 ~ the temperature being controlled within ---1 ~ and ---5 ~ respectively. Quench rates were measured by introducing very thin (0.2 mm diameter) Chromel-Alumel thermocouples into the samples, and recording the data by means of a Hewlett-Packard Data Acquisition System, model 3497A. In machining and punching the DSC samples, uncontrolled effects, such as heating or deformation, were kept as low as possible. Machining was performed in a lathe at 575 rpm. A pure hydrocarbon (C14) was used as coolantlubricant. Punching was carried out in a press designed and assembled in the laboratory; it was of the drop-through die type,12 and was operated manually. The bottom of the punch was flat and finely ground. Two different cutting steels (55 HRC and 46 HRC) were used for the die and punch, respectively. A clearance of 0.02 mm was allowed. ~2 The samples were not ground after punching. The DSC measurements were performed using a Perkin Elmer DSC-2C apparatus controlled through a minicomputer. High purity aluminum was used as reference. The runs were carried out at a heating rate of 20 ~ per minute from 25 ~ to 580 ~ All experiments were carried out under a dynamic nitrogen atmosphere (1 1 per hour). The discussion of the results of the present work should start with a consideration of the quench sensitivity of the alloy (samples a, b, c, and d of Table I and Figure 1). The main change noticed in the DSC curves in going from lower to higher quench rates is the shift of the exothermic peak (formation of the 0' phase 8'11) to lower temperatures. The maximum shift found in this work (27 ~ is rather large, as a consequence of the wide range of quench rates (over three orders of magnitude) explored. This shift might be understood in terms of residual stresses and the associated dislocation density which increases with cooling rate. Therefore, as dislocations are known to accelerate the 0' phase formation, 8'11'~3the related peak occurs at lower temperatures for the higher cooling rate (Table I). On the other hand, very small differences are noticed in the first endothermic peak, implying that phase formation during aging at 100 ~ (GP zones) does not noticeably depend on quench rate. This result seems to be in conflict with the high quench sensitivity found by Vigier et a1.9 in a pure A1-4 wt pct Cu.
Table I. Peak Temperatures Tp (~ for the Exothermic Reaction (Formation of the 0' Phase) for Samples of Alloy AA2011 Aged at 100 ~ for 24 Hours Tp Q.R.
a 355 5000
b 363 1000
c 378 250
d 382 2
e 330 1500
f 333 700
Data are given for six different samples, namely, a, b, c, and d: DSC samples either directly quenched, from 525 ~ in water at 25 ~ or inside a tube wit
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