The evolution of texture as a function of recrystallization has been characterized for hot-rolled AA1050. Samples prepared from a hot rolled sheet were annealed.
Microstructural evolution during recrystallization in hot rolled Aluminum Alloy 1050 Mohammed H. Alvi1, Sunwook Cheong2, Hasso Weiland2 and Anthony D. Rollett1 1 Materials Science and Engineering Carnegie Mellon University Pittsburgh, PA 2 Alcoa Technical Center, Alcoa Center, PA, 15069, USA
Abstract The evolution of texture as a function of recrystallization has been characterized for hot-rolled AA1050. Samples prepared from a hot rolled sheet were annealed isothermally for sufficient time to allow complete recrystallization. The spatial orientation variation within the deformed microstructure of nucleation, growth and orientations of recrystallized grains is examined. The microstructural variation and texture evolution in the samples is observed by automatic indexing of Electron Back Scatter Diffraction (EBSD) patterns in a Scanning Electron Microscope (SEM). The effect of deformation texture on the evolution and growth of various recrystallization texture components is also analyzed by EBSD. The analysis is aimed at obtaining a correlation between the deformation microstructure, texture development and recrystallization kinetics in the hot-rolled condition.
Introduction The microstructural evolution during annealing of rolled aluminum alloy is an important way of changing properties of the rolled sheets. The important forming properties can be restored after annealing of rolled sheets, allowing for the further reduction in thickness. The microstructural features of deformed and recrystallized regions differ considerably. While the dislocation content of a deformed grain is high thereby giving a large intragranular orientation spread, the recrystallized grains are characterized by low dislocation content having a small intragranular orientation spread. The main aspect analyzed in the present study is the intragranular orientation spread of deformed and recrystallized
grains. The orientation spread is defined as the misorientation angle between all the points in a grain and Grain Orientation Spread (GOS) is the average value of orientation spread in a grain. The difference in GOS value for deformed and recrystallized grains provides an important way of partitioning the deformed and recrystallized regions in partially recrystallized and deformed samples scanned through Orientation Imaging Microscopy (OIM) based on automated Electron Back Scattered Diffraction (EBSD). Different methods have been analyzed previously for obtaining recrystallized fraction from EBSD data, namely Image Quality (IQ) and Confidence Index (CI). (1, 2) The main advantage of using GOS as a criterion is its independence from the user dependent settings affecting the IQ and CI value during the scanning of an area. The IQ and CI values can show a large variation on even on a single sample, whereas a single GOS value can be used for all recrystallized grains on different samples. The regions partitioned in this way can be further analyzed for texture evolution of deformed and recrystallized grains during annealing of deformed samples. Experimental Procedure The chemical composition of hot rolled Aluminum Alloy 1050 (AA1050) used in the present analysis is given in Table 1. Samples of size 20mm × 10mm × 6mm were obtained from the hot rolled sheet and were annealed at 375 o C and 400 o C for time intervals ranging from 30s to 1800s for complete recrystallization of deformed samples. These samples were then polished with SiC carbides papers upto 1200 grade and 1µm alumina suspension to prepare them for
microhardness tests. On an average, 8 to 10 microhardness indentations were obtained on each sample with two samples
1
three scans were obtained from each sample surface. A step size of 1µm is used for deformed samples and 2µm for partially recrystallized and fully recrystallized samples.
Table 1: Chemical composition for AA1050 (mass % element) Element Mass (%) Element Mass (%) Si 0.08 Mg 0.004 Fe 0.31 Zn 0.009 Cu 0.003 Ti 0.008 Mn 0.036 Al 99.54
Recrystallization kinetics from microhardness variation The kinetics of recrystallization were determined from the microhardness values by considering the maximum and minimum values of microhardness, indicating deformed and completely recrystallized samples respectively. The maximum and minimum values for microhardness obtained in the present analysis are around 380MPa and 200MPa respectively. A small variation in microhardness values observed after complete recrystallization can be attributed to local variations within samples and heterogeneity of the microstructure. The fraction recrystallized is obtained from the microhardness value by using the following formula: X =
Where H
max
H max − H i H max − H min
is maximum hardness corresponding to deformed
sample (t = 0), H
min
is minimum hardness corresponding to
1.0 0.9 0.8 Fraction Recrystallied (X)
prepared for each annealing time to obtain the average microhardness value. The samples scanned on a surface containing RD and TD in a previous analysis indicated an elongated microstructure which cannot be characterized in single scan (1). The samples were, therefore, sectioned perpendicular to the rolling direction after microhardness testing to obtain a uniform microstructure. The samples were then electropolished to scan in a Philips FEI XL40 Field Emission Gun scanning electron microscope using EBSD software. A typical scan area of 800 µm × 800 µm was selected for a scan and on an average
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
0
200 400 600 800 1000 1200 1400 1600 1800 2000 Time (s)
Figure 1 Recrystallization kinetics obtained from microhardness data. The microhardness variation shown in Fig. 1 includes the contributions to the overall decrease in microhardness from recovery as well as recrystallization processes. The determination of recrystallization kinetics, therefore, involves removal of the recovery contribution from the total decrease in microhardness. The recovery contribution in the present study was analyzed in an earlier study of recrystallization kinetics for AA1050 (1), however, and found to be negligible. The kinetics of recrystallization can be represented in a mathematical form by using the JMAK relationship. The variation of fraction recrystallized with annealing time in JMAK relationship is given as X = 1 − exp[ −(kt ) n ]
Here n and k are the JMAK exponent and temperature dependent constant, respectively. This equation can be rearranged to a linear relationship by using a logarithmic expression. 1 ln[ln ] = n ln(t ) + n ln(k ) 1− X
The slope of this linear expression will yield the exponent n and the parameter k can be obtained from the ordinate as shown in Fig. 2.
fully recrystallized sample and Hi is microhardness after a given annealing time. Fig 1 shows the variation of microhardness and fraction recrystallized obtained from microhardness values for samples annealed at 375 o C .
2
2.5
Recrystallization kinetics obtained from GOS
2.0
Measurements of intra-granular misorientation in metals and alloys have revealed the presence of variations in orientations in deformed as well as recrystallized samples (3). The orientation spread in a metallic sample corresponds directly to the level of deformation and dislocation content in that sample. A high value of orientation spread indicates a high geometrically necessary dislocation (GND) content and more deformation in the sample whereas the recrystallized samples are characterized by low dislocation content and a corresponding lower value of grain orientation spread. A suitable value for grain orientation spread is required to characterize the deformed as well as the recrystallized regions in a sample. This value can obtain by analyzing fully recrystallized samples and observing the variation of the fraction of points with grain orientation spread. A plot showing the variation of fraction of points with GOS value for eight different samples is presented in the Fig 4. A GOS value for distinguishing a recrystallized grain from an unrecrystallized one is obtained from this plot as the one which corresponds to the inclusion of 95% points in a fully recrystallized sample. This criterion yields the threshold value of GOS as 3° for the present analysis. Grains with GOS>3° are considered unrecrystallized whereas grains with GOS
