Static and In-Situ HRTEM Investigations of the Atomic Structure and Dynamics of Massive Transformation Interfaces in a TiAl Alloy
James M. Howe, William T. Reynolds, Jr.# and Vijay K. Vasudevan*
Department of Materials Science & Engineering, University of Virginia
Charlottesville, VA 22904-4745, USA
#Department of Materials Science & Engineering, Virginia Polytechnic Institute
& State University, Blacksburg, VA 24061, USA
*Department of Materials Science & Engineering, University of Cincinnati
Cincinnati, OH 45221-0012, USA
Determining the atomic structure, mechanisms and dynamics of interface motion between two phases with high-index or irrational orientation relationships (ORs) is of central importance for understanding the mechanisms of the massive transformation in solids, as well as for obtaining a fundamental understanding of the motion of interphase boundaries with limited or no coherency . Obtaining such information is challenging, because the lack of a relatively low-index rational OR between the matrix and massive transformation product makes high-resolution transmission electron microscope (HRTEM) imaging and simulation of the interfaces difficult. Such HRTEM experiments are often used to determine the atomic structure and dynamics of interphase boundaries between coherent and partly coherent phases in other phase transformations .
In this investigation, static and in-situ high-resolution transmission electron microscopy (HRTEM) and three-dimensional near-coincident site (NCS) atom modeling were used to determine the atomic structure, growth mechanisms and dynamics of massive transformation interfaces in a TiAl alloy. Results from these experiments showed the following :
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2. J. M. Howe, Mater. Trans. JIM, 39, 3 (1998).
3. J. M. Howe, W. T. Reynolds, Jr. and V. K. Vasudevan, Metall. Mater. Trans., 33A, (2002).
Figure 1. Pairs of HRTEM images of an a2/gM interface, each taken in a slightly different imaging orientations: (a) shows the a2/gM interface oriented edge-on with both crystals diffracting, but tilted slightly off the exact a2 and gM zone axes, (b) shows the same interface with the gM phase on a gM zone axis, and (c) shows the interface with the a2 grain oriented exactly on the a2 zone axis. White boxes in the low-magnification images show where the higher-magnification images were obtained.
Figure 2. Digitally processed images from Figures 2(b) and (c), showing the roughness of the a2 and gM phases across the (-241)gM interface and the nearly one-to-one correspondence between the (0-21)a2 and (001)gM planes across the interface, leading to one-dimensional commensurability.
Figure 3. (a) Pattern of NCSs obtained from the experimentally determined OR for the a2 and gM phases given in Figure 2 using a calculation block that is 8 x 10 x 4 nm along the x, y and z directions, respectively, with the viewing direction (x) oriented along the [1.00, -4.65, 10.91] direction in the gM phase. (b) The NCSs contained between the two parallel lines (spaced 0.4 nm apart), viewed along the z-axis over an expanded area of interface 13 x 13 nm wide. (c) The same NCS positions as in Figure 3(b), but with atoms in the gM phase also plotted as small dots. (d) The same NCSs in Figures 3(b) and (c) viewed from the edge of the simulation block along the x axis.
Figure 4. (a-f) A series of in-situ HRTEM images taken 10 s apart along a bulge in an a2/gM interface, where the interface is nearly parallel to a (2-20)gM plane and edge-on in the foil. The images were obtained after the specimen had been at 575oC for about 5400 s (1.5 hr). The OR between the a2 and gM phases is approximately: a2||gM within 1.4o; (-223)a2||(11-1)gM.
Figure 5. (a-d) A series of in-situ HRTEM images taken 2 s apart, of a second area of the (2-20)gM interface in Figure 4 observed about 10,800 s (3 hr) later in the experiment, after the temperature was raised to approximately 650oC. The OR is still: a2||gM within 1.4o; (-223)a2||(11-1)gM, but the IP is now inclined through the foil thickness.