The effect of hydrostatic pressurization on the microhardness and compressive behavior of the porous Mn-Modified Ll 2 ti
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INTRODUCTION
TERNARY titanium trialuminides (Al3Ti 1 X) modified with X 5 Cu, Ni, Fe, Mn, or Cr exhibit an ordered face-centered cubic (fcc) crystallographic structure (Ll2). It has a required number of five independent slip systems for a homogeneous plastic deformation of a polycrystal according to the von Mises criterion.[1] Unfortunately, despite the cubic Ll2 structure, the tensile ductility at room temperature is still near zero (0.2 pct).[2,3] Some authors reported significant ductilities of the Ll2 titanium trialuminides in compression.[4,5,6] However, such a statement may be grossly misleading. It is shown that the compressive deformation of the Ll2 titanium trialuminides is accompanied by a continuous development of internal microcracking.[4,7,8] Therefore, a certain portion of so-called ‘‘compressive ductility’’ (apparent) in these intermetallics can be a result of the internal microcracking (cataclastic deformation[9]). The causes of the brittle (or quasibrittle) nature of the Ll2 titanium trialuminides have not been ascertained yet. In particular, the development of a cataclastic deformation is of importance and its existence has recently been confirmed by compressive testing of the Ll2 titanium trialuminides under a superimposed hydrostatic pressure.[9] However, since most of the Ll2 titanium trialuminides develop porosity during casting and/or Kirkendall porosity during subsequent homogenization,[7,10,11] it is important to understand the reZ. WITCZAK, Senior Scientist, is with the High Pressure Research Center, Polish Academy of Sciences, 01-142 Warsaw, Poland. R.A. VARIN, Professor of Materials Science and Engineering, is with the Department of Mechanical Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1. Manuscript submitted April 23, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
lationship between porosity and mechanical behavior of these materials, which may shed some light on the causes of their brittleness. A valuable tool in such studies is mechanical testing under a superimposed hydrostatic pressure or testing after the hydrostatic pressurization of a material. Under the application of a sufficient hydrostatic pressure, pores (voids) behave as the elastic discontinuities at which the shear stresses are developed (in the mechanical sense, a pore can be approximated as a second phase of an infinite compressibility), and as a result, dislocations can be generated within the material.[12] In the macroscale, these microscopic phenomena lead to an increase in the density of a material (due to partial closing of pores) and simultaneously to an increase in the work hardening of the matrix. The resulting mechanical properties of such a material densified by hydrostatic pressurization are due to two factors: (a) reduction in the volume fraction of pores resulting from the reduction in their average size and (b) the extent of the work hardening of the matrix. The aim of this work is to examine the effect of hydrostatic pressurization on the compressive behavior of the Mn-modifie
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