Materials Science and Engineering A261 (1999) 261 – 269 Residual thermal stresses in MoSi 2 –Mo 5 Si 3 in-situ composites P. Peralta a, *, R. Dickerson a , J.R. Michael b , K.J. McClellan a , F. Chu a , T.E. Mitchell a a Los Alamos National Laboratory, Mail Stop K765, Los Alamos, NM 87545, USA b Sandia National Laboratory, Mail Stop 1405, Albuquerque, NM 87185, USA Abstract Residual thermal stresses in MoSi 2 –Mo 5 Si 3 in-situ composites are calculated for a dilute concentration of particles of one phase embedded in a matrix of the other, using the fields of anisotropic ellipsoidal inclusions. Additionally, the eutectic interfaces are modeled as boundaries between two anisotropic half-spaces. The misorientation between MoSi 2 –Mo 5 Si 3 is obtained from the literature for Mo 5 Si 3 precipitates in MoSi 2 and by electron diffraction in the scanning electron microscope (SEM) for the opposite case. Tensile stresses of up to 3 GPa can develop after cooling from the eutectic temperature due to the thermal expansion mismatch between the phases. Electron microscopy of arc-melted Si-rich Mo 5 Si 3 shows that stresses are relieved by intergranular fracture in Mo 5 Si 3 and either dislocation plasticity or transgranular cracks in MoSi 2 , in a manner consistent with the calculations. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Residual thermal stress; Eutectic interfaces; Dislocation plasticity; Molybdenum disilicide; In-situ composites 1. Introduction Molybdenum disilicide (MoSi 2 ) is a candidate mate- rial for structural applications at high temperatures [1]. However, its creep resistance is a major issue due to metal-like plasticity present above the brittle – ductile transition temperature (BDTT) [1]. Mo 5 Si 3 has been considered a good reinforcement to improve the creep properties of MoSi 2 , due to its thermodynamic stability, good creep strength at high temperatures and eutectic reaction with MoSi 2 , from which in-situ composites can be obtained [2,3]. Mason and coworkers [2,3] have shown that directionally solidified MoSi 2 –Mo 5 Si 3 eu- tectics have better creep properties than other MoSi 2 - based composites. However, they reported the presence of cracks in their samples after processing them from the melt. These cracks are the likely result of residual stresses developed during cooling due to thermal expan- sion mismatch. Unusual cracking in MoSi 2 single crys- tals has been attributed to Mo 5 Si 3 particles [4], and intergranular cracking in polycrystalline Mo 5 Si 3 has been connected to thermal stresses at grain boundaries [5]. Therefore, residual thermal stresses are calculated for a dilute concentration of particles of Mo 5 Si 3 in a MoSi 2 matrix and vice versa, modeling the particles as anisotropic ellipsoidal inclusions. Additionally, eutectic interfaces are modeled as boundaries between two an- isotropic half-spaces. Moreover, a Si-rich Mo 5 Si 3 alloy was prepared to determine the microstructure and the orientation relationship (OR) of MoSi 2 particles em- bedded in Mo 5 Si 3 . Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) are used to study the failure modes due to the stresses. 2. Modeling 2.1. Anisotropic ellipsoidal inclusions and eutectic microstructure The elastic fields of anisotropic ellipsoidal inclusions were obtained by Asaro and Bamett [6], who showed that there is a linear relationship between a stress free transformation strain  ij t and the final strain  ij c in the inclusion, i.e.:  ij c =K ijkl C klmn  mn t (1) * Corresponding author. 0921-5093/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S09 21- 5093(98)0 1 07 4- 0