Effect of Ductile Phase Reinforcement Morphology on Toughening of MoSi 2
- PDF / 2,822,624 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 109 Downloads / 243 Views
EFFECT OF DUCTILE PHASE REINFORCEMENT MORPHOLOGY ON TOUGHENING OF MoSi 2 D.E. Alman and N.S. Stoloff Materials Engineering Department, Rensselaer Polytechnic Institute, Troy NY 12180
ABSTRACT Niobium was added to MoSi 2 in the form of particles, random short fibers and continuous aligned fibers. It was found that the morphology of Nb played a role in the toughening that occurred (as measured by the area under load displacement curves from room temperature three point bend tests and the examination of fracture surfaces). The Nb particles did not toughen MoSi 2 . The random short fibers appeared to toughen MoSi2 via crack deflection along the fiber matrix interface. Aligned fibers imparted the greatest toughness improvements, as toughening resulted from fiber deformation. However, larger diameter fibers displayed a greater ability to toughen MoSi 2 than smaller diameter fibers. This was attributed to the constraint resulting from the interfacial layer between the MoSi2 matrix and the Nb fiber. Maximum toughness occurs when the fiber is able to separate from the matrix and freely deform. INTRODUCTION The attractive combination of low density (6.25 g/cm 3 ), high melting temperature (20301C) and excellent resistance to oxidation (up to 1700 0 C) makes the compound MoSi2 very attractive as a potential elevated temperature structural material. Recent research employing MoSi 2 as a matrix material for composites has resulted in improving the inadequate creep resistance and poor low temperature fracture toughness of monolithic MoSi2. Petrovic and co-workers [1] have reinforced MoSi 2 with SiC whiskers. The SiC whiskers markedly improved the mechanicaý behavior of MoSi2 at elevated temperatures. More importantly, the room temperature fracture resistance of MoSi2 has been improved by the addition of Nb fibers, first reported by Fitzer and Remmele [2] in 1985. Room temperature fracture toughness for MoSi122 with Nb fibers has been reported to be 12 MPa-m 1/2, compared to 3.3 MPaom / for monolithic MoSi2 [3]. These successes have generated enormous research activity on MoSi 2 and other silicides over the past few years, as evidenced by a recent workshop devoted solely to structural silicides [4]. Other researchers have published results on MoSi2/Nb foil composites tested in tension in situ in the scanning electron microscope [3,5]. In these studies, the Nb foils would deform during testing. However, when powder-consolidated composites with Nb filaments [3] or particles [5] were tested in bending at room temperature, the Nb reinforcement showed limited ductility. The explanation for the behavior of Nb was a combination of both embrittlement of the Nb by oxygen and the stress state around the Nb reinforcement [5]. The goal of the present effort was to systematically study the role of Nb reinforcement morphology on the mechanical behavior of MoSi 2 /Nb composites. Thus, MoSi 2 was fabricated reinforced with Nb particles, random Nb fibers, and aligned Nb fibers with two different diameters. EXPERIMENTAL PROCEDURE The characterist
Data Loading...
