The incorporation of self-propagating, high-temperature synthesis-formed Fe-TiB 2 into ferrous melts
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ALUMINIUM-BASED metal-matrix composites (MMCs) have received considerable interest due to their attractive combination of improved mechanical properties and weight-saving potential. Recently, however, a limited number of iron- and steel-based systems have emerged, which are low cost, versatile, and exhibit relatively good mechanical properties. A majority of these composite materials focus on using TiC as the reinforcing particulate phase, primarily due to its capability of greatly improving the wear resistance of the material.[1,2,3] Methods of fabricating these by a liquid route include in-situ reactions between Ti powder and C-rich liquid Fe[4] or C powder and liquid Fe-Ti alloys,[5] direct dispersion of TiC powder in liquid steel,[6] and carbothermic reduction processes.[7] Another liquid processing route employed, which is of particular relevance to the present study, is the addition of Fe-TiC and Fe-(W,TiC) master alloy powders to molten iron and steel matrices.[8,9] These master alloys are prepared by a self-propagating, high-temperature synthesis (SHS) reaction and are reported to be impurity-free and readily wetted by the liquid ferrous matrix, resulting in high yields and good dispersions of TiC. Although steel has a high Youngs modulus, its specific modulus (Youngs modulus divided by density) is comparable to that of lightweight alloys of aluminum. Improvements in the Youngs and specific modulus of steels would, therefore, be desirable in applications where component weight, as well as stiffness, was important. TiB2 is considered the best reinforcement for achieving this, since it exhibits an extremely high specific modulus of 120 GPa/ C.C. DEGNAN, Research Fellow, and P.H. SHIPWAY, Senior Lecturer, are with the Advanced Materials Group, School of Mechanical, Materials, Manufacturing Engineering and Management, University of Nottingham, NG7 2RD, United Kingdom. Contact e-mail: [email protected]. ac.uk or [email protected] Manuscript submitted December 10, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
Mg.m⫺3.[10] Titanium diboride is also well known for its high hardness and outstanding tribological properties. Furthermore, it has a high thermal conductivity (⬃110 W m⫺1 K⫺1 at 25 ⬚C) and a significantly lower coefficient of thermal expansion than steel (⬃7.2 ⫻ 10⫺6 K⫺1 for TiB2 and ⬃13 ⫻ 10⫺6 K⫺1 for steel). Thus, steel-matrix composites, which incorporate TiB2 as the reinforcing phase, have increased stiffness, hardness, and wear resistance, along with reduced coefficients of thermal expansion and only a moderate decrease in thermal conductivity properties. Unlike most other ceramic reinforcements, which are reactive in molten iron, TiB2 is stable in liquid Fe;[11,12,13] however, if C (and/or oxygen) is present, as in the case of steels, then the formation of iron boride (tetragonal Fe2B) can also occur. Controversy exists regarding the mechanism by which this phase originates. Sigl and Schwetz[12] and Ju¨ngling et al.[13] propose that C reacts with TiB2 and Fe, which resul
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