Stoichiometric and Off-Stoichiometric Alloying in Silicide Compound Ti 5 Si 3 By Nb or Cr Additions
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ABSTRACT Titanium silicide Ti5 Si 3 whose melting temperature is 2130'C bears the potential for very high temperature application. This paper reports our results on the alloying behaviour of Nb or Cr addition to this compound. Total energy calculation shows that the substitution of Ti by Nb or Cr atoms in Ti5 Si 3 crystal stiffens the bonding between the atoms. In experiment, two means of alloying are considered : stoichiometric and off-stoichiometric alloying. Stoichiometric alloying in Ti5 Si 3 results in compounds consisting of single Ti5 Si 3 phase while off-stoichiometric alloying yields hypereutectic microstructure with Ti5 Si 3 being the primary phase. The Ti5 Si 3 phase in both cases dissolves certain amount of Nb or Cr alloying element and its composition agrees with the stoichiometric composition of (Ti,Nb)5 Si 3 or (Ti,Cr)5 Si 3. The moduli of the stoichiometric alloys increase with the increase of alloying element addition, indicating an enhancement in Ti5 Si 3 crystal.
INTRODUCTION A need for high performance materials at temperatures up to 16000 C leads to the interest in the study of silicon compounds. Compared with the aluminide materials, silicides have the advantages of higher melting points, lighter weight and better oxidation resistance, etc. Among them, Ti5 Si 3 is expected to be a potential candidate for very-high-temperature structural application due to its high melting temperature (2130 0 C) and low density (4.32 g/cm 3 ). G. Frommeyer is one of the first researchers who carried out investigations to evaluate the basic properties of Ti5 Si 3 intermetallic compound [1]. In the years followed, a few studies on the physical and mechanical properties of Ti5 Si 3 were reported by other researchers [2-5]. Still, there is much left for us to know about Ti5 Si 3. For instance, the alloying behaviour of Ti5 Si 3 remains unreported, since most of the previous works dealt with the binary Ti5 Si 3. Alloying addition to Ti5 Si 3 is a possible way to improve the properties of this compound. In this paper, the alloying behaviour of Ti5 Si 3 with Nb and Cr additions was carefully investigated through two ways of alloy design in order to gain a further understanding in this compound.
THEORETICAL CALCULATION Theoretical calculation of the alloying effect in Ti5 Si 3 compound is based on the ideal cleavage model in which the crystal cell is divided into two halves along a selected crystallographic plane (hkl), i.e. the cleavage plane. By increasing the inter-planar spacing between the two cleavage planes, total energy is calculated as a function of the inter-planar spacing. Thus cleavage energy is expressed as : Ecleavage = Eideal - E_ (1) where Eideal is the total energy of the complete crystal and E_ is the total energy of the two separated halves when the spacing between the cleavage planes is infinite. Ti5 Si 3 crystal belongs to complex hexagonal structure (D8.), whose stacking density is estimated to be around 71%-76%. The interstitual between the atoms is around 0.16 nm thus it does not KK5.24.1 Mat.
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