A comparison of the reciprocating sliding wear behaviour of steel based metal matrix composites processed from self-propagating high-temperature synthesised Fe-TiC and Fe-TiB2 masteralloys

TiB2 and TiC stainless steel matrix composites

An application of thermodynamic calculations to the phase equilibria in TiB2-reinforced high modulus steel (HMS) is described. Titanium diboride (TiB2) has been verified as the best reinforcement for improving isotropic Young’s modulus of steels. The TiB2 particles can be synthesized in-situ in a ferrite matrix using a combination of ferrotitanium and ferroboron powders. The calculations reveal that impurities, such as oxygen, could promote the iron boride (Fe2B) formation in a Fe-TiB2 pseudobinary system, especially for the HMS processed through this powder metallurgical technique. Modification of the steel matrix of HMS by an addition of ferrite formers is encouraged based on the calculated (Fe-TiB2)-X diagrams that show no significant change in the stability of TiB2. In the development of HMS with an Fe-Cr matrix, the phase constitution was predicted successfully, and the highest Young’s modulus is reached with a proper control of titanium content.

Phase equilibria in TiB2-Reinforced high modulus steel

The degree of reaction and dispersion achieved when TiB2 powders are melted in contact with liquid iron based alloys has been assessed via a levitation dispersion test which had been developed earlier. Both Fe2B and TiC were observed to form as a result of dissolution and reaction of TiB2. The formation of TiC occurs during the reaction of commercial grade TiB2 with liquid iron alloys containing as little as 0·08 wt-%C. The reaction of high purity TiB2 with liquid iron alloys containing 0·24 wt-%C does not however lead to TiC formation. The formation of Fe2B was observed for all conditions tested, owing to the effectively zero solubility of boron in solid iron. The TiB2 remaining after dissolution and reaction was found to produce relatively good dispersions in the iron matrix and therefore additions of TiB2 to liquid iron alloys may provide a means of producing Fe–TiB2 composite materials. However, the brittle properties of Fe2B will mean that, whereas such materials may be very hard, they are li...

Dispersion and reaction of TiC in liquid iron alloys

Steel-matrix particulate composites were processed by direct addition of an Fe-TiB2 master alloy powder to a BS970:080M30 medium-carbon steel. This powder was produced using a self-propagating, high-temperature synthesis (SHS) reaction and consisted of a dispersion of fine TiB2 particles (2 to 5 µm), respectively, in an iron binder. The addition of the Fe-TiB2 powder resulted in the formation a parasitic Fe2B phase and TiC within the steel microstructure. In response to this, an SHS master alloy composed of Fe-(50 pct TiB2+50 pct Ti) was manufactured, which, when added to the steel, prevented the formation of Fe2B and resulted in a composite containing a mixture of TiB2 and TiC particles. The effect of master alloy composition and addition level on the microstructural phases generated is discussed in detail. The response to heat treatment of composite materials manufactured in this way was also investigated. It was found that an isothermal hold at 840 °C leads to a substantial softening of the material processed using the Fe-TiB2 additive, while at 1000 °C, a hardness level equivalent to that of the as-cast material was maintained. The same heat treatment of samples in which the formation of Fe2B was suppressed resulted in no appreciable difference in hardness level or microstructure.

The incorporation of self-propagating, high-temperature synthesis-formed Fe-TiB2 into ferrous melts

Adhesion of iron-based alloys (Fe-Ni, Fe-Cr within the concentration range of 2 to 30 wt.% Ni or Cr) to titanium diboride has been studied using the sessile drop technique. The energy parameters of wetting, work of adhesion and interfacial surface tension have been calculated. The structure and phase compositions of the contact area of interaction have been investigated by metallographic methods and energy dispersive X-ray microanalysis. The given iron-based alloys have been found to be promising as binding metals for composite materials based on titanium diboride.

A.P. Umansky

Papers

Physicochemical principles of the formation of composite materials based on titanium diboride