Metal matrix composites(MMCs) are being considered for structural materials where potential weight savings can be realized by using light-weight matrices strengthened by ceramic phases. TiBp reinforced composites based on titanium alloys are particularly attractive because of the very high specific modulus and specific strength of both the matrix and the reinforcing phase. Since most MMCs are exposed to engineering service, their structural integrity is often limited by their mechanical performance under creep, fatigue and creep-fatigue interaction at high temperature. Thus in order to ensure proper design and safe use of the metal matrix composites, the high temperature mechanical properties and their effects incorporated in life-prediction procedures must be comprehensively characterized. While several studies have been conducted to both understand and document the processing method of particulate reinforced Ti - base composite, there is little information in the published literature on the aspects relating to high temperature mechanical properties such as creep, fatigue and creep-fatigue interaction behaviors. As such, the need to examine the creep behavior of this composite has prompted the current research. In this study, the effect of TiBp particle reinforcement on the creep deformation and fracture behavior of Ti-6Al-2Sn-4Zr-2Mo (Ti6242) titanium alloy made by blended elemental powder metallurgy is investigated at the test conditions over the temperature range from 525 to 675℃ under the constant stress range from 100 to 300MPa in air. Creep deformation behavior is examined based on the activation energy and stress exponent analysis. In addition, the results of microstructural observations with optical microscope and SEM in the conditions of creep fracture are presented. All of the creep curves obtained at the test conditions conducted in this research show the normal primary creep and well defined steady state creep region. At the higher temperature and lower stress condition of 650℃/150MPa, for all stresses investigated, steady state creep follows a power law stress dependence with the stress exponent, n, equal to 4.0 and 4.9. for Ti6242 alloy and Ti6242/10vol%TiBp composite, respectively. The steady state creep rates of Ti6242 alloy and Ti6242/10vol%TiBp composite are not so different and the activation energies for creep deformation of both materials have the value of about 320±20kJ/mol. At lower temperature and high stress level of 550℃/300MPa, the values of the stress exponent are measured to be equal to 6.5 and 5.0 for Ti6242 alloy and Ti6242/10vol%TiBp composite, respectively. Measured stress exponents are in good agreement with the values obtained by other researchers[3-7] in Ti alloy base metal matix composites made by blend elemental powder metallurgy. The apparent activation energies for creep deformation during secondary stage is founded to be 360±20kJ/mol for Ti6242/10vol%TiBp composite and 320±20kJ/mol for Ti6242 alloy, respectively.
The measured activation energy and stress exponent values of Ti alloy base composite are quite different from those values obtained from other studies on the creep deformation behavior of those high temperature application material systems such as dispersion strengthening alloy, precipitate strengthening alloy materials and Al alloy base particle reinforced metal matrix composites. From the experimental results, TiBp particle reinforced Ti base metal matrix composite made by blend elemental powder metallurgy has a superior creep resistance than Ti6242 matrix alloy. It seems that TiBp particles significantly increase creep deformation resistance and also give strong retardation effect on the dislocation movement at these test conditions. The retardation effect of TiBp particles on the dislocation movement is slightly reduced up to the higher temperature and lower stress condition.
Since TiBp particles act as an obstacle for dislocation gliding during creep, dislocation mean path might be limited by the TiBp particle spacing. The measured high apparent activation energy for creep at lower temperature high stress test condition in Ti6242/10vol%TiBp composite is responsible for TiBp reinforcement blocking effect. While creep deformation, gliding dislocation might be piled up to TiBp reinforcement and dislocation should break the particles to move forward. The energy for breaking the particles seems to be the origin of the measured high activation energy for creep of Ti6242 /10vol%TiBp composite. While grain boundary cavity formation and coalescence seems to be responsible for creep fracture mechanism for Ti6242 alloy, homogeneously dispersed TiBp particle reinforcement fracture is responsible for Ti6242/10vol%TiBp composite.
The rate controlling process of the steady state creep rate might be identical for the both alloy systems. The addition of TiBp particles gives a significant enhancement effect on the creep resistance in the primary and secondary creep regime in addition to the creep life by load transfer and microstructural strengthening effect.
This evaluation can be helpful to qualify these alloys for the use in light weight application components such as exguast valve in automobile industry and to maximize the operating time with good safety and performance of these alloys under operating condition. These results can be also bases of developing new titanium base composite which have good mechanical properties at high temperature. The knowhow of evaluating and developing new composite used for automobile industry is important because the interest is taken in automobile industry.
