Diffraction Study on the Thermal Stability of Ti3SiC2/TiC/TiSi2 Composites in Vacuum

Titanium silicon carbide (Ti3SiC2) possesses a unique combination of properties of both metals and ceramics, for it is thermally shock resistant, thermally and electrically conductive, damage tolerant, lightweight, highly oxidation resistant, elastically stiff, and mechanically machinable. In this paper, the effect of high vacuum annealing on the phase stability and phase transitions of Ti3SiC2/TiC/TiSi2 composites at up to 1550° C was studied using in-situ neutron diffraction. The role of TiC and TiSi2 on the thermal stability of Ti3SiC2 during vacuum annealing is discussed. TiC reacts with TiSi2 between 1400–1450°C to form Ti3SiC2. Above 1400° C, decomposition of Ti3SiC2 into TiC commenced and the rate increased with increased temperature and dwell time. Furthermore, the activation energy for the formation and decomposition of Ti3SiC2 was determined

In situ neutron diffraction study on the effect of aluminium fluoride on phase transformation of mullite from alumina/clay

The effect of aluminium fluoride (AIF3) on the phase transformation sequence of mullite (3AI2032Si02) from two different types of kaolin (kaolinite and halloysite) (AI2Si20s(OH)4-2H20) within an alumina (A1203) matrix for a temperature range of 20 - 1500 C was investigated using in situ neutron diffraction. Samples containing a mixture of A1F3 (0 - 5 wt%), AI203 and kaolin were heated up to 1500 C and then furnace cooled. During the heating procedure, one hour neutron diffraction scans were conducted at 600, 900, 1100, 1200, 1300 and 1400 C, followed by six consecutive one hour scans at 1500 C and finally a one hour scan at room temperature upon cooling. The diffraction patterns collected between 1100 and 1500 C were analyzed by Rietveld analysis. The observed phase transformations exhibited a typical sequence found inclay/alumina ceramics. Corundum, mullite and cristobalite were observed. A common feature among the specimens containing different amounts of AIF3 and kaolin was that the content of corundum decreased as the amount of mullite increased, whilst the cristobalite content tended to peak near the temperature where the amounts of corundum and mullite were approximately equal. The mullitization temperature was reduced as the AIF3 content increased for both kaolinite and halloysite. The presence of AIF3 appeared to reduce the onset temperature for mullite nucleation, which is at a much lower temperature compared to that of grain growth. However, AIF3 also seemed to lower densification. Likewise mechanical properties of the resulting specimens were determined

Indentation Responses of Functionally-Graded Al2TiO5-Based Ceramics

Aluminium titanate, Al2TiO5 (AT) with the pseudobrookite structure is the only compound in the alumina-titania system. It is an excellent refractory and thermal shock resistant material due to its relatively low thermal expansion coefficient (1 ×10-6 ºC –1) and high melting point (1860ºC). However, its low mechanical strength, hardness and fracture resistance together with susceptibility to decomposition in the temperature range 900–1200ºC has limited its wider application. In this paper, the innovative tailored design of functionally- graded Al2TiO5 – based ceramics system was presented. This involves the use of a vacuum heat-treatment or die-pressing to form hard graded layers of alumina on Al2TiO5. These hard outer layers will provide hardness and wear resistance to protect the softer but damage resistant underlayers. The results will also explore unresolved issues concerning the effect of graded interfaces on their physical and mechanical performance properties

Kinetics of Decomposition in MAX Phases at Elevated Temperature

The susceptibility of MAX phases to thermal dissociation at 1300-1800 °C in high vacuum has been studied using in-situ neutron diffraction. Above 1400 °C, MAX phases decomposed to binary carbide (e.g. TiCx) or binary nitride (e.g. TiNx), primarily through the sublimation of A-elements such as Al or Si, which results in a porous surface layer of MXx being formed. Positive activation energies were determined for the decomposition of MAX phases except for Ti3AlC2 where negative activation energy of 71.9 kJ mol-1 was obtained due to formation of fine pores on TiCx. The kinetics of isothermal phase decomposition at 1550 °C was modelled using a modified Avrami equation. An Avrami exponent (n)of < 1.0 was determined, indicative of the highly restricted diffusion of Al or Si between the channels ofM6X octohedra. The characteristics of thermal stability and phase transition are discussed

Kinetics of Phase Decomposition in MAX Phases - A Comparative Diffraction Study

The susceptibility of MAX phases to thermal dissociation at 1300-1550 °C in high vacuum has been studied using in-situ neutron diffraction. Above 1400 °C, MAX phases decomposed to binary carbide (e.g. TiCx) or binary nitride (e.g. TiNx), primarily through the sublimation of A-elements such as Al or Si, which results in a porous surface layer of MXx being formed. Positive activation energies were determined for the decomposition of MAX phases except for Ti3AlC2 where negative activation energy of 71.9 kJ mol-1 was obtained due to formation of fine pores on TiCx. The kinetics of isothermal phase decomposition at 1550 °C was modelled using a modified Avrami equation. An Avrami exponent (n) of < 1.0 was determined, indicative of the highly restricted diffusion of Al or Si between the channels of M6X octahedra. The characteristics of thermal stability and phase transition are discussed

Diffraction Study of Self-Recovery in Decomposed Al2TiO5 During Vacuum Annealing

The ability of decomposed Al2TiO5 to undergo self-recovery or reformation during vacuum annealing was characterised by in-situ neutron diffraction. It is shown that the process of phase decomposition in Al2TiO5 was reversible and that reformation occurred readily when decomposed Al2TiO5 was re-heated above 1300°C. The kinetics of isothermal and temperature-dependent self-recovery was modelled using the Avrami equation. The influence of grain-size on the Avramic kinetics of self-recovery was also evident

Mapping of Phase Compositions and Air-Oxidized Titanium Silicon Carbide (Ti3SiC2)

Ternary carbides such as Ti3AlC2 and Ti3SiC2 are nano-layered ceramics with the general formula Mn+1AXn (n=1-3), where M is an early transition metal, A is a group A element, and X is either carbon and/or nitrogen. These ceramics exhibit a unique combination of mechanical, electrical, thermal and physical properties such as good high-temperature strength, and excellent corrosion and damage resistance. For instance, the electrical and thermal conductivities of Ti3SiC2 are greater than that of titanium and its machinability is similar to graphite. However, these ceramics are susceptible to thermal dissociation at ~1400°C in inert environments (e.g., vacuum or argon) to form TiC and Ti5Si3C. The chemistry and kinetics of the dissociation processes involved are not yet fully understood. Surprisingly, the study of thermal stability in ternary carbides has received relatively little attention despite its importance in applications such as heating elements or the feasibility of designing functionally-graded Ti3SiC2-TiC with unique wear resistance and damage tolerance

Physical, Flammability and Mechanical Properties of Polymer Eco-Nanocomposites

When compared to their synthetic counterparts, natural fibres represent an environmentally friendly alternative by virtue of several attractive attributes that include lower density, lower cost, non-toxicity, ease of processing, renewability and recyclability. In addition, the use of natural fibres in polymer matrix composites has the potential to produce materials with higher specific strength and specific modulus due to their low density. In this chapter, epoxy and vinyl-ester resin matrix composites reinforced with recycled cellulose fibre (RCF) have been fabricated and characterised. Nano-composites from both polymers reinforced with different concentration (1%, 5% and 10%) of nano-clay platelets (30B) and halloysite nano-tubes (HNTs) have been synthesized. The mechanical properties, such as flexural strength, impact toughness, fracture toughness and the effect of water absorption on impact toughness were evaluated. Porosity and flammability properties have been investigated. Results indicated that flexural strength decreased for the majority of samples due to the poor dispersion of nano-fillers and the formation of micro-voids within the samples. In contrast, impact toughness and fracture toughness were improved for all reinforced samples. Water absorption resulted in enhanced impact toughness as a result of local plasticization at the crack-tip. Addition of nanoclay increased the porosities but improved the flammability of all nanocomposites. The effects of RCF and nanoclay additions on the physical and mechanical properties have been discussed in terms of the observed microstructures

Study of Indentation Response of Functional Graded Al2TiO5- Based Bioceramics

Aluminium titanate, Al2TiO5 (AT) with the pseudobrookite structure is the only compound in the alumina-titania system. It is an excellent refractory and thermal shock resistant material due to its relatively low thermal expansion coefficient (1 ×10-6 ºC –1) and high melting point (1860ºC). However, its low mechanical strength, hardness and fracture resistance together with susceptibility to decomposition in the temperature range 900–1200ºC has limited its wider application. In this paper, the innovative tailored design of functionally- graded Al2TiO5 – based ceramics system is presented. This involves the use of a vacuum heat-treatment or die-pressing to form hard graded layers of alumina on Al2TiO5. These hard outer layers will provide hardness and wear resistance to protect the softer but damage resistant underlayers. The results will also explore unresolved issues concerning the effect of graded interfaces on their physical and mechanical performance properties

Kinetics of Phase Decomposition in Ti4AlN3 and Ti2Aln - A Comparative Diffraction Study

A method for determining the kinetics of and the activation energy for the thermal dissociation of Ti2AlN and Ti4AlN3 in vacuum using in-situ time-of-flight (ToF) neutron diffraction is described. We discuss the thermal stability and phase transitions in Ti2AlN and Ti4AlN3. The 4th order polynomial function of time [y(t)=At4+Bt3+Ct2+Dt+E] is used to describe the isothermal decomposition and to calculate the mean value of the reaction rate constant at 1400, 1450, 1500,and 1550 °C. The rate constants at these four temperatures are used to determine the activation energy for the decomposition of Ti4AlN3 and Ti2AlN in vacuum using the Arrhenius equation, which are found to be 410.8 ± 50.0 and 573.8 ± 130 kJ/mol, respectively