Michael J. Koczak

Fabrication of Al matrix in situ composites via self-propagating synthesis

Abstract Al matrix composite materials with 30 vol.% TiC, TiB2 and TiC + TiB2 ceramic reinforcements were processed in situ via self-propagating high temperature synthesis (SHS) followed by high pressure consolidation to full density. Non-steady-state oscillatory motion of the combustion wave was observed during the SHS processing, resulting in a typical layered structure of the reaction products. The microstructure and phase composition of the materials obtained were studied using X-ray diffraction, optical microscopy and scanning (SEM) and transmission (TEM) electron microscopy. Very-fine-scale ceramic particles ranging from tens of nanometers up to 1–2 μm were obtained in the Al matrix. Microstructural analysis of the reaction products showed that the TiB2/Al and (TiB2 + TiC)/Al composites contained the Al3Ti phase, indicating that full conversion of Ti had not been achieved. In the TiC/Al composite a certain amount of Al4C3 was detected. High room and elevated temperature mechanical properties (yield stress, microhardness) were obtained in the high-pressure-consolidated SHS-processed TiC/Al and TiB2/Al composites, comparable with the best rapidly solidified Al-base alloys. These high properties were attributed to the high density of the nanoscale ceramic particles and matrix grain refinement.

Feasibility of aluminium nitride formation in aluminum alloys

Abstract The feasibility of forming aluminum nitride by in situ reactive nitrogen gas injection into molten aluminum alloys has been evaluated both analytically and experimentally over the temperature range from 700 to 1500°C. It is shown that aluminum nitride can be melt formed in the presence of Mg and Si, with nitrogen and/or ammonia as the reactive gases at temperature above 1100°C. In this role, magnesium serves as a catalyst. Magnesium niride is first formed in the vapor phase by the reaction of vaporized magnesium and nitrogen gas, followed by incorporation of magnesium nitride particles into the molten aluminum. Via an in situ substitution reaction, aluminum nitride forms between magnesium nitride and aluminum. Up to 17 wt.% aluminum nitride in an aluminum alloy has been formed with an average reinforcement size of 3 μm. The potential for this process permits economical liquid phase processing of aluminum nitride-aluminum metal matrix composite with nitrogen gas injection for structural, thermal and wear applications.

Microstructure-property relationships of in situ reacted TiC/AlCu metal matrix composites

A novel technique was utilized for the fabrication of in situ titanium carbide reinforced aluminum alloy metal matrix composites. The reacted, cast, extruded and heat-treated samples exhibited a homogeneous distribution of fine (0.1–3 μm) TiC platelets in a fine-grained recrystallized Al4.5wt.%Cu matrix. Elevated temperature tensile testing indicated that the composite retains its room temperature strengths up to 250 °C and compared favorably with composites fabricated by more complex and costly processes. When compared with Al4.5wt.%Cu alloy processed similarly but without the TiC reinforcement, the additions of TiC resulted in a yield strength and tensile strength increase of 130% and 65% respectively. Fractographic analysis indicated ductile failure, although the ductility and strength were limited by the presence of coarse titanium aluminides (Al3Ti).

Emerging technologies for the in-situ production of MMCs

Among the numerous methods of producing discontinuously reinforced metal-matrix composites, technologies allowing insitu production of the reinforcing phase offer significant advantages from a technical and economic standpoint. The in-situ formation of a ceramic second phase provides greater control of the size and level of reinforcement, yielding better tailorability of the composite properties. As an example, significantly finer ceramic particulates are possible, which minimizes toughness degradation that is traditionally associated with composites containing relatively large particles. Several emerging, innovative technologies in this area are under development. As with any new technology there are technical challenges, but it is believed that these processes have unique capabilities and, thus, they present cost-effective production processes for metal-matrix composites.

Analysis of in situ formation of titanium carbide in aluminum alloys

Abstract A novel technique to generate fine single-crystal TiC platelets in an Al-4.4wt.%Cu matrix has been developed. The highly exothermic process is moderated by means of a carrier gas and leads to a fine (0.1–2.0 μm) and homogeneous distribution of the stoichiometric carbides. An analysis is provided to examine the carbonaceous gas decomposition reaction, the reaction products and reaction paths, the rate-limiting steps in the formation of TiC and the solidification route from analytical and experimental evidence it is suggested that nucleation of the carbide occurs via titanium diffusing to the carbon and precipitating out as a carbide via a solid-liquid chemical reaction. In the initial stage of the process, the rate-limiting step is the supply of carbon in the carrier gas. Towards the latter part of the process, with titanium depletion, diffusion of titanium across the boundary layer is the rate-limiting stepp. Theoretical thermodynamic considerations indicate that TiC is stable during solidification and Al 4 C 3 formation is precluded.

Formation of TiC in in situ processed composites via solid-gas, solid-liquid and liquid-gas reaction in molten AlTi

Abstract A novel technique to generate fine single crystal TiC platelets in an aluminum based matrix has been developed (M.J. Koczak and K.S. Kumar, US Patent 4,808,372 (1989)). The process involves decomposition of a carbonaceous gas (CH4) and reaction of nascent carbon with a strong carbide former such as Ti in an aluminum matrix at a relatively high temperature (1200–1400 °C). The highly exothermic process is moderated by means of a carrier gas and leads to a fine distribution of carbides of size 0.1–3 μm. A nucleation and growth study was carried out to understand the decomposition of the methane, distribution and subsequent reaction with an aluminum-titanium alloy to form the titanium carbide. It was observed that formation of TiC occurs in stages. Following the CH4 decomposition, the solid carbon particles are distributed and trapped in the alloy. The reaction to form TiC is probably limited by diffusion of titanium to carbon and thereafter the carbide. After inoculating carbon in the alloy, the reaction can be completed in solid or liquid state. Transmission electron microscopy studies confirmed the presence of 40–50 nm amorphous carbon particulates in the alloy. It is also postulated that, in liquid state, once the reaction proceeds, the first phase to form is aluminum carbide or an aluminum-titanium carbide of the form. Given sufficient time for completion, the reaction proceeds to form the most stable carbide, i.e. TiC.

Creep and microstructural stability of dispersion strengthened AlFeVSiEr alloy

Abstract A planar flow cast AlFeSiV alloy modified with 0.75 wt.% Er was evaluated in terms of creep response and elevated temperature stability. Creep tests were conducted at temperature and stresses ranging from 291 to 426 °C and 71.86 to 186.4 MPa respectively. In addition, isothermal coarsening was assessed at temperatures of 375, 475 and 525 °C for times up to 456 h. Steady state creep rates, at 375 °C were in the range 1.7 × 10 −10 to 1.2 × 10 −5 s −1 . Creep properties were analyzed in terms of Arrheninius-type equations and the creep response was found to be controlled by the self diffusion of aluminium. Coarsening was monitored by hardness measurements and dispersoid size and was found to be insignificant at 375 °C. At 475 and 525 °C the average precipitate size increased from 0.087 μm to 0.163 μm and 0.195 μm respectively after 456 h; this resulted in softening of the alloy. The measured coarsening rates were in good agreement with the rates predicted by the Lifshitz-Slyozov-Wagner (LSW) theory.

Work of fracture in aluminum metal-matrix composites

The effect of isothermal exposure and thermal cycling on the toughness of B/Al (1100), B/Al (6061), and A12O3/A1 composites has been investigated. In B/Al (1100), isothermal exposure at 773 K for 45 × 104 s (125 hours) reduced toughness, measured by the work of fracture, from 76 kJm-2 to 10 kJm-2, and a similar reduction occurred after equivalent thermal cycling. The corresponding reduction in toughness after isothermal exposure in B/Al (6061) was from 44.5 kJm-2 to 8 kJm-2; however, the effect of thermal cycling was less detrimental. In the FP-A12O3/A1 composite, the work of fracture was insensitive to both forms of thermal treatment. Changes in the toughness of the B/Al composites have been correlated with and analyzed in terms of modifications to matrix, fiber, and interface properties, in particular, matrix softening, interface reaction products, and fiber notch sensitivity.

Thick-section AS4-graphite/E-glass/PPS hybrid composites: Part I. Tensile behavior

Abstract In this study, thick-section (16 and 40 plies) hybrid composites with a poly(phenylene sulfide) (PPS) matrix were evaluated for tensile response. The hybridization of the composite was achieved by using E-glass and AS4-graphite fibers. Composites of five different AS4-graphite volume fractions (1.0, 0.75, 0.5, 0.25 and 0) and three stacking sequences ([0], [ 0 90 ] s and [0/ ± 45 90 ] s ) have been evaluated and analysed. Theoretical predictions based on the classical laminate theory were used to analyse the experimental data. The effect of residual thermal processing stresses was incorporated in the predictions and found to be negligible. The hybrid effect in the composites was quantified by measuring the increase in the failure strains of the hybrid composites as compared to that of 100% AS4-graphite/PPS composites. Experimental results showed a distinct strain enhancement in tensile tests of 2–8% of hybrid composites. The hybrid effect is due to the crack arresting characteristics of the E-glass fibers. The hybridization of the composites resulted in changes in fracture modes from the brittle failure mode of AS4-graphite composites and confirmed the crack arresting properties of the E-glass/PPS fibers.

Pressure-assisted reactive synthesis of titanium aluminides from dense 50Al-50Ti elemental powder blends

In the present research, dense γ-TiAl based intermetallic samples were fabricated by reactive synthesis of fully dense elemental 50 at. pct Al-50 at. pct Ti powder blends. Two different processing routes were attempted: thermal explosion under pressure (combustion consolidation) and reactive hot pressing. In both approaches, relatively low processing or preheating temperatures (900 °C) were used. The entire procedure of thermal explosion under pressure could be performed in open air without noticeable oxidation damage to the final product. The application of a moderate external pressure (≤250 MPa) during synthesis was shown to be enough to accommodate the negative volume change associated with TiAl formation from the elemental components and, thereby, to ensure full density of the final product. Microstructure and phase composition of the materials obtained were characterized employing X-ray diffraction and scanning electron microscopy with energy dispersive analysis. It was found that at elevated temperatures(e.g., 900 °C), the equiatomic 50Al-50Ti alloy lies beyond the homogeneity range of the y-TiAl phase in the Ti-Al binary and contains, in addition to γ-TiAl, Al-rich Ti3Al. Mechanical properties of the materials synthesized were evaluated in compression tests at different temperatures and by microhardness measurements. Due to its very fine microstructure, the Ti-Al material synthesizedvia reactive hot pressing exhibited superplastic behavior at temperatures as low as 800 °C.