Elemental Compacts Research Articles

Investigation of the effect of SiC additions on the high-temperature oxidation behavior of combustion-synthesized MoSi2

In this study, the effect of the weight percentage of SiC on the high-temperature oxidation behavior of MoSi2-Xwt. % SiC composite (X = 0, 10, 20, 30) was studied. The composites were produced from elemental powder compacts of molybdenum, silicon and carbon (where applicable) by combustion synthesis and subsequent sintering method. Additionally, a series of each synthesized powder was mixed with 5 wt% bentonite and consequently sintered. The sintered specimens were then subjected to oxidation at 1100 °C. Microstructure and phase analysis of the oxidized samples were investigated by X-ray diffraction (XRD) and a scanning electron microscope equipped with energy-dispersive spectroscopy (SEM-EDS). Surface and cross-sectional examinations of the samples after oxidation revealed that all samples possessed good oxidation resistance in the way that no spallation was observed between the oxide layer and the substrate. The highest oxidation resistance at 1100 °C was observed in the sample with MoSi2 (without SiC) containing bentonite (AB0). This sample showed only 0.75% weight gain after 50 h of oxidation, which is probably due to the effective role of bentonite in decreasing the open porosities, and further reducing the oxygen penetration path during the oxidation process.

The effect of alloying elements on the properties of pressed and non-pressed biodegradable Fe–Mn–Ag powder metallurgy alloys

Current trends in the biodegradable scaffold industry call for powder metallurgy methods in which compression cannot be applied due to the nature of the scaffold template itself and the need to retain the shape of an underlying template throughout the fabrication process. Iron alloys have been shown to be good candidates for biomedical applications where load support is required. Fe–Mn alloys were researched extensively for this purpose. Current research shows that all metallurgical characterisation and corrosion test on Fe–Mn and Fe–Mn–Ag non pre-alloyed powder alloys are performed on alloys which are initially pressed into greens and subsequently sintered. In order to combine the cutting-edge field of biodegradable metallic alloys with scaffold production, metallurgical characterisation of pressed and non-pressed Fe, Fe–Mn and Fe–Mn–Ag sintered elemental powder compacts was carried out in this study. This was performed along with determination of the corrosion rate of the same alloys in in vitro mimicking solutions. These solutions were synthesised to mimic the osteo environment in which the final scaffolds are to be used.Both pressed and non-pressed alloys formed an austenite phase under the right sintering conditions. The corrosion rate of the non-pressed alloy was greater than that of its pressed counterpart. In a potentiodynamic testing scenario, addition of silver to the alloy formed a separate silver phase which galvanically increased the corrosion rate of the pressed alloy. This result wasn't replicated in the non-pressed alloys in which the corrosion rate was seen to remain similar to the non-silver-bearing alloy counterparts.

Rietveld refinement on in-situ neutron diffraction during sintering of NiTi from elemental powders

Rietveld refinement of neutron diffraction patterns obtained in-situ during the reactive sintering of Ni-Ti (49 at% Ni: 51 at% Ti) elemental powder compacts for two sintering profiles was performed. The atomic displacement parameter (Uiso) for each phase that occurs during reactive sintering (α-Ti, β-Ti(Ni), Ti2Ni(O), NiTi, Ni3Ti and Ni) were obtained from separate in-situ experiments. The phase fractions determined from the two sintering runs were found to be in excellent agreement, with an uncertainty estimated at ± 1.6 wt%. Intermetallic phases do not begin to form until 600 °C. Initially, Ti2Ni(O) grows faster than NiTi and Ni3Ti. By 780 °C, NiTi becomes the dominant phase, growing at the expense of all the other phases. Homogenization occurs at 1100 °C and the microstructure consists of 93 wt% NiTi and 7 wt% Ti2Ni(O). Experimental evidence along with the refinement of the O concentration in Ti2Ni(O) during sintering was used to explain the persistence of Ti2Ni(O) in the microstructure. The oxygen raises the melting point of the phase such that it is in equilibrium with NiTi at 1200 °C, despite Ti2Ni having a melting temperature of 984 °C.

Combustion Synthesis of MAX Phase Solid Solution Ti<sub>3</sub>(Al,Sn)C<sub>2</sub>

Formation of the Ti3(Al,Sn)C2 solid solutions was studied by combustion synthesis from elemental powder compacts with Al4C3 and TiC additions. Combustion temperatures of 1590–1700 °C and flame-front speeds of 14.2–18.8 mm/s were measured for the Al4C3-added samples. Due to the dilution effect of TiC, the combustion temperature and flame velocity were significantly reduced to 1220–1280 °C and 7.1–9.6 mm/s, respectively, for the TiC-adopted samples. The XRD analysis indicated that MAX solid solutions Ti3(Al1–xSnx)C2 with x = 0.2, 0.4, and 0.6 were produced from the Al4C3-adopted samples. Because of the low reaction temperature, the extent of Sn substitution for Al in Ti3(Al1–ySny)C2 was narrowed down to y = 0.4 for the TiC-containing samples. The as-synthesized Ti3(Al,Sn)C2 grains were plate-like and closely stacked into a laminated microstructure.

Formation of Ti5Si3 and V5Si3 by self-propagating high-temperature synthesis and evaluation of combustion wave kinetics

Fabrication of Ti5Si3 and V5Si3 was conducted by self-propagating high-temperature synthesis (SHS) from the elemental powder compacts of their stoichiometries. Combustion wave kinetics was experimentally studied by measuring the combustion front velocity and temperature, and numerically investigated to determine the Arrhenius factor of the rate function (K0) and to evaluate the effective thermal conductivity (keff). Experimental results showed that the 5Ti + 3Si reaction is more exothermic than the 5V + 3Si reaction. With the increase of sample density from 50% to 60% TMD, the flame-front velocity increased from 28 to 50 mm/s for the formation of Ti5Si3 and from 6.9 to 8.5 mm/s for V5Si3. Based upon numerical simulation and experimental validation, K0 = 3 × 109 s−1 and 5 × 107 s−1 were deduced for combustion synthesis of Ti5Si3 and V5Si3, respectively. The ratio of keff/kbulk increasing from 0.02 to 0.05 was employed in the numerical calculations to simulate the effect of sample compaction density on combustion wave kinetics. Moreover, the measured and calculated combustion front temperatures were comparable, which confirmed the reaction exothermicity of the numerical model.

Effects of metal carbides as additives on combustion synthesis of Ti3(Al,Sn)C2 solid solutions

Formation of the Ti3(Al,Sn)C2 solid solutions with a broad substitutional range was investigated by combustion synthesis from elemental powder compacts with Al4C3 and TiC additions. High combustion temperatures of 1590–1700 °C and rapid flame-front speeds of 14.2–18.8 mm/s were measured for the Al4C3-adopted samples. Due to the dilution effect of TiC, the combustion temperature and velocity were significantly reduced for the TiC-added samples and approximated to 1220–1280 °C and 7.1–9.6 mm/s, respectively. The XRD analysis indicated that Ti3(Al1–xSnx)C2 solid solutions with x = 0.2, 0.4, and 0.6 were produced from the Al4C3-adopted samples. Further increase of Sn to x = 0.8 led to the formation of a large amount of Ti2SnC and TiC. Because of the low reaction temperature for the TiC-based reaction scheme, the effective substitution of Sn for Al in Ti3(Al1–ySny)C2 was only achieved at y = 0.2 and 0.4. Incomplete phase formation at y = 0.6 and 0.8 yielded Sn-lean Ti3(Al,Sn)C2 along with Ti2SnC, TiC, and free Sn. The as-synthesized Ti3(Al,Sn)C2 grains were formed into the shape of platelets and most of them were closely stacked into a laminated microstructure.

Use of TiH2 as a reactant in combustion synthesis of porous Ti5Si3 and Ti5Si3/TiAl intermetallics

Porous Ti5Si3 and Ti5Si3/TiAl intermetallics were fabricated by self-propagating high-temperature synthesis (SHS) from elemental powder compacts with addition of TiH2. The TiH2 content ranged from 1.25 to 6.25 mol.% for formation of the Ti5Si3 monoliths and from 2 to 10 mol.% for the Ti5Si3/TiAl composites. Due to endothermic decomposition of TiH2, the increase of TiH2 decreased the reaction temperature and combustion wave velocity. Moreover, dehydrogenation of TiH2 released hydrogen gas to act as a pore-forming agent and generated highly reactive Ti to facilitate the reaction. The porosity of the intermetallic product increased with increasing TiH2 content and the microstructure with a uniform distribution of pores was attained. As a consequence, the porosity of as-synthesized Ti5Si3 monoliths was controllable and increased from 38% to 60% with TiH2 additions. Likewise, Ti5Si3/TiAl composites with an increasing porosity from 31% to 56% were obtained from the TiH2-added samples.

Experimental and Numerical Studies on Self-Propagating High-Temperature Synthesis of Ta5Si3 Intermetallics

Formation of Ta5Si3 by self-propagating high-temperature synthesis (SHS) from elemental powder compacts of Ta:Si = 5:3 was experimentally and numerically studied. Experimental evidence showed that the increase of either sample density or preheating temperature led to the increase of combustion wave velocity and reaction temperature. The apparent activation energy, Ea ≈ 108 kJ/mol, was determined for the synthesis reaction. Based upon numerical simulation, the Arrhenius factor of the rate function, K0 = 2.5 × 107 s−1, was obtained for the 5Ta + 3Si combustion system. In addition, the influence of sample density on combustion wave kinetics was correlated with the effective thermal conductivity (keff) of the powder compact. By adopting 0.005 ≤ keff/kbulk ≤ 0.016 in the computation model, the calculated combustion velocity and temperature were in good agreement with experimental data of the samples with compaction densities between 35% and 45% theoretical maximum density (TMD).

Combustion characteristics of molybdenum–silicon mixtures

Molybdenum disilicide was produced from elemental powder compacts of molybdenum and silicon by self-propagating high-temperature synthesis. The effects of the reactant ratio, particle size of silicon, and density of the compact were investigated in a high-pressure microreactor. In order to get the relationships among them, the atomic ratio of silicon to molybdenum (Si/Mo) was changed from the values of 1.0 to 2.6 with steps of 0.2. Stable combustion was observed for values of atomic ratio of silicon to molybdenum of 1.8, 2.0, and 2.2. The SHS-produced material consisted of uniform and single-phased molybdenum disilicide (MoSi2) for the atomic ratio of silicon to molybdenum of 2.0. In the meantime unstable combustion such as oscillatory and spinning combustion was detected for values of atomic ratio of silicon to molybdenum beyond the limits of stable combustion. The SHS-produced material under unstable combustion includes impurities of Mo5Si3, unreacted Mo and Si resulting from the layered structure which gives a lower degree of reaction and poor electrical properties of MoSi2 heating element. A significant amount of unreacted molybdenum (≈17%) was found for the atomic ratio of silicon to molybdenum of 1.0. This may result from the spinning combustion as well as from the low combustion temperature below the melting point of silicon (≈1414°C). The value of criterion α suggested by Shkadinskii et al. to differentiate stable combustion from unstable one, is found to be 0.74 for the production of molybdenum disilicide by self-propagating high-temperature synthesis. Stable combustion was observed for α≥0.74 to give uniform products while unstable combustion was detected for α<0.74 to result in non-uniform and multi-phased products. This critical value should be useful for the industrial production of uniform molybdenum disilicide by self-propagating high-temperature synthesis.

Synthesis of γ-TiAl by Reactive Spark Plasma Sintering of Cryomilled Ti and Al Powder Blend: Part II: Effects of Electric Field and Microstructure on Sintering Kinetics

The current study shows the dramatic effect of an electric field (EF) and use of nanosized cryomilled grains on accelerating sintering kinetics during spark plasma sintering of blended elemental powder compacts of Ti53Al47 targeted to produce γ-TiAl intermetallic compounds. The EF had the dominating effect since it reduced the activation barrier for diffusion through Al3Ti leading to faster growth of Al3Ti; the precursor to γ-TiAl. The Avrami exponent (n) determined for the micrograin compact lies between 1.0 and 1.5, which indicates that reaction sintering is controlled by bulk diffusion in these compacts, while for cryomilled compacts this is between 0.7 and 1.0 suggesting the important role of dislocations and grain boundaries on the transformation during reaction sintering. The activation energies were found to be in increasing order as: cryomilled compacts with EF (182 kJ/mol); micrograin compacts with EF (290 kJ/mol); cryomilled compacts without EF (331 kJ/mol); and micrograin compacts without EF (379 kJ/mol). The cryomilled microstructure also enhanced the sintering kinetics because of the availability of faster diffusing paths in Al and Ti including larger grain boundary area and dislocation density.

The Effect of Pelletizing Pressure on SHS Synthesis of MoSi&lt;sub&gt;2&lt;/sub&gt;

An experimental study on the preparation of molybdenum disilicide (MoSi2) was conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts with different pelletizing pressures (0, 50, 100 and 200MPa). The composition of test specimens is Mo:Si = 1:2. The results showed that the propagation time of the reaction front, the density of the combustion products, the particle size and the morphology of MoSi2 were significantly influenced by pelletizing pressure of the reactant compact. In addition, all the samples obtained with different pelletizing pressures were single-phase MoSi2.

Modeling evaluation of Arrhenius factor and thermal conductivity for combustion synthesis of transition metal aluminides

Two important rate-governing parameters, Arrhenius factor and thermal conductivity, in the process of combustion synthesis are numerically evaluated for the formation of transition metal aluminides, NiAl and CoAl. Correlation of the effective thermal conductivity (keff) with sample compaction density and dependence of the Arrhenius factor (K0) on the grain size of the reactant powders were established. Experimental data employed to validate the numerical model were obtained from the sample compacts composed of reactant powders with different grain sizes (Ni: <45 and 3–7 μm, Al: <40 and 10 μm, and Co: <45 μm) and formed by different densities (50%–70% TMD). For the Ni–Al samples of 50%–65% TMD, the calculated combustion velocity and temperature based upon keff/kbulk = 0.016–0.052 and K0 = 3.0 × 108–2.0 × 109 s−1 are in good agreement with those experimentally measured. The magnitudes of keff/kbulk = 0.028–0.08 and K0 = 1.5 × 108–4.0 × 108 s−1 were justified for combustion synthesis of CoAl from elemental powder compacts of 55%–70% TMD.

Field-activated sintering of blended elemental γ-TiAl powder compacts: Porosity analysis and growth kinetics of Al3Ti

Various powder compacts with targeted compositions of TiAl and Ti3Al were consolidated by field-activated sintering, specifically spark plasma sintering (SPS) at various temperatures, to examine phase growth under a superimposed electric field. While compacts with a composition of TiAl showed significant porosity after sintering, Ti3Al compacts showed no porosity. The porosity is attributed principally to volume changes associated with the formation of Al3Ti and the Kirkendall effect. The kinetics underlying Al3Ti growth during SPS are also explained.

Microstructure and mechanical properties of TiAl alloys produced by rapid heating and open die forging of blended elemental powder compacts

Ti–43Al–5V–4Nb (at%) intermetallic compounds were fabricated by a rapid heating and open die forging method using blended elemental powders. The process route consisted of powder blending, compacting, rapid heating, open die forging and heat treatment. During heating there is porosity in the blended and warm compacted powder billets which still persists after the first forging. In order to remove this difficulty a two stage forging process was used. A fully lamellar structure was seen in the TiAl alloy after heat treatment. Overall, the TiAl alloy showed moderately good mechanical properties at room temperature and good mechanical properties, with reasonable strength and good ductility at high temperatures.

Effect of Ti/Si ratio on the products of laser igniting self-propagating high-temperature synthesis in Cu–Ti–Si system

The production of titanium silicide, from elemental powder compacts was successfully synthesized by laser ignited self-propagating high-temperature synthesis (SHS) of Cu–Ti–Si system in this study. Effects of Ti/Si ratio on the combustion characteristics, combustion temperature, the composition and microstructure of final products were studied. The results showed that the combustion process was preceded by the fast propagation of a self-sustained combustion front, followed by vigorous bulk combustion, furthermore, the combustion temperature increased with the increasing of Ti/Si ratio. When Ti/Si ratio is 1:2, the polygon-like shape TiSi2 phase was the dominant phase, minor amounts of Ti5Si3 and Cu3Si were detected in the final product; when Ti/Si ratio is over 1:2, the amounts of TiSi2 and Ti5Si3 phases decreased and increased, respectively. Especially, the cobblestone-like shape Ti5Si3 as the dominant phase was identified in the final product when Ti/Si ratio is 5:3.

Preparation of (Mo,Nb)Si&lt;sub&gt;2&lt;/sub&gt; Ternary Alloys by Self−Propagating High−Temperature Synthesis

An experimental study on the preparation of (Mo,Nb)Si2 ternary alloys was conducted by self-propagating high-temperature synthesis method from elemental powder compacts of different stoichiometries. And the combustion mode, combustion temperature, flame-front propagation velocity and product structure were discussed. The results show that (Mo,Nb)Si2 ternary alloys are characterized by an unsteady state combustion mode with a spiral−trajectory reaction front from top to bottom. The combustion temperature and flame−front propagation velocity decrease with the addition of coarse niobium powder. The combustion temperature and flame-front propagation velocity of MoSi2 are 1629K and 3.13mm/s respectively. However, those of (Mo0.8Nb0.2)Si2 alloy are 1460K and 1.97mm/s. The solid solubility of niobium in MoSi2 is less than 2.5at.%, and the combustion synthesis produce still remains Cllb single-phase structure in (Mo1-x,Nbx)Si2(x&lt;0.075) sample. The C40-type structure appears in (Mo0.925,Nb0.075)Si2 compact and the intensity of diffraction peaks of C40-type phase gradually reinforces with the increase of niobium content. Combustion synthesis is an effective technology for producing (Mo,Nb)Si2 ternary alloys.

Combustion synthesis of (Ti 1− xNb x) 2AlC solid solutions from elemental and Nb 2O 5/Al 4C 3-containing powder compacts

Preparation of the (Ti 1− x Nb x ) 2AlC solid solution (formed from the M n+1 AX n or MAX carbides, where n = 1, 2, or 3, M is an early transition metal, A is an A-group element, and X is C) with x = 0.2–0.8 was investigated by self-propagating high-temperature synthesis (SHS). Nearly single-phase (Ti,Nb) 2AlC was produced through direct combustion of constituent elements. Due to the decrease of reaction exothermicity, the combustion temperature and reaction front velocity decreased with increasing Nb content of (Ti 1− x Nb x ) 2AlC formed from the elemental powder compacts. In addition, the samples composed of Ti, Al, Nb 2O 5, and Al 4C 3 were adopted for the in situ formation of Al 2O 3-added (Ti,Nb) 2AlC. The SHS process of the Nb 2O 5/Al 4C 3-containing sample involved aluminothermic reduction of Nb 2O 5, which not only enhanced the reaction exothermicity but also facilitated the evolution of (Ti,Nb) 2AlC. Based upon the XRD analysis, two intermediates, TiC and Nb 2Al, were detected in the (Ti,Nb) 2AlC/Al 2O 3 composite and their amounts were reduced by increasing the extent of thermite reduction involved in the SHS process. The laminated microstructure characteristic of the MAX carbide was observed for both monolithic and Al 2O 3-added (Ti,Nb) 2AlC solid solutions synthesized in this study.

A comparative study on combustion synthesis of Ta–B compounds

A comparative study on the preparation of various tantalum borides (including Ta 2B, Ta 3B 2, TaB, Ta 5B 6, Ta 3B 4, and TaB 2) in the Ta–B system was experimentally conducted by self-propagating high-temperature synthesis (SHS) from the elemental powder compacts of their corresponding stoichiometries. Both combustion temperature and reaction front velocity increased and then decreased with increasing boron content in the powder mixture. The fastest flame front with a reaction temperature of 1732 °C and a propagation rate of 11.2 mm/s was observed in the sample of Ta:B = 1:1. The combustion temperature (1205 °C) and flame-front velocity (3.82 mm/s) for the powder compact of Ta:B = 2:1 were the lowest. According to the XRD analysis, single-phase TaB and TaB 2 were produced from the samples of Ta:B = 1:1 and 1:2, respectively. However, multiphase products were synthesized from the samples of other stoichiometries. In the final products from Ta-rich samples of Ta:B = 2:1 and 3:2, two boride phases, Ta 2B and TaB, along with a large amount of residual Ta were detected. The products yielded from boron-rich reactants of Ta:B = 5:6 and 3:4 were composed of TaB, Ta 3B 4, and TaB 2. Based upon the temperature dependence of combustion wave velocity, the activation energies associated with the formation of TaB and TaB 2 by solid state combustion were determined to be 131.1 and 181.4 kJ/mol, respectively.

Effect of composition of starting materials of Mo–Si on self-propagating high temperature SHS reaction

An experimental study of the self-propagating high temperature synthesis of Mo–Si alloys was conducted from elemental powder compacts. Test specimens with seven compositions, including Mo/Si = 1∶1·25, 1∶1·50, 1∶1·75, 1∶2·00, 1∶2·25, 1∶2·50 and 1∶2·75 respectively, were employed. Experimental evidence showed that a combustion wave featuring a spinning reaction zone can be observed. When the powder compacts are from Mo∶1·25Si to Mo∶1·75Si, the combustion temperature and the propagation velocity of combustion wave increase with increasing silicon in the sample, and the combustion products are composed of MoSi2, Mo5Si3 and Mo. However, when the powder compacts are from Mo∶2·25Si to Mo∶2·75Si, the combustion temperature and the propagation velocity decrease rapidly as the silicon in the compact increases, and the combustion products are composed of MoSi2 and Si. The sample with Mo/Si = 1∶2·00 possesses the highest combustion temperature (1628·9 K) and propagation velocity (3·13 mm s−1). A single-phase MoSi2 is synthesised by the Mo/Si = 1∶2·00 sample.

Combustion synthesis of (Mo1 − xCrx)Si2 (x = 0.00–0.30) alloys in SHS mode

Combustion synthesis was adopted to successfully synthesize molybdenum–silicon–chromium (Mo−Si−Cr) alloys by the mode of self-propagating high-temperature synthesis (SHS). The experimental study of combustion synthesis of Mo−Si−Cr alloys was conducted on elemental powder compacts. Powder compacts with nominal compositions including MoSi2, (Mo0.95Cr0.05)Si2, (Mo0.90Cr0.10)Si2, (Mo0.85Cr0.15)Si2, (Mo0.80Cr0.20)Si2, (Mo0.75Cr0.25)Si2 and (Mo0.70Cr0.30)Si2 were employed in combustion synthesis experiments. The combustion mode, combustion temperature, flame-front propagation velocity and product structure were investigated. The results showed that Mo−Si−Cr alloys were synthesized by an unsteady state combustion mode with a spiral-trajectory reaction front. The peak combustion temperature reduced with the addition of Cr to Mo–Si system. The flame-front propagation velocity decreased with an increase in Cr content of the powder compact. The X-ray diffraction (XRD) results showed that the crystal structure of the combustion product changed from Cllb-type structure (Mo0.90Cr0.10)Si2 to C40-type structure (Mo0.85Cr0.15)Si2 with increase in Cr content of Mo–Cr–Si alloys. The intensities of diffraction peaks of the C40-type phase gradually increased with increase in Cr content.