Thermal Explosion Reaction Research Articles

Interfacial microstructure and mechanical properties of Ti/Cu joint manufactured by Ni-Al thermal explosion reaction

Abstract The dissimilar metals joining of Ti/Cu was conducted using a rapid thermal explosion (TE) reaction. The Ni-Al powder mixture was used as TE reaction system, and the effect of temperature (600, 700, 800 and 900 °C) on interfacial diffusions, microstructure and mechanical properties of Ti/Cu joint were investigated. The maximum combustion temperature of Ni-Al interlayer during heating process was 720 °C and a significant exothermic phenomenon was observed for 5 s. The infiltration of melted Al into the Ti/Cu substrate and formed a continuous transition region at the interface of the joints were the dominant joining mechanisms. Microhardness of the interlayer after heated at 900 °C for 30 min possessed prominent improvement from 32.6 to 101.7 HV due to the completely phase transformation of Ni-Al interlayer, which composed of main phase NiAl and minor phase Ni3Al. The bond strength of the joint was checked to assess the joint quality. For the joint processed by 900 °C for 30 min, the maximum shear strength of Ti/NiAl and Cu/NiAl were 33.4 MPa and 39 MPa, respectively.

Visible Observation and Formation Mechanism of Porous TiAl3 Intermetallics During the Continuous Sintering Process

The formation mechanism of porous TiAl3 intermetallics from Ti-75 at.% Al powder compact was explored through a continuous sintering process. The exothermic behavior, temperature changes and open porosity were investigated by ranging the sintering temperature from 600°C to 700°C. Results show that a thermal explosion (TE) occurs once the sample temperature reaches to 675°C, upon which it rapidly climbs to 1148°C within 10 s. Visual observation was achieved and revealed the exothermic state at every step, showing the overall heating and instantaneous reaction of TE. The open porosity increased significantly from 24.8% to 58.7% by increasing the sintering temperature from 670°C to 675°C. The reaction mechanism of porous TiAl3 materials in the Ti-75 at.% Al system indicates that sintering at 600°C, 650°C, and 670°C was controlled by atomic diffusion, diffusion-induced TE, and liquid-induced TE reaction, respectively. The diffusion reaction activation energy was 161.5 kJ mol−1 for diffusion-controlled growth of the TiAl3 reaction layer.

Fe–Al Phase Formation Studied by Internal Friction during Heating Process**Supported by the National Natural Science Foundation of China under Grant Nos. 51661032 and 51301150, the Special Program of Science and Technology New Star of Shaanxi Province under Grant No. 2013KJXX-11.

We systematically investigate the internal friction properties of a Fe–(43 at.%)Al powder mixture compact during the heating process with the expectation to understand the phase formation and transition process. Three internal friction peaks are successively observed during the heating process from room temperature to 750°C, but almost completely disappear in the subsequent cooling process. Three internal friction peaks exhibit obvious measuring frequency dependence, which increases with decreasing the frequency. The first internal friction peak originates from the micro-sliding of weak bonding interface between Al particles corresponding to a recrystallization process of deformed Al particles. The second internal friction peak is attributed to a phase formation process associated with the formation of the intermediate phase Fe2Al5. The third internal friction peak is considered to result from the formation of the FeAl intermetallic compound owing to the reaction of Fe2Al5 and residual Fe initiated by a dramatic thermal explosion reaction.

Interfacial microstructure of Ti/Ni joints with Ti–Al interlayer by rapid thermal explosion bonding in vacuum

The present study investigated the joining between Ti and Ni by rapid thermal explosion (TE) in vacuum of <10 Pa. The TE temperature of Ti–Al interlayer reaction was measured and the effects of hold time on microstructure and micro-hardness of Ti/Ti–Al/Ni joint were discussed. The TE temperature of Ti–Al interlayer reaction is about 940.3 °C and an obvious exothermic phenomenon lasting 13s during TE reaction process is observed. Obvious diffusion layers at both interfaces are observed due to the inter-diffusion of Ti, Ni and Al elements. With the increasing of hold time, thickness of diffusion layers was increased from 7.6 to 15 μm at the Ti/Ti–Al interface and from 16.7 to 23.7 μm at the Ti–Al/Ni interface. Furthermore, the micro-hardness of substrates and interlayer are not obviously changed with increasing of hold time ranging from 30 to 90 min.

Brazing of Gr/2024Al composite and TC4 using AgCuTi filler with the assist of Ti-Al-C-Cu interlayer

In order to braze Gr/2024Al composite at low temperature to keep its heat insulation property, a novel thermal explosion assisting brazing method was designed. Through this method, the thermal explosion reaction of Ti-Al-C-Cu interlayer offered additional heat, and AgCuTi filler reacted with Gr/2024Al composite under the melting point of AgCuTi. Based on the theoretical calculation and DSC analysis, the composition of Ti-Al-C-Cu interlayer was designed. The microstructure and mechanical properties of brazed joint was investigated, and the brazing parameters were optimized. The results showed that AgCuTi reacted with the Gr/2024Al composite to form Ti3AlC2 layer, and a continuous TiAl3 layer acted as the reaction layer near the TC4 alloy. The high preheating temperature and long holding time would promote the reaction of AgCuTi filler, Gr/2024Al and Ti-Al-C-Cu interlayer, and Al5CuTi2 phase volume in the brazing zone increased. The Al5CuTi2 distribution affected the joint strength significantly. The optimized joint strength reached 11 MPa when the joint was brazed at 650 °C for 15 min.

Real-Time Measurement on the Heat Release Property of Titanium Blended with Different Carbon Allotropes, under Externally Constant Heat Flux

Ti/C blended powder is commonly employed as an initiating combustion agent for preparing calcium aluminate; a dedicated test system is exploited for real-time examining of the heat release of Ti/C blended powder during combustion under atmosphere conditions with an externally constant heat flux of 973 K, which is comprised of cone calorimeter, thermal-gravimetry/differential scanning calorimetry, X-ray diffraction (XRD), scanning electron microscope/energy dispersive spectrometer, and a theoretical thermal calculation, with the aim of quantitatively illuminating its combustion mechanism in depth. Furthermore, a comparison of the heat release property of titanium powder blended with different carbon allotropes, including natural flaky graphite (FG), carbon black (CB), expandable graphite (EG), and vermicular graphite (VG) is preliminarily investigated, to clarify the effect of different carbon allotropes on the heat release property of Ti/C blended powder. It reveals that the oxidation reaction between Ti and O2 initiates the subsequent combination of TiC through a thermal explosion reaction, using graphite (FG, VG, or EG) and Ti powder as the starting materials, respectively. Moreover, EG facilitates an accelerated (fire growth index of 0.42 kW·m−2·s−1) and enhanced peak heat release rate (pHRR) of 30.7 kW·m−2 at 73 s, while VG suppresses the heat release with the pHRR of 5.2 kW·m−2 at 64 s and fire growth index of 0.08 kW·m−2·s−1, and FG favors the formation of TiC with a higher crystallinity from XRD. Additively, the prior NaOH-impregnation is favorable for the formation of TiC for Ti/CB blended powder, although the TiO2 predominates final combustion production. It reveals the chemical evolution and mechanisms evolved in the formation of TiC during ignition.

Microstructure and oxidation resistance of porous NbAl3 intermetallic prepared by thermal explosion reaction

ABSTRACTPorous Nb–Al intermetallic was prepared by thermal explosion (TE) of combustion synthesis (CS). The temperature profile, phase composition, open porosity and oxidation resistance of Nb–Al compact were investigated. The results showed that the significant exothermic reaction occurred, which means that an obvious TE appeared during heating. The volume expansion of 190% was observed, and the prepared sample exhibited interconnected pores with an open porosity of 65.7%. The porous NbAl3 intermetallic showed ‘pest’ oxidation in the temperature range of 500–600°C, and followed the parabolic oxidation law at 400°C. Moreover, due to the highly porous structure, this material had great potential for application to separation and heat insulation at a temperature range of room temperature to 400°C.

Exothermic behavior and thermodynamic analysis for the formation of porous TiAl3 intermetallics sintering with different heating rates

Porous TiAl3 intermetallcis are of great interest due to its excellent properties and widely applied in filtering apparatus, separation material and catalyst carrier. In this study, interconnected pore-structures have been synthesized by diffusion or thermal explosion (TE) reaction sintering with different heating rates. The thermal characteristics such as temperature-time curves, exothermic change and visual images indicate that the sample experienced a significant TE reaction at higher heating rates. Results shown that the sample was ignited at 672 °C and then rapidly increased to combustion temperature of 1169, 1110 and 933 °C in tens of seconds with the heating rate of 15, 10 and 5 °C∙min−1 respectively. Meanwhile, TE represented the uniformity of volume combustion, instantaneous reaction and rapid cooling to furnace temperature, the amount of heat released during TE reaction dropped from 1303 to 963 J g−1. This indicates that the entire sintering process was controlled by TE and the pre-diffusion reaction before the melting temperature of Al atom, which would affect the subsequent combustion reaction. Thermodynamic data explained that the reaction mechanism is mainly step-controlled diffusion reaction at a low heating rate (1 °C∙min−1), while the energy gradually accumulated and thermal explosion (TE) reaction become obvious with the increasing of heating rate (from 2 to 15 °C∙min−1).

Rapid reactive synthesis of TiAl3 intermetallics by thermal explosion and its oxidation resistance at high temperature

Porous TiAl3 intermetallics were synthesized from Ti-75 at.% Al elemental powder mixtures using an energy-saving and rapid reactive method of thermal explosion (TE). The results demonstrated that the actual temperature of the compact climbed rapidly from 673 °C to 1036 °C within 24 s, indicating that an obvious TE reaction occurred during sintering process. The video graphs suggested that the TE in Ti–Al system behaved instant occurrence and overall heating whether from axial or radial direction. The silver wires and NaCl particles that pressed on the surface of the sample disappeared due to the heavy heat released during TE reaction. Only pure TiAl3 phases were synthesized in TE products and the open porosity of 55.4% was easy to obtain. After high-temperature treatment at 1000 °C, large amounts of sintering-neck formed and then improved the compressive strength of porous TiAl3 materials. Moreover, the mass gain curve of porous TiAl3 intermetallics oxidized at 650 °C for 120 h exhibited the parabolic oxidation rate law. XPS analysis confirmed that the strong O 1s peak was 531.4 eV which was the typical binding energy of Al2O3. Therefore, the excellent oxidation resistance of porous TiAl3 foams would be considered as good candidate materials for prolonging the service life at high temperatures.

Fast and facile synthesis of Sb-doped Mg2Si0.5Sn0.5 solid solutions with decent thermoelectric performance

Sb-doped Mg2Si0.5Sn0.5 materials were successfully prepared by a fast thermal explosion reaction for the first time. X-ray diffraction (XRD) patterns indicate that this new method easily converts the immiscible composition of 0.5Mg2Si-0.5Mg2Sn into a single-phase Mg2Si0.5Sn0.5 solid solution. With Sb content increasing, the dimensionless thermoelectric figure of merit ZTpeak (peak value) and ZTave (average value) are greatly improved, which are quite comparable to or even better than the values obtained by other methods. More importantly, this facile fabrication technique possesses attractive merits, such as high time- and energy-efficiency, low cost and the potential of mass production. Hence, a significant foundation has been laid for the future synthesis of other Mg-based and functional materials.

Porous NbAl3/TiAl3 intermetallic composites with controllable porosity and pore morphology prepared by two-step thermal explosion

Porous NbAl3/TiAl3 with adjustable porosity was prepared by the thermal explosion (TE) mode of combustion synthesis (CS). This was achieved by using NaCl as temporary space holder, which was completely removed from specimens before TE reaction. The combustion image of this process provided the visual evidence for the observation of TE. The temperature profile, course of reaction as well as the effect of NaCl content on the phase composition, pore morphology and open porosity of porous NbAl3/TiAl3 were investigated. The results showed that the addition of TiH2 prevent the samples from cracking and deforming, and in situ synthesis of NbAl3/TiAl3 intermetallic composites. The interconnected channel was formed when using NaCl as space holder, which made the porous NbAl3/TiAl3 an open cellular structure. The open porosity can be improved from 48% to 79% by adding 70vol.% NaCl particles. The small pores (<20μm) in porous NbAl3/TiAl3 originated from the intergranular pores in green compact and the decomposition of TiH2, whereas the large pores (200–300μm) were attributed to the replication of NaCl particles. This leachable space holder route provides a flexible way to adjust the pore morphology and porosity to multifarious applications.

Influence of Al12Mg17 Additive on Performance of Cold-Sprayed Ni-Al Reactive Material

Ternary Ni-Al-Al12Mg17 and binary Ni-Al reactive materials were obtained by cold spraying. The reactive performance of both materials was compared to explore the influence of the Al12Mg17 additive. According to differential scanning calorimetry–thermogravimetric analysis–derivative thermogravimetry, the Ni-Al-Al12Mg17 thermal explosion reaction produced a peak at 480 °C, being 160 °C lower than that of binary Ni-Al. The high-resolution element surface distribution clearly showed the microstructure evolution after controlled annealing and revealed that the rapid oxidation reaction of liquid Mg played a critical role in the occurrence of the thermal explosion reaction in air. During the self-propagating combustion process, the Al12Mg17 additive acted as “ignition spots,” and the Ni-Al intermetallic reaction was ignited by combustion of Mg. The sustained power provided by the Mg combustion promoted the continuous reaction in the Ni-Al matrix. The combustion wave velocity of the Ni-Al-Al12Mg17 material was almost doubled, from 60-100 to 150-200 mm s−1, demonstrating the suitability of the Al12Mg17 additive for enhancing the performance of Ni-Al-based reactive material.

Fabrication and Characterization of Highly Porous FeAl‐Based Intermetallics by Thermal Explosion Reaction

Porous FeAl‐based intermetallics with different nominal compositions ranging from Fe–40 at% Al to Fe–60 at% Al are prepared by a novel process of thermal explosion (TE) mode. The results show that the Al content significantly affects the combustion behavior of the specimens, the ignition temperature of the Fe–Al intermetallics varies from 641 to 633 °C and the combustion temperature from 978 to 1179 °C. The porous materials exhibit uniform pore structures with porosities and average pore sizes of 52–61% and 20–25 µm, respectively. The TE reaction is the dominant pore formation mechanism regardless of the alloy composition. However, differences in the porosity and average pore size are observed depending on the Al content. The compressive strength of porous Fe–Al intermetallics is in the range of 23–34 MPa, duly applied as filters. Additionally, a surface alumina layer is formed at the early stage and both of the oxidation process and the sulfidation process follows the familiar parabolic rate law in the given atmosphere, exhibiting excellent resistance to oxidation and sulfidation. These results suggest that the porous Fe–Al intermetallics are promising materials for applications in harsh environments with a high‐temperature sulfide‐bearing atmosphere, such as in the coal chemical industry.

Grain Refinement of Al-5Ti-0.62C-0.2Nd Grain Refiner for Commercial Pure Aluminum and Its Effect on Mechanical Properties

It is well known that the mechanical properties of commercial pure Al are influenced by size of α-Al dendrites. In the present work, Al-5Ti-0.62C-0.2Nd grain refiner was prepared by a pure molten aluminum thermal explosion reaction, and its effect on grain refinement and mechanical properties of commercial pure Al was investigated. Microstructure and phase composition show that Al-5Ti-0.62C-0.2Nd grain refiner consists of α-Al, granular TiC, lump-like TiAl3, and block-like Ti2Al20Nd. Grain-refining tests on commercial pure Al show that an Al-5Ti-0.62C-0.2Nd grain refiner has better refining performance compared with Al-5Ti-0.62C grain refiner. With addition of 0.2-wt.%-Al-5Ti-0.62C-0.2Nd grain refiner, the average grain size of commercial pure Al can be refined from roughly 2800 to 155±5 µm effectively, and it has higher resistance to grain-refinement fading. On account of the grain refinement, the tensile strength and elongation are increased by approximately 18.3% and 83.5%, respectively.

Effects of Al–Ti–C–Ce Master Alloy on Microstructure and Mechanical Properties of Hypoeutectic Al–7%Si Alloy

It is well known that the mechanical properties of hypoeutectic Al–7%Si alloys are influenced by the size, morphology, and distribution of primary α-Al and eutectic Si crystals. In the present work, a novel Al–Ti–C–Ce master alloy was prepared by the pure molten aluminum thermal explosion reaction, and its effects on the microstructure and mechanical properties of hypoeutectic Al–7%Si alloy were investigated. The results show that the Al–Ti–C–Ce master alloy containing α-Al, granular TiC, lump-like TiAl3, and block-like Ti2Al20Ce has excellent refining and modification properties for hypoeutectic Al–7%Si alloy. When 1.5 wt.% Al–Ti–C–Ce master alloy is added, the coarse dendritic α-Al in hypoeutectic Al–7%Si alloy is refined into equiaxed grains. The secondary dendritic arm spacing (SDAS) was also reduced, and the coarse needle-flake eutectic Si phase was transformed into a fibrous and granular phase. It was found that the ultimate tensile strength and elongation increased by 59% and 56%, respectively, due to the decrease in the SDAS of primary α-Al dendrites and the modification of eutectic Si crystals. Moreover, the change of mechanical properties corresponds to the evolution of microstructure.

Fabrication of highly porous TiAl3 intermetallics using titanium hydride as a reactant in the thermal explosion reaction

Abstract

Porous TiAl3 intermetallics with symmetrical graded pore-structure fabricated by leaching space holder and thermal explosion process

High porosity of porous TiAl3 intermetallics were prepared from Ti-75Al at.% elemental powders by NaCl as temporary space holder, combining with multilayer stacking technology and thermal explosion reaction. Results showed that space holder particles in green compact were completely removed before sintering and only single TiAl3 phases were synthesized in sintered product. The porosity (>70%) can be easily obtained when adding 0, 30 or 50 vol.% NaCl in designed layers. SEM image indicated that the distribution of small pores (<80 μm) was uniform and embedded in pore walls of the skeleton without NaCl space holder. Large pores (300–500 μm) began to appear when added the NaCl particles and the number of large pores improved with the increase of space holder content. The sintered discs with three or five sintered layers exhibited a symmetrical graded pore size distribution: large pores replicating from space holder and small pores generated during the sintering process. The graded pore-structure can be designed by adjusting the compact structure in each layer, allowing for a widespread applications in specific filtration units and liquid-gas separation.

Microstructure and properties of Al-Cr porous intermetallics fabricated by thermal explosion reaction

Al-Cr porous intermetallics were fabricated by thermal explosion (TE) reaction. Microstructure (phase composition and pore characteristics) and properties (expansion behavior, density and oxidation resistance) of Al-Cr sintered products were investigated. Results showed that the sintered products retained their original cylindrical shape with an obvious expansion behavior and high open porosity. The interconnected pores were mainly inherited from green compacts or come from the diffusion of liquid Al and volume expansion. The phase compositions of the specimen with Al:Cr = 8:5 and 9:4 were mainly Al8Cr5 with minor Al4Cr (in the specimen with Al:Cr = 9:4), Al2O3 and unreacted Cr. The intermetallics of the specimen with Al:Cr = 8:5 had superior oxidation resistance.

A novel fabrication strategy for highly porous FeAl/Al2O3 composite by thermal explosion in vacuum

The high porosity of FeAl/Al2O3 composites was successfully prepared via a low-energy consumption method of thermal explosion (TE) in vacuum from reactant mixtures of Fe, Al and Fe2O3. The temperature profiles, phase compositions, microstructure, porosity and pore size of the products were investigated. The TE reactions were ignited between 639 and 648 °C and maximum combustion temperatures reached to 1196–1867 °C. XRD patterns showed that FeAl, Fe2Al5 and Al2O3 were formed via TE reaction, and FeAl and Al2O3 were evolved as dominant phase after the final sintering at 1100 °C. The FeAl/Al2O3 composites exhibited an interconnected pore structure with porosities and pore size of 52–61% and 27–32 μm, respectively.

Microstructure Evolution and Pore Formation Mechanism of Porous TiAl3 Intermetallics via Reactive Sintering

Porous TiAl3 intermetallics were fabricated through vacuum reactive sintering from Ti–75Al at.% elemental powder mixture. The phase compositions, expansion behaviors, pore characteristics and microstructure evolution of TiAl3 intermetallics were investigated, and the pore formation mechanism was also proposed. It was found that the actual temperature of compacts showed an acute climb from 668 to 1244 °C in 166s, while the furnace temperature maintained the linear growth of 5 °C/min, which indicated that an obvious thermal explosion (TE) reaction occurred during sintering, and only single-phase TiAl3 intermetallic was synthesized in TE products. The open porosity increased from 22.2 (green compact) to 32.8% after reactive diffusion sintering at 600 °C and rised to 58.7% after TE, then decreased to 51.2% after high-temperature homogenization at 1100 °C. Therefore, TE reaction is the dominated pore formation mechanism of porous TiAl3 intermetallics. The pore evolution in porous TiAl3 intermetallics occurred by the following mechanisms: certain intergranular pores remained among powder particles of green compact, then low-temperature sintering resulted in a further increase in porosity due to the Kirkendall effect. Moreover, TE reaction gave rise to a dramatic volume expansion because of the rapid increase in temperature, and high-temperature sintering caused densification and a slight shrinkage.