Long-term Heat Treatments Research Articles

Energy-Dispersive X-Ray Microanalysis – as a Method for Study the Aluminium-Polysilicon Interface after Exposure with Long-Term and Rapid Thermal Annealing

Energy dispersive X-ray microanalysis is one of the main methods for determining the elemental composition of matter. Possessing high locality and a relatively shallow penetration depth of the electron beam (< 1 μm), this method has found wide application in the field of microelectronics, as the main method for analyzing the elemental composition of matter. The method allows to study the surface of a substance both pointwise and over an area with the construction of element distribution maps. In the paper we investigated the influence of long-term and rapid heat treatments on the formation of the aluminum-polysilicon interface in order to study the formation of ohmic contacts in the element base of integrated circuits. The aluminumpolysilicon interface was studied using energy-dispersive X-ray microanalysis. It has been established that during long-term thermal annealing (450 °C, 20 min) polysilicon is completely dissolved in aluminum followed by its segregation in the form of separate agglomerates in the aluminum film, which can lead to a complete failure of the integrated circuit. During rapid thermal annealing (450 °C, 7 s) such a phenomenon was not detected. Thus it is advisable to use rapid thermal annealing as an alternative to traditional long-term thermal annealing in microelectronics. This makes it possible to significantly reduce the dissolution of polysilicon in aluminum, avoid the destruction of ohmic contacts and increase the percentage of yield of workable 0products in the process of integrated circuits' manufacturing.

Dependence of deformation mechanisms on micro-pores in the fourth-generation single crystal superalloy during out-of-phase thermal-mechanical fatigue

The high-temperature and long-term solution heat treatments of fourth-generation single crystal (SX) superalloys have sparked an increasing number of homogenization pores in the alloy. Out-of-phase (OP) thermal–mechanical fatigue (TMF) experiments were conducted to elucidate the effect of pores on the fatigue behaviors and deformation mechanisms. The results showed that the plastic deformation was mainly concentrated upon γ matrix in regions away from pores, more specifically, intrinsic stacking fault (ISF) and extrinsic stacking fault (ESF) as well as dislocation cross-slipping were detected. In areas near micro-pores, however, the γ/γ′-structure was subjected to severe twinning shearing. High-resolution observation illustrated that the amount of ESF and a/6 〈112〉 twinning dislocations had remarkably increased. Besides, the partial dislocations were more prone to move in different slipping systems at these regions with considerable stress concentration. These factors collectively enabled the formation of deformation twins from the edge of micro-pores. Moreover, the twinning nucleation could occur near both H-pores and S-pores, while the increasing pore size would enhance the nucleation and propagation of pore-induced twins by a large margin. These findings reported have provided important guidance for the future application of the fourth-generation SX superalloys.

Accelerated coarsening behavior of the γ´-phase in CMSX-4 during non-isothermal heat treatment

In literature, microstructural investigations can be found comparing the evolution of the well-known γ/γ ´ -phase morphology of nickel-based superalloys for long-term heat treatments with and without mechanical load, as well as for isothermal and non-isothermal creep tests. However, data of experiments conducted under non-isothermal conditions without an applied load are hard to find. To close this gap, isothermal and non-isothermal heat treatments without a mechanical load were conducted. Conductive heating was applied to samples that consisted of segments with different cross sections and allowed the simultaneous testing at different temperatures. The microstructural changes were examined by scanning electron microscopy after 500 h and 1000 h. Several established methods for the quantification of the γ ´ -phase coarsening were evaluated and employed for comparison of the two heating conditions studied. The coarsening was more distinct in samples heat-treated under non-isothermal conditions at all temperatures investigated. This was also verified by calculating the connectivity number NA, which was found to be the most useful parameter for quantification of microstructural changes observed. The γ ´ -precipitates became more connected during the heat-treatment, but no preferred direction of coarsening was observed like during rafting under an applied load. Therefore a more general particle agglomeration mechanism is assumed to be responsible for coarsening of the γ ´ -phase.

Thermal ageing effect on solute segregation and precipitation in the heat-affected-zone of dissimilar metal welds for nuclear power plants

Solute segregation in the carbon-depleted (CDZ) heat-affected zone of dissimilar metal welds (DMW) designed for nuclear power plant EPRTM reactors has been investigated by atom probe tomography (APT) and related to the mechanical properties (toughness test). The analysis of grain boundaries (GBs) by APT allowed the quantification of the P segregation and other solute elements, in all the analyzed samples, before and after long-term ageing heat treatments. The post-weld stress-relief heat treatment applied to the samples before the ageing heat treatments leads to important GBs segregation levels. Following thermal ageing treatments allow a relative increase of GB segregation. P segregation increases with the various thermal ageing conditions. The experimental data were used to model P segregation in high-angle GBs. The simulations show that P segregation is kinetically limited by bulk diffusion, explaining the observed P segregation increase with temperature. P segregation in GBs in the CDZ of the DMW should remain low compared to the expected equilibrium segregation level during the entire reactor service. In addition to solute segregation in extended defects, APT measurements revealed the presence of Cu-rich clusters in GBs, dislocations, and grains in all the aged samples. These clusters were not observed in the sample before ageing.

Thermal Load Influence during the Formation of Al-Al Contacts on the Electrical Parameters of the Integrated Circuits with Aluminum-Polysilicon Contacts

This work is devoted to establishing the effect of using rapid thermal processing (RTP) method (450 °C, 7 s) to form an ohmic contact between two layers of aluminum metallization on the electrical parameters and reliability of integrated circuits. The resistance values of contact chains aluminum-silicon, aluminum-polysilicon, polysilicon-silicon n+, aluminum-silicon n+, current-voltage characteristics of the tested bipolar transistors, as well as the results of the reliability analyses by conducting thermal field tests were chosen as the analyzed parameters of this microcircuit. Comparison of these parameters was carried out with respect to the microcircuits manufactured using standard RTP method (450 °C, 20 min) to form this contact. An analysis of the results of the resistance value of various contact chains showed that, regardless of the type of thermal treatment, all contact chains, with the exception of the aluminum-polysilicon contact chain, have almost the same resistance. By analyzing the elemental composition of the cleavage in the area of this contact by scanning electron microscopy, it was found that during rapid heat treatment, the depth of penetration of aluminum into polysilicon is 2 times less than during its standard formation due to a 2-fold reduction in the time of exposure to high temperature compared to the standard process. This leads to a lower concentration of the aluminum in the silicon and as a result to a higher contact resistance between the aluminum and polysilicon. An analysis of the currentvoltage characteristics showed that they are all identical, except for the course of the direct branch of the base current value from the emitter-base voltage. The deviation of the linear nature of this dependence in the region of their low voltage values (£ 200 mV) in the case of the formation of ohmic Al-Si and Al-Al contacts with the use of long-term heat treatments is due to the predominance of the generation-recombination current in this region associated with an increased density of traps in the depleted region and on the surface of the semiconductor. The ideal behavior of the base current versus the emitter-based voltage is maintained by applying rapid RTP method to form an Al-Al contact by eliminating traps both in the depletion layer and on the surface of the semiconductor. The tests carried out on the reliability of these products showed that it does not depend on the type of formation of ohmic contacts between the metallization layers.

Electrolyzed reduced water could regulate muscular energy producing ability of heat-exposed broiler chickens

We have previously shown that alleviation in ROS-induced oxidative damage to skeletal muscle and improvement in production efficiencies are observed in broiler chickens supplied with electrolyzed-reduced water when exposed to 5 days-long chronic heat exposure. In this study, we investigated whether this reduced water may also improve those parameters in broiler chickens exposed to 14 days-long chronic heat exposure. Broiler chickens (Gallus gallus) were fed either a control diet with tape water or control diet supplied with electrolyzed-reduced water for 14 d after hatch, and then exposed either to heat stress (34oC for 14 days), or kept at a thermoneutral temperature (24oC). Skeletal muscle substrate metabolism (3HADH and CS activities), adenine nucleotides concentration, enzymatic scavenging systems were studied. We confirmed that heat-stressed broilers decreased weight gain and feed consumption compared to control broilers. In contrast, heat-exposed broilers supplied with electrolyzed-reduced water significantly and definitely improved performance when compared to heat-stressed control broilers. On exposure to long-term heat treatments, broiler supplied with reduced water showed higher CS activity, indicating that the reduced water could improve energy-producing ability via modulating TCA cycle. We also found that the reduced water could definitely improve the total adenine nucleotide (TAN), adenylated energy charge (AEC), and ATP to ADP ratios under such conditions. On the other hand, broiler chickens supplied with reduced water kept relatively constant the SOD, catalase, and GPx activities, implying that reduced water could support the enzymatic scavenging systems. Taken together, these results suggest that broiler chickens supplied with electrolyzed-reduced water could improve energy producing ability via modulating TCA-cycle enzyme, and lead to improving performance.

Phase stability of V- based multi-principal element alloys

The success of fusion power requires alloys with good mechanical properties and the ability to withstand extreme thermal and irradiation conditions without prohibitive levels of activation or structural degradation. Body-centred cubic multi-principal component alloys, V–Cr–Mn and Ti–V–Cr–Mn have shown promising high-temperature stability at . For the alloys to be suitable for use in nuclear fusion reactors, they must be stable across a wide range of temperatures relevant to fusion applications. Here, we assess alloy microstructural stability in these systems following long-term heat treatments at various temperatures. Encouragingly, most of the alloys showed no significant change in microstructure across all temperatures. This contrasts with many other multi-principal component alloys, which develop complex microstructures after exposure to intermediate temperatures.

Phase transformations and phase stability in the Ti–44 at.%Al–(0–7 at.%)Mo system

Molybdenum is an excellent candidate to study the effect of β-stabilizing elements on the phase transitions in intermetallic γ-TiAl based alloys. In the present work, two model alloys based on the Ti–44Al (at.%) system containing 1 at.% and 5 at.% Mo are investigated. Examinations of the microstructures after casting and subsequent heat treatments reveal significant differences in the morphology and amount of the phases present and, thus, highlight the effect of Mo on the different phase transformations near and far from thermodynamic equilibrium. In-situ high-energy X-ray diffraction and differential scanning calorimetry were employed to identify the occurring phases and determine the phase transition temperatures in the two model alloys. Especially, the β phase was found to be strongly impacted by the Mo content of the alloys. While the β phase is only present at high temperatures in the Ti–44Al–1Mo alloy, it is thermodynamically stable from the liquidus temperature downwards to room temperature in the Ti–44Al–5Mo alloy. Long-term heat treatments for 30 days at 1000 °C were conducted to determine the phase equilibria at intermediate temperatures. Ultimately, the findings of this work were combined with data from literature to reassess the quasi-binary Ti–44Al–(0–7)Mo (at.%) phase diagram.

Quantitative Nanostructure and Hardness Evolution in Duplex Stainless Steels: Under Real Low-Temperature Service Conditions

Duplex stainless steels are a group of widely used stainless steels, because of their attractive combination of strength and corrosion resistance. However, these steels embrittle because of a phase separation phenomenon in the ferrite phase when exposed to temperatures within the miscibility gap. This manuscript investigates the phase separation in two commercial stainless steels, the duplex stainless steel (DSS) 22Cr-5Ni (2205 or UNS S32205), and the super-duplex stainless steel (SDSS) 25Cr-7Ni (2507 or UNS S32750), and its subsequent effect on mechanical property evolution. Long-term isothermal aging heat treatments were carried out at industrially relevant temperatures between 250 °C and 350 °C for up to 48,000 hours, and quantitative measurements of the amplitude and wavelength of the phase separated nanostructure were obtained using Small-Angle Neutron Scattering (SANS). These quantifications were used as input parameters in hardness models to predict the hardness evolution. It is concluded that the quantitative information from SANS combined with these hardness models enables the prediction of hardness evolution in DSS at low temperatures, which in turn correlates with the embrittlement of the DSS.

Design of a Co\u2013Al\u2013W\u2013Ta Alloy Series with Varying \u03b3\u2032 Volume Fraction and Their Thermophysical Properties

The γ′ volume fraction is a key parameter in precipitation-strengthened Co- and Ni-base superalloys and mainly determines the alloys’ properties. However, systematic studies with varying γ′ volume fractions are rare and the influence on thermal expansion has not been studied in detail. Therefore, a series of six Ta-containing Co-based alloys was designed with compositions on a γ–γ′ tie-line, where the γ′ volume fraction changes systematically. During solidification, Laves (C14-type) and µ (D85-type) phases formed in alloys with high levels of W and Ta. Single-phase γ or two-phase γ/γ′ microstructures were obtained in four experimental alloys after heat treatment as designed, whereas secondary precipitates, such as χ (D019-type), Laves, and μ, existed in alloys containing high levels of γ′-forming elements. However, long-term heat treatments for 1000 hours revealed the formation of the χ phase also in the former χ-free alloys. The investigation of the thermal expansion behavior revealed a significant anomaly related to the dissolution of γ′, which can be used to determine the γ′ solvus temperature with high accuracy. Compared to thermodynamic calculations, differential scanning calorimetry (DSC) and thermal expansion analysis revealed a larger increase of the γ′ solvus temperatures and a lesser decline of the solidus temperatures when the alloy composition approached the composition of the pure γ′ phase.

A comparative analysis of Inconel 718 made by additive manufacturing and suction casting: Microstructure evolution in homogenization

Homogenization is one of the critical stages in the post-heat treatment of additive manufacturing (AM) component to achieve uniform microstructure. During homogenization, grain coarsening could be an issue to reserve strength, which requires careful design of both time and temperature. Therefore, a proper design of homogenization becomes particularly important for AM design, for which work hardening is usually no longer an option. In this work, we discovered an intriguing phenomenon during homogenization of suction-cast and AM Inconel 718 superalloys. Through both short and long-term isothermal heat treatments at 1180 °C, we observed an abnormal grain growth in the suction-cast alloy but continuous recrystallization in the alloy made by laser powder bed fusion (LPBF). The grain size of AM samples keeps as small as 130 μm and is even slightly reduced after homogenization for 12 h. The homogeneity of Nb in the AM alloys is identified as the critical factor for NbC formation, which further influences the recrystallization kinetics at 1180 °C. Multi-type dislocation behaviors are studied to elucidate the grain refinement observed in homogenized alloys after LPBF. This work provides a new pathway on microstructure engineering of AM alloys for improved mechanical performance superior to traditionally manufactured ones.

Effect of Boron on the Thermal Stability of the Precipitate Phase of FC92B and FC92N Steels During Aging and Creep

In this study, precipitation strengthening alloying elements (C, N, V, Nb, Ta) and solution strengthening alloying elements (Cr, W, Mo, B) were appropriately combined to fabricate FC92B and FC92N alloys. Long-term aging heat treatments at 650 °C of up to 20,000 h were conducted to compare the high-temperature properties of each alloy, investigate the effects of microstructure stability on creep performance characteristics, and analyze the precipitate growth behavior in relation to the long-term thermal stability of the precipitate phase. Transmission electron microscopy and nanoscale secondary ion mass spectrometry were carried out to investigate microstructural changes according to the aging treatment time and the microstructure of creep specimens in relation to the addition of trace alloying elements. Although FC92B contains a higher B content than FC92N, FC92B did not exhibit significant changes in hardness or tensile strength regardless of the aging time, which extended to a maximum of 20,000 h. The FC92B alloy with an adequate addition of B showed the highest creep performance and exhibited rupture durations that were more than twice those of FC92N under stress conditions of 100 MPa and 140 MPa at 650 °C. The precipitate size of FC92N was significantly coarser than that of FC92B. In FC92B aged for 20,000 h, B was distributed in the grain boundaries, as shown by Nano-Secondary Ion Mass Spectrometry. B content was found to be high within the M23C6 precipitate.

High-Temperature Resistant and Gas-Tight Coatings for Long-Term Heat Treatments using the Example of α-Cu-Al-Zn

Abstract Cu alloys with relatively small contents of Zn and Al will oxidize heavily during heat treatment in air. Using shielding gas, however, will lead to the evaporation of Zn. A gas-tight coating, which also prevents oxidation, is required when heat treatments are carried out over several days or weeks. In the context of experiments that are aiming at the determination of diffusion parameters such long-term heat treatments are carried out. Different coatings on Cu-Zn12-Al4 (wt.%) heat-treated at 850 °C over 72 h were evaluated. A suitable system of layers consisting of an Y2O3-based and a MgO-based coating was successfully applied using the example of a heat treatment on Cu-Zn15-Al1 at 850 °C over 27 days.

The Effectiveness of Al-Si Coatings for Preventing Interfacial Reaction in Al-Mg Dissimilar Metal Welding

The dissimilar welding of aluminum to magnesium is challenging because of the rapid formation of brittle intermetallic compounds (IMC) at the weld interface. An Al-Si coating interlayer was selected to address this problem, based on thermodynamic calculations which predicted that silicon would change the reaction path to avoid formation of the normally observed binary Al-Mg IMC phases (β-Al3Mg2 and γ-Al12Mg17). Long-term static heat treatments confirmed that a Si-rich coating will preferentially produce the Mg2Si phase in competition with the less stable, β-Al3Mg2 and γ-Al12Mg17 binary IMC phases, and this reduced the overall reaction layer thickness. However, when an Al-Si clad sheet was tested in a real welding scenario, using the Refill™ friction stir spot welding (FSSW) technique, Mg2Si was only produced in very small amounts owing to the much shorter reaction time. Surprisingly, the coating still led to a significant reduction in the IMC reaction layer thickness and the welds exhibited enhanced mechanical performance, with improved strength and fracture energy. This beneficial behavior has been attributed to the softer coating material both reducing the welding temperature and giving rise to the incorporation of Si particles into the reaction layer, which toughened the brittle interfacial IMC phases during crack propagation.

Role of Nanostructuring and Microstructuring in Silver Antimony Telluride Compounds for Thermoelectric Applications.

Thermoelectric (TE) materials are of utmost significance for conversion of heat flux into electrical power in the low-power regime. Their conversion efficiency depends strongly on the microstructure. AgSbTe2-based compounds are high-efficiency TE materials suitable for the mid-temperature range. Herein, we explore an Ag16.7Sb30Te53.3 alloy (at %) subjected to heat treatments at 380 °C for different durations aimed at nucleation and coarsening of Sb2Te3-precipitates. To characterize the Sb2Te3-precipitation, we use a set of methods combining thermal and electrical measurements in concert with transmission electron microscopy and atom probe tomography. We find correlations between the measured TE transport coefficients and the applied heat treatments. Specifically, the lowest electrical and thermal conductivity values are obtained for the as-quenched state, whereas the highest values are observed for alloys aged for 8 h. In turn, long-term heat treatments result in intermediate values of transport coefficients. We explain these findings in terms of interplay between precipitate formation and variations in the matrix composition, highlighting the importance of thermal stability of the material under service conditions.

Comparative Evaluation of Cast Aluminum Alloys for Automotive Cylinder Heads: Part I—Microstructure Evolution

The present study stages a comparative evaluation of microstructure and associated mechanical and thermal response for common cast aluminum alloys that are used for manufacturing automotive cylinder heads. The systems considered are Al-Cu (206-T6), Al-Si-Cu (319-T7), and Al-Si (356-T6, A356-T6, and A356 + 0.5Cu-T6). The focus of the present manuscript is on the evaluation of microstructure at various length scales after aging, while the second manuscript will deal with the mechanical and thermal response of these alloys due to short-term (aging) and long-term (pre-conditioning) heat treatments. At the grain-scale, the Al-Cu alloy possessed an equiaxed microstructure as opposed to the dendritic structure for the Al-Si-Cu or Al-Si alloys which is related to the individual solidification conditions for these alloy systems. The composition and morphology of intermetallic precipitates within the grain and at the grain/dendritic boundary are dictated by the alloy chemistry, solidification, and heat treatment conditions. At the nanoscale, these alloys contain various metastable strengthening precipitates (GPI and $$ \theta^{\prime\prime} $$ in Al-Cu alloy, $$ \theta^{\prime} $$ in Al-Si-Cu alloy, and $$ \beta^{\prime} $$ in Al-Si alloys) with varying size, morphology, coherency, and thermal stability.

Planar, Polysilazane‐Derived Porous Ceramic Supports for Membrane and Catalysis Applications

Porous, silicon carbonitride‐based ceramic support structures for potential membrane and catalysis applications were generated from a preceramic polysilazane precursor in combination with spherical, ultrahigh‐molecular weight polyethylene microparticles through a sacrificial filler approach. A screening evaluation was used for the determination of the impact of both porogen content and porogen size on pore structure, strength, and permeability characteristics of planar specimens. By optimizing both the composition as well as cross‐linking parameters, maximum characteristic biaxial flexural strengths of 65 MPa and porosities of 42% were achieved. The evolution of an interconnected, open‐pore network during thermal porogen removal and conversion of the preceramic polymer led to air permeabilities in the order of 10−14 m². The materials were further exposed to long‐term heat treatments to demonstrate the stability of properties after 100 h at 800°C in oxidizing, inert, and reducing environments. The determined performance, in combination with the versatile preparation method, illustrates the feasibility of this processing approach for the generation of porous ceramic support structures for applications at elevated temperatures in a variety of fields, including membrane and catalysis science.

Thermodynamic Calculations of Phase Equilibria and Phase Fractions of a β-Solidifying TiAl Alloy using the CALPHAD Approach

ABSTRACTThe CALPHAD (CALculation of PHAse Diagrams) method is widely recognized as a powerful tool in both scientific and industrial development of new materials and processes. For the implementation of consistent databases, where each phase is described separately, models are used which are based on physical principles and parameters assessed from experimental data. Such a database makes it possible to perform realistic calculations of thermodynamic properties of multi-component systems. However, a commercial available TiAl database can be applied for thermodynamic calculations to both conventional Ti-base alloys and complex intermetallic TiAl alloys to describe experimentally evaluated phase fractions as a function of temperature. In the present study calculations were done for a β-solidifying TiAl alloy with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at. %), termed TNMTM alloy. At room temperature this alloy consists of ordered γ-TiAl, α2-Ti3Al and β0-TiAl phases. At a certain temperature α2 and β0 disorder to α and β, respectively. Using the commercial database the thermodynamic calculations reflect only qualitative trends of phase fractions as a function of temperature. For more exact quantitative calculations the commercial available thermodynamic database had to be improved for TiAl alloys with high Nb (and Mo) contents, as recently reported for Nb-rich γ-TiAl alloys. Therefore, the database was modified by experimentally evaluated phase fractions obtained from quantitative microstructure analysis of light-optical and scanning electron micrographs as well as conventional X-ray diffraction after long-term heat treatments and by means of in-situ highenergy X-ray diffraction experiments. Based on the CALPHAD-conform thermodynamic assessment, the optimized database can now be used to correctly predict the phase equilibria of this multi-component alloying system, which is of interest for applications in automotive and aircraft engine industry.

On Phase Equilibria and Phase Transformations in β/γ-TiAl Alloys – A Short Review

Due to the fact that Molybdenum is a strong β stabilizer it plays an important role among TiAl-based alloys, especially since a significant volume fraction of the disordered body-centered cubic β-phase improves the processing characteristics during hot-working at elevated temperatures. Unfortunately the effect of Mo on the individual phases and their transition temperatures is insufficiently known though it is essential for designing engineering components. In this paper sections of the Ti-Al-Mo ternary phase diagram derived from thermodynamic calculations as well as experimental data are presented. Furthermore the effect of Mo on the phase transformation behavior during slow heating and rapid cooling in the course of isothermal short and long-term heat treatments is elaborated. Below a certain amount of Mo the high-temperature cubic β-phase transforms into ordered hexagonal α2'-martensite during water quenching. At higher contents the high-temperature β-phase can be retained upon quenching; only the ordering reaction β → βo takes place. The subsequent tempering at 1000 °C leads to a strong grain refinement as well as to a reduction in hardness.

Micro-Chemistry and Mechanical Behaviour of Ti6Al4V-SiC<sub>f</sub> Composite Produced by HIP for Aeronautical Applications

The paper reports the results of an extensive characterization of the Ti6Al4V-SiCfcomposite produced by hot isostatic pressing (HIP) to assess its capability to withstand the in-service conditions of turbine blades operating at middle temperatures in aeronautical engines. The microstructure of composite, in as-fabricated condition and after long-term heat treatments (up to 1,000 hours) in the temperature range 673-873 K, has been investigated by means of different techniques. Particular attention was paid to the micro-chemical evolution of fibre-matrix interface which is scarcely affected also by the most severe heat treatments examined here. This leads to stable mechanical properties as evidenced by hardness, tensile and FIMEC instrumented indentation tests. Therefore, the composite can operate at the maximum temperature (873 K) foreseen for its aeronautical applications without remarkable modifications of its microstructure and degradation of mechanical properties. The mechanical characterization has been completed by internal friction and dynamic modulus measurements carried out both at constant and increasing temperature, from 80 to 1173 K.