High Temperature Microstructure Research Articles

Investigation of microstructural and thermal stability of Ni-Y-Zr ternary nanocrystalline alloy

This investigation focused on the thermal stability of nanocrystalline (NC) binary and ternary Ni-Y-Zr alloys synthesized through ball-milling. The microstructural changes following annealing, conducted up to 1200 °C, were studied using various techniques, including X-ray-line-broadening, micro-hardness, and transmission electron microscopy. The results revealed that the rate of grain growth observed in the Ni-Y-Zr ternary alloy at 600 °C resembled that in pure NC-Ni at 100 °C. Moreover, the Ni-1.4Y-1.1Zr ternary system exhibited a maximum hardness of 753 HV (average) at 600 °C, which was approximately 60 HV higher than the Ni-1.2Y/1.9Y and Ni-1.5Zr/2.7Zr binary alloys and more than double that of pure NC-Ni for similar grain sizes. These characteristics were linked to the formation of nano-sized oxides and nitrides of Y and Zr within the microstructure. In summary, this study emphasizes the notable high-temperature microstructure stability of Ni-based ternary alloys, attributed to the additive effects of Y and Zr. Due to this extended stability, this ternary system could find potential applications at elevated temperatures in areas such as jet engine turbine blades and power plants.

Improved understanding of the growth of blocky alpha in welded Zircaloy-4

Zirconium alloys are widely used in nuclear reactors as fuel cladding materials. Fuel cladding is used to contain the nuclear fuel and cladding tubes are typically sealed using welds. Welding of zirconium alloys can result in changes in the local microstructure, with the potential to grow so called ‘blocky α’ grains in the welded region during subsequent thermal processing and these blocky α grains have the potential to be detrimental to the integrity of the component. In this work, complimentary heating experiments with ex situ and in situ electron backscatter diffraction (EBSD) analysis are used to aid understanding of the blocky α grain growth within the weld region. These experiments reveal that blocky α grain growth is related to the prior-β (high temperature) microstructure, as grains grow adjacent to a prior-β grain boundary and the orientation of the growing α grain can be explained using neighbourhood orientations from this prior-β grain. This growth mechanism is explained via a simple mechanism which is related to the α grain orientations, grain boundary structures and local stored energy. Ultimately, our findings indicate that the likely grain growth (size and morphology) across the weld region can now be predicted from the initial as-welded microstructure.

Kinematic analysis of the Caçapava do Sul Granitic Complex, southern Brazil: Ediacaran syntectonic magmatism and strain partitioning during inclined ductile extrusion along a transpressional shear zone, Dom Feliciano Belt

The Caçapava do Sul Granitic Complex is a ca. 30 km-long syntectonic batholith emplaced at 580-560 Ma along the N–S to NNE-SSW striking Caçapava do Sul Shear Zone, configuring the core of a double-plunging regional asymmetric antiformal fold, with the Passo Feio Metamorphic Complex metavolcanosedimentary wallrocks occurring at the fold limbs. Structural mapping and kinematic analysis along its whole margins allowed to subdivide the granite complex into seven structural domains, each with its own geometric characteristics and kinematics. The observed microstructures included abundant K-feldspar and plagioclase porphyroclasts with asymmetric recrystallization tails, S–C and S–C–C’ fabrics, and more rarely synmagmatic fractures and asymmetrically developed myrmekites. In general, high temperature microstructures are abundant in the southern half of the granitic complex, while these features are overprinted by low temperature brittle-ductile to brittle microstructures in its northern half. The Southern Domain presents foliations with medium angle dip towards the south, high rake lineation towards the SSW and normal shear sense indicators. The Northwestern and Southwestern domains present mainly subvertical N–S striking foliations with sub-horizontal lineations mostly plunging towards SSW and abundant sinistral shear sense indicators. On the other hand, Southeastern and Santos Ferreira domains presents predominantly NE-SW striking foliations with variably dip and sub-horizontal lineations with abundant dextral shear sense indicators. The Northeastern Domain presents low angle foliation dipping towards the NE, which is also locally described in the Southeastern Domain. Dominantly normal movement with top to NE is described in the Northeastern Domain, also locally reported in the Southeastern Domain. The Northern Domain presents low-angle foliation dipping towards the N, with high rake lineation. No shear sense indicator was observed in the Northern Domain due to brittle deformation overprint, but normal kinematics is inferred. The opposite kinematics of the distinct Caçapava do Sul Shear Zone margins are corroborated by punctual data from the Passo Feio Metamorphic Complex rocks. This is compatibilized by the resultant syntectonic granite flow towards N/NE, which is interpreted in a model of oblique transpression with inclined ductile extrusion. The model suggests that transpression of the middle crust had an important pure shear component which was effectively partitioned into structural domains along the Caçapava do Sul Shear Zone in consequence of post-collisional syntectonic magmatism, and coexisted with extensional tectonics in the upper crust (Camaquã Basin) through a detachment along the brittle-ductile transition.

Effect of deep cryogenic treatment on improving the high-temperature impact resistance of Al–Zn–Mg–Cu alloy

The aging Al–Zn–Mg–Cu alloy exhibits a significant decrease in performance after undergoing thermal shock. In order to improve the resistance of this type of alloy to thermal shocks, the influence of deep cryogenic treatment (DCT) on the high temperature mechanical properties and microstructure of thermal shocked Al–Zn–Mg–Cu alloy is investigated in this paper. It was found that the mechanical properties of Al–Zn–Mg–Cu alloys decrease significantly with the increase of electric heating temperature. The deep cryogenic treatment reduced the grain size of Al–Zn–Mg–Cu alloy, increased the density of the precipitates, and also inhibited the coarsening of the precipitates. Therefore, the deep cryogenic treatment can slow down the weakening effect of Al–Zn–Mg–Cu alloy at high temperatures.

High temperature oxidation behavior and microstructure evolution of medium Mn steel

The phase transition, microstructure evolution and oxidation behavior of Fe-0.2C-12Mn-3Al-2Cu-xNi steel during the oxidation process for 120 min at 1050 °C-1200 °C were systematically studied. The twin boundaries will provide rapid channels for the oxygen atoms diffusion, while the (110) and (111) grain orientations show a low oxidation resistance property, which also then promote the oxygen atoms diffuse along grain boundaries. Cu and Ni enrichment areas were formed on the subsurface in the 1100 °C-1200 °C oxidation stage, which limited the ion diffusion rate on the subsurface and prevented the further growth of the oxide layer. The oxidation temperature, Mn concentration gradient and [CuNi] rich region on the subsurface contribute to multi-oxide layer structure: Mn deficient region, Mn rich region, transition layer and inner oxide layer. The Mn deficient region with rough surface is mainly composed of Fe2O3, Fe3O4 and Mn3O4, while the Mn rich region is mainly composed of FeO and MnO. The honeycomb transition layer and inner oxide layer is composed of FeO, MnO, MnAl2O4, and Al2O3.

Castable eutectic Ni–Ce high temperature alloys strengthened by γ/γʹ microstructure

Ni-based superalloys are the premier material for high-temperature applications but are often difficult to manufacture via solidification-based processing. Eutectic alloys typically possess good castability due to narrow solidification ranges. The Ni–Ce system possesses a eutectic reaction but has never been examined as a potential high-temperature material. This work investigates the microstructure and mechanical properties of near-eutectic alloys Ni–17Ce and Ni–17Ce–5Al (wt.%). Both alloys show outstanding high-temperature microstructure and mechanical property stability at 900 °C. The formation of Ni3Al (γʹ) precipitates is observed in the primary FCC-Ni phase of the ternary Ni–Ce–Al alloy. The precipitation hardened ternary alloy exhibits superior hardness to Inconel 718 at elevated temperatures, retaining its strength to 750 °C, or about 50 °C higher than Inconel 718. The novel hierarchical microstructure consisting of thermally stable eutectic regions combined with γʹ-strengthened primary FCC-Ni and eutectic phases show great promise as a castable material solution for high-temperature applications.

G phase and ferrite in a high Si austenitic stainless steel during 510 °C aging

Except the excellent oxidation and corrosion resistance at elevated temperatures, the high temperature microstructure stability of high Si austenitic stainless steels is also of attention. In this work, the microstructure and the corresponding mechanical properties evolutions of a high Si austenitic stainless steel during long-term aging at 510 °C were investigated. The results showed that Si has promoted the formation of a multi-phase, composed by G phase and ferrite. It is revealed that G phase precipitates at grain boundaries prior to ferrite and it is because G phase formation has changed the composition in the surrounding austenite that ferrite was induced. Such ferrite is proved to be responsible for the dramatically decreased impact toughness during aging, since the impact toughness could be rejuvenated by only reducing the ferrite content through high temperature annealing.

High Temperature Deformation Behavior and Microstructure Evolution of Harmonic Structure Composites with WC-Co and High Speed Steel

The high-temperature deformation behavior and microstructural changes of harmonic structure composites with WC-Co alloys and high-speed steel (HSS) were investigated in detail. A harmonic structure composite was fabricated by consolidating the mechanically milled powder having WC-Co and HSS powder. The harmonic structure composite demonstrates the microstructure composed of network area (WC-Co) and dispersed area (HSS). The harmonic structure composite shows a sufficient compressive strength in the compression tests at 773 K, but the compression strength decreases at temperatures of above 873 K. The 0.2% proof stress at high temperature almost unchanged even if the network area fraction changed. Furthermore, the network area plays an important role in the high temperature deformation of harmonic structure composites. These results suggest that the formation of voids for WC-Co boundary sliding and poor sintering is an important factor in stress reduction in the high-temperature compression of harmonic structure composites.

High Temperature Tensile Properties and Microstructure of Ni20Cr Alloy Fabricated by Laser Powder Bed Fusion

Additive Manufacturing (AM) brings about an array of modifications in microstructure with respect to conventional routes transforming mechanical performances. These new microstructure features depend on process parameters and especially on volume energy-density delivered by the laser on powder layer. Among the different alloys manufactured by AM, Ni-alloys exhibit high-strength at elevated temperature opening the way of fabrication of gas turbines and jet-engine parts. Ni-superalloys experience precipitation hardening due to the formation of γ′ and γ′′ phases leading to complex microstructures. To better study the influence of the AM microstructure on Ni-alloys mechanical properties, in particular at elevated temperatures, a theoretically monophasic and binary Ni20Cr-alloy manufactured by laser powder-bed fusion was studied in this work. Remarkable Yield Strength (400 MPa) and Ultimate Tensile Strength (UTS) (600 MPa) were observed at 500°C with hardly any loss of properties from room temperature, owing to the thermal stability of cellular dendrites till 700°C. Ductility drop was reported at 700°C due to anomalous brittle behaviour of Ni-alloys. Hardening behaviour vanished at 900°C signifying the deletion of dendrites, disappearance of dislocations, diffusion of Cr from dendritic walls and growth of oxides.

High-temperature microstructure evolution of an advanced intermetallic nano-lamellar γ-TiAl-based alloy and associated diffusion processes

Nano-lamellar advanced γ-TiAl based alloys doped with small amounts of C and Si are being developed to improve the creep resistance in order to increase the performances of this kind of alloys applied in the low-pressure turbine of aircraft engines. In order to extend the service temperature up to 1073 K or even above, the control of the microstructure stability is key. In this work, a complete study of the microstructure evolution during high-temperature exposure up to 1153 K has been approached through different electron microscopy techniques including HRTEM and HRSTEM with microanalysis. The nucleation and growth of the ordered βo precipitates and the ζ silicides inside the α2 lamellae has been carefully characterized and new orientation relationships and the misfit between all crystalline lattices has been determined, as well as the chemical concentration of the different atomic species on each phase. The electron microscopy study shows that βo and ζ precipitates inside α2 prevents or retards the dissolution of the α2 lamellae and its final disintegration in favor of the γ lamellae. This phenomenon has been discussed in terms of the phase coherence and diffusion processes. These important results allow conclude that the coarsening of the γ lamellae is delayed because of the βo and ζ precipitation, allowing to explain the observed enhancement of the creep resistance in this γ-TiAl based alloy exhibiting a nano-lamellar microstructure.

High Temperature Flexural Strength, Microstructure, and Phase Evolution of Quartz Fiber/Boron Phenolic Resin Ceramizable Composite Modified with W and B4C

In order to investigate the effect of refractory metal on the high temperature properties of phenolic resin matrix composites, modified quartz fiber reinforced ceramizable composites were prepared by a molding process with a refractory component, tungsten, as the functional component and boron carbide as the ceramic forming agent. The effects of the tungsten and the boron carbide on the heat resistance of the composite were investigated. The results showed that the introduced refractory metal tungsten and the boron carbide can react to form tungsten borides and tungsten carbides at high temperature, and form a ceramic layer on the surface of the composite, which can fill the defects caused by pyrolysis of matrix and improve the high temperature performance of the composite. When the content of boron carbide was 10 wt% and the content of tungsten powder was 30 wt%, the flexural strength of the composite before and after heat treatment at 1200 °C were increased by 54.6% and 30.2% respectively compared with that without filler.

Collaboratively enhancing the strength and ductility of GH3600 Nickel-Based Ultra-Thin strips via the Pre-Precipitation process

Here, the effects of pre-precipitation annealing on the microstructural evolution and room-temperature mechanical properties of nickel-based ultra-thin strips after the subsequent high-temperature annealing were analyzed. By pre-precipitation annealing, discontinuous grain-boundary carbide was introduced into the cold-rolled 67 μm GH3600 nickel-based ultra-thin strips, which hindered the rapid migration of grain boundaries. Thus, the grain size was maintained at only 5.5 μm after holding at 1050 °C for 1 h, indicating that the high-temperature microstructure stability of the strips was significantly improved. Further, the phenomenon of local single-layer crystal and size effect caused by the abnormal coarsened grains, which were more than 100 μm in size during the single-stage high-temperature annealing (1050 °C for 1 h), was eliminated. Owing to the weakened free surface influence and an increase in the strengthening effect, such as grain boundary strengthening, as well as intragranular and intergranular back stress strengthening, the work hardening ability of the strips was enhanced after pre-precipitation annealing, thereby resulting in excellent comprehensive mechanical properties whose flow stress and elongation were 1.9 and 4.8 times those of single-stage high-temperature annealing. This study successfully provides an economical and easy-to-implement effective method to prepare solid-solution-strengthened high-performance ultra-thin superalloy strips.

Mechanical properties, high temperature resistance and microstructure of eco-friendly ultra-high performance geopolymer concrete: Role of ceramic waste addition

This study aims to develop ultra-high performance geopolymer concrete (UHPGC) using fine ceramic waste (FCW) as a partial fine aggregate. In this research, 12 mixes were designed, prepared, and tested to evaluate the influence of ceramic waste as a partial replacement of fine aggregate up to 22.5% on the performance of UHPGC. The fresh properties, mechanical properties, durability, and high temperature resistance of the developed material were investigated. The ecological assessment of UHPGC incorporating FCW has also been analyzed via life cycle assessment. The experimental results showed that the incorporation of FCW accelerates the setting time and reduces the slump of the fresh concrete. For hardened concrete, it has a relatively negative effect on mechanical properties to a certain extent. The compressive strength decreased to 123 MPa with the incorporation of 22.5% FCW. However, with smaller replacement percentages, 7.5 and 15%, the concrete exhibited better strength than 22.5%. This reduction may be attributable to the lower bond between the geopolymer gel and the ceramic particles. It was additionally found that the UHPGC's sorptivity increased simultaneously with increasing FCW content. For, high temperature resistance, inclusion of fine ceramic waste showed enhancement of residual strength as compared to the control mix after 700 °C exposures. A microscopic investigation showed that samples utilizing ceramic aggregate had fewer micropores and cracks after being subjected to high temperatures. Besides, TGA analysis revealed that mixtures using FCW were more stable up to 650 °C. Finally, using FCW can reduce carbon emissions and energy consumption to acceptable levels of 267 kg CO2eq/t and 571 MJ/m3, respectively, with low economic targets. Based on the obtained results, ceramic waste can be optimized and exploited as potential sources for manufacturing eco-friendly UHPGC with improved resistance to high temperatures and minimum harmful effects on mechanical properties and durability.

High temperature microstructure stability of Waspaloy produced by Wire Arc Additive Manufacturing

The microstructural stability of Waspaloy produced by wire arc-based Cold Metal Transfer (CMT) was studied in the 700–1050 °C temperature range. Major process-induced chemical segregation resulted in heterogeneous γ' precipitation between dendrite cores and interdendritic spacings up to 1050 °C. The coarsening behavior of γ' followed the Lifshitz-Slyozov-Wagner theory between 760 and 900 °C. Diffusion activation energies revealed that kinetics in the dendrite cores are faster than within the interdendritic spacings, although precipitates in the latter appear more stable at higher temperatures. Fine globular (Cr,Mo)23C6 and blocky (Ti,Mo)C carbides were observed to decorate grain boundaries. The formation of plate-like (Cr,Mo)23C6 was found in both interdendritic spacings and grain boundaries. The laths were predominantly aligned along the <110>γ directions and precipitated as a result of (Ti,Mo)C degeneration associated with the presence of lattice defects such as stacking faults and dislocations. Thermo-Calc® calculations were performed and correlated well with experimental Time-Temperature-Transformation diagrams.

High temperature deformation behaviour and microstructure evolution in Cu containing High Strength Low Alloy (HSLA) steel

ABSTRACT The hot deformation behaviour of a Cu containing high strength low alloy (HSLA) steel has been evaluated by carrying out hot compression tests over a temperature range of 900 °C to 1100 °C and constant true strain rates of 0.0003 to 1 s−1. Although single peak flow curve is observed predominantly, a tendency to exhibit multiple peaks at low strain rates (0.0003 and/or 0.001 s−1) at temperature greater than 950 °C. The important parameters pertaining to dynamic recrystallisation (DRX), such as critical, peak, steady state stress and strain were ascertained, and the DRX state diagram depicting the variation of critical parameters governing DRX with Zener-Hollomon parameter Z was established. The volume fraction of recrystallised grains, computed from flow stress, as expected, follows a sigmoidal trend indicating higher kinetics at low strain rates for a given temperature. Reconstruction of prior austenite grain boundaries (PAGB) based on electron back scattered diffraction (EBSD) technique for a few select specimens clearly elucidated the effect of temperature and strain rate on the DRX behaviour of the steel. The DRX state diagram established here clearly demarcates the deformation space i.e. temperature-strain rate-stain space into regimes where the material exhibits full DRX, hardening/softening + DRX and only hardening (no DRX) behaviour. This would be useful for designing suitable forging sequence to achieve the desired microstructure and mechanical properties.

High Temperature Deformation and Microstructural Evolution of Homogenized AA 2026 Alloy

AA 2026 is an improved version of AA 2024, an alloy with added Zr to reduce Fe and Si content and inhibit recrystallization during hot working. Al 2026 alloy has high strength and high damage resistance, so it is widely used in aircraft parts. In this study, in order to investigate the hot workability of AA 2026 and to optimize the hot forming parameters, hot compression tests were conducted in the temperature range of 300 to 450 o C, at a strain rate of 0.01 to 10 and in the 50% strain section. The true stress–true strain curve showed a dynamic softening phenomenon while the stress increased rapidly at a small strain and then remained steady. In order to evaluate its high temperature processability, the constitutive equations for flow stress, temperature, and strain were quantified based on the Arrhenius equation, and a process strain map was prepared. The peak stress accuracy of the constitutive equation was about 98.2%, which was consistent with the experimental data of AA 2026 under strain rate and temperature conditions. In addition, scanning electron microscopy (SEM) and backscattered electron diffraction pattern analyzer (EBSD) analyses were conducted to confirm the mechanism of the dynamic softening phenomenon. The CDRX phenomenon was confirmed under the high strain condition in the low temperature region and the DDRX phenomenon in the low strain condition in the high temperature region.

High temperature flexural strength, microstructure, phase evolution and anti-oxidation mechanism of Al-coated carbon fiber/boron phenolic resin ceramizable composite modified with TiB2 and B4C

Carbon fiber/phenolic resin composites (CF/Ph) have attracted great interests in the field of thermal protection materials for their characteristics of high specific strength and easy manufacturing. However, CF/Ph are inherently susceptible to oxidation failure at elevated temperatures. In this study, a novel Al-coated carbon fiber/boron phenolic resin ceramizable composite modified with TiB2 and B4C was fabricated by an impregnating and compression molding route. Thermal stability, flexural strength, microstructure and phase evolution of the resulting ceramizable composite were studied. The residue yield at 1400 °C and flexural strength after treated at 1400 °C for 15min was 90.4% and 53.1 MPa, respectively, which was increased by 15.9% and 532.1% than that without ceramizable fillers. Surface defects generated by matrix pyrolysis were well healed, and PyC and carbon fibers were covered with dense ceramic layers while the fracture surface was covered with relatively continuous ceramic layers without visible pores. Multiphase ceramics composed of TiB2, TiO2, TiC and PyC were identified. Furthermore, oxidation failure and anti-oxidation mechanism was revealed based on the aforementioned characterizations and thermodynamic calculation results. Oxidation resistance got enhanced markedly for synergistic effects of oxygen consuming, carbon fixation, oxygen barrier and endothermic effect, which were derived from ceramization reactions between TiB2, B4C, O2, Al and PyC.

Effect of the Deposition Process on High-Temperature Microstructure and Properties of the Direct Energy Deposition Al-Cu Alloy

The excellent microstructure and mechanical properties of Al-Cu alloy deposited using the Direct Energy Deposition (DED) process has been shown in previous studies, which is the most likely aluminum alloy material to meet the load-bearing requirements of hypersonic aircraft. However, its high-temperature performance and the law of microstructure transformation during the high-temperature process still need to be determined. In this paper, DED Al-Cu alloy samples were prepared using the Cold Metal Transfer (CMT) process, CMT Pulse process, and CMT Pulse Advanced process. The effect of different deposition processes on the microstructure and mechanical properties of the deposits was investigated at 200 °C for 30 min. The results show that the Al-Cu alloy’s main strengthening phase θ′ is excessively transformed into the equilibrium θ phase after the high-temperature process, which is the main reason for the degradation in the DED Al-Cu alloy’s properties at high temperatures. Different deposition processes have almost no effect on the high-temperature performance of the DED Al-Cu alloy, and the deposition process can be selected according to the product’s structure.

Softening mechanisms and microstructure evolution of 42CrMo steel during hot compressive deformation

Hot compression experiments were conducted on 42CrMo steel under different conditions using a Gleeble-3500 thermomechanical simulator. Systematic studies on the microstructure evolution and flow behavior under different deformation conditions were conducted. The results demonstrate that the flow curves exhibit significant peaks at low strain rates and high temperatures, and that the peak stress increases with decreasing deformation temperature and increasing strain rate. The dynamic recrystallization (DRX) critical points under various deformation conditions were derived from the work-hardening curves. The relationship between the peak (εp) and critical (εc) strains satisfies εc = 0.65εp. The value of εc decreased as the strain rate decreased and deformation temperature increased. The softening mechanism under different deformation conditions was determined in conjunction with the high-temperature microstructure and critical strains. When deformed at low temperature, the softening mechanism was a combination of dynamic recovery (DRV) and DRX; when deformed at high temperature, the softening mechanism was mainly DRX, and at low strain rates, the deformed DRX grains coarsened. Analysis of the room-temperature microstructure revealed that the substructure introduced during hot compression can affect the phase transition. Electron backscatter diffraction results show that room-temperature microstructure have a higher proportion of medium orientation angle boundaries and greater average local strain when DRX is incomplete. The higher dislocation density in the tissue with incomplete DRX disrupts the martensitic multilevel structure, whereas with complete DRX, it tends to form a typical martensitic multilevel structure.

High Temperature Mechanical Properties and Microstructure in 9Cr or 12Cr Oxide Dispersion Strengthened Steels

Oxide Dispersion Strengthened (ODS) ferritic steel, a candidate material for fast reactor fuel cladding, has low thermal expansion, good thermal conductivity, and excellent resistance to irradiation damage and high temperature strength. The origin of the excellent high-temperature strength lies in the dispersion of fine oxides. In this study, creep tests at 700°C or 750°C, which are close to the operating temperatures of fast reactors, and high-temperature tensile tests at 900°C to 1350°C, which simulate accident conditions, were conducted on 9Cr ODS ferritic steels, M11 and MP23, and 12Cr ODS ferritic steel, F14, to confirm the growth behavior of oxides. In the M11 and F14 creep test samples, there was little oxide growth or decrease in number density from the initial state, indicating that dispersion strengthening by oxides was effective during deformation. After creep deformation of F14, the development of dislocation substructures such as dislocation walls and subgrain boundaries was hardly observed, and mobile dislocations were homogeneously distributed in the grains. The dislocation density increased with increasing stress during the creep test. In the high-temperature ring tensile tests of MP23 and F14, the strength of both steels decreased at higher temperatures. In MP23, elongation decreased with increasing test temperature from 900°C to 1100°C, but increased at 1200°C, decreased drastically at 1250°C, and increased again at 1300°C. In F14, elongation decreased with increasing temperature. It was inferred that the formation of the δ-ferrite phase was responsible for this complex change in mechanical properties of MP23 from 1200 to 1300°C.