High Temperature Tensile Properties Research Articles

Additive manufacturing Ti-22Al-25 Nb alloy with excellent high temperature tensile properties by electron beam powder bed fusion

Additive manufacturing of TiAl alloys remains challenging due to their poor high-temperature tensile properties. In this work, electron beam powder bed fusion (PBF-EB) was applied to fabricate Ti-22Al-25 Nb alloy with a high pre-heating temperature (830℃), achieving a breakthrough in improving the tensile properties of TiAl alloys by additive manufacturing. The microstructure and tensile properties of Ti-22Al-25 Nb fabricated by laser beam powder bed fusion (PBF-LB) and PBF-EB were investigated. Excellent tensile strength (803 MPa) at 650℃ was obtained by PBF-EB, which was attributable to the precipitation and growth of the O phase at a suitable pre-heating temperature. An in-depth microstructure analysis of the deformation and fracture mechanisms of the as-printed samples was performed via Scanning Electron Microscopy (SEM), Electron Backscatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM). The area fraction of the O phase in the PBF-EB sample was approximately 6 times higher than that of the PBF-LB sample. The PBF-EB sample exhibited transgranular fracture at 650 °C, in contrast to the PBF-LB samples. The shape and quantity of the O phase and β phase played an important role in improving the tensile properties of the Ti-22Al-25 Nb alloys at high temperature.

High-temperature tensile properties and deformation behavior of a multi-phase FeNiCrAlTi alloy

Single-phase multi-principal elements alloys (MPEAs) within the elevated temperature experience particularly severe strength loss, and even a distinct ductile-to-brittle transition. Herein, we report on a non-equiatomic, multi-phase FeNiCrAlTi MPEA which possesses remarkable combinations of mechanical properties across a wide range of temperatures from 400 °C to 800 °C, especially its specific yield strength possesses nearly 100 MPa/g∙cm−3 at 600 °C. The excellent mechanical behavior benefits from the synergistic effect of the special phase structure and precipitation strengthening by the ordered Ni (Al, Ti) B2 nanoparticles. The transformation of deformation mode at 600 °C should be considered the dominant of the resistance to intermediate or high-temperature embrittlement. This work provides a new methodology for regulating mechanical properties of superior alloys that involve high-temperature conditions.

Constructing high-density dislocations by primary (Nb,Ti)(C,N) to induce massive secondary precipitations in austenitic heat-resistant cast steel

The microstructural evolution and the high-temperature tensile properties of a novel Nb-Ti-N alloyed austenitic heat-resistant cast steel were investigated during the isothermal aging at 900 °C for up to 1000 h. The mechanisms of enhancing strength-ductility synergy and the high-temperature tensile fracture behaviors after the aging process were analyzed in detail through tensile tests at 900 °C. Obtained results show that the high-density dislocation regions and the Cr segregation, formed around the intragranular primary (Nb,Ti)(C,N), provided numerous nucleation sites for submicron-sized secondary M23C6 precipitates. The density of secondary M23C6 significantly increased, while the coarsening rate was notably reduced during the aging process. Simultaneously, the uniform dispersed precipitation of nano-sized secondary (Nb,Ti)C was promoted by massive dislocations, which exhibited high resistance to coarsening during long-term aging. The improvement of high-temperature strength was attributed to the precipitation strengthening effects induced by the submicron-sized M23C6 and nano-sized (Nb,Ti)C particles. Additionally, the enhancement of ductility during aging was mainly due to the more effective stress transmission and the ameliorated interface relationship between primary (Nb,Ti)(C,N) and austenitic matrix, which was facilitated by the abundant precipitation of secondary phases. The current findings indicate that high-density dislocations constructed by the intragranular primary (Nb,Ti)(C,N) induce the extensive dispersion of secondary strengthening phases, which enhanced both ultimate tensile strength of 158 MPa and elongation of 45.3% at 900 °C after isothermal aging for 1000 h.

Effect of fan-shaped structure on high temperature tensile properties of nickel-based deformed superalloys

GH4742 alloy is a kind of precipitation-hardened superalloy, which is widely used in turbine disc, compressor disc, bearing ring and other parts due to its excellent high temperature mechanical properties. In this paper, high-temperature tensile comparison experiments were carried out at four temperatures, 1050 °C, 1080 °C, 1110 °C and 1140 °C, between alloy A containing a fan-shaped structure and alloy B without a fan-shaped structure. The results show that the tensile strength and plasticity of alloy A at 1050 °C are superior to those of alloy B. With the increase of temperature, the difference of tensile strength and plasticity between alloy A and B decreases gradually. By analyzing the fracture, both alloy A and B were found to have a large number of tough dimples, which showed ductile fracture characteristics. But alloy B was found to have some icing-like fracture at 1050 °C, which showed some brittle fracture characteristics. After 1110 °C, the difference in tensile strength and plasticity between alloy A and B almost disappears because γ′ phase is completely dissolved in the matrix. TEM results show that dislocations gather around the large-size γ′ phase, and for the small-size γ′ phase, dislocations tend to cut into γ′ phase directly. The results show that the existence of fan-shaped structure is beneficial to the high temperature tensile properties of GH4742 alloy, which has a certain reference effect on the production of GH4742 alloy in the future.

Effect of pre-precipitation thermomechanical treatment on precipitation behavior of CLAM steel

China Low Activation Martensitic steel (CLAM) is a cladding material used in thermonuclear fusion reactors. Its high temperature performance is closely related to precipitation strengthening. The MX phase with strong thermal stability is the key to improving the high temperature performance of CLAM steel, and the M23C6 phase which is easy to coarsen under long-term high temperature service conditions is one of the important factors causing creep failure. In order to increase the content of MX phase and reduce the content of M23C6 phase, this paper first pre-precipitates near the precipitation temperature of M23C6 phase (which is also close to the maximum precipitation temperature of MX phase), which limits the precipitation of M23C6 phase and promotes the maximum precipitation of MX phase. Then the subsequent thermomechanical treatment (TMT) is carried out. The results show that the content of M23C6 phase in the samples after pre-precipitation TMT is greatly reduced, and the size is slightly reduced compared with the samples with only thermal deformation. The size of the MX phase is reduced by 10.6%, the volume density is increased by 88%, the volume fraction is increased by 34.1%, and the dislocation density is slightly increased. At the same time, it was found that a large number of MX phases precipitated at the lath boundary and pinned the boundary, which delayed the coarsening of the lath during tempering. The changes of these microstructures improve the precipitation strengthening, dislocation strengthening and boundary strengthening, so the room temperature tensile properties and high temperature tensile properties are improved, but the work hardening makes the total elongation and room temperature impact properties decrease.

Microstructure, tribological property and high temperature tensile property of Co-Cr-Ni-W alloy parts fabricated by laser directed energy deposition

In this study, Co-Cr-Ni-W alloy parts were fabricated on the surface of 304 stainless steel (SS) substrate by laser directed energy deposition (LDED). Microstructure, microhardness and tribological behavior of as-deposited samples and 750 °C–950 °C aging-treated samples were tested. Carbides precipitated from Co-Cr-Ni-W alloy, and martensite transformation from γ-Co (FCC) to ε-Co (HCP) occurred in high temperature. The changes of microstructure caused by aging treatment could increase the microhardness and change wear mechanism. Tensile property of Co-Cr-Ni-W alloy at 750 °C–950 °C was also investigated. Co-Cr-Ni-W alloy formed a hard-sliding structure under high temperature, leading to high tensile strength. The main fracture mode of Co-Cr-Ni-W alloy was brittle cleavage fracture.

Temperature dependence tensile behaviors of additively manufactured GH4099 Ni-based superalloy

Additive manufacturing was currently widely used to fabricate complex-shaped superalloy components for extreme environment applications. To replace the components prepared by the traditional approaches, it is crucial to ensure the reliability of AMed superalloys, especially in a thermal-stress environment. In this work, the high-temperature tensile properties of AMed GH4099 nickel-based superalloy are studied to establish the relationships among deformation temperatures, mechanisms, and failure responses. The results show that the deformation mechanisms at 600 °C are mainly cross-slip of the slip bands and arrays, which induced a ductile fracture. Increasing the tensile temperature will cause a shift of the dislocation movement from cross-slip to Orowan looping and stacking fault shearing to dislocation entangles-recovery-recrystallization, and the fracture mode transformed from pure ductile to mixed ductile-brittle. Additionally, the neighboring grain boundaries and the carbide-grain boundary bondings will also change with the temperature, which is further linked to the mediate-temperature brittleness and high-temperature abnormal plasticity of this superalloy.

High temperature tensile and creep properties of the new cladding steel of 15-15Ti-Y

The 15-15Ti austenitic stainless steel is commonly used as a fast reactor cladding material due to its favorable comprehensive properties. To further enhance its high temperature strength, yttrium (Y) modified 15-15Ti (15-15Ti-Y) steel was developed. In this study, the tensile properties of 15-15Ti-Y were evaluated across the temperature range of 200–700 °C, and the constant load creep properties of 20 % cold worked (CW) 15-15Ti-Y were measured at temperatures of 550–700 °C under various applied stresses. The results indicated that Y addition led to enhanced intragranular precipitates and delayed intergranular precipitates, which resulted in a YS (YS) that was ∼ 100 MPa higher than that of Y-free steel at temperatures ranging from 200 to 600 °C, without compromising the plasticity and ductility. The creep fracture life of 15-15Ti-Y steel ranged from 21 to 5761 h, and the deformation and damage processes were found to adhere to the Norton power-law and Modified Monkman Grant relationship. The stress exponent revealed that the deformation was dislocation creep mechanism, while the creep damage tolerance parameter suggested that the creep fracture was caused by necking. These findings provide data and theoretical support for the application of Y-modified 15-15Ti steel as a fast reactor cladding material in high temperature environments.

Different Heat-Exposure Temperatures on the Microstructure and Properties of Dissimilar GH4169/IC10 Superalloy Vacuum Electron Beam Welded Joint

Vacuum electron-beam welding (EBW) was used to join the precipitation-strengthened GH4169 superalloy and a new nickel-based superalloy IC10 to fabricate the turbine blade discs. In this study, a solid solution (1050 °C/2 h for GH4169 and 1150 °C/2 h for IC10) and different heat-exposure temperatures (650 °C, 750 °C, 950 °C and 1050 °C/200 h, respectively) were used to study the high-temperature tensile properties and microstructure evolution of welded joints; meanwhile, the formation and evolution of the second phases of the joints were analyzed. After EBW, the welded joint exhibited a typical nail morphology, and the fusion zone (FZ) consisted of columnar and cellular structures. During the solidification process of the molten pool, Mo elements are enriched in the dendrites and inter-dendrites, and that of Nb and Ti elements was enriched in the dendrites, which lead to forming a non-uniform distribution of Laves eutectic and MC carbides in the FZ. The microhardness of the FZ gradually increased during thermal exposure at 650 °C and reached 300–320 HV, and the γ′ and γ″ phases were gradually precipitated with size of about 50 nm. Meanwhile, the microhardness of the FZ decreased to 260–280 HV at 750 °C, and the higher temperature resulted in the coarsening of the γ″ phase (with a final size of about 100 nm) and the formation of the acicular δ-phase. At 950 °C and 1050 °C, the microhardness of FZ decreased sharply, reaching up to 170~190 HV and 160~180 HV, respectively. Moreover, the Laves eutectic and MC carbides are dissolved to a greater extent without the formation of γ″ and δ phases; as a result, the absent of γ″ and δ phases are attributed to the significant improvement of segregation at higher temperatures.

Effect of Mn/Ag Ratio on Microstructure and Mechanical Properties of Heat-Resistant Al-Cu Alloys.

This paper mainly investigated the effect of the Mn/Ag ratio on the microstructure and room temperature and high-temperature (350 °C) tensile mechanical properties of the as-cast and heat-treated Al-6Cu-xMn-yAg (x + y = 0.8, wt.%) alloys. The as-cast alloy has α-Al, Al2Cu, and a small amount of Al7Cu2 (Fe, Mn) and Al20Cu2 (Mn, Fe)3 phases. After T6 heat treatment, a massive dispersive and fine θ'-Al2Cu phase (100~400 nm) is precipitated from the matrix. The Mn/Ag ratio influences the quantity and size of the precipitates; when the Mn/Ag ratio is 1:1, the θ'-Al2Cu precipitation quantity reaches the highest and smallest. Compared with the as-cast alloy, the tensile strength of the heat-treated alloy at room temperature and high temperature is greatly improved. The strengthening effect of the alloy is mainly attributed to the nanoparticles precipitated from the matrix. The Mn/Ag ratio also affects the high-temperature tensile mechanical properties of the alloy. The high-temperature tensile strength of the alloy with a 1:1 Mn/Ag ratio is the highest, reaching 135.89 MPa, 42.95% higher than that of the as-cast alloy. The analysis shows that a synergistic effect between Mn and Ag elements can promote the precipitation and refinement of the θ'-Al2Cu phase, and there is an optimal ratio (1:1) that obtains the lowest interfacial energy for co-segregation of Mn and Ag at the θ'/Al interface that makes θ'-Al2Cu have the best resistance to coarsening.

Study on the High Temperature Tensile Properties and Strengthening Mechanism of Nitrogen Containing Nickel‐Based Deposited Metals

This article investigates the tensile properties and strengthening mechanism of nitrogen‐containing nickel‐based flux cored welding wire deposited metal from room temperature to 900 °C. The results showed that the high‐temperature strength of the deposited metal increased with the addition of Tungsten and Nitrogen. When the temperature reaches 700 °C, M23C6 carbides begin to decompose. The alloying elements dissolve back into the matrix, and the solid solution strengthening effect minimizes the plasticity of the alloy. More carbonitrides are precipitated within the crystal with temperature. During high‐temperature deformation, carbonitrides fractures at the interface with the substrate due to difficulty in coordinating deformation. Therefore, the crack source formed greatly reduces the strength of the material. In addition, the stacking faults present in the low‐temperature zone have a significant impact on the work hardening of materials. The main deformation mechanism is the unit dislocation a/2 < 110> cutting precipitation phase. It can be observed that the dislocation cutting precipitation mechanism system has transformed into a dislocation bypassing precipitation mechanism system at intermediate. Finally, the anomalous yield strength (YS) and intermediate temperature brittleness phenomenon are reasonably explained. The main deformation mechanism gradually transforms into dislocation slip and climbing with temperature.

High-Temperature Creep and Microstructure Evolution of Alloy 800H Weldments with Inconel 625 and Haynes 230 Filler Materials

Alloy 800H stands as one of the few code-qualified materials for fabricating in-core and out-of-core components operating in high-temperature reactors. Welding is a common practice for assembling these components; however, the selection of a suitable filler material is essential for enhancing the high-temperature creep resistance of Alloy 800H weldments in high-temperature applications. In this study, Inconel 625 and Haynes 230 filler materials were used to weld Alloy 800H plates by employing the gas tungsten arc welding technique. The high-temperature tensile and creep rupture properties, microstructural stability, and evolution of the weldments after high-temperature exposure were investigated and compared with those of Alloy 800H. The results show that both weldments exhibit enhanced tensile and creep behavior at 760 °C. The creep rupture times of the weldments with Inconel 625 filler and Haynes 230 filler materials were about two and three time longer, respectively, than those of Alloy 800H base metal when tested at 80 MPa and 760 °C. Carbides (MC and M23C6) were commonly observed in the microstructures of both the weld and base metals in the two weldments after high-temperature creep tests. However, the Inconel 625 filler weldment displayed detrimental δ and Laves phases in the fusion zone, and these precipitates could be potential sites for initiating cracks following prolonged high-temperature exposure. This study shows that the weldment with Haynes 230 filler material exhibit better phase stability and creep rupture properties than the one with Inconel 625, suggesting its potential for use as a candidate filler material for Alloy 800H for further investigation. This finding also emphasizes the critical consideration of microstructural evolutions and phase stability in evaluating high-temperature materials and their weldments in high-temperature reactor applications.

Effect of the subsolvus and supersolvus solution treatments on the basket-weave microstructure, room and high temperature properties of TC21 alloy

In this study, two different microstructures were obtained after subsolvus solution treatment (910 °C + 550 °C) and supersolvus solution treatment (1010 °C + 550 °C) in TC21 alloy. The relationship of microstructure with room and high temperature properties was studied. The results showed that at room temperature, both samples after two treatments had high hardness. At room temperature, the subsolvus sample had high tensile strength of 1070 MPa and moderate plasticity and impact toughness, while the supersolvus sample was hard and brittle. The high strength and toughness of subsolvus sample was owing to its clear basket-weave structure within finer β grains which contributes to fine grain strengthening and the increase of crack tortuosity. Poor plasticity and toughness of supersolvus sample could be mainly attributed to its incomplete basket-weave structure, coarse β grains, Widmannstatten α and some coarse α-phase in the grain boundary. Both samples showed excellent high temperature tensile properties and stability at 400 °C, the tensile strength was still as high as 1000 MPa while plasticity was significantly improved. With the tensile temperature increasing to 600 °C, the alloys showed obvious softening as the tensile strength significantly decreased and elongation increased.

Characterization of the microstructure, texture, and tensile behavior of additively manufactured Hastelloy X superalloy: Insights into heat treatment at 900 °C

This research aimed at the microstructural characterization and elucidation of room- and high-temperature tensile properties of additively manufactured Hastelloy X Ni-based superalloy heat-treated at 900 °C. The samples were fabricated via the laser powder bed fusion method utilizing two scan strategies known as island and meander types. The results indicated that the cellular structure embedded in the columnar grains of the as-built microstructure disappeared after applying the heat treatment, linking to the reduction in dislocation density while keeping the former morphology of the grains unchanged. However, the intensities of the Brass, Goss, Goss-Brass, and rotated Goss texture components were reduced, leading to the generation of more homogenous fibers in the heat-treated microstructure. Additionally, it caused the formation of a Mo-rich phase on the grain boundaries, which improved the room temperature's ultimate tensile strength. However, it was elucidated that the Mo-rich phase-induced void formation and intergranular cracking failure caused a slight decrease in hot ductility. Overall, the obtained results revealed that the heat-treated island samples showed superior hot tensile behavior than the meander sample due to having a larger grain size.

Effect of Heat Treatment Temperature on Microstructure, Tensile Properties and δ-Precipitate Phase in Ni-based Superalloy

Here, we investigated the influence of δ-precipitate (orthorhombic D0a Ni<sub>3</sub>Nb-ordered phase) on the room- and high-temperature tensile properties in wrought nickel-based Inconel 625 superalloys subjected to solution and aging heat treatment. Typically, solution heat-treatment temperatures in these alloys affect the solid-state precipitation of δ-phase, which governs high-temperature tensile properties. While precipitation of fine D0<sub>a</sub> δ-phase is known to have beneficial effects on the mechanical properties owing to the retardation of grain coarsening, Widmanstätten δ precipitation plays a deleterious influence on the fracture toughness, tensile ductility, and fatigue resistance. Therefore, to enhance the mechanical properties of this alloy series, it is key to generate a high number density of fine D0<sub>a</sub> δ precipitate by adjusting solid solution treatment temperatures. In this study, solution heat treatments were conducted above and below δ-phase solvus temperatures. By applying solution heat treatment at 900°C and 970°C, this alloy was confirmed to have a Widmanstätten δ phase and is composed similarly to the annealed microstructure. This Widmanstätten δ precipitate was densely distributed at both intergranular and intragranular grains. On the other hand, when solution treatment was applied at 1040 and 1100°C, more coarse particles (approximately 30 μm) with a significant reduction of Widmanstätten type δ phase were obtained. We found that grain size and Widmanstätten δ-phases have an important role in the high-temperature tensile properties of Inconel 625 superalloy series.

Effect of variation in Zr content on microstructure and high-temperature tensile properties of a γ-TiAl alloy

A series of Ti-46Al-xZr (x = 0, 1, 2, 3, 4, 5 at.%) alloys were prepared to systematically investigate the effect of variation in Zr content on the microstructure and high-temperature tensile properties. The results show that the addition of Zr intensifies the solidification segregation and promotes the formation of blocky γ phase in TiAl alloys. With the increase in Zr content from 1 to 5 at.%, the volume fraction of the blocky γ phase increases from 0.2 to 12.8 %, and the solidification microstructure is transformed from a fully lamellar to a nearly lamellar microstructure composed of the blocky γ phase and lamellar colonies. Because Zr stabilises the γ phase, continuous and discontinuous coarsening occur simultaneously; the former is observed inside the colony, whereas the latter occurs between two adjacent lamellar colonies. The high-temperature tensile results show that the ultimate tensile strength at 800 °C of the Ti-46Al-4Zr alloy (692 MPa) is significantly greater than that of the Ti-46Al alloy (351 MPa), which is attributed to solid solution strengthening and the coordinated deformation of blocky γ phase.

High-temperature mechanical properties and dynamic recrystallization of Mo-14Re alloy

The Mo-14Re alloy is currently utilized as a candidate structural material in space nuclear reactors and exhibits significant potential in other high-temperature fields. However, due to the high cost of rhenium, limited research has been conducted on the high-temperature properties of Mo-14Re. This study aims to investigate the high-temperature tensile properties of the stress-relieved Mo-14Re within the temperature range of 1200 K to 1773 K, specifically focusing on the microstructure of dynamically recrystallized grains and their influence on high-temperature deformation. The results demonstrate that the ultimate tensile strength of Mo-14Re reaches 399 MPa, 358 MPa, 311 MPa, and 33 MPa at 1200 K, 1350 K, 1500 K, and 1773 K, respectively. Above 1350 K, the softening caused by dynamic recovery and dynamic recrystallization becomes more pronounced, surpassing the influence of work hardening on high-temperature deformation. Continuous dynamic recrystallization is identified as the primary mechanism driving the dynamic recrystallization. Moreover, a new texture emerges when Mo-14Re undergoes dynamic recrystallization above 1500 K. Notably, no precipitated phases or element segregation are observed in Mo-14Re following high-temperature deformation.

Enhancing high-temperature mechanical property of Ti4822 alloy with in-situ Ti2AlC precipitates

Ti4822/xC composites with excellent high-temperature performance were prepared by spark plasma sintering technique. As the carbon content increased from 0.5 wt% to 1.5 wt%, the volume fraction of the Ti2AlC precipitates increased from 6.5% to 14.0%. The average size of the lamellar colony decreased from 305.5 ± 5 µm to 76.5 ± 5 µm. When the carbon content was 0.5 wt%, the lamellar size reached the minimum value of 75.5 ± 5 µm. Meanwhile, the equiaxed γ phase precipitated at the grain boundaries. With the carbon content of 0.5 wt%, the tensile properties reached the maximum value of 582 MPa, and the fracture elongation of 7.2% at 800 ℃. Compared with Ti4822 alloy, the ultimate strength and elongation of the composite increased by approximately 28% and 1.9% correspondingly. The dominant mechanism for high-temperature tensile properties of the composites was the combination of grain refinement strengthening, second phase strengthening, and γ phase weakening.

Microstructural Characteristics and Material Failure Mechanism of SLM Ti-6Al-4V-Zn Alloy

This study focuses on the additive manufacturing technique of selective laser melting (SLM) to produce Ti-6Al-4V-Zn titanium alloy. The addition of zinc at 0.3 wt.% was investigated to improve the strength and ductility of SLM Ti-6Al-4V alloys. The microstructure and mechanical properties were analyzed using different vacuum heat treatment processes, with the 800-4-FC specimen exhibiting the most favorable overall mechanical properties. Additionally, zinc serves as a stabilizing element for the β phase, enhancing the resistance to particle erosion and corrosion impedance of Ti-6Al-4V-Zn alloy. Furthermore, the incorporation of trace amounts of Zn imparts improved impact toughness and stabilized high-temperature tensile mechanical properties to SLM Ti-6Al-4V-Zn alloy. The data obtained serve as valuable references for the application of SLM-64Ti.

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.