2Cr 2Nb Research Articles

Significant enhancement in tensile strength and work hardening rate in CoCrFeMnNi by adding TiAl particles via selective laser melting

In this study, 4 at.% Ti–48Al–2Cr–2Nb (TiAl) powder particles were added into CoCrFeMnNi high entropy alloy (HEA) and processed by selective laser melting (SLM) followed by hot isostatic pressing and ageing with the aim of enhancing its strengths. It was found that the as-developed HEA-TiAl sample shows a complex microstructure which is composed of near-equiaxed γ grains embedded with long-range ordered (LRO) L12 domains, various intragranular precipitates including Al2O3, B2, Heusler particles and γ-TiAl particles and grain boundary precipitates such as σ. With such a unique microstructure, the samples show remarkably enhanced 0.2% yield strength and significantly improved ultimate tensile strength and strain hardening rates. Precipitates such as Al2O3, Heusler and σ were found to have acted as effective dislocation motion obstacles. Dislocations tended to cut through the LRO L12 domains by forming stacking faults with a number of them intersecting to form immobile Lomer-Cottrell locks which are beneficial for strain hardening.

The sintering densification, microstructure and mechanical properties of Ti–48Al–2Cr–2Nb by a small addition of Sn–Al powder

TiAl alloys are difficult to obtain high density through traditional pressing and sintering process. Sn is a promising liquid-phase sintering addition for TiAl alloys; however, the further studies have elaborated that the liquid phase sintering of Sn is limited due to poor wettability between TiAl and melt Sn. Based on it, this study adopts Sn–Al powder addition instead of pure Sn powder. The results describe that a small amount of active Al enhanced the wettability of the Sn droplets to the TiAl matrix, thereby further promoting the liquid phase sintering effect of the addition of Sn. The sample sintered at 1320–1350 °C has a density excess of 95%, near γ or duplex microstructure with small proportion of lamellar colonies. Through subsequent non-encapsulation HIP, the fine-grained structure is maintained at the same time to reach full density, which enables the as-sintered samples to obtain moderate tensile properties at room temperatures (UST = 520 ± 5 MPa, YS = 401 ± 7 MPa, EL = 1.33 ± 0.1).

Trace element geochemistry and U–Pb ages of alluvial rutiles from a neoproterozoic coastal area, southwestern Cameroon: Implication for source rocks

This study focuses on alluvial rutile from Nkolembonda in the southwestern Cameroon. The studied rutile grains were analyzed using mineral chemistry and geochronology. Rutile chemistry shows Cr, Nb and Zr concentrations varying from 418 to 582 ppm, from 201 to 4070 ppm and from 60 to 272 ppm, respectively. The source area discriminant diagram based on the Cr–Nb concentrations indicates that the majority of the investigated alluvial rutile is originated from metapelitic rocks, whereas few alluvial rutile grains are derived from metamafic rocks. The calculated rutile formation temperatures at P = 10 kbar (based on the Zr-in rutile geothermometry) vary from 528 to 635 °C, with an average temperature of 595 °C. Zr-in-rutile geothermometer yielded coincident temperatures for both metapelitic-and metamafic-derived rutiles. This demonstrates that the source rocks of the alluvial rutiles underwent similar metamorphic conditions and have similar metamorphic history.The alluvial rutile LA-ICP-MS U–Pb dating yielded ages in the time range of ca 534 to 760 Ma, consistent with the age of the Pan-African orogeny. Trace-element compositions, Zr-in-rutile geothermometry and rutile LA-ICP-MS U–Pb geochronology show that alluvial rutile grains were derived from Middle to Late Neoproterozoic rocks that underwent amphibolite-facies metamorphism. Amphibolite-facies metamorphic rocks in the Yaoundé Group and nepheline metasyenite outcropping in the southwestern Cameroon coastal zone seem to be the primary source rocks for the alluvial rutile in the study area.

Effects of tungsten alloying and fluorination on the oxidation behavior of intermetallic titanium aluminides for aerospace applications

Current limitations to a wider use of intermetallic TiAl alloys in aircraft and automotive engines arise from an insufficient oxidation resistance at temperatures above approximately 800 °C. In this paper, the high temperature oxidation behavior of three engineering γ-TiAl-based alloys at 900 °C in air is reported. The performance of the TNM alloy (Ti-43.5Al–4Nb–1Mo-0.1B), the 4822 alloy (Ti–48Al–2Cr–2Nb), and the Nb-free IRIS alloy (Ti–48Al–2W-0.08B) is compared (all chemical compositions are given in at.%). During testing in air non-protective mixed oxide scales developed on all untreated samples, but with different compositions and thicknesses. These different oxide layers are characterized and their formation mechanisms are discussed. The presence of W in the IRIS alloy leads to a better oxidation behavior compared to untreated TNM and 4822. This behavior was changed in the direction of a protective alumina layer formation via the so-called “fluorine effect”. The above-mentioned alloys were treated with fluorine via a liquid phase process by evenly spraying a fluorine containing polymer on all faces of the specimens. The oxidation resistance of the fluorine treated samples was significantly improved compared to the untreated specimens. Due to the fluorination all treated test coupons exhibited slow oxidation kinetics. The results of isothermal as well as thermocyclic exposure tests are presented and discussed in the view of the chemical composition and processing conditioned microstructure of the three investigated γ-TiAl-based alloys.

Influence of addition of TiAl particles on microstructural and mechanical property development in a selectively laser melted stainless steel

316L stainless steel is well known for its excellent corrosion resistance and ductility. However, its relatively low strengths restrict its application in many load-bearing fields. In this study, Ti–48Al–2Cr–2Nb powder particles were mixed with 316L powder particles and then processed by selective laser melting (SLM) with the aim of developing intermetallic nano-particles reinforced 316L. It was found that the addition of 2 wt.% TiAl particles led to formation of numerous nano-sized cuboidal γ-TiAl precipitates in the austenite matrix and ferrite in the bottom regions of solidified melt pools. The majority of ferrite shows no particular orientation relationships with austenite that are characteristic of austenite → ferrite (γ-Fe → α-Fe) solid phase transformation, indicating that they should have formed during solidification. The α-Fe domains and surrounding γ-Fe regions were found to have experienced significant dynamic recrystallization during thermal cycling, resulting in fine α-Fe and γ-Fe equiaxed grains and massive twins in the γ-Fe. The addition of TiAl has moderately improved 0.2% yield strength and significantly increased ultimate tensile strength, thanks to the refined grain structure and massive γ-TiAl nano-particles which have acted as effective dislocation motion barriers. Some un-melted TiAl particles acted as preferential crack initiation and propagation sites during deformation, leading to reduction in ductility.

Examination of the Oxidation and Metal–Oxide Layer Interface of a Cr–Nb–Ta–V–W High Entropy Alloy at Elevated Temperatures

As the aerospace industry continues to grow, so does the demand for materials to withstand extreme environments. Refractory high entropy alloys (HEA) that utilize refractory elements are a rising class of materials with tremendous potential for use in these applications. Chintalapalle V. Ramana, and co-workers examine in their article, number 2100164, the oxidation behavior of a novel refractory HEA at different temperatures and highlight the differences.

Metastable phase and microstructural degradation of a TiAl alloy produced via selective electron beam melting

Selective electron beam melting (SEBM) is an attractive rapid prototyping technology to fabricate TiAl alloys with a complex structure. A near fully dense Ti–47Al–2Cr–2Nb alloy was fabricated by SEBM by using a low-energy density of 20.04 J/mm3. The phase transformation, microstructural degradation and microhardness of the SEBM-produced TiAl alloy were investigated. The results showed that the microstructure was inhomogeneous along the building direction. Multiple phase transformations were observed during the in-situ thermal cycle. Disordered α phase was observed in the last solidifying tracks in the top surface due to the rapid cooling rate. A metastable Ti2Al phase was observed due to the relatively high residual stress that was caused by a quick cooling rate when fabrication occurred under a low energy density of 20.04 J/mm3. The decomposition of α2 lamellae, continuous coarsening and discontinuous coarsening of lamellar structure were the three main routes for microstructural degradation for SEBM-produced TiAl alloys. The microhardness exhibited an increasing tendency from a height of 18 mm to the top surface due to the increasing content of the lamellar colony. Finally, the degradation mechanism of the lamellar colony was discussed in detail.

Effects of Stress‐Aging Pretreatment on Hot Deformation Behavior and Microstructure Evolution of a Ni–Cr–Nb–Mo–Ti Alloy

Hot deformation behavior and microstructure evolution of stress and stress‐free aged NiCrNbMoTi alloy are investigated by hot compressive tests. The impacts of stress aging upon the flow softening and dynamic recrystallization (DRX) behavior are analyzed. Also, the variations of dislocation substructures are studied. The basket‐weave δ‐phases induced by stress‐aging pretreatment can refine grains during hot deformation. On the one hand, the basket‐weave δ‐phases mutually intersect and form some enclosure spaces. Then, the dislocations which are restrained in these enclosure spaces can increase the potential sites for DRX nucleation. On the other hand, the growth of DRX grain is significantly hindered by the basket‐weave δ‐phases. The peak stress increases substantially as the stress‐aging stress rises. The dislocations pinned near the basket‐weave δ‐phases can hinder the migration of grain boundaries and accelerate the DRX process during hot deformation.

Enhancing high-temperature strength and ductility of γ-TiAl matrix composites with controllable dual alloy structure

Gamma-TiAl matrix composites can function as promising high-temperature structural materials. In this study, to improve the poor ductility and insufficient strength of composites at high temperatures, they were successfully prepared via powder metallurgy by adding the Ti–6Al–4V alloy. The experimental results indicate that the soft phases of Ti–6Al–4V were surrounded by the hard phase of Ti–48Al–2Cr–2Nb, and the special structure exhibited a unique strain hardening mechanism, which resulted in the high strength and ductility of the composites at high temperature. The 8 wt% Ti–6Al–4V composites were fractured with an elongation approximately 90.65% and tensile strength of 427.75 MPa at 800 °C and 5 × 10−5 s−1; the elongation and tensile strength were both higher than those of single unreinforced Ti–48Al–2Cr–2Nb alloy. Moreover, with an increase in the deformation strain, the deformation behavior started from the Ti–6Al–4V area, which had good ductility. The dislocation movement was hindered by the Ti–48Al–2Cr–2Nb area, and dislocation accumulation and grain deformation occurred, leading to strength increase. The stress concentration and high-density dislocations were stored in the soft phases, which delayed the initiation and propagation of pores and cracks. Moreover, besides grain sliding, evident dislocation activities occurred; therefore, the high ductility of the composites was obtained.

Структура зварних з’єднань багатокомпонентного високоентропійного сплаву системи Nb–Cr–Ti–Al–Zr, одержаних лазерним зварюванням

Automatic Welding, 2021, №06. Journal «AVTOMATICHESKAYA SVARKA» Monthly scientific-technical and production journal. ISSN: 0005-111X, Since 1948. Russian version of «The PATON Welding Journal» Text in Russian, contents and summaries in English. The Journal presents a wide range of scientific-technical and promotion information in the field of technologies, equipment and materials for welding, surfacing, brazing, and deposition of protective coatings.

Structure of laser welded joints of multicomponent high-entropy alloy of Nb–Cr–Ti–Al–Zr system

Structure of laser welded joints of multicomponent high-entropy alloy of Nb–Cr–Ti–Al–Zr system

Formation of Al–Al2O3 core–shell nanosphere chains during electron beam melting of γ-TiAl

Several core–shell nanosphere chains were observed in the electron beam melted Ti–48Al–2Cr–2Nb samples. The formation mechanism of such core–shell nanospheres, induced by Al vaporization, during the electron beam melting of γ-TiAl was investigated using the HRTEM, FESEM, STEM, and XRD analyses. The solidification of TiAl/Ti3Al lamellae, in the vicinity of the Al-diminished zones, was disclosed by microstructural studies. The core and shell of the nanospheres were characterized as porous Al and α-Al2O3, respectively. Finally, the formation of core–shell nanosphere chains in the additive manufactured γ-TiAl was schematically illustrated.

Effect of multi-stage heat treatment on mechanical properties and microstructure transformation of Ti–48Al–2Cr–2Nb alloy

Refining the grain size of γ -TiAl alloys to improve their strength and ductility is of academic interest. Thus, we report a multi-stage heat treatment method consisting of solution treatment, cyclic heat treatment, annealing, short heat treatment, and aging to refine the microstructure of the Ti–48Al–2Cr–2Nb alloy. The solution-treated microstructure was refined from 1100 to 191 μm by cyclic heat treatment, promoting feathery γ packets. Fine duplex grains below 23 μm were obtained through discontinuous coarsening, which was accelerated in the cyclic-heat-treated alloys owing to the high driving force resulting from the presence of feathery γ packets and fine interlamellar spacing. Through the short heat treatment and aging at single α and α 2 + γ fields, a tailored duplex structure with a grain size of 24 μm and interlamellar spacing of 42 nm was achieved. Through microstructure refinement, tensile strength and elongation were improved to 697 MPa and 2.1%, respectively, compared to those of the conventional forged specimen (622 MPa and 1.3%, respectively). We believe that our study provides a simple pathway to refine the grain size and interlamellar spacing of γ -TiAl alloys.

Examination of the Oxidation and Metal–Oxide Layer Interface of a Cr–Nb–Ta–V–W High Entropy Alloy at Elevated Temperatures

The authors report on the evaluation of the oxide scale and the interface microstructure of a Cr–Nb–Ta–V–W refractory high entropy alloy (HEA) at elevated temperatures. The Cr–Nb–Ta–V–W HEA is oxidized at 700 and 800 °C in lab air and the substrate/oxide interface is investigated. Combined in situ X‐ray diffraction (XRD) coupled with ex situ scanning electron microscopy (SEM) and energy dispersive X‐ray spectrometry (EDS) analyses characterize the oxide scale and confirm the phases present in the substrate which have been previously identified in this alloy. The microstructure near the interface is studied for an indication of selective oxidation of this alloy. Cracking and porosity are found along the interface layer which grows directionally outward. Two main oxides are identified: a W‐based oxide with a needle‐like structure and a Cr oxide containing Ta that has a granular structure, primarily found in clusters. The oxide layers are porous, and no dense protective oxide is identified. It is found that when the temperature is increased to 800 °C, the oxide layer exhibits an increase in thickness. In situ XRD indicates that V is the first element to oxidize.

Laser fabricated nickel-based coating with different overlap modes

ABSTRACT Ni204 cladding layers were prepared on the surface of 45# steel alloys via laser cladding under different overlap modes. The effects of overlap modes on microstructure evolution, microhardness, wear resistance, compressive and tensile strength were investigated. The intensity of the diffraction peaks is related to the cladding strategy. The remelting mode introduced by “double vertical cross” reduces the cooling rate and improves the uniformity of microstructure, exhibiting a stability of the microhardness and wear resistance. Cr0.19Fe0.7Ni0.11, [Fe, Ni], and Ni–Cr–Nb–Mo are primary detected phases in the cladding layer. With the increase in overlap uniformity, the content of [Nb, Mo] which precipitated in grain boundary increases and the microstructure is refined. The uniform microstructure distribution promotes the stable distribution of microhardness and friction coefficient. The angle between the extension direction of the overlap area and the applied loading directly determines the performance of the Ni204 alloy after yielding. The inter-laminar overlap mode weakens the effect of shear stress induced by microstructure differences.

Tribological properties of the uncoated and aluminized Ti–48Al–2Cr–2Nb TiAl alloy at high temperatures

Al-rich phases are often recommended as potential coating candidates for TiAl alloys and their application as a turbine blade material in aero engines. As high wear resistance is also required for this application, the tribological properties of Al-rich phases need to be investigated with a focus on the influence of temperature. Therefore, a comparative investigation of the dry sliding wear behavior of the uncoated and aluminized Ti–48Al–2Cr–2Nb TiAl alloy was conducted between room temperature and 600 °C in air. While the uncoated TiAl alloy suffers from successively increasing wear rates with increasing temperature, the aluminized alloy shows lower wear rates up to 400 °C. However, barely any positive influence on the required wear protection can be observed at 600 °C due to partial failure of the coating. Wear mechanisms of both the uncoated and aluminized samples are characterized by high material transfer to the counterpart material, the Ni-based alloy IN718. This phenomenon is caused by the tendency for strong adhesive bonding between the two metallic counterparts and becomes even more pronounced with increasing temperature.

Evolution of rapidly grown cellular microstructure during heat treatment of TiAl-based intermetallic and its effect on micromechanical properties

Continuous growth of rapidly grown cellular microstructure (i.e., cellular microstructure obtained by rapid solidification) could be realized by the method of laser powder bed fusion. However, there is still lack of research about how to adopt post heat treatment further optimizing this type of homogeneous and fine microstructure. Herein, the phase and microstructure evolution of rapidly grown cellular microstructure of TiAl-based intermetallic during heat treatment was specifically investigated; besides, both the micromechanical properties and behavior of the heat-treated microstructures were analyzed. The phase and microstructure evolution of Ti–48Al–2Cr–2Nb (in at.% unless otherwise specified) rapidly grown cellular microstructure during heating at 694–1180 °C was: the α2 rapidly grown cellular microstructure transformed into cellular microstructure with α2/γ nanolamellae substructures at 694 °C; at 900 °C, almost all of the rapidly grown cellular microstructure transformed into γ phase, and began to transform into equiaxed microstructures by recrystallization; when the temperature increased to 936 °C, α2 phase began to be precipitated in or between the equiaxed γ grains; after being held at 1000 °C for 30 min, the rapidly grown cellular microstructure completely decomposed into equiaxed near-γ microstructures. The nanohardness of the heat-treated microstructures first decreased then increased with increasing heat-treatment temperature, the maximum nanohardness (8.697 GPa on average) was obtained after heat treatment at 700 °C. The cellular microstructure with nanolamellae substructures can further improve the strength and toughness of the rapidly grown cellular microstructure, effectively realizing the strengthening and toughening of TiAl-based alloy.

Structure Formation and Mechanical Characteristics of Dispersion-Hardening Alloy Based on Ni–Cr–Nb–Mo System

Structure Formation and Mechanical Characteristics of Dispersion-Hardening Alloy Based on Ni–Cr–Nb–Mo System

Thermodynamic assessments of the U–Nb–Mo and U–Nb–Cr ternary systems

The thermodynamic assessments of the U–Nb–Mo and U–Nb–Cr systems have been performed by using the CALPHAD (Calculation of Phase Diagrams) method on the basis of critical evaluation of phase diagram data reported in literature. The reported individual solution phases, i.e. liquid, (αU), (βU), γ, δ and two intermetallic compounds, i.e. MoU2 and NbCr2, have been modeled. The modeling covers the whole composition range and a wide temperature range. By utilizing the available thermodynamic parameters of the sub-binary systems, the U–Nb–Mo and U–Nb–Cr systems have been thermodynamically assessed and a series of self-consistent parameters have been obtained for the first time, which can reproduce most of the phase diagram and thermodynamic data to provide guidance for the design of nuclear fuels.

Nitrogen Solubility in Liquid Fe–Nb, Fe–Cr–Nb, Fe–Ni–Nb and Fe–Cr–Ni–Nb Alloys

Nitrogen Solubility in Liquid Fe–Nb, Fe–Cr–Nb, Fe–Ni–Nb and Fe–Cr–Ni–Nb Alloys