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11 pages, 5373 KiB

Open AccessArticle

High-Temperature Friction and Wear Properties of NiCr/hBN Self-Lubricating Composites

by Xuewei Zhu, Xiaofeng Wei, Yuxiang Huang, Fu Wang and Pengpeng Yan

Metals 2019, 9(3), 356; https://doi.org/10.3390/met9030356 - 20 Mar 2019

Cited by 26 | Viewed by 4122

NiCr/hBN self-lubricating composites with hBN solid-lubricant contents of 8%, 9%, 10%, 11%, and 12% (mass fractions) were prepared by powder-metallurgy method. Their tribological properties at room temperature and 600, 700, and 800 °C were studied. SEM and XRD were performed to analyze their [...] Read more.

NiCr/hBN self-lubricating composites with hBN solid-lubricant contents of 8%, 9%, 10%, 11%, and 12% (mass fractions) were prepared by powder-metallurgy method. Their tribological properties at room temperature and 600, 700, and 800 °C were studied. SEM and XRD were performed to analyze their wear-surface morphology and abrasive-dust composition. Results indicate that the NiCr/hBN self-lubricating composites have favorable tribological properties at room temperature and 600 °C. The optimum comprehensive friction and wear performance are achieved at 10% hBN, and the main wear mechanism is abrasive wear. At 700 and 800 °C, the friction and wear properties of NiCr/hBN self-lubricating composites obviously decline. The wear surface experiences oxidation and the spinel oxides Cr2O3, NiCr2O4, and CrBO3 are formed. The main wear mechanism becomes combined scratching and glue form of adhesive wear, leading to greatly increased wear rate for the NiCr/hBN sel-lubricating composites. Full article

(This article belongs to the Special Issue Alloys for High-Temperature Applications)

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13 pages, 10405 KiB

Open AccessArticle

Mechanical Behavior of Inconel 625 at Elevated Temperatures

by Mauro M. de Oliveira, Antônio A. Couto, Gisele F. C. Almeida, Danieli A. P. Reis, Nelson B. de Lima and Renato Baldan

Metals 2019, 9(3), 301; https://doi.org/10.3390/met9030301 - 7 Mar 2019

Cited by 70 | Viewed by 13250

Abstract

Inconel 625 is a nickel-based alloy that is mainly used in high-temperature applications. Inconel 625 exhibits an unstable plastic flow at elevated temperatures characterized by serrated yielding, well-known as the Portevin-Le Chatelier effect. The evaluation of the mechanical properties of Inconel 625 at [...] Read more.

Inconel 625 is a nickel-based alloy that is mainly used in high-temperature applications. Inconel 625 exhibits an unstable plastic flow at elevated temperatures characterized by serrated yielding, well-known as the Portevin-Le Chatelier effect. The evaluation of the mechanical properties of Inconel 625 at high temperatures is the aim of this work. The tensile tests were executed in temperatures ranging from room temperature to 1000 °C with strain rates of 2 × 10⁻⁴ to 2 × 10⁻³ s⁻¹. The creep tests were executed in the temperature range of 600–700 °C and in the stress range of 500–600 MPa in a constant load mode. The optical and scanning electron microscopes were used for surface fracture observation. In the curves obtained at 200–700 °C the serrated stress-strain behavior was observed, which was related to the dynamic strain aging effect. The yield strength and the elongation values show anomalous behavior as a function of the test temperature. An intergranular cracking was observed for a specimen tensile tested at 500 °C that can be attributed to the decohesion of the carbides along the grain boundaries. The fracture surface of the specimen tensile tested at 700 °C showed the predominance of transgranular cracking with tear dimples with a parabolic shape. Full article

(This article belongs to the Special Issue Alloys for High-Temperature Applications)

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18 pages, 12734 KiB

Open AccessArticle

Dynamic Recrystallization and Hot-Working Characteristics of Ni-Based Alloy with Different Tungsten Content

by Zhihua Gong, Hansheng Bao and Gang Yang

Metals 2019, 9(3), 298; https://doi.org/10.3390/met9030298 - 6 Mar 2019

Cited by 10 | Viewed by 3233

Abstract

The hot deformation behavior of the GY200 Ni-based alloys with different tungsten (W) content were investigated by means of hot compression tests, microscopic observations, and processing maps at temperatures between 950 °C and 1200 °C, strain rate between 0.01 s⁻¹ and 10 [...] Read more.

The hot deformation behavior of the GY200 Ni-based alloys with different tungsten (W) content were investigated by means of hot compression tests, microscopic observations, and processing maps at temperatures between 950 °C and 1200 °C, strain rate between 0.01 s⁻¹ and 10 s⁻¹ with strain of 0.9. The hyperbolic-sine type constitutive equations were established between peaks tress and deformation conditions through Z parameters, and for alloys with higher W content results in higher activation energy and complete recrystallization temperature. The hot-working maps were exploited based on the experimental data. The hot-working maps showed that the instability zone extended with increasing W content. The stable domain of alloys are located in the temperature range between 1025 °C and 1200 °C and strain rate range between 0.01 s⁻¹ and 1 s⁻¹, dominated by the dynamic recrystallization (DRX) microstructural evolution, suited for hot deformation. The cracking on the surface of the sample compressed at 950 °C was resulted from the tensile stress, while the fracture of the sample compressed at 1200 °C was triggered by the melting of grain boundaries. Full article

(This article belongs to the Special Issue Alloys for High-Temperature Applications)

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10 pages, 4918 KiB

Open AccessArticle

The Influence of Laser Nitriding on Creep Behavior of Ti-4Al-4V Alloy with Widmanstätten Microstructure

by Luciana Aparecida Narciso da Silva Briguente, Javier Oñoro, Flávio Perpétuo Briguente, Fabrícia Assis Resende, Joares Lidovino dos Reis, Danieli Aparecida Pereira Reis and Aline Capella de Oliveira

Metals 2019, 9(2), 236; https://doi.org/10.3390/met9020236 - 16 Feb 2019

Cited by 9 | Viewed by 3255

Abstract

Ti-6Al-4V alloy has been considered in applications of aeronautical and aerospace industries, due to its properties such as high specific resistance, good creep resistance and metallurgical stability. However, its use in applications for high temperatures is restricted due to its great affinity with [...] Read more.

Ti-6Al-4V alloy has been considered in applications of aeronautical and aerospace industries, due to its properties such as high specific resistance, good creep resistance and metallurgical stability. However, its use in applications for high temperatures is restricted due to its great affinity with the oxygen, which results in the formation of oxide layers and limits its mechanical resistance at these conditions. Thus, specific treatments have been employed in the material to work as surface barriers to avoid the oxygen diffusion in the alloy under high temperature conditions. One surface treatment that can be used is laser nitriding. In the present work, the surface of Ti-6Al-4V alloy with Widmanstätten microstructure was nitrided by applying Nd:YAG laser focal with 0.6 mm diameter, at laser power of 700, 750 and 800 W, process speed of 100 mm/s and 20 L/min of N₂ flow. Creep tests were performed at constant load at 600 °C and 125 MPa, to verify the influence of treatment on the Ti-6Al-4V alloy. Results have indicated a lower stationary creep rate for the titanium alloy with Widmanstätten laser-nitrided structure when compared to the non-nitriding material. Besides that, the surface hardness increased from 368 HV of base material to 1000 HV after laser nitriding. Full article

(This article belongs to the Special Issue Alloys for High-Temperature Applications)

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10 pages, 3478 KiB

Open AccessFeature PaperArticle

Effect of Zr Addition on Overaging and Tensile Behavior of 2618 Aluminum Alloy

by Stefania Toschi, Eleonora Balducci, Lorella Ceschini, Eva Anne Mørtsell, Alessandro Morri and Marisa Di Sabatino

Metals 2019, 9(2), 130; https://doi.org/10.3390/met9020130 - 26 Jan 2019

Cited by 11 | Viewed by 4379

Abstract

The effect of Zr addition on overaging and tensile behavior in a 2618 Al–Cu–Mg–Ni–Fe alloy has been investigated in this study. The chemical composition of the base 2618 alloy, containing ~0.1 wt % of Zr, was modified by adding Zr to reach the [...] Read more.

The effect of Zr addition on overaging and tensile behavior in a 2618 Al–Cu–Mg–Ni–Fe alloy has been investigated in this study. The chemical composition of the base 2618 alloy, containing ~0.1 wt % of Zr, was modified by adding Zr to reach the target content of 0.25 wt %. Cast bars were T6 heat-treated according to industrial parameters, involving soaking at 525 °C for 8 h, quenching in hot water (50 °C), and artificial aging at 200 °C for 20 h. Both the T6 2618 and 2618 + Zr alloys were overaged at 250 and 300 °C for up to 192 h, to evaluate the decrease in hardness with high temperature exposure time. The tensile behavior of the alloys was investigated in the overaged condition, both at room temperature and at 250 °C. The microstructure of the as-cast and solution-treated samples was investigated by optical and scanning electron microscopy, while the precipitate microstructure at the nanoscale was analyzed by transmission electron microscopy in overaged condition. Experimental data revealed that the presence of 0.25 wt % Zr does not induce modifications at the macroscale on the microstructure of 2618 alloy while, at the nanoscale, the presence of Zr-based precipitates was observed. The overaged Zr-enriched alloy showed increased yield and ultimate tensile strength in comparison to the base alloy, at equal heat treatment condition, both at room temperature and 250 °C. Full article

(This article belongs to the Special Issue Alloys for High-Temperature Applications)

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17 pages, 12423 KiB

Open AccessArticle

Characterization of Microstructural Evolution for a Near-α Titanium Alloy with Different Initial Lamellar Microstructures

by Hui Li, Zhanglong Zhao, Yongquan Ning, Hongzhen Guo and Zekun Yao

Metals 2018, 8(12), 1045; https://doi.org/10.3390/met8121045 - 10 Dec 2018

Cited by 12 | Viewed by 7458

Abstract

The effects of initial lamellar thickness on microstructural evolution and deformation behaviors of a near-α Ti-5.4Al-3.7Sn-3.3Zr-0.5Mo-0.4Si alloy were investigated during isothermal compression in α + β phase field. Special attention was paid to microstructural conversion mechanisms for α lamellae with different initial thicknesses. [...] Read more.

The effects of initial lamellar thickness on microstructural evolution and deformation behaviors of a near-α Ti-5.4Al-3.7Sn-3.3Zr-0.5Mo-0.4Si alloy were investigated during isothermal compression in α + β phase field. Special attention was paid to microstructural conversion mechanisms for α lamellae with different initial thicknesses. The deformation behaviors, including flow stress, temperature sensitivity, and strain rate sensitivity, and processing maps and their dependence on initial lamellar thickness were discussed. The detailed microstructural characterizations in different domains of the developed processing maps were analyzed. The results showed that the peak efficiency of power dissipation decreased with increasing initial lamellar thickness. The interaction effects with different extents of globularization, elongating, kinking, and phase transformation of lamellar α accounted for the variation in power dissipation. The flow instability region appeared to expand more widely for thicker initial lamellar microstructures during high strain rate deformation due to flow localization and local lamellae kinking. The electron backscatter diffraction (EBSD) analyses revealed that the collaborative mechanism of continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) promoted the rapid globularization behavior for the thinnest acicular initial microstructure, whereas in case of the initial thick lamellar microstructure, CDRX leading to the fragmentation of lamellae was the dominant mechanism throughout the deformation process. Full article

(This article belongs to the Special Issue Alloys for High-Temperature Applications)

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15 pages, 6086 KiB

Open AccessArticle

Effect of Plasma Nitriding on the Creep and Tensile Properties of the Ti-6Al-4V Alloy

by Gisele F. C. Almeida, Antônio A. Couto, Danieli A. P. Reis, Marcos Massi, Argemiro S. Da Silva Sobrinho and Nelson B. De Lima

Metals 2018, 8(8), 618; https://doi.org/10.3390/met8080618 - 7 Aug 2018

Cited by 22 | Viewed by 4891

Abstract

This work aimed to enhance the creep resistance of Ti-6Al-4V alloy treated by plasma nitriding. The nitriding was performed on specimens with a Widmanstätten microstructure for four hours at 690 °C under a gas atmosphere containing Ar:N₂:H₂ (0.455:0.455:0.090). X-ray diffraction [...] Read more.

This work aimed to enhance the creep resistance of Ti-6Al-4V alloy treated by plasma nitriding. The nitriding was performed on specimens with a Widmanstätten microstructure for four hours at 690 °C under a gas atmosphere containing Ar:N₂:H₂ (0.455:0.455:0.090). X-ray diffraction analysis showed that the ε-Ti₂N and δ-TiN formed on the nitrided sample, in addition to the α-Ti and β-Ti matrix phases. The layer thickness of this sample was about 1 µm. Hot tensile tests were performed in the temperature range of 500 to 700 °C on nitrided and non-nitrided samples, which indicated an increased strength of the nitrided samples. The same temperature range was used for the creep tests in a stress range of 125 to 319 MPa. The plasma-nitrided samples exhibited better creep resistance when compared to the untreated samples. This result was demonstrated by the decreased secondary creep rate and the increased final creep time. This improvement in the creep resistance appeared to be associated with the formation of the nitrided layer, which worked as a barrier to oxygen diffusion into the material and due to the formation of a surface residual compressive stress. Full article

(This article belongs to the Special Issue Alloys for High-Temperature Applications)

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16 pages, 7646 KiB

Open AccessArticle

Effect of Initial Microstructures on Hot Deformation Behavior and Workability of Ti₂AlNb-Based Alloy

by Sibing Wang, Wenchen Xu, Yingying Zong, Xunmao Zhong and Debin Shan

Metals 2018, 8(6), 382; https://doi.org/10.3390/met8060382 - 25 May 2018

Cited by 15 | Viewed by 3701

Abstract

In order to study the effect of initial microstructures on the hot deformation behavior and workability of Ti₂AlNb alloy, the isothermal compression experiments of as-rolled and solution-treated Ti–19Al–23Nb–0.5Mo alloys were conducted in the temperature range of 900–1100 °C and strain rate [...] Read more.

In order to study the effect of initial microstructures on the hot deformation behavior and workability of Ti₂AlNb alloy, the isothermal compression experiments of as-rolled and solution-treated Ti–19Al–23Nb–0.5Mo alloys were conducted in the temperature range of 900–1100 °C and strain rate range of 0.001–10 s⁻¹. Subsequently, the processing maps of different state materials were established based on dynamic material model (DMM) and Prasad’s instability criterion. The suitable regions for hot working were determined in the processing maps, which was verified through high-temperature tensile test. The results show that although the solution-treatment could be used to improve the ductility of as-rolled Ti₂AlNb alloy at room temperature, the as-rolled microstructure exhibited better hot workability at high temperature compared to the solution-treated microstructure. Full article

(This article belongs to the Special Issue Alloys for High-Temperature Applications)

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13 pages, 15018 KiB

Open AccessArticle

The Evolution of Cast Microstructures on the HAZ Liquation Cracking of Mar-M004 Weld

by Yi-Hsin Cheng, Jyun-Ting Chen, Ren-Kae Shiue and Leu-Wen Tsay

Metals 2018, 8(1), 35; https://doi.org/10.3390/met8010035 - 5 Jan 2018

Cited by 6 | Viewed by 4691

Abstract

The causes of liquation cracking in the heat-affected zone (HAZ) of a cast Mar-M004 superalloy weld were investigated. X-ray diffraction (XRD), electron probe microanalyzer (EPMA), and electron backscatter diffraction (EBSD) were applied to identify the final microconstituents at the solidification boundaries of the [...] Read more.

The causes of liquation cracking in the heat-affected zone (HAZ) of a cast Mar-M004 superalloy weld were investigated. X-ray diffraction (XRD), electron probe microanalyzer (EPMA), and electron backscatter diffraction (EBSD) were applied to identify the final microconstituents at the solidification boundaries of the cast alloy. Fine borides and lamellar eutectics were present in front of some γ-γ′ colonies, which were expected to be liquefied prematurely during welding. The metal carbide (MC) enriched in Nb, Hf; M₃B₂ and M₅B₃ borides enriched in Cr and Mo; and lamellar Ni-Hf intermetallics were mainly responsible for the induced liquation cracking of the Mar-M004 weld, especially the MC carbides. Scanning electron microscope (SEM) fractographs showed that the fracture features of those liquation cracks were associated with the interdendritic constituents in the cast superalloy. Full article

(This article belongs to the Special Issue Alloys for High-Temperature Applications)

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