Research

13 pages, 3193 KiB

Open AccessArticle

Study on the Microstructure and Mechanical Properties of Non-Equimolar NiCoFeAlTi High Entropy Alloy Doped with Trace Elements

by Chunfen Wu, Shuzhi Zhang, Jianchao Han, Changjiang Zhang and Fantao Kong

Metals 2023, 13(4), 646; https://doi.org/10.3390/met13040646 - 24 Mar 2023

Viewed by 1021

Abstract

The method of improving the microstructure and thus the properties of alloys by adjusting their composition has been widely used in the study of high entropy alloys (HEAs). However, most studies have focused on improving the properties of HEAs with face-centered cubic (FCC) [...] Read more.

The method of improving the microstructure and thus the properties of alloys by adjusting their composition has been widely used in the study of high entropy alloys (HEAs). However, most studies have focused on improving the properties of HEAs with face-centered cubic (FCC) or body-centered cubic (BCC) structures by adjusting the contents of elements such as Ni, Al, Ti, Cr, Mn and Mo. The doping of B, Mg and Zr also has a certain effect on the mechanical properties of HEAs. In this paper, the phase structure, microstructure, and mechanical properties of Ni_45.5Co₂₂Fe₂₂Al₅Ti₅ HEA doped with B, Mg, and Zr were investigated. The results demonstrated that the three-phase structures of FCC matrix, L1₂ precipitate, and BCC phase were present in all the as-cast HEAs of Ni_45.5Co₂₂Fe₂₂Al₅Ti_5×0.5 (X = B, Mg, and Zr). The microstructures of the as-cast alloys showed typical dendritic and inter-dendritic architecture. The maximum hardness was found in the alloy doped with B element, with a value of 433 HV. During the compressive test at room temperature, neither the Mg0.5 HEA nor the Zr0.5 HEA cracked until the load limit, but the B0.5 HEA cracked at a compressive strain of about 12%. B0.5 HEA had the highest compressive yield strength of the three alloys, followed by Zr0.5 HEA, while Mg0.5 HEA had the lowest, with values of 1030 MPa, 754 MPa, and 628 MPa, respectively. The work is expected to provide a boost for the research on the optimization of the properties of new HEAs reinforced by precipitation of L1₂ phase by providing a simple solution-microalloying method. Full article

(This article belongs to the Special Issue Hot Forming/Processing of Metallic Materials)

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10 pages, 3475 KiB

Open AccessArticle

Effect of Cryogenic Treatment on Microstructure and Mechanical Properties of Al_0.6CrFe₂Ni₂ Dual-Phase High-Entropy Alloy

by Sichen Xie, Qingyao Lv, Wei Zhang, Yingdong Qu, Hao Qi, Bo Yu, Rongde Li, Guanglong Li and Fei Yang

Metals 2023, 13(2), 195; https://doi.org/10.3390/met13020195 - 18 Jan 2023

Cited by 1 | Viewed by 1397

Abstract

The contradiction between strength and ductility limits the application of high-entropy alloys (HEAs). To simultaneously improve the strength and ductility of HEAs, the cryogenic treatment was proposed and applied in this paper. The Al_0.6CrFe₂Ni₂ HEA with dual-phase structure [...] Read more.

The contradiction between strength and ductility limits the application of high-entropy alloys (HEAs). To simultaneously improve the strength and ductility of HEAs, the cryogenic treatment was proposed and applied in this paper. The Al_0.6CrFe₂Ni₂ HEA with dual-phase structure was selected as the experimental material for cryogenic treatment. The microstructure and mechanical properties of the HEA in an as-cast and cryogenically treated state were analyzed in detail. The results showed that the grain size of equiaxed crystal in the alloy decreased continuously by prolonging the cryogenic treatment time, and the average value was 44.6 μm for the cryogenically treated HEA at the time of 48 h, which was 46.5% lower than that of the as-cast alloy. The number and size of ordered body-centered cubic (B2) spherical nanophases embedded in the body-centered cubic (BCC) structured inter-dendritic region, however, increased continuously by extending the cryogenic treatment time. The cryogenic treatment also made more slip systems activate, cross-slip occurred in the alloy, and a large number of stacking faults were found in the transmission electron microscopy (TEM) microstructure for the alloy that underwent a long time in cryogenic treatment. The yield strength of the Al_0.6CrFe₂Ni₂ HEA was gradually increased with the increase in cryogenic treatment time, and the maximum yield strength of the 48 h cryogenically treated alloy was 390 MPa, which was 39.3% higher than that of the as-cast. This increase in mechanical properties after cryogenic treatment was attributed to the refinement of grains and the large precipitation of nanophases, as well as the appearance of cross-slips and stacking faults caused by cryogenic treatment. Full article

(This article belongs to the Special Issue Hot Forming/Processing of Metallic Materials)

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10 pages, 1551 KiB

Open AccessFeature PaperArticle

Yield of Annealing on the Properties of the Ti-5Al-2.5Fe Alloy Produced by Powder Forging

by Mingtu Jia, Yousef Alshammari, Fei Yang and Leandro Bolzoni

Metals 2023, 13(2), 189; https://doi.org/10.3390/met13020189 - 17 Jan 2023

Cited by 1 | Viewed by 980

Abstract

The high cost of titanium alloys can be reduced using alternative manufacturing techniques and using cheap alloying elements. In this study, the Ti-5Al-2.5Fe alloy, a cheaper α + β alloy than the workhorse Ti-6Al-4V, was produced by powder metallurgy combined with hot thermomechanical [...] Read more.

The high cost of titanium alloys can be reduced using alternative manufacturing techniques and using cheap alloying elements. In this study, the Ti-5Al-2.5Fe alloy, a cheaper α + β alloy than the workhorse Ti-6Al-4V, was produced by powder metallurgy combined with hot thermomechanical deformation by means of forging. The forged alloy was subsequently subjected to a heat treatment at 750 °C for several annealing times in order to modify the microstructure and tailor the mechanical properties. This study demonstrates that, regardless of the forging temperature used, annealing of the forged Ti-5Al-2.5Fe alloy improves both the strength and the ductility. Generally, the longer the annealing time, the higher the gain in strength and ductility with respect to the forged alloy. Moreover, annealing is significantly more beneficial to improve the ductility rather than the strength of the powder-forged Ti-5Al-2.5Fe alloy. Full article

(This article belongs to the Special Issue Hot Forming/Processing of Metallic Materials)

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14 pages, 2893 KiB

Open AccessArticle

Effect of Heat Treatments on Microstructure and Mechanical Properties of Low-Cost Ti-6Al-4V Alloy Produced by Thermomechanical Powder Consolidation Route

by Ajit Pal Singh, Rob Torrens, Brian Gabbitas and Giribaskar Sivaswamy

Metals 2023, 13(1), 173; https://doi.org/10.3390/met13010173 - 14 Jan 2023

Viewed by 1522

Abstract

This paper investigates the level of properties enhancement achievable by heat-treating Ti-6Al-4V alloy produced from a blended powder mixture using a thermomechanical powder consolidation route involving warm uniaxial pressing and vacuum sintering followed by extrusion at super transus temperature (1150 °C). The as-extruded [...] Read more.

This paper investigates the level of properties enhancement achievable by heat-treating Ti-6Al-4V alloy produced from a blended powder mixture using a thermomechanical powder consolidation route involving warm uniaxial pressing and vacuum sintering followed by extrusion at super transus temperature (1150 °C). The as-extruded material with a higher oxygen content of 0.55 wt.% was subjected to two different sub-transus annealing treatments: HT-A: 955 °C/1 h-furnace cooling and HT-B: 925 °C/4 h-cooling @ 50 °C/h to 760 °C-furnace cooling. Room temperature Charpy v-notch impact toughness tests and tensile tests were performed to ascertain the effect of microstructural changes during post-extrusion annealing treatments. After impact tests, analysis of microstructures and fracture surfaces of samples was carried out using optical and scanning electron microscopy. The as-extruded material displayed mean impact toughness of 4 J along with a yield strength of 956 MPa, an ultimate tensile strength of 1150 MPa, and an elongation to fracture of 2.4%. The annealing treatments gave a noticeable enhancement in the impact toughness (average values 5–6 J obtained) while maintaining a yield strength and ultimate tensile strength level of about 992 MPa and 1164–1181 MPa, respectively. Additionally, the level of change in ductility was limited for each sub-transus annealing treatment, and HT-A has given only a 30% increase compared to as-extruded material. Full article

(This article belongs to the Special Issue Hot Forming/Processing of Metallic Materials)

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16 pages, 25870 KiB

Open AccessArticle

Hall-Petch Strengthening by Grain Boundaries and Annealing Twin Boundaries in Non-Equiatomic Ni₂FeCr Medium-Entropy Alloy

by Zhiwen Li, Liang Wang, Chen Liu, Junbo Zhao, Binbin Wang, Zhe Li, Liangshun Luo, Ruirun Chen, Yanqing Su and Jingjie Guo

Metals 2023, 13(1), 134; https://doi.org/10.3390/met13010134 - 09 Jan 2023

Cited by 3 | Viewed by 1545

Abstract

A novel Co-free non-equiatomic Ni₂FeCr medium-entropy alloy (MEA) was designed, and the Hall–Petch strengthening by grain boundaries and annealing twin boundaries was investigated. For this purpose, the alloy was prepared by cold rolling and recrystallization at 873–1323 K for 40 min–6 [...] Read more.

A novel Co-free non-equiatomic Ni₂FeCr medium-entropy alloy (MEA) was designed, and the Hall–Petch strengthening by grain boundaries and annealing twin boundaries was investigated. For this purpose, the alloy was prepared by cold rolling and recrystallization at 873–1323 K for 40 min–6 h. Annealing at different temperatures revealed that Ni₂CrFe alloy forms a stable face-centered cubic (FCC) solid solution. Mean grain sizes (excluding annealing twin boundaries) and mean crystallite sizes (including both grain and annealing twin boundaries) were determined using the linear intercept method and the equivalent circle diameter in electron back-scattered diffraction (EBSD) soft. Tensile tests at 293 K indicated that the Hall-Petch slopes of grain sizes and crystallite sizes are 673 and 544 MPa μm^1/2, respectively, and this contribution was then subtracted from the overall strength to calculate the intrinsic uniaxial lattice strength (90 MPa). Additionally, tensile tests, performed between 293 K and 873 K, revealed that the Ni₂CrFe MEA has a stronger resistance to softening at high temperatures. Transmission electron microscopy of deformed specimens revealed the formation of dislocation pile-ups at annealing twin boundaries, indicating that it is also an obstacle to dislocation slip. Furthermore, the thickening of the annealing twin boundary after deformation was observed and illustrated by the interaction between different dislocations and annealing twin boundaries. Full article

(This article belongs to the Special Issue Hot Forming/Processing of Metallic Materials)

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16 pages, 6575 KiB

Open AccessArticle

Numerical Simulation of Temperature Field and Melt Pool Characteristics of CP-Ti Manufactured by Laser Powder Bed Fusion

by Kai Guo, Yunping Ji, Yiming Li, Xueliang Kang, Huiyi Bai and Huiping Ren

Metals 2023, 13(1), 11; https://doi.org/10.3390/met13010011 - 20 Dec 2022

Cited by 1 | Viewed by 1417

Abstract

A coupled heat source model that combined a Gauss surface heat source with a Gauss cylindrical volumetric heat source was introduced to simulate temperature field distribution and melt pool characteristics using a finite element simulation (FEM) method for the deep and narrow melt [...] Read more.

A coupled heat source model that combined a Gauss surface heat source with a Gauss cylindrical volumetric heat source was introduced to simulate temperature field distribution and melt pool characteristics using a finite element simulation (FEM) method for the deep and narrow melt pools formed in laser powder bed fusion (L-PBF) aiming at commercial pure titanium (CP-Ti). For comparison, the same simulations using the Gauss surface heat source model and the double ellipsoid heat source model were also performed. The simulated melt pool geometries using the coupled heat source model match well with the measurements, with an average error of 1% for the melt pool depth and 7% for the width. Based on the single-track experimental results, it was found by comparing the simulated results from the three heat source models that the coupled heat source model had better accuracy than the other two. Then, the temperature field and the melt pool geometries of CP-Ti fabricated at different laser power levels from 300 W to 500 W and scanning speeds from 600 mm/s to 4000 mm/s were simulated. According to the simulated maximum temperature and geometries of the melt pool, a suitable process parameters map for CP-Ti was obtained. The reported experimental results agree well with the simulated map. The coupled heat source model is more accurate and applicable for the deep and narrow melt pools formed during L-PBF. Full article

(This article belongs to the Special Issue Hot Forming/Processing of Metallic Materials)

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19 pages, 12380 KiB

Open AccessArticle

Fabrication and Characterization of In Situ Ti-6Al-4V/TiB Composites by the Hot-Pressing Method using Recycled Metal Chips

by Yutao Zhai, Ajit Pal Singh, Leandro Bolzoni, Yingdong Qu, Yiming Li and Fei Yang

Metals 2022, 12(12), 2038; https://doi.org/10.3390/met12122038 - 27 Nov 2022

Cited by 3 | Viewed by 1365

Abstract

In this study, a Ti-6Al-4V matrix composite reinforced with in situ synthesized TiB whiskers has been successfully produced from wasted chips by a cost-effective powder metallurgy route combining rapid heating and hot pressing. The effect of boron powder addition (1 vol.%, 3 vol.%, [...] Read more.

In this study, a Ti-6Al-4V matrix composite reinforced with in situ synthesized TiB whiskers has been successfully produced from wasted chips by a cost-effective powder metallurgy route combining rapid heating and hot pressing. The effect of boron powder addition (1 vol.%, 3 vol.%, 5 vol.%, and 7 vol.%) on phase constituents, microstructures, and mechanical properties of the fabricated composites was investigated. Additionally, the effect of two different post-heat treatments on the 3 vol.% boron-containing composites was also explored. The results show that the ultimate tensile strength (UTS) and yield strength (YS) of the composite first increase when the content of boron is less than 3 vol.%, and then start to decrease with the further increase of boron content. The TiB reinforcement layers change from discontinuous to quasi-continuous and then to continuous with increasing boron content, which is attributed to a higher fraction of agglomerated TiB reinforcement phases. The composites with 1 vol.% (UTS of 1085 MPa, strain to fracture of 5.83%) and 3 vol.% (UTS of 1127 MPa, strain to fracture of 3.98%) boron powder addition show optimized tensile properties. The mechanical properties of 3 vol.% boron-containing composites are not significantly improved after heat treatment. Our experimental results demonstrate the feasibility of fabricating low-cost, high-performance titanium alloy matrix composites from Ti-6Al-4V machining chips. It can serve as a promising and cost-effective method to directly utilize Ti-6Al-4V chips to fabricate strong and ductile Ti-6Al-4V composites for niche applications. Full article

(This article belongs to the Special Issue Hot Forming/Processing of Metallic Materials)

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15 pages, 8676 KiB

Open AccessArticle

A Modified Johnson–Cook Model and Microstructure Evolution of As-Extruded AA 2055 Alloy during Isothermal Compression

by Dongmei Jiang, Jian Zhang, Tiejun Liu, Wei Li, Zhong Wan, Tingzhuang Han, Chaojie Che and Liren Cheng

Metals 2022, 12(11), 1787; https://doi.org/10.3390/met12111787 - 23 Oct 2022

Cited by 5 | Viewed by 1137

Abstract

Isothermal compression behaviors of as-extruded AA 2055 alloy (T6 state) were studied at temperature of 320, 380, 440 and 500 °C with strain rate of 0.001, 0.01, 0.1 and 1 s⁻¹ by a Gleeble-3800 testing machine. A modified Johnson–Cook model fitted by [...] Read more.

Isothermal compression behaviors of as-extruded AA 2055 alloy (T6 state) were studied at temperature of 320, 380, 440 and 500 °C with strain rate of 0.001, 0.01, 0.1 and 1 s⁻¹ by a Gleeble-3800 testing machine. A modified Johnson–Cook model fitted by polynomial and power-exponential functions were established to describe the flow stress of the alloy. The constitutive models fitted by higher-order polynomials were more accurate than the ones fitted by second-order polynomial and power-exponential functions. The constitutive model fitted by a fourth-order polynomial was chosen for the optimal constitutive model in order to balance the prediction accuracy and model complexity. The modified Johnson–Cook constitutive model could predict the flow stress well, especially in high-temperature zone (around 500 °C) and low-temperature zone (around 320 °C). The dynamic precipitation and dissolution of the T₁ phase during hot compression were discussed. The unusual dynamic precipitation of the T₂ phase was investigated during hot compression by XRD and TEM. The massive dense fine precipitates effectively pinned dislocations or subgrain boundaries to accelerate DRV but suppressed DRX, leading to a low frequency of HAGBs in compressed samples. Full article

(This article belongs to the Special Issue Hot Forming/Processing of Metallic Materials)

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11 pages, 3997 KiB

Open AccessArticle

The Microstructure, Mechanical Properties, and Corrosion Resistance of a Novel Extruded Titanium Alloy

by Ning Cui, Saihao Chen, Tiewei Xu, Wei Sun, Binjiang Lv, Shuling Zhang, Hongzhi Niu and Fantao Kong

Metals 2022, 12(10), 1564; https://doi.org/10.3390/met12101564 - 21 Sep 2022

Cited by 5 | Viewed by 1391

Abstract

Titanium alloys are widely used in marine engineering and other industries. To broaden their application, a novel (α + β) titanium alloy Ti−6Al−3Mo−2Zr−2Fe was studied in this work. A tube with an outer diameter of 60mm, inner diameter of 38 mm, and length [...] Read more.

Titanium alloys are widely used in marine engineering and other industries. To broaden their application, a novel (α + β) titanium alloy Ti−6Al−3Mo−2Zr−2Fe was studied in this work. A tube with an outer diameter of 60mm, inner diameter of 38 mm, and length of 500 mm was produced by ingot metallurgy and hot extrusion. The microstructure, mechanical properties, and corrosion resistance of the tube were systematically analyzed. The as−extruded Ti−6Al−3Mo−2Zr−2Fe alloy exhibited a typical duplex microstructure. EBSD observation showed that a strong <

10 \bar{1} 0

>//RD fiber texture of an α phase with a close−packed hexagonal structure was formed in the radial direction. The transformation temperature T_β was determined to be 880–890 °C. A duplex microstructure with fine α platelets was obtained when the alloy was solution−treated at 850 °C for 1 h and underwent an aging treatment at 550 °C for 6 h. The room−temperature tensile strength and elongation of the aged alloy reached 1081.5 MPa and 6.5%, respectively. The corrosion resistance was tested by open circuit potential and a potentiodynamic polarization curve. The results show that the corrosion resistance of the Ti−6Al−3Mo−2Zr−2Fe alloy was better than that of the commonly used TC4 alloy in both a 3.5% NaCl solution and an acidic solution. Full article

(This article belongs to the Special Issue Hot Forming/Processing of Metallic Materials)

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20 pages, 5881 KiB

Open AccessArticle

Microstructural Evolution and Stability of Coarse-Grained S31254 Super Austenitic Stainless Steel during Hot Deformation

by Jia Xing, Chengzhi Liu, Aimin Li, Shouming Wang, Xinjie Zhang and Yongxin Shi

Metals 2022, 12(8), 1319; https://doi.org/10.3390/met12081319 - 06 Aug 2022

Cited by 3 | Viewed by 1475

Abstract

The ingot of S31254 super austenitic stainless steel (SASS) was annealed at 1220 °C for 70 h to eliminate the segregation of Mo element, and the grain size grows to the level of millimeter. The stress–strain response and microstructural evolution of coarse-grained S31254 [...] Read more.

The ingot of S31254 super austenitic stainless steel (SASS) was annealed at 1220 °C for 70 h to eliminate the segregation of Mo element, and the grain size grows to the level of millimeter. The stress–strain response and microstructural evolution of coarse-grained S31254 SASS were investigated by hot compression tests in the temperature range of 950–1250 °C and strain rate range of 0.001–10 s⁻¹. The results showed that the energy required for plastic deformation improved with the increase of strain rate and the decrease of deformation temperature. The hot deformation activation energy was calculated to be 542.91 kJ·mol⁻¹ through the regression analysis of hyperbolic-sine function, and the constitutive equation was established. Processing maps were constructed, and two optimal hot working parameters ranges were clarified. Due to the low fraction of grain boundaries, the main deformation mechanism of coarse-grained S31254 SASS was dynamic recovery. However, when the deformation temperature improved to 1250 °C, recrystallized grains began to nucleate and grow along with the band-like structure within the austenitic grains. When the deformation temperature is 950–1150 °C, the microstructural stability of S31254 SASS under tension stress was excellent. However, when the temperature and the strain rate were 1250 °C and 0.5 s⁻¹ respectively, the microstructural stability deteriorated resulting from the formation of δ-ferrite phase and local melting of austenitic grain boundaries. Full article

(This article belongs to the Special Issue Hot Forming/Processing of Metallic Materials)

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14 pages, 11300 KiB

Open AccessArticle

Effects of Pulsed Magnetic Field Melt Treatment on Grain Refinement of Al-Si-Mg-Cu-Ni Alloy Direct-Chill Casting Billet

by Xinyu Bao, Yonglin Ma, Shuqing Xing, Yongzhen Liu and Weiwei Shi

Metals 2022, 12(7), 1080; https://doi.org/10.3390/met12071080 - 24 Jun 2022

Cited by 9 | Viewed by 1390

Abstract

Al-Si-Mg-Cu-Ni alloy is widely used in the manufacture of high-performance car engine parts. Coarse, dendritic α-Al and large primary Si are common in Al-Si-Mg-Cu-Ni alloy DC casting billet, which is harmful to the performance of the final product. In this paper, a pulsed [...] Read more.

Al-Si-Mg-Cu-Ni alloy is widely used in the manufacture of high-performance car engine parts. Coarse, dendritic α-Al and large primary Si are common in Al-Si-Mg-Cu-Ni alloy DC casting billet, which is harmful to the performance of the final product. In this paper, a pulsed magnetic field melt treatment technique was applied to the melt in the launder of a DC casting platform to modify the α-Al and primary Si in the billet. A transient numerical model was established to analyze the electromagnetic field, flow field and temperature field in the melt during the pulsed magnetic field treatment. The effect of the magnetic energy on the clusters in the melt was analyzed. We found that during the pulsed magnetic field melt treatment, the number of clusters close to the critical size was increased due to the cluster formation work being reduced by the magnetic energy, which facilitated nucleation and refined the solidification structure. Furthermore, the flow velocity increased, and temperature homogenized in the melt during the pulsed magnetic field melt treatment, which benefitted the clusters close to the critical size distributed and maintained in the melt uniformly. The experimental results show that the α-Al and primary Si were small and homogeneous following the pulsed magnetic field melt treatment. The size of α-Al and primary Si was reduced by 25.6–44.4% and 32.2–54.1%, respectively, in the billet center compared to the conventional process. Full article

(This article belongs to the Special Issue Hot Forming/Processing of Metallic Materials)

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11 pages, 57972 KiB

Open AccessArticle

Static Recrystallization Behavior and Texture Evolution during Annealing in a Cold Rolling Beta Titanium Alloy Sheet

by Shuzhi Zhang, Qibin Wang, Xing Cheng, Jianchao Han, Wanggang Zhang, Changjiang Zhang and Jie Wu

Metals 2022, 12(6), 899; https://doi.org/10.3390/met12060899 - 25 May 2022

Cited by 3 | Viewed by 1856

Abstract

In this study, the cold rolling microstructure and static recrystallization mechanism of the high strength titanium alloy Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe were systematically investigated. Results show that the cold rolling microstructure is mainly composed of the elongated deformed β grains containing micro-shear bands. After annealing at [...] Read more.

In this study, the cold rolling microstructure and static recrystallization mechanism of the high strength titanium alloy Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe were systematically investigated. Results show that the cold rolling microstructure is mainly composed of the elongated deformed β grains containing micro-shear bands. After annealing at 815 °C for 2 min, the fine SRXed grains are observed, mainly concentrated in the micro-shear band, the grain boundary and the interior of the deformed grain. The sub-grain structure obtained by static recovery inside the deformed grain produces continuous SRX during the annealing treatment. Meanwhile, geometric and discontinued SRXed grains are also observed in the large deformed β grain and at the trigeminal grain boundaries, respectively. Many ultra-fine grains appear inside the micro-shear band, exhibiting a phenomenon of the micro-shear band assisting SRX. With the increase in the annealing holding time, the elongated β grains are significantly refined and the degree of recrystallization is continuously improved. In addition, the recrystallization behavior also results in a significant change in the fiber texture. With the extension of the annealing holding time, the rolling texture type evolves gradually, with the {111} <112> γ-fiber texture to weak α-fiber, γ-fiber, and Goss-fiber. Full article

(This article belongs to the Special Issue Hot Forming/Processing of Metallic Materials)

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12 pages, 6622 KiB

Open AccessArticle

Effects of Heat Treatment on Microstructures and Properties of Cold Rolled Ti-0.3Ni Sheets as Bipolar Plates for PEMFC

by Haifeng Zhu, Xiaopeng Wang, Wei Meng and Fantao Kong

Metals 2022, 12(5), 792; https://doi.org/10.3390/met12050792 - 04 May 2022

Cited by 2 | Viewed by 1403

Abstract

As promising materials for bipolar plates substrate, as-cold rolled Ti-0.3Ni (wt.%) sheets were heat treated with three different processes in this work. As-cold rolled sheets consist of α matrix and dispersed Ti₂Ni intermetallic precipitates, and typical Widmanstatten microstructure can be observed [...] Read more.

As promising materials for bipolar plates substrate, as-cold rolled Ti-0.3Ni (wt.%) sheets were heat treated with three different processes in this work. As-cold rolled sheets consist of α matrix and dispersed Ti₂Ni intermetallic precipitates, and typical Widmanstatten microstructure can be observed after heat treatment. Lamellar Ti₂Ni precipitates inside the colonies. Elongation of as-cold rolled sheets equals less than 7% while this value rises up to around 20%, and tensile strength decreases by more than 47% after heat treatment. Open circuit potentials of as-cold rolled sheets treated at 950 °C for 1 h followed by wind cooling (950 °C/1 h/WC), sheets aged at 500 °C for 3 h followed by air cooling (950 °C/1 h/WC + 500 °C/3 h/AC), and sheets treated at 950 °C for 1 h followed by furnace cooling (950 °C/1 h/FC) equals −0.536 V, −0.476 V, −0.486 V, −0.518 V, respectively. A potentiodynamic polarization test reveals that all of the specimens exhibit typical active–passive transition behavior. Sheets treated at 950 °C/1 h/WC possess the lowest corrosion current density (155.4 μA·cm⁻²). Results of electrochemical impedance spectroscopy (EIS) show that 950 °C/1 h/WC treated sheets possess the largest polarization resistance (R_pol), 122.6 Ω·cm². Moreover, steady-state current densities (I_ss) increase in the order of 950 °C/1 h/WC, 950 °C/1 h/WC + 500 °C/3 h/AC, 950 °C/1 h/FC according to the results of potentiostatic polarization. This can be attributed to various amounts of Ti₂Ni precipitation caused by different cooling rates. Full article

(This article belongs to the Special Issue Hot Forming/Processing of Metallic Materials)

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19 pages, 9314 KiB

Open AccessArticle

Fabrication, Microstructure, Mechanical, and Electrochemical Properties of NiMnFeCu High Entropy Alloy from Elemental Powders

by Ashok Kumar, Michael Mucalo, Leandro Bolzoni, Yiming Li, Fantao Kong and Fei Yang

Metals 2022, 12(1), 167; https://doi.org/10.3390/met12010167 - 17 Jan 2022

Cited by 5 | Viewed by 2594

Abstract

Transition metal based high entropy alloys (HEAs) are often used in electrocatalytic (water electrolysis) applications due to the synergistic effect operating among its constituent elements and unpaired electrons in d orbitals of the concerned metal. In this study, a low cost NiMnFeCu high [...] Read more.

Transition metal based high entropy alloys (HEAs) are often used in electrocatalytic (water electrolysis) applications due to the synergistic effect operating among its constituent elements and unpaired electrons in d orbitals of the concerned metal. In this study, a low cost NiMnFeCu high entropy alloy was successfully synthesised using the combined techniques of mechanical milling (MA) and vacuum sintering. X-ray diffraction was used to analyse the phase composition, optical microscopy, and scanning electron microscopy were used to characterise the fabricated material’s microstructure and chemical homogeneity, thermal, and mechanical properties were tested using the differential scanning calorimetry method and a universal testing machine, respectively. Electrochemical workstation was used to carry out preliminary electrochemical studies such as linear sweep voltammetry (LSV), cyclic voltammetry (CV) and chronoamperometry. The results showed that the as- sintered NiMnFeCu HEA possessed a single- phase FCC structure. The HEA NiMnFeCu sintered at 1050 °C (S4) and 1000 °C (S2) with a holding time of 2 h showed a yield strength of 516.3 MPa and 389.8 MPa, respectively, and the micro-hardness values were measured to be 233.45 ± 9 HV and 198.7 ± 8 HV, respectively. Preliminary electrochemical studies proved that the alloy sintered at 1000 °C (S2) with a holding time of 2 h exhibited excellent electrocatalytic properties with a measured overpotential of 322 mV at 10 mA cm⁻² at 100 cycles of CV and good stability for 10 h when compared to state-of-the-art electrocatalytic materials

{IrO}_{2}

and Ru

O_{2}

. This suggested that the HEA NiMnFeCu fabricated under the condition S2 could potentially be used for industrial-scale water electrolysis as it possesses permissible mechanical and good electrochemical properties. Full article

(This article belongs to the Special Issue Hot Forming/Processing of Metallic Materials)

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10 pages, 10136 KiB

Open AccessArticle

Effects of Graphene on the Wear and Corrosion Resistance of Micro-Arc Oxidation Coating on a Titanium Alloy

by Ruifang Zhang, Kai Lv, Zhaoxin Du, Weidong Chen, Pengfei Ji and Mingli Wang

Metals 2022, 12(1), 70; https://doi.org/10.3390/met12010070 - 31 Dec 2021

Cited by 5 | Viewed by 1929

Abstract

In order to improve the wear and corrosion resistance of micro-arc oxidation (MAO) coating on a Ti-5Al-1V-1Sn-1Zr-0.8Mo alloy, 0–0.20 g/L graphene was added to the electrolyte to prepare micro-arc oxidation coating. The thickness, roughness, micro-morphology, and composition of the MAO coating were characterized, [...] Read more.

In order to improve the wear and corrosion resistance of micro-arc oxidation (MAO) coating on a Ti-5Al-1V-1Sn-1Zr-0.8Mo alloy, 0–0.20 g/L graphene was added to the electrolyte to prepare micro-arc oxidation coating. The thickness, roughness, micro-morphology, and composition of the MAO coating were characterized, and the wear and corrosion resistance of the coating was tested and analyzed. The results show that with 0.05 g/L of graphene in the electrolyte, the roughness of the coating decreased from 56.76 μm to 31.81 μm. With the increase in the addition of graphene, the microstructure of the coating became more compact, the diameter of micro-holes and micro-cracks decreased, and the corrosion resistance of the coating improved. The wear tests showed that the mass loss of the coating at the early wear stage (0~100 revolutions) was greater than that at the later stage (100~250 revolutions), and the wear resistance of the coating obtained by the addition of 0.10 g/L of graphene was the highest. With 0.10 g/L of graphene, the adhesion force between the coating and the substrate alloy is the largest, reaching 57.1 N, which is 9.98 N higher than that without graphene. After salt spray corrosion for 480 h, the coating with graphene has better corrosion resistance than that of a graphene-free coating. Full article

(This article belongs to the Special Issue Hot Forming/Processing of Metallic Materials)

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13 pages, 6501 KiB

Open AccessArticle

Preparation of Multiscale α Phase by Heat Treatments and Its Effect on Tensile Properties in Metastable β Titanium Alloy Sheet

by Hanyu Jiang, Zhaoxin Du, Da Wang, Tianhao Gong, Xiaoming Cui, Fei Liu, Jun Cheng and Wenzhen Chen

Metals 2021, 11(11), 1708; https://doi.org/10.3390/met11111708 - 26 Oct 2021

Cited by 7 | Viewed by 1494

Abstract

This study presents a strategy for the preparation of multiscale α phase by high/low-temperature two-step aging in metastable β titanium alloy, Ti-15Mo-3Al-2.7Nb-0.2Si. The purpose of high-temperature aging is to obtain a number of micro-scale secondary α phases (α_S), besides precipitation-free zones [...] Read more.

This study presents a strategy for the preparation of multiscale α phase by high/low-temperature two-step aging in metastable β titanium alloy, Ti-15Mo-3Al-2.7Nb-0.2Si. The purpose of high-temperature aging is to obtain a number of micro-scale secondary α phases (α_S), besides precipitation-free zones (PFZs). After that, in the second stage of low-temperature aging, the nano-scale tertiary α phase (α_T) precipitates in the PFZs. The second-step aging temperature and the time at low temperature will have an important effect on α_T. On the one hand, with the extension of aging time, the morphology of α_T changes from dot-like to needle-like. Secondly, the increase of the second-step aging temperature will promote precipitation of α_T in PFZs. The strength increases gradually with the accelerated precipitation of α_T, and good plasticity is still maintained. The effect of multiscale α on fracture behavior was investigated by in situ tensile testing. Results show that cracks form a transgranular fracture along the slip line after solution treatment. After first-step aging, the crack easily passes through α_S and forms an intergranular fracture. After second-step aging, the formation of multiscale α phase has a stronger effect in hindering the crack growth, which leads to crack propagation mainly by transgranular fracture. Overall, this study demonstrates that the alloy can be heat-treated to achieve an excellent match between strength and ductility, which is expected to meet the application requirements of structural materials of the new generation of aircraft. Full article

(This article belongs to the Special Issue Hot Forming/Processing of Metallic Materials)

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