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Microstructural Characterization of Metallic Materials

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A special issue of Metals (ISSN 2075-4701). This special issue belongs to the section "Structural Integrity of Metals".

Deadline for manuscript submissions: closed (30 December 2022) | Viewed by 18160

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Prof. Dr. Jer-Ren Yang

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Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
Interests: exploration and characterization of atomic/nano/micro/macro-structure in newly advanced steels and thin-films for energy-saving structural/functional materials application; Micro-alloy Steels; Stainless Steels; Allotriomorphic Ferrite; Idiomorphic Ferrite; Massive ferrite; Pearlite; Widmanstätten Ferrite; Acicular Ferrite; Bainite; Martensite; Austenite; Carbides and Intermetallic Compound Particles in Steels

Prof. Dr. R. Devesh K. Misra

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Department of Metallurgical and Materials Engineering, University of Texas, 500 W. University Avenue, El Paso, TX 79968, USA
Interests: advanced engineering steels; processing; deformation and fracture; biomaterials; nanostructured materials
Special Issues, Collections and Topics in MDPI journals

Prof. Dr. Chengjia Shang

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Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, China
Interests: bainite; bainitic transformations; martensite; heat affected zone; high-strength low-alloy steel; submerged Arc welding; transformation induced plasticity steel; manganese steel; austenite

Special Issue Information

Dear Colleagues,

The classification of microstructures of metallic materials based on morphology is of great significance for the study of structure–property relationships. Further insight into the details of substructures is usually necessary for the researchers to evaluate the relevant attributes. The development of metallic materials is always required to achieve the improved combinations of toughness and strength (or hardness). Careful alloy design and advances in process technology can make ordinary materials become extraordinary. This Special Issue aims to explore the special microstructures that are produced by some new processes/alloy additions and exhibit excellent properties. Through optical metallography, scanning electron microscopy/electron backscatter diffraction, transmission electron microscopy, scanning transmission electron microscopy, etc., microstructural characterization brings about new findings. The related metallic materials for the subjects will include (but not limited to) alloy steels, stainless steels, light metals: aluminum alloys, magnesium alloys and titanium alloys, high entropy alloys, etc.

Prof. Dr. Jer-Ren Yang
Prof. Dr. R. Devesh K. Misra
Prof. Dr. Chengjia Shang
Guest Editors

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Keywords

microstructural characterization
steels
light metals
high entropy alloys

Published Papers (10 papers)

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Research

13 pages, 9237 KiB

Open AccessArticle

Distinguishing Features of Quenched Nanoprecipitates in Allotriomorphic Ferrite and Reverted γ during Aging for Dual-Phase PHSS

by Ping-Jui Yu, Tzu-Ching Tsao, Cheng-An Hsu, Neng-Hao Gan, Shing-Hoa Wang, Jer-Ren Yang, Horng-Yi Chang and Tsai-Fu Chung

Metals 2023, 13(3), 625; https://doi.org/10.3390/met13030625 - 21 Mar 2023

Cited by 1 | Viewed by 1258

Abstract

A novel dual-phase PHSS consisting of lath martensite plus allotriomorphic δ ferrite (ALF) with nanoprecipitates was characterized by high-resolution field emission transmission electron microscopy for quenched, solid-solution-treated, and aged stainless steel. The effects of aging at various durations prior to H₂O or liquid N₂ quenching were investigated. Cu-rich nanoprecipitates evolve from body-centered cubic clusters to 9R Cu under quenching to 3R Cu and subsequently to face-centered cubic ε-Cu at various aging durations. Maximum hardness was observed after aging at 600 °C for 1 h. However, after this aging, both reversed austenite and Cu-rich nanoprecipitates coexisted in the martensite matrix. The segregation and diffusion of austenite-stabilizing elements promoted the nucleation of reversed austenite. Full article

(This article belongs to the Special Issue Microstructural Characterization of Metallic Materials)

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15 pages, 5795 KiB

Open AccessArticle

The Influence of Creep Ageing on the Hardening Behavior and Microstructure of 7050 Aluminum Alloy

by Yo-Lun Yang, Tsai-Fu Chung, Md Sadique Ali, Dilshad and Pengjing Zhao

Metals 2023, 13(2), 196; https://doi.org/10.3390/met13020196 - 18 Jan 2023

Viewed by 1482

Abstract

The creep ageing process can have a significant influence on the mechanical properties of aluminum alloys. In the present work, microstructural analysis and mechanical testing were implemented to characterize the age hardening effect and microstructure evolution, and to investigate how the stress applied under creep ageing conditions can affect a material’s microstructure. The curves depicting yield strength in relation to creep ageing time suggested that the stress applied in creep ageing can result in a reduction of the strength of aluminum alloy 7050; the yield strength decreases with increasing applied stress. Microstructural analysis by transmission electron microscopy (TEM) revealed that by applying stress, the growth and coarsening rate of the Guinier-Preston (GP) zones and η′ precipitates can be sped up. Even after pure/creep ageing for 8 h, there are still some GP zones in the aluminum matrix, demonstrating that the GP zones’ nucleation is a continuous process. Full article

(This article belongs to the Special Issue Microstructural Characterization of Metallic Materials)

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16 pages, 13942 KiB

Open AccessArticle

Effect of Cu Additions on the Evolution of Eta-prime Precipitates in Aged AA 7075 Al–Zn–Mg–Cu Alloys

by Ting-Jung Hsiao, Po-Han Chiu, Cheng-Lin Tai, Tzu-Ching Tsao, Chien-Yu Tseng, Yi-Xian Lin, Hsueh-Ren Chen, Tsai-Fu Chung, Chih-Yuan Chen, Shing-Hoa Wang and Jer-Ren Yang

Metals 2022, 12(12), 2120; https://doi.org/10.3390/met12122120 - 09 Dec 2022

Cited by 3 | Viewed by 1550

Abstract

In the present study, after solid solution treatment, four different artificial aging treatments (100, 120, 140 and 160 °C) were performed on Al-5.98Zn-2.86Mg-1.61Cu (wt.%) alloy, denoted as 7075-LCu, and Al-5.91Zn-2.83Mg-1.98Cu (wt.%) alloy, denoted as 7075-HCu. Peak aging conditions were determined for each aging temperature at various hold time intervals of up to 24 h. It was found that both alloys possessed the optimal strengths after artificial aging at 120 °C for 24 h. Under this condition, the ultimate tensile strengths (UTSs) were 618 MPa (7075-LCu) and 623 MPa (7075-HCu), respectively. Moreover, a method was used to calculate the average sizes and number density of the major strengthening precipitates, η^′, under peak aging conditions in these two alloys from transmission electron microscopy (TEM) images and electron energy loss spectroscopy (EELS). The above results indicated that for the 7075-LCu and 7075-HCu samples with the optimal UTS strengths, the former possessed an average thickness of 2.15 nm, and a number density of 3.27 × 10¹⁷ cm⁻³; the latter, 2.04 nm and 3.52 × 10¹⁷ cm⁻³. Full article

(This article belongs to the Special Issue Microstructural Characterization of Metallic Materials)

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19 pages, 6689 KiB

Open AccessArticle

Alternative Approach for the Intercritical Annealing of (Cr, Mo, V)-Alloyed TRIP-Assisted Steel before Austempering

by Vadym Zurnadzhy, Vasily Efremenko, Ivan Petryshynets, Manuele Dabalà, Mattia Franceschi, Kaiming Wu, František Kováč, Yuliia Chabak, Viktor Puchy and Michail Brykov

Metals 2022, 12(11), 1814; https://doi.org/10.3390/met12111814 - 26 Oct 2022

Cited by 3 | Viewed by 1222

Abstract

TRIP-assisted C-Si-Mn steels are usually subjected to austempering with a preliminary intercritical annealing that is targeted at the multi-phase structure with 40–60 vol.% of proeutectoid ferrite. The kinetics and the mechanism of phase-structural transformations can be impacted due to the additional alloying of TRIP-assisted steel by the strong carbide forming elements, thus necessitating an alternative approach for the selection of intercritical annealing parameters. This issue is analyzed in the present work, which investigates the effect of the temperature of intercritical annealing on the “Structure/Properties” correlations in 0.2 wt.% C-Si-Mn-Nb steel additionally alloyed by 0.55 wt.% Cr, 0.20% Mo, and 0.11 wt.% V. The annealing temperature ranged from 770 °C to 950 °C, and austempering was performed at 350 °C for 20 min. It was observed that the addition of the (Cr, Mo, and V) complex significantly improved the steel hardenability. However, the annealing of steel at 770 °C (to gain 50 vol.% of proeutectoid ferrite) resulted in the precipitation of coarse cementite lamellas during bainite transformation, thus lowering the amount of retained austenite (RA) and decreasing the strength and ductility of the steel. At higher annealing temperatures, carbide-free bainite was formed, which presented a 2.5–3.5 times increase in the RA volume fraction and a 1.5 times increase in the RA carbon content. The optimal combination of the mechanical properties (UTS of 1040 MPa, TEL of 23%, V-notch impact toughness of 95 J/cm², PSE of 23.9 GPa·%) referred to annealing at a temperature close to the Ac₃ point, resulting in a structure with 5 vol.% ferrite and 9 vol.% RA (the residue was carbide-free bainite). This structure presented an extended manifestation of the TRIP effect with an enhanced strain hardening rate due to strain-induced martensite transformation. The impact of the alloying elements on the carbon activity in austenite served as the basis for the analysis of structure formation. Full article

(This article belongs to the Special Issue Microstructural Characterization of Metallic Materials)

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9 pages, 2037 KiB

Open AccessArticle

Effect of Natural Ageing on Subsequent Artificial Ageing of AA7075 Aluminum Alloy

by Cheng-Ling Tai, Po-Jui Tai, Ting-Jung Hsiao, Po-Han Chiu, Chien-Yu Tseng, Tzu-Ching Tsao, Tsai-Fu Chung, Yo-Lun Yang, Chih-Yuan Chen, Shing-Hoa Wang and Jer-Ren Yang

Metals 2022, 12(10), 1766; https://doi.org/10.3390/met12101766 - 20 Oct 2022

Cited by 4 | Viewed by 1666

Abstract

The effects of natural ageing treatment prior to artificial ageing treatment on the microstructures and mechanical properties of AA7075 Al-5.7Zn-2.6Mg-1.5Cu-0.18Cr-0.08Mn-0.05Si-0.17Fe (wt.%) aluminum alloy have been investigated. The hardness of solution-treated samples (91.0 HV) profoundly increased to 146.8 HV after 7 days of natural ageing. The purpose of the present work was to examine the kinetic hardening evolution in subsequent artificial ageing treatments of samples naturally aged for 7 days and their counterparts without natural ageing. The former were labelled as NA-7d samples, and the latter, NA-0d samples. After artificial ageing at 120 °C for 2 h, the hardness of NA-0d samples increased rapidly to 148.2 HV, which was approximately the same as that of the specimens with natural ageing for 7 days, compensating for the prior state of lower hardness without natural ageing. After being treated at 120 °C for 16 h, the ultimate tensile strength (UTS) and yield strength (YS) of NA-7d reached the highest value, respectively, 601 MPa and 539 MPa, followed by a slight decrement of UTS when aged to 24 h. On the other hand, NA-0d specimens aged at 120 °C for 16 and 24 h showed nearly the same UTS (598 MPa); the former possessed YS of 538 MPa, and the latter, 545 MPa. The results presumably reveal that the peak ageing condition for NA-0d samples can be achieved under 24 h ageing at 120 °C. Under the same treatment at 120 °C for 24 h, the size of η’ phase in NA-7d sample (with a length of 4.96 nm) coarsened and grew larger than that in NA-0d sample (with a length of 3.46 nm). In addition, some η’ phase in the NA-7d sample was found to be transformed into the η₂ phase. The results indicated that the naturally aged specimens (NA-7d) reached the peak ageing condition earlier, but did not significantly enhance the UTS in AA7075 aluminum alloy, as compared to the samples without prior natural ageing (NA-0d). Full article

(This article belongs to the Special Issue Microstructural Characterization of Metallic Materials)

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13 pages, 3343 KiB

Open AccessArticle

Investigation of Strain-Induced Precipitation of Niobium Carbide in Niobium Micro-Alloyed Steels at Elevated Temperatures

by Tzu-Ching Tsao, Po-Han Chiu, Chien-Yu Tseng, Cheng-Lin Tai, Hsueh-Ren Chen, Tsai-Fu Chung, Chih-Yuan Chen, Shing-Hoa Wang, Yu-Ting Tsai and Jer-Ren Yang

Metals 2022, 12(10), 1619; https://doi.org/10.3390/met12101619 - 27 Sep 2022

Viewed by 1328

Abstract

Two steels with a base composition of Fe-0.2C-0.8Mn-1.2Cr (wt%) but with different niobium (Nb) contents (0.02 and 0.03 wt%) were employed to study the effect of precipitate evolution on the softening resistance in the austenite region under elevated temperature deformation. The thermomechanical procedure was executed by a deformation-dilatometer and involved double deformation processes with 25% strain at a 0.25 s⁻¹ strain rate at 900, 925, 950, and 1000 °C. The softening ratios, reflecting the competition between recrystallization and precipitation, were evaluated. The results indicated that both steels showed better softening resistance at 900 °C than at other temperatures. However, the softening ratio of 0.03 wt% Nb-containing steel (Steel 3N) rose after 100 s at 900 °C, while 0.02 wt% Nb-containing steel (Steel 2N) maintained a low softening ratio within 300 s at 900 °C, indicating that Steel 3N was relatively non-durable. A microstructural characterization showed that in the Steel 3N sample deformed at 900 °C, recrystallization occurred more strongly than for Steel 2N after a 1000 s holding time. A follow-up analysis then showed that Steel 3N treated at 900 °C revealed a faster coarsening of the carbides than Steel 2N even in the early stage of precipitation, evidencing that Steel 2N exhibited a lower softening resistance at 900 °C. Full article

(This article belongs to the Special Issue Microstructural Characterization of Metallic Materials)

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11 pages, 3077 KiB

Open AccessArticle

Investigation of Co-Cr-Fe-Mn-Ni Non-Equiatomic High-Entropy Alloy Fabricated by Wire Arc Additive Manufacturing

by Kirill Osintsev, Sergey Konovalov, Dmitrii Zaguliaev, Yurii Ivanov, Victor Gromov and Irina Panchenko

Metals 2022, 12(2), 197; https://doi.org/10.3390/met12020197 - 21 Jan 2022

Cited by 14 | Viewed by 2926

Abstract

Fabrication of high-entropy alloys (HEAs) is a crucial area of interest for materials scientists since these metallic materials may have many practical uses. Wire arc additive manufacturing (WAAM), unlike other additive technologies, has tangible benefits for making large-sized components, but manufacturing the wire from HEAs is still very limited. Recent studies suggested tackling this problem using a combined cable composed of wires consisting of pure elements as feeding material. However, not all compositions of HEAs can be obtained by the pure elements’ wires because the number of them is limited. This study aims to examine phase composition, chemical elements distribution, microstructure, and mechanical properties of a Co-Cr-Fe-Mn-Ni HEA, which was not previously obtained by the WAAM. The cable-type wire used in this study is composed of two wires which consist of Cr, Fe, Mn, and Ni, and one pure Co wire. The phase composition, chemical elements distribution, microstructure, and mechanical properties were investigated. The prepared high-entropy alloy has non-equiatomic chemical composition with a single-phase FCC crystal structure with homogeneously distributed elements inside the grains. The microstructure examinations showed dendrite structure which is typical for WAAM processes. The compressive yield strength of the alloy is ~279 MPa, the ultimate compressive strength is ~1689 MPa, the elongation is 63%, and the microhardness is ~150 HV, which was found to be similar to the previously fabricated Co-Cr-Fe-Mn-Ni alloys by other methods. Fracture analysis confirmed the ductile behavior of deformation by the presence of dimples. Full article

(This article belongs to the Special Issue Microstructural Characterization of Metallic Materials)

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14 pages, 7980 KiB

Open AccessArticle

Microstructures and High-Temperature Properties of Sm-Modified Mg-4Al-4RE Alloy

by Hongkui Mao, Xiaoyu Bai, Yu Wang, Hong Xu, Jibo Hou, Liangliang Wei and Raja Devesh Kumar Misra

Metals 2022, 12(1), 135; https://doi.org/10.3390/met12010135 - 11 Jan 2022

Viewed by 1189

Abstract

The effect of Sm on the microstructure and tensile properties of Mg-4Al-4 (La, Ce) alloy was studied. The Mg-4Al-4 (La, Ce) alloy was mainly composed of α-Mg and Al₁₁(La, Ce)₃. With the addition of Sm, a new phase of Al₂(La, Ce, Sm) was revealed in the alloy. The results showed that at room temperature (RT), after Sm addition, the ultimate tensile stress and the elongation decreased, while the yield stress increased slightly; the elongation increased with the Sm addition and the yield stress was basically the same, but the ultimate tensile stress decreased at an elevated temperature of 150 °C. The change in the mechanical properties of the alloy was mainly related to the change in microstructure and phase. With the increase in Sm content, the volume fraction of Al₂(La, Ce, Sm) phase increased and the Al₁₁(La, Ce)₃ eutectic volume fraction decreased significantly, which led to a change in the mechanical properties of the alloy. The 1 wt.%Sm-addition alloy exhibited greater elongation than the Sm-free alloys. Full article

(This article belongs to the Special Issue Microstructural Characterization of Metallic Materials)

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11 pages, 5924 KiB

Open AccessArticle

Morphology and Crystallography Analyses of HSLA Steels with Hardenability Enhanced by Tailored C–Ni Collocation

by Zhipeng Liu, Yishuang Yu, Jie Yang, Zhiquan Wang, Hui Guo and Chengjia Shang

Metals 2022, 12(1), 32; https://doi.org/10.3390/met12010032 - 24 Dec 2021

Cited by 3 | Viewed by 2316

Abstract

High hardenability is of great importance to ultra-heavy steel plates and can be achieved by tailoring the composition of steel. In this study, the continuous cooling transformation (CCT) curves of two high-strength low-alloy (HSLA) steels (0.16C-0.92Ni steel and 0.12C-1.86Ni steel) were elucidated to reveal the significance of C–Ni collocation on hardenability from the perspective of morphology and crystallography. At a low cooling rate (0.5 °C/s), the 0.12C-1.86Ni steel showed higher microhardness than 0.16C-0.92Ni steel. The microstructure in 0.16C-0.92Ni steel was mainly granular bainite with block-shaped martensite/austenite islands (M/A islands), while that in 0.12C-1.86Ni steel was typically lath bainite with film-shaped M/A islands, denoting that the 0.12C-1.86Ni steel is of higher hardenability. Moreover, the 0.12C-1.86Ni steel exhibited a higher density of block boundaries, especially V1/V2 boundaries. The higher density of block boundaries resulted from the weakened variant selection due to the larger transformation driving force and more self-accommodation of transformation strain induced by the reduced carbon and increased nickel content. Full article

(This article belongs to the Special Issue Microstructural Characterization of Metallic Materials)

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16 pages, 9175 KiB

Open AccessArticle

Tailoring Heterogeneous Microstructure in a High-Strength Low-Alloy Steel for Enhanced Strength-Toughness Balance

by Yishuang Yu, Minliang Gao, Bin Hu, Chang Tian, Xuequan Rong, Zhenjia Xie, Hui Guo and Chengjia Shang

Metals 2021, 11(12), 1983; https://doi.org/10.3390/met11121983 - 09 Dec 2021

Cited by 2 | Viewed by 2162

Abstract

The attainment of both strength and toughness is of vital importance to most structural materials, although unfortunately they are generally mutually exclusive. Here, we report that simultaneous increases in strength and toughness in a high-strength low-alloy (HSLA) steel were achieved by tailoring the heterogeneous microstructure consisting of soft intercritical ferrite and hard martensite via intercritical heat treatment. The heterogeneous microstructure features were studied from the perspective of morphology and crystallography to uncover the effect on mechanical properties. Specifically, the volume fraction of martensite increased with increasing annealing temperature, which resulted in increased back stress and effective stress, and thereby an improved strength-ductility combination. The enrichment of carbon and alloying elements in the martensite was lowered with the increase in annealing temperature. As a result, the hardness difference between the intercritical ferrite and martensite was reduced. In addition, the globular reversed austenite preferentially grew into the adjacent austenite grain that held no Kurdjumov-Sachs (K-S) orientation relationship with it, which effectively refined the coarse prior austenite grains and increased the density of high angle grain boundaries. The synergy of these two factors contributed to the improved low-temperature toughness. This work demonstrates a strategy for designing heterostructured HSLA steels with superior mechanical properties. Full article

(This article belongs to the Special Issue Microstructural Characterization of Metallic Materials)

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