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Innovations in Heat Treatment of Metallic Materials

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A special issue of Metals (ISSN 2075-4701). This special issue belongs to the section "Metal Casting, Forming and Heat Treatment".

Deadline for manuscript submissions: 30 November 2026 | Viewed by 7060

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Dr. Zhenli Mi

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Institute of Engineering Technology, University of Science and Technology Beijing, Beijing 100083, China
Interests: advanced steels; deformation mechanism; microstructure; mechanical properties; advanced processing; service safety
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Special Issue Information

Dear Colleagues,

The accelerating transition toward zero-defect manufacturing (ZDM) in metal components is driving an 8.2% compound annual growth rate (CAGR) in advanced heat treatment technologies (2024-2030), positioning phase transformation engineering as the cornerstone of next-generation metallic systems. Advanced thermomechanical technologies, including austempering/Quenching and Partitioning (Q&P), Carburizing/Nitriding, intercritical annealing, and flash heat treatment, have been developed. Core innovations in advanced steels and metals such as high-strength low-alloy (HSLA) steels, third-generation advanced high-strength steels (3G-AHSS), additively manufactured alloys, and nanostructured bainitic steels have been realized. Efficient experimental characterization and simulation, through, for example, phase-field modeling of phase transformations, crystallographic texture evolution, in situ synchrotron diffraction, and machine learning for heat treatment optimization, contribute significantly to the innovative design of metallic materials.

This Special Issue will focus on cutting-edge advances in thermal processing technologies for metallic materials, bridging fundamental thermodynamics and industrial implementation through precision-controlled thermal processes. We seek contributions elucidating microstructure–property relationships under novel heat treatment regimes, including but not limited to the following: 1. phase transformation kinetics in ultrahigh-carbon steels and lightweight alloys; 2. energy-efficient processes (e.g., induction heating, laser-assisted treatments); 3. residual stress engineering through computational thermodynamics; 4. AI-guided design of heat treatment cycles; 5. sustainable approaches to reducing CO₂ emissions in industrial heat treatment; 6. in situ characterization of phase evolution during rapid heating.

Dr. Zhenli Mi
Guest Editor

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Keywords

heat treatment optimization
phase transformations
residual stress control
sustainable manufacturing
multi-scale modeling

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Published Papers (9 papers)

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Research

33 pages, 19108 KB

Open AccessArticle

A Transfer-Learning and Continuous Optimization-Based Framework for Predicting Heat Treatment-Dependent Mechanical Properties of DED-Processed Low-Alloy Steels

by Atiqur Rahman, Sung-Heng Wu, Ranjit Joy and Frank Liou

Metals 2026, 16(8), 892; https://doi.org/10.3390/met16080892 - 10 Aug 2026

Viewed by 225

Abstract

Directed energy deposition (DED) of low-alloy steels involves strongly coupled effects among alloy composition, solidification behavior, and post-deposition heat treatment, making mechanical property prediction difficult when target-domain data are limited. This study develops a transfer-learning and continuous optimization framework for predicting heat treatment-dependent yield strength (YS), ultimate tensile strength (UTS), hardness (HV), and as-solidified phase fractions of martensite, ferrite, and austenite in DED-processed low-alloy steels. A CALPHAD-based dataset was generated for 125 low-alloy steel compositions. A multilayer perceptron (MLP) surrogate was first trained as a baseline model, then fine-tuned through transfer learning and progressively updated as staged continuous optimization; the composition pool increased from 72 to 125 compositions using Random, Greedy, and Bayesian upper-confidence-bound acquisition strategies. The heat treatment prediction accuracy improved from an average R² of 0.757 for the baseline model to 0.929 after transfer learning and to approximately 0.997 after continuous optimization, with a nearly 78% reduction in RMSE relative to transfer learning. For the solidification outputs, the average R² increased from 0.770 after transfer learning to approximately 0.859 after optimization. Bayesian-UCB provided the most stable and data-efficient improvement by balancing predicted performance with model uncertainty. The optimized prediction system showed low case-study errors for both solidification and heat treatment properties, demonstrating its potential as a rapid screening tool for alloy composition and tempering-condition selection in DED low-alloy steel development. Full article

(This article belongs to the Special Issue Innovations in Heat Treatment of Metallic Materials)

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12 pages, 4527 KB

Open AccessArticle

Effect of Zn/Mg Ratio on the Microstructure and Coarsening Resistance of Al–Zn–Mg Alloys Aged at 150 °C

by Xueqin Zhang, Xiaolan Wu, Peihao Zhao, Xiangyuan Xiong, Zhi Zheng, Gaoteng Zhang, Shanglong Ao, Guishan Shi, Kunyuan Gao, Wu Wei, Shengping Wen, Hui Huang, Li Rong and Zuoren Nie

Metals 2026, 16(8), 885; https://doi.org/10.3390/met16080885 - 10 Aug 2026

Viewed by 250

Abstract

The role of the Zn/Mg ratio in regulating microstructure, precipitation evolution and coarsening resistance in Al–Zn–Mg-based alloys was investigated by microhardness testing, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) during isothermal aging at 150 °C. Three alloy compositions were designed with Zn/Mg ratios of 1.6, 2.4, and 3.9, whereas the combined Zn and Mg level was fixed at 6.0 wt%. All alloys exhibited a typical age-hardening response, whereas the maximum peak hardness was obtained at an intermediate Zn/Mg level rather than at the two extremes. The optimal composition (Zn/Mg = 2.4) reached 137 HV, which is attributable to the formation of the finest precipitates (~3 nm) and the highest number density. Moreover, this alloy exhibited the smallest hardness loss (ΔH = 19 HV) after prolonged aging (192 h). TEM analysis indicated that this alloy exhibited the lowest coarsening rate constant, Kr = 0.43 at 192 h. Furthermore, the variation in Zn/Mg ratio affected grain boundary precipitation, leading to a minimized PFZ width at Zn/Mg = 2.4 while maintaining a similar discontinuous distribution of grain boundary precipitates among the alloys. Overall, tailoring the Zn/Mg balance offers an effective strategy to achieve refined precipitates, improved coarsening resistance, and enhanced mechanical performance with superior thermal stability. Full article

(This article belongs to the Special Issue Innovations in Heat Treatment of Metallic Materials)

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23 pages, 22220 KB

Open AccessArticle

Pre-Aging Prior to Delayed Artificial Aging Modulates Precipitation Behavior and Strengthening Response in an Al-Mg-Si-Cu Alloy

by Qingyong Zheng, Huixue Jiang, Yuxing Tian, Guanmei Niu, Mingchao Yu, Xiaobin Guo, Zhihao Zhao, Gaowu Qin and Hutian Li

Metals 2026, 16(8), 842; https://doi.org/10.3390/met16080842 - 3 Aug 2026

Viewed by 296

Abstract

Room-temperature storage after solution treatment and quenching is often unavoidable during the industrial processing of Al-Mg-Si-Cu alloys, but its interaction with pre-aging and the subsequent artificial aging response remains insufficiently quantified. Here, we demonstrate a pre-aging-assisted delayed artificial aging strategy to regulate β″/L precipitation and improve the strengthening response of an Al-Mg-Si-Cu alloy after short-term room-temperature storage. After solution treatment and quenching, the alloy was stored at room temperature for 6–48 h with or without prior pre-aging at 120 °C, followed by artificial aging at 170 °C. It is found that both the delayed artificial aging (DA) and pre-aging-assisted delayed artificial aging (PDA) routes show limited sensitivity to room-temperature storage time within 6–48 h, whereas PDA consistently increases the yield strength and ultimate tensile strength by approximately 10 MPa compared with the corresponding DA route, with a slight reduction in elongation. Based on Avrami–Johnson–Mehl analysis, the activation energy for β″ precipitation decreases from 102.74 kJ/mol in the DA-pre state and 107.13 kJ/mol in the solution-treated state to 70.63 kJ/mol in the PDA-pre state, indicating that pre-aging modifies the initial microstructural state in a manner that facilitates subsequent β″ precipitation. Quantitative TEM characterization reveals that PDA has little influence on the average sizes of β″ and L precipitates but increases their number densities. In particular, the number density of β″ precipitates increases by approximately 15% relative to the corresponding DA samples. Strengthening-model calculations further show that β″ precipitates make the dominant contribution to precipitation strengthening, and that the strength improvement under the PDA route mainly originates from the increased β″ number density rather than changes in precipitate size. These results establish a quantitative link between pre-aging, precipitation kinetics, β″/L precipitate population, and strengthening response, providing guidance for optimizing heat-treatment schedules of Al-Mg-Si-Cu extrusion alloys that experience unavoidable room-temperature storage before final artificial aging. Full article

(This article belongs to the Special Issue Innovations in Heat Treatment of Metallic Materials)

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17 pages, 32274 KB

Open AccessArticle

Heat Treatment Enables β-Mediated Strain Accommodation and Interfacial Stress Redistribution in Zr–2.5Nb Alloy Fabricated by Laser Powder Bed Fusion

by Hongwen Deng, Aiwen Li, Chenkai Zhou, Lingyi Cao, Jun Du and Xu Cheng

Metals 2026, 16(8), 839; https://doi.org/10.3390/met16080839 - 1 Aug 2026

Viewed by 259

Abstract

Heat treatment significantly improves the ductility of additively manufactured Zr–2.5Nb alloy, but the mechanisms responsible for this improvement remain poorly understood. In this study, the effects of heat treatment on the microstructure and room-temperature tensile behavior of Zr–2.5Nb fabricated by laser powder bed fusion (LPBF) were investigated by comparing as-built (AB) and heat-treated (HT) specimens. The HT specimens were held at 800 °C for 2 h and subsequently air-cooled. A microstructure-based crystal plasticity fast Fourier transform (CPFFT) model was constructed directly from two-dimensional electron backscatter diffraction (EBSD) orientation and phase maps to quantify the local stress, strain, and slip responses of the α and β phases in the HT microstructure. Heat treatment caused the acicular α′ martensite to decompose, producing a coarser lamellar α + β microstructure. In a representative EBSD field of the HT specimen, β-Zr accounted for 6.0% of the analyzed area and was distributed predominantly between the α lamellae. Compared with the AB condition, heat treatment reduced the mean 0.2% proof stress and ultimate tensile strength from 840 and 1033 MPa to 792 and 881 MPa, respectively, while increasing the mean uniform strain from 4.02% to 6.94%. At an applied axial strain of 3.2%, the β/α ratios of phase-averaged equivalent strain and accumulated absolute slip were 1.59 and 2.51, respectively, whereas the corresponding ratios for von Mises stress and axial stress were 0.57 and 0.81. These results reveal pronounced stress–strain partitioning between the phases: β-Zr accommodated greater equivalent strain and more extensive slip, whereas α-Zr carried higher stresses. This interphase partitioning helps explain the increased uniform strain of the HT specimens, while the reduction in strength is primarily associated with α′-martensite decomposition and α-lamella coarsening. Full article

(This article belongs to the Special Issue Innovations in Heat Treatment of Metallic Materials)

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15 pages, 26537 KB

Open AccessArticle

Effect of Hot Rolling Temperature on the Microstructure and Macro-Texture Evolution Laws of TC2 Titanium Alloy and Their Influence on Mechanical Properties

by Jiazhi Yuan, Qingfu Qian, Zaijiu Li, Qinglin Jin, Zhongxue Feng, Yanying Li and Zhaosong Chen

Metals 2026, 16(6), 651; https://doi.org/10.3390/met16060651 - 13 Jun 2026

Viewed by 343

Abstract

TC2 titanium alloy (Ti-4Al-1.5Mn, wt.%) is a near-α titanium alloy with promising aerospace and biomedical applications, but its limited room temperature ductility and strong texture sensitivity hinder the fabrication of high-performance sheets. In this study, the effects of hot rolling at 830 °C and 930 °C on the microstructure, macro-texture, mechanical properties, and fracture behavior of TC2 alloy were investigated. Compared with the 830 °C rolled sample, the 930 °C rolled sample exhibited finer primary α grains, a higher volume fraction of fine and dispersed secondary α_s phase, and more uniform Mn distribution, while both samples retained an α + β phase constitution. Texture and ODF (orientation distribution function) analyses revealed that increasing the rolling temperature reduced the maximum intensity of the (0001) pole figure from 6.68 to 5.23 m.r.d. (multiples of a random distribution) and increased that of the (10-10) pole figure to 9.62 m.r.d., indicating weakened basal texture, enhanced prismatic texture, and more dispersed orientation distribution. Consequently, although the tensile strength slightly decreased to approximately 730 MPa, the elongation increased from approximately 24% to 28%. The finer and denser dimples observed after 930 °C rolling further confirmed improved plastic deformation coordination. Full article

(This article belongs to the Special Issue Innovations in Heat Treatment of Metallic Materials)

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12 pages, 2974 KB

Open AccessArticle

Study on the Microstructure Evolution of Mg-1Ca-(2Ag) Alloys During Hot Rolling and Its Corrosion Properties

by Qingfu Qian, Daliang Sun, Zaijiu Li, Qinglin Jin and Yikai Sun

Metals 2026, 16(2), 218; https://doi.org/10.3390/met16020218 - 13 Feb 2026

Viewed by 522

Abstract

Magnesium alloys’ poor corrosion resistance limits their applications as biodegradable bone repair materials. Alloying tailors Mg alloys’ microstructure and properties. The present study investigates the effect of 2 wt.% Ag addition on the microstructure and initial corrosion behavior of hot-rolled Mg-1Ca alloy. Mg-1Ca and Mg-1Ca-2Ag alloys were prepared by melting using Mg-2Ca and Mg-4Ag master alloys, followed by homogenization at 400 °C for 2 h, hot rolling, and stress-relief annealing at 400 °C for 6 h. The alloys were systematically characterized using field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD). Initial corrosion behavior was evaluated via 3 h immersion tests in simulated body fluid (SBF). Results reveal Ag’s high thermal diffusivity promotes segregation at tensile twin boundaries, forming Ag₃Mg nanoparticles. These nanoparticles hinder grain boundary migration and, with increased deformation, facilitate grain rotation and high-angle grain boundary formation, weakening texture. Internal stress accumulation near twin boundaries—driven by grain orientation variation and nanoparticles—induces ~86° rotation of {10–12} tensile twins around the c-axis, forming double twins. During corrosion, nanoparticles and double twins synergistically promote dense protective film formation, significantly reducing corrosion rates. Full article

(This article belongs to the Special Issue Innovations in Heat Treatment of Metallic Materials)

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21 pages, 19627 KB

Open AccessArticle

Microstructural Evolution and Fatigue Behavior of Laser-Welded Joints in Air-Cooled Steel

by Xing Fang, Yan-Xin Wu, Xin-Yue Liu, Wang-Nan Zuo, Xiao-Yu Yang, Qi Zhang, Fei-Da Chen, Yong-Gang Yang and Zhen-Li Mi

Metals 2026, 16(1), 65; https://doi.org/10.3390/met16010065 - 5 Jan 2026

Cited by 1 | Viewed by 981

Abstract

Air-cooled steel is a high-strength steel widely used in automotive subframe applications. In this study, the microstructural evolution and fatigue performance of air-cooled steel welded joints subjected to a one-step heat treatment and an additional two-step heat treatment were systematically investigated. The results indicate that, after the one-step heat treatment, the microstructure of the welded joints transformed from coarse lath martensite to a mixture of tempered martensite and newly formed martensite. After the two-step heat treatment, the microstructure of the welded joints evolved from coarse lath martensite to newly formed martensite, M-A islands, bainitic ferrite, and a small amount of polygonal ferrite. Under both heat treatment conditions, the fatigue limits of the welded specimens were lower than those of the base metal, which can be attributed to the reduced overall deformation compatibility induced by the welded joints. In addition, the welded joints exhibited superior crack propagation resistance compared with the base metal after both heat treatment processes, which is likely related to the enhanced ability of dislocation structures and grain boundaries to impede crack propagation. Full article

(This article belongs to the Special Issue Innovations in Heat Treatment of Metallic Materials)

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15 pages, 3921 KB

Open AccessArticle

Aging Kinetics and Activation Energy-Based Modeling of Electrical Conductivity Evolution in a Cu–4Ti Alloy

by Guojin Sun, Hong Liu, Yingtang Zhang, Wenbin Wu and Qi Wang

Metals 2026, 16(1), 61; https://doi.org/10.3390/met16010061 - 4 Jan 2026

Viewed by 881

Abstract

The aging behavior and electrical performance evolution of Cu–4Ti alloy were systematically investigated through experimental characterization and theoretical modeling. A series of solution and aging treatments were conducted at temperatures ranging from 450 °C to 600 °C for durations of 1–420 min, with and without 50% cold deformation. Based on solid-state diffusion theory, the activation energy of the aging process was determined using the Arrhenius relationship combined with regression analysis. The calculated activation energies were 298.5 kcal·mol⁻¹ for the solution-treated alloy and 136.1 kcal·mol⁻¹ for the cold-deformed alloy, indicating that deformation-induced lattice defects substantially accelerate atomic diffusion and precipitation kinetics. A predictive model was further established to describe electrical conductivity as a function of aging temperature and time, with high correlation coefficients (R² = 0.90 for the non-deformed and R² = 0.89 for the deformed condition). The model accurately captures the conductivity evolution under various heat treatment conditions, demonstrating its strong predictive capability. Moreover, kinetic curves were constructed to intuitively represent the relationship between conductivity, temperature, and time, providing a rapid and visual tool for process optimization. Full article

(This article belongs to the Special Issue Innovations in Heat Treatment of Metallic Materials)

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16 pages, 7278 KB

Open AccessArticle

Study on Cold Cracking in 430Cb Ferritic Stainless Steel Castings Based on Multiscale Characterization and Simulation Analysis

by Siyu Qiu, Jun Xiao and Aimin Zhao

Metals 2025, 15(12), 1310; https://doi.org/10.3390/met15121310 - 28 Nov 2025

Cited by 1 | Viewed by 2627

Abstract

Cracks were found at the gate of the 430Cb ferritic stainless steel exhaust system jet base produced by investment casting. In this paper, the cracks of failed stainless steel castings were comprehensively analyzed by means of macroscopic inspection, laser confocal microscopy, field emission scanning electron microscopy, electron backscatter diffraction, X-ray diffractometer, ProCAST (version 2018, ESI Group, Paris, France) simulation and Thermo-Calc (TCFE10 database, 2022a, Thermo-Calc Software AB, Solna, Sweden) thermodynamic calculation. It can be concluded that all the cracks originate from the gate on the surface of the casting, and the fracture surface shows brittle intergranular characteristics, which can be determined as cold cracks. The formation of cold cracks can be attributed to the fact that the local stress generated during cooling after the casting solidifies exceeds the strength limit of the material itself. As the gate is the final solidification zone, shrinkage is limited and stress is concentrated. The grains are coarse, and the microstructure defects such as shrinkage porosity, pores and needle-like NbC further weaken the plasticity of the grain boundaries, promoting the crack to propagate along the direction of the maximum principal stress. The uneven cooling rate and shell constraint during the investment casting process make it difficult to release stress, and the existence of microstructure defects are the fundamental causes of crack generation. Full article

(This article belongs to the Special Issue Innovations in Heat Treatment of Metallic Materials)

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