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Effect of low-temperature ausforming on bainitic transformation and mechanical properties in ultrahigh-strength bainitic steel

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Journal of Iron and Steel Research(International) 2023年第9期30卷 1843-1853页

作者： Tian-yu Zhang Ling-yu Wang Yu Wang Jun Hu Hong-shuang Di Wei Xu State Key Laboratory of Rolling and Automation Northeastern UniversityShenyang 110819LiaoningChina

Low-temperature ausforming(LT-AF)prior to bainitic transformation leads to a noticeable acceleration of bainitic transformation kinetics;however,the effect of LT-AF on the retained austenite(RA)features and the resulting mechanical properties is still unclear.LT-AF was applied to ultrahigh-strength bainitic steel before austempering.The deformation behavior and the resulting dislocation substructures were investigated by thermomechanical simulator and transmission electron microscopy(TEM).The planar dislocation structures produced during deformation at 350℃ accelerate the bainitic transformation kinetics during isothermal holding.The effect of LT-AF on the bainitic transformation kinetics and the features of RA was elucidated via dilatometer measurement,TEM,scanning electron microscopy,and X-ray diffraction.It is observed that LT-AF not only retains more RA content but also facilitates improved RA stability.This trend is mainly due to the large amounts of planar dislocations in RA and bainitic laths inherited from undercooled austenite caused by LT-AF,the decrease in bainitic sheaves size,and the increase in filmy RA content compared to the sample without ausforming.A large fraction of filmy RA with high stability and the refinement of bainitic sheaves obtained by LT-AF remarkably enhance the strain hardening capacity and achieve significantly better ductility compared to the directly austempered sample.

关键词： Ultrahigh-strength bainitic steel Bainitic transformation Low-temperature ausforming Retained austenite strain hardening rate

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Effects of Grain Size and Cryogenic Temperature on the strain hardening Behavior of VCoNi Medium‑Entropy Alloys

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Acta Metallurgica Sinica(English Letters) 2023年第6期36卷 973-986页

作者： Guo‑Dong Liu Xue‑Mei Luo Ji‑Peng Zou Bin Zhang Guang‑Ping Zhang Shenyang National Laboratory for Materials Science Institute of Metal ResearchChinese Academy of SciencesShenyang 110016China School of Materials Science and Engineering University of Science and Technology of ChinaShenyang 110016China Key Laboratory for Anisotropy and Texture of Materials Ministry of EducationSchool of Materials Science and EngineeringNortheastern UniversityShenyang 110819China

The mechanical behavior of VCoNi medium-entropy alloys with five different grain sizes at three different temperatures was investigated.The VCoNi alloys with different grain sizes exhibit a traditional strength–ductility trade-off at 77 K,194 K and 293 K.Both the yield strength and the uniform elongation of the VCoNi alloys with similar grain size increase with decreasing the deformation temperature from 293 to 77 K.Obvious strain hardening rate recovery characterized by an evident up-turn behavior at stage II is observed in VCoNi alloys with the grain size above 11.1μm.It is found that the extent of the strain hardening rate recovery increases with increasing grain size or decreasing deformation temperature.This may mainly result from the faster increase in the dislocation multiplication rate caused by the decrease in the dislocation mean free path,the decrease in the absorption of dislocations by grain boundaries and the dynamic recovery from the cross-slip with increasing grain size,as well as the suppressed dynamic recovery at cryogenic temperatures.The critical grain sizes for the occurrence of the recovery of strain hardening rate are determined to be around 9.5μm,8.3μm and 3μm for alloys deformed at 293 K,194 K and 77 K,respectively.The basic mechanism for the strain hardening behavior of the VCoNi alloys associated with grain size and deformation temperature is analyzed.

关键词： Medium-entropy alloy strain hardening rate Cryogenic temperature Grain size Slip band refinement

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Characterization of the Hot Deformation Behavior of Cu–Cr–Zr Alloy by Processing Maps

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Acta Metallurgica Sinica(English Letters) 2016年第5期29卷 422-430页

作者： Yi Zhang Hui-Li Sun Alex A.Volinsky Bao-Hong Tian Zhe Chai Ping Liu Yong Liu School of Materials Science and Engineering Henan University of Science and TechnologyLuoyang 471003China Department of Mechanical Engineering University of South FloridaTampaFL 33620USA School of Materials Science and Engineering University of Shanghai for Science and TechnologyShanghai 200093China Collaborative Innovation Center of Nonferrous Metals Luoyang 471003China

Hot deformation behavior of the Cu-Cr-Zr alloy was investigated using hot compressive tests in the tem- perature range of 650-850℃ and strain rate range of 0.001-10 s-1. The constitutive equation of the alloy based on the hyperbolic-sine equation was established to characterize the flow stress as a function of strain rate and deformation temperature. The critical conditions for the occurrence of dynamic recrystallization were determined based on the alloy strain hardening rate curves. Based on the dynamic material model, the processing maps at the strains of 0.3, 0.4 and 0.5 were obtained. When the true strain was 0.5, greater power dissipation efficiency was observed at 800-850 ℃ and under 0.001-0.1 s-1, with the peak efficiency of 47%. The evolution of DRX microstructure strongly depends on the deformation temperature and the strain rate. Based on the processing maps and microstructure evolution, the optimal hot working conditions for the Cu-Cr-Zr alloy are in the temperature range of 800-850 ℃ and the strain rate range of 0.001-0.1 s-1.

关键词： Cu-Cr-Zr alloy Hot deformation behavior strain hardening rate Constitutive equation Processing map

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Comparison between the Methods of Determining the Critical Stress for Initiation of Dynamic Recrystallization in 316 Stainless Steel

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Journal of Materials Science & Technology 2008年第6期24卷 840-844页

作者： M.Jafari A.Najafizadeh Department of Materials Engineering Isfahan University of TechnologyIsfahanIran

Several methods have been proposed to calculate the critical stress for initiation of dynamic recrystallization （σc） on the basis of mathematical methods. One＇ of them is proposed by Stewart et al. in which this critical point appears as a distinct minimum in the （-dθ/dσ vs σ） through differentiating from θ vs σ. Another one is presented by Najafizadeh and Jonas by modifying the Poliak and Jonas method. According to this method, the strain hardening rate was plotted against flow stress, and the value of σc was attained numerically from the coefficients of the third-order equation that was the best fit from the experimental θ-σ data. Hot compression tests were used in the range of 1000 to 1100℃ with strain rates of 0.01^-1 s^-1 and strain of I on 316 stainless steel. The result shows that Najafizadeh and Jonas method is simpler than the previous one, and has a good agreement with microstructures. Furthermore, the value of normalized critical stress for this steel was obtained uc=σc/σp=0.92.

关键词： Dynamic recrystallization Critical stress strain hardening rate

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DEFORMATION AND MICROFRACTURE OF GRANULAR BAINITE

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Acta Metallurgica Sinica(English Letters) 1992年第6期5卷 432-435页

作者： CHEN Huangpu Xi’an Jiaotong University,Xi’an,China Lecturer,The Research Institute for Strength of Metals,Xi’an Jiaotong University,Xi’an 710049,China Xi'an Jiaotong Univ Xi'an China

Relationship among deformation history,fracture process and stress distribution of granular bainite has been investigated.The main process of fracture of granular bainite is the forma- tion.growth and coalescence of the microvoids.Even though the microcracks have formed at the earlier stage of deformation,they are not fateful for fracture because in the successive deformation stage the microcracks change their orientation toward the tensile axis.The strain hardening rate of granular bainite has a minimal value during the deformation process. Before and after the strain of the minimal value,the material shows different stress distribu- tion and microfracture mechanism.

关键词： granular bainite microfracture strain hardening rate

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Influence of grain size on the tensile ductility and deformation modes of rolled Mg–1.02 wt.%Zn alloy

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Journal of Magnesium and Alloys 2013年第3期1卷 210-216页

作者： B.Q.Shi R.S.Chen W.Ke State Key Laboratory for Corrosion and Protection Institute of Metal ResearchChina The University of the Chinese Academy of Sciences Beijing 100049China

The influence of grain size on the tensile deformation and ductility for Mg–1.02%Zn(wt.%)alloy was investigated.The uniform elongation is nearly insensitive to the increase of grain size,but the post-uniform elongation is significantly decreased with increasing grain size.The high ductility in the fine-grained samples is due to the lower frequency of twins and increased dynamic recovery from the enhanced activation of prismaticslip.

关键词： Magnesium Ductility Texture strain hardening rate Prismatic slip

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