High-temperature properties and microstructural stability of hot-work tool steels

Indexable insert tools for machining operations are in service exposed to high temperatures and cyclic mechanical loads. Secondary hardening steels such as hot-work steels are commonly used for tools subjected to thermal exposure. However, these steels, highly alloyed with strong carbide forming elements as Cr, V and Mo, are generally difficult to machine and machining represents a large fraction of the production cost of a tool. Thus, the present study concerns the development of a new steel with improved machinability and meeting the requirements for high-temperature properties.Softening resistance of the THG2000 and QRO90 tool steels, commonly used in hot-work applications, and a newly developed tool steel MCG2006 with lower alloying content of carbide forming elements, was investigated by tempering and isothermal fatigue testing. Mechanisms of high-temperature softening of the tested tool steels were discussed with respect to their microstructure and high-temperature mechanical properties. Carbide morphology and precipitation as well as dislocation structure were determined using transmission electron microscopy and X-ray line broadening analysis.No difference in softening behaviour was found among the QRO90 and MCG2006 regarding hot hardness measurements. The THG2000 indicated some stabilization of the hardness between 450 and 550 °C and a considerable hardness decrease at higher temperatures.The short-time cyclic softening in isothermal fatigue was controlled by dislocation rearrangement and annihilation. The alloying composition of the steels presently tested had no influence on the dislocation density decrease.The long-time softening was affected by the material's temper resistance and strongly depended on the carbide morphology and their over-ageing resistance. The QRO90 with greater molybdenum and lower chromium contents than in the THG2000 show the best resistance to softening among the tested grades at all temperatures. The MCG2006, leaner alloyed with the carbide forming elements and alloyed with 4 wt% nickel, has better temper resistance than THG2000 at higher temperatures and longer tempering times.     

On the evolution of flow stress during constrained groove pressing of pure copper sheet

Using a mechanical model and dislocation density based model, the evolutions of dislocation density and flow stress of pure copper during constrained groove pressing (CGP) process are investigated. In this regard, the strain and strain rate are achieved from the mechanical model and then input into the dislocation model. To verify the predicted flow stress, the process of constrained groove pressing is performed on the sheets of pure copper from one to three passes. The predicted flow stresses are compared with the experimental data and a good agreement is observed. Also, it is found that during the straining of the copper sheet in CGP process, the dislocation density and strength dropping occur in lower strain than that in other severe plastic deformation processes.     

镁及镁合金阻尼特性的机理研究

基于金属力学性质的微观理论,结合合金元素影响镁合金晶粒性质的分析结果,重点研究镁及镁合金晶体在振动变形过程中的阻尼特性机理,通过对阻尼特性机理的理论推导和试验验证,表明：其在宏观屈服前,随应变振幅增大,阻尼机理依次为位错弹性变形时的共振型阻尼、位错脱钉扎(滑移增殖)时、位错线相互纠结钉扎时和滑移带出现时的静滞后型阻尼四个机理阶段;杂质或合金元素的质量分数及分布影响位错源的数量及其平衡状态,进而影响各阶段的阻尼性能并决定各阻尼机理阶段的特征参数值。该研究为设计和制备高阻尼镁合金材料提供了理论基础。     

Microstructure of Epitaxial La_(0.7)Ca_(0.3)MnO_3 Thin Films Deposited by Direct Current Magnetron Sputtering on LaAlO_3 Substrate

Transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) have been used to study the microstructural properties of La0.7Ca0.3MnO3 films on (001) LaAlO3 substrates prepared by direct current magnetron sputtering technique.The as-grown thin films with different thickness are perfectly coherent with the substrates.The film suffers a tetragonal deformation in the area near the interface between the film and the substrate.With increasing thickness, the film is partially relaxed.It was found that La0.7Ca0.3MnO3 films consist of two types of oriented domains described as: (1) (110)f[001]f||(001)s[100]sand (110)f[001]f||(001)s[100]s and (2) (110)f[001]f||(001)s[010]s and (110)f[001]f//(001)s[010]s.Upon annealing, the film is relaxed by the formation of misfit dislocations.Other than misfit dislocations, two types of threading dislocations with Burgers vector of <100> and <110> were also identified.     

Microstructure of Epitaxial La0.7Ca0.3MnO3 Thin Films Deposited by Direct Current Magnetron Sputtering on LaAlO3 Substrate

Transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) have been used to study the microstructural properties of La_0.7Ca_0.3MnO₃ films on (001) LaAlO₃ substrates prepared by direct current magnetron sputtering technique. The as-grown thin films with different thickness are perfectly coherent with the substrates. The film suffers a tetragonal deformation in the area near the interface between the film and the substrate. With increasing thickness, the film is partially relaxed. It was found that La_0.7Ca_0.3MnO₃ films consist of two types of oriented domains described as: (1) (110)_f [001]_f||(001)_s[100]_s and (1¹10)_f [001]_f||(001)_s[100]_s and (2) (110)_f [001]_f||(001)_s[010]_s and (1¹10)_f [001]_f//(001)_s[010]_s. Upon annealing, the film is relaxed by the formation of mis¯t dislocations. Other than mis¯t dislocations, two types of threading dislocations with Burgers vector of <100> and <110> were also identified.     

Partial dislocation configurations in a low-angle boundary in α-Al2O3

The dislocation structures of a low-angle tilt grain boundary in alumina bicrystal were investigated by transmission electron microscopy. The grain boundary was found to consist of two regions: an area with pairs of partial dislocations and an area with groups of odd numbered partial dislocations (multiple-partial-structure). Eight kinds of multiple-partial-structures were found in the fabricated grain boundary. The Burgers vectors of each partial dislocation in the grain boundary can be distinguished by dark-field imaging, and thus the arrangement of partial dislocations in the multiple-partial-structures are determined. It is concluded that a slight twist component of the boundary is the origin of the characteristic multiple-partial-structures.     

工业纯钛位错界面的高速形变响应

以工业纯钛为密排六方金属的模型材料。通过多道次冷轧工艺制备具有不同位错界面类型的工业纯钛板材。利用分离式霍普金森压杆(SHPB)实现高速形变,采用透射电子显微分析技术观察位错界面结构的变化,从而分析出不同类型位错界面的高速形变响应。结果表明:在应变速率为1000 s~(-1)时,初始位错界面成为高速形变过程中位错滑移的主要障碍。几何必须位错界面间距为0.5μm的板材冲击后会出现与原始界面交截的新生位错界面,初始几何必须位错界面(GNB)间距为0.3μm以下的工业纯钛在高速形变后会出现位错团结构;初始位错界面在0.1μm或以下,局部剪切的组织模式只是初始位错界面的扭折和位错塞积,在高度局域化的组织中,基体扭折位错界面并未产生,但有位错塞积和亚晶结构。     

Combined finite element method and dislocation density method solution to residual stress induced by water cavitation peening

Water cavitation peening is a technique similar to shot peening that induces compressive residual stresses in materials for improved fatigue resistance. Generally, residual stress is of two types: macro-residual stress and micro-residual stress. In this paper, a novel combined finite element method and dislocation density method (FEM/DDM), proposed for predicting macro and micro-residual stresses induced on the material subsurface treated with water cavitation peening, is presented. A bilinear elastic–plastic finite element method was conducted to predict macro-residual stresses and a dislocation density method was conducted to predict micro-residual stresses. These approaches made possible the prediction of the magnitude and depth of residual stress fields in pure titanium. The effect of applied impact pressures on the residual stresses was also presented. The results of the FEM/DDM modeling were in good agreement with those of the experimental measurements.     

Thermo-mechanical fatigue behavior of nickel-base powder metallurgy superalloy FGH96 under tension-tension loading

Thermo-mechanical fatigue (TMF) behavior of FGH96, a nickel-base powder metallurgy superalloy, has been studied under tension-tension loading at the temperature range from 550℃ to 720℃. The results show that TMF fracture mode is intergranular for the in-phase (IP), but transgranular cleavage-like for the out-of-phase (OP) samples. The total content of Al, Ti and Nb in the γ' phases for the IP or OP samples and the partitioning ratio of γ'/γ in these elements for the IP samples are relatively higher at the lower strain amplitude, which is consistent with the case of the γ' size that is larger at the lower strain amplitude, the lattice parameter misfit is negative and the absolute value is lower at the lower strain amplitude that is correlative with the change of the γ' morphology. The deformation at the lower strain amplitude is mainly dominated by the dislocation lines and dislocation pairs in the matrix channels, at the higher strain amplitude dominated by the large numbers of superlattice stacking faults within the γ' phases.     

Mechanical Behavior of fcc Crystal Simulated by 3-D Discrete Dislocation Dynamics

To simulate the mechanical behavior of the FCC crystal with the lower Peierls stress, the stiff property and physical meaning of the differential equation group consisting of dislocation evolution and mechanical state was investigated based on the 3-D discrete dislocation dynamics; the results indicate that the differential equation group is serious stiff, namely the external stress changes more quickly than dislocation evolution. Using the established numerical algorithm, the mechanical behavior of FCC crystal was simulated with the dislocations located in the parallel slip planes, and the effect of strain rate on the dislocation configuration and mechanical behavior, and the sat- uration process of mobile dislocation were discussed. The simulation results indicate that the numerical algorithm can efficiently simulate the dislocation dipole and the low strain rate loading.