锻造中铬合金白口铸铁组织和性能研究

利用锻造方法并结合锻后适当的热处理工艺可大幅度提高铬合金白口铸铁的冲击韧性．采用热望性镦粗试验方法,对不同碳含量中铬合金白口铸铁的热塑性、碳对中铬白口铸铁热塑性的影响规律进行了研究,分析了不同热处理工艺对其锻后组织和性能的影响,并提出了可明显提高锻造中铬合金白口铸铁抗冲击磨损性能的热处理工艺参数．研究表明：含碳量为1.85％-2．59％的中铬合金自口铸铁在850-1130℃的温度范围内,具有良好的塑性变形能力,且随碳含量的增加,中铬白口铸铁的热塑性下降,其主要原因是碳含量不同所引起的组织中共晶碳化物的数量的改变所致．     

Characterization of intermetallic precipitates in a Nimonic alloy by ultrasonic velocity measurements

Ultrasonic velocity measurements have been carried out in Nimonic 263 specimens thermally aged at 923 and 1073 K for durations up to 75 h and correlated with the results of hardness measurements and electron microscopy studies. The ultrasonic velocities and hardness results obtained in the specimens thermally aged at both temperatures clearly indicated that ultrasonic velocity is more sensitive to the initiation of the precipitation, whereas the influence of precipitation on hardness can be observed only after the precipitates attain a minimum size to influence the movement of dislocations. Further, ultrasonic velocity measurements also revealed faster kinetics and a lesser amount of precipitation at 1073 K compared to 923 K due to higher solubility of precipitate-forming elements.     

Characterisation of heterogeneous microstructure in large forged products using nonlinear ultrasonic method

ABSTRACT

A comparative study between the ultrasonic attenuation and the nonlinearity is presented for the characterisation of microstructure in large dimension forgings. Results are provided for two austenitic stainless steel forgings of AISI type 304L with a diameter 200?mm, where microscopic observations reveal abnormal grain growth near the surface of one of the forgings. Frequency dependence of the nonlinearity parameter is used to discern variations in grain size in these forgings more precisely than attenuation measurements. The distribution of grain size is shown to have a significant influence on the nonlinearity parameter. Relative changes in the nonlinearity parameter with applied frequency were correlative with the microstructural variations in both the forgings.     

High-cycle fatigue behaviour and mechanism analysis of a forged TiZr-based alloy

Fatigue resistance, particularly the endurance limit, is an important design consideration in engineering applications for TiZr-based alloys. The investigated Ti–20Zr–6Al–4?V (wt-%) alloy exhibited a high fatigue endurance limit of 775?MPa. Results showed that severe local stress concentration due to extensive dislocation pile-up at α/β interfaces was responsible for the crack initiation. A transition from a tensile mode to a shear mode crack was observed during crack propagation. Many striations as well as some micro-cracks which can improve the resistance to crack propagation exist in the stable crack-propagation region. A localised deviation between the crack-growth direction was also found, and this outcome combined with micro-cracks and tear ridge may be attributed to varied crystallographic orientations between different phases.     

Effect of aging on work hardening behaviour of cold rolled Nimonic C-263 alloy

Abstract

Experimental true stress–true strain data of Nimonic C-263 alloy in solution treated as well as aged condition have been analysed using different flow relationships. Ludwigson relationship provides the best fit of the data for all the conditions investigated. The transition in macroscopic flow behaviour of the alloy with plastic strain, in solution treated condition, can be correlated with the transition in deformation mode from low strain regime to high strain regime. Although aging does not appear to alter the macroscopic flow behaviour, it causes a considerable change in flow parameters of the Ludwigson relationship and substructural evolution. On the other hand, the effect of sheet thickness is marginal. The flow data of the aged alloys fitted according to Ludwigson model not only yield a unique set of flow parameters for each aging condition but also exhibit a systematic trend with aging time. The transition in macroscopic flow behaviour of the alloy with strain, in aged conditions, can be correlated with a change in dislocation mechanism from dislocation–precipitate interaction at lower strains to dislocation–dislocation interaction at higher strains leading to formation of a dense dislocation tangled networks in the matrix regions surrounding the precipitates. The alloy in both solution treated and aged conditions exhibits three fairly distinct stages of strain hardening. The strain hardening rate decreases in regime I, remains constant in regime II and begins to fall again in regime III. Furthermore, it is observed that the alloy specimen with longitudinal orientation (L, i.e. parallel to rolling direction), exhibits marginally highest strain hardening rates, while specimens with long transverse orientation exhibit lowest strain hardening rates both in solution treated and aged conditions. However, for all other in-plane orientations (i.e. L+30°, L+45° and L+60°), the strain hardening rate data are fairly very close and lie in between those of longitudinal and long transverse orientations.     

Solidification microstructure of Cr4Mo4V steel forged in the semi-solid state

Semi-solid forging of iron-based alloys during solidification has unique characteristics distinct from those of the classical hot forging. With the aim of acquiring precise knowledge concerning the microstructural evolution of bearing steel Cr4 Mo4 V in this process, a series of semi-solid forging experiments were carried out in which samples were wrapped in a designed pure iron sheath. The effects of forging temperature and forging reduction on the grain morphology and liquid flow behavior were investigated, respectively.By forging solidifying metal(FSM), bulky primary dendrites were broken and spheroidal grains with an average shape factor of 0.87 were obtained at 1360?C. With the decreasing forging temperature to 1340?C,the microstructural homogeneity can be improved. On the other hand, it shows that a higher forging reduction(50%) is essential for the spheroidization of grains and elimination of liquid segregation. Those microstructural characteristics are related to different motion mechanisms of solid and liquid phases at different forging temperatures. Additionally, the effect of semi-solid forging on the eutectic carbides was also investigated, and the results demonstrate that the higher diffusion capacity and less liquid segregation jointly lower the large eutectic carbides and consequently cause its uniform distribution during FSM.     

Ultrasonic characterization of microstructure in powder metal alloy

The ultrasonic wave propagation characteristics were measured for IN-100, a powder metallurgy alloy used for aircraft engine components. This material was selected as a model system for testing the feasibility of characterizing the microstructure of a variety of inhomogeneous media including powder metals, ceramics, castings, and composites. The data were obtained for a frequency range from about 2–20 MHz and were statistically averaged over numerous volume elements of the samples. Micrographical examination provided size and number distributions for grain and pore structure. The results showed that the predominant source for the ultrasonic attenuation and backscatter was a dense (100/mm³) distribution of small micropores (10 µm radius). Two samples with different micropore densities were studied in detail to test the feasibility of calculating from observed microstructural parameters the frequency dependence of the microstructural backscatter in the regime for which the wavelength is much larger than the size of the individual scattering centers. Excellent agreement was found between predicted and observed values so as to demonstrate the feasibility of solving the forward problem. The results suggest a way towards the nondestructive detection and characterization of anomalous distributions of micropores when conventional ultrasonics imaging is difficult. The findings are potentially significant toward the application of the early detection of porosity during the materials fabrication process and after manufacturing of potential sites for stress-induced void coalescence leading to crack initiation and subsequent failure.This article is dedicated to Professor Dr. Paul Höller on the occasion of his 65th birthday.     

Failure analysis of cold forged 37Cr4 alloy M10x28 bolts

This paper covers experimental and numerical studies on the crack evolution on cold forged ³⁷Cr₄ high strength steel M10x28 bolts. Numerical simulations of forging operations were prepared on SIMUFACT FORMING finite element software. Failure evolution in simulations was predicted by using Cockroft-Latham damage model. Possible surface defects on work-pieces were also considered in the study. Artificial surface defects were generated on work-pieces by using electric discharge machine to determine the effects of defects on crack formation and propagation. As followings, numerical predictions on crack evolution were verified by conducting forging experiments. It was determined that deviation of the punch during forging led formation of shear cracks on the bolt head. Artificial surface defect was found to trigger crack evolution and affect the shape and propagation of the crack. Shear cracks propagated though the head of the bolt with 52° inclination angle while cracks triggered by surface defect propagated perpendicularly though longitudinal axis of the bolt.     

Crack initiation in VHCF regime on forged titanium alloy under tensile and torsion loading modes

This paper is focused on the VHCF behavior of aeronautical titanium alloy under tensile and torsion fatigue loadings. Tensile tests were carried out with two different stress ratios: R = −1 and R = 0.1. Both surface and subsurface crack initiations were observed. In the case of subsurface crack initiation several fatigue life controlling mechanisms of crack initiation were found under fully-reversed loading conditions: initiation from (1) strong defects; (2) ‘macro-zone’ borders; (3) quasi-smooth facets and (4) smooth facets. Tests with stress ratio R = 0.1, have shown that initiation from the borders of ‘macro-zones’ becomes the dominant crack initiation mechanism in presence of positive mean stress. Like for the tensile results, surface and subsurface crack initiations were observed under ultrasonic torsion in spite of the maximum shear stress location on the specimen surface. But the real reason for the subsurface crack initiation under torsion was not found.     

热处理条件对锻造ZK60-Y镁合金力学性能的影响

研究了不同热处理条件下锻造ZK60-Y镁合金微观组织的变化对其力学性能的影响.结果表明,直接进行人工时效的合金具有优越的强度和塑性.XRD分析表明,析出相主要有Mg2Zn3、Mg24Y5、Zn2Zr3和w-Mg3Y2Zn3.Mg2Zn3和w-Mg3Y2Zn3等析出相的尺寸、数量及其在基体中的分布状态对合金的力学性能影响很大.锻造态下大块破碎呈带状分布的Mg3Y2Zn3相及T4和T6态下粗化呈片层状的Mg2Zn3相是合金力学性能降低的主要原因.细小呈带状分布的Mg3Y2Zn3相和细层片状分布的Mg2Zn3相及其在此状态下细小的晶粒使T5态合金具有优越的抗拉强度和塑性.     

Microstructure,texture and mechanical behavior characterization of hot forged cast ZK60 magnesium alloy

structure with a significant reduction in casting porosity, while the texture changed to sharp basaMeasured mechanical properties of the forged alloy showed that strength did not change, howductility improved by 75%. The analysis of the fracture surface of the forged alloy under tension rea ductile fracture with dimple morphology, while the as-cast alloy displayed a brittle fracture with pores. This demonstrated that the reduction of casting defects and dendritic morphology, as well evolution of recrystallized grains, enhanced ductility, while partial dynamic recrystallization throuforging process resulted in only marginal modification of strength in the forged condition.. 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials SciTechno     

Weld microstructure refinement in a 1441 grade aluminium-lithium alloy

Clad 2 mm thick sheets of Russian 1441 grade Al-Li alloys were welded using a gas tungsten arc welding process (GTAW). Comparisons were made between the weld beads obtained under (i) continuous current (CC), (ii) pulsed current (PC), and (iii) arc oscillation (AO) conditions for their macro- and microstructural details. In the case of CC GTAW, sound welds could be produced only under a narrow range of welding parameters. Centre line cracks, which occurred in CC GTAW welds under certain conditions, were halted by switching to PC or AO conditions while the welding was in progress. Microstructural refinement was significant in the case of PC and AO GTA welding. This revised version was published online in November 2006 with corrections to the Cover Date.     

The phase and microstructure of TC21 alloy

TC21 alloy is a new alpha–beta damage tolerance titanium alloy with high strength and high toughness. Little work has been done in the field of microstructure and phase analysis since this alloy was developed. The phase and microstructure of TC21 alloy under different heat-treatment were investigated in this paper. Different experiment methods such as OM, SEM, TEM and XRD were adopted to investigate the microstructure and phase of TC21. The result showed that heat-treatment parameters (solution temperature, cooling rate and cooling mode) had influence on the feature of α phase, β phase and secondary α phase. Two precipitated phases (α₂-Ti₃Al and B₂-Ti₂AlNb) were discovered by XRD analysis.     

A1ZnSnSbPbMnMg高熵合金显微组织和耐热性的研究

AlZnMnSnSbPbMg高熵合金按照等原子比例配制熔炼而成.合金内主要相为面心立方固溶体,还有少量密排六方相以及未知相.合金具有比组成元素Mg、Al、Zn、Sn、Sb、Pb更高的熔点,在750℃下合金不融化.同时,还发现AlZnMnSnSbPbMg高熵合金在750℃下具有良好的抗氧化性,热重增加率为0.04%,与其对比的纯镁热重增加率为2.74%.     

Numerical simulation for determination of limit strains of a cold rolled and solution treated Nimonic C-263 alloy sheet

Nimonic alloys are Ni-base superalloys used for several high temperature applications, notable among them are the components in space vehicles, rocket engines, submarines, nuclear reactors, chemical processing vessels and heat exchange tubing as they exhibit excellent mechanical strength and creep resistance at high temperatures. Hence, evaluation of their formability characteristics is of utmost importance to make them into several useful components. Limit strains or forming limit curve is one of the parameters that indicates the formability, especially the drawability of sheet metal for deep drawing applications. In this paper, the limit strains of Nimonic C-263 alloy is investigated and presented using an explicit finite element code LSDYNA 3D. The material properties and the material model are evaluated by conducting tensile tests. The limit strains obtained from the simulation are verified by the analytical equations developed using vertex theory. The results tally within ±10% error.