Thermal fatigue and wear of compacted graphite iron brake discs with various thermomechanical properties

The increase in payload capacity of trucks has heightened the demand for cost-effective yet high performance brake discs.In this work,the thermal fatigue and wear of compacted graphite iron brake discs were investigated,aiming to provide an experimental foundation for achieving a balance between their thermal and mechanical properties.Compacted graphite iron brake discs with different tensile strengths,macrohardnesses,specific heat capacities and thermal diffusion coefficients were produced by changing the proportion and strength of ferrite.The peak temperature,pressure load and friction coefficient of compacted graphite iron brake discs were analyzed through inertia friction tests.The morphology of thermal cracks and 3D profiles of the worn surfaces were also discussed.It is found that the thermal fatigue of compacted graphite iron discs is determined by their thermal properties.A compacted graphite iron with the highest specific heat capacity and thermal diffusion coefficient exhibits optimal thermal fatigue resistance.Oxidization of the matrix at low temperatures significantly weakens the function of alloy strengthening in hindering the propagation of thermal cracks.Despite the reduced hardness,increasing the ferrite proportion can mitigate wear loss resulting from low disc temperatures and the absence of abrasive wear.

Simulation of space heavy-ion induced primary knock-on atoms in bipolar devices

Bipolar junction transistors(BJTs) are often used in spacecraft due to their excellent working characteristics. However,the complex space radiation environment induces primary knock-on atoms(PKAs) in BJTs through collisions, resulting in hard-to-recover displacement damage and affecting the performance of electronic components. In this paper, the properties of PKAs induced by typical space heavy ions(C, N, O, Fe) in BJTs are investigated using Monte Carlo simulations. The simulated results show that the energy spectrum of ion-induced PKAs is primarily concentrated in the low-energy range(17eV–100eV) and displays similar features across all tested ions. The PKAs induced by the collision of energetic ions have large forward scattering angles, mainly around 88°. Moreover, the distribution of PKAs within a transistor as a function of depth displays a peak characteristic, and the peak position is linearly proportional to the incident energy at a certain energy range. These simulation outcomes serve as crucial theoretical support for long-term semiconductor material defect evolution and ground testing of semiconductor devices.

First-principles calculations of high pressure and temperature properties of Fe_(7)C_(3)

Eckstrom-adcock iron carbide(Fe_(7)C_(3))is considered to be the main constituent of the Earth’s inner core due to its low shear wave velocity.However,the crystal structure of Fe_(7)C_(3)remains controversial and its thermoelastic properties are not well constrained at high temperature and pressure.Based on the first-principles simulation method,we calculate the relative phase stability,equation of state,and sound velocity of Fe_(7)C_(3)under core condition.The results indicate that the orthorhombic phase of Fe_(7)C_(3)is stable under the core condition.While Fe_(7)C_(3)does reproduce the low shear wave velocity and high Poisson’s ratio of the inner core,its compressional wave velocity and density are 12%higher and 6%lower than those observed in seismic data,respectively.Therefore,we argue that carbon alone cannot completely explain the thermal properties of the inner core and the inclusion of other light elements may be required.

Sc添加对Al0.2CoCrFeNi高熵合金显微组织、相演变及力学性能的影响

采用电弧熔炼法制备Al_(0.2)CoCrFeNiSc_(x)(x=0,0.1,0.2,0.3,摩尔分数)合金,研究Sc添加对合金显微组织、相演变和力学性能的影响。结果表明,Sc能细化晶粒、改变相类型以及提高合金的力学性能。相较于Al_(0.2)CoCrFeNiSc_(0.1)合金的晶粒尺寸(17.2μm),Al_(0.2)CoCrFeNiSc_(0.3)合金的晶粒尺寸仅为8.5μm,减小了约50.6%。合金的晶体结构由FCC相演变为包括两种FCC相和一种BCC相的混合相。FCC相主要出现在富Co、Cr和Fe枝晶区域和富Ni和Sc枝晶间区域,而BCC相主要出现在富Al和Ni枝晶间区域。当Al_(0.2)CoCrFeNiSc_(x)合金中x值由0增加到0.3时,合金的屈服强度由167 MPa增加至717 MPa,提高了329.3%,这主要归因于晶粒强化、固溶强化和相演变。

Different thermal fatigue behaviors between gray cast iron and vermicular graphite cast iron

The initiation and propagation of thermal fatigue cracks in gray cast iron and vemicular graphite cast iron were investigated by Uddeholm method to reveal the complex thermal fatigue behaviors of cast iron.Differences of thermal fatigue behaviors of gray cast iron and vemicular graphite cast iron were observed and analyzed.It is found that the observed differences are related to the combination of graphite morphology and the oxidization of matrix.More oxidized matrix is observed in gray cast iron due to its large specific surface area.The brittle oxidized matrix facilitates the propagation of microcracks along the oxidization layer.By contrast,the radial microcracks are formed in vermicular graphite at the edge of graphite due to fewer oxidization layers.It indicates that the thermal fatigue resistance of gray cast iron is dominated by graphite content and morphology while that of vermicular graphite cast iron strongly relates to the strength of the matrix.

Phenomena at three-phase electroslag remelting

The electroslag remelting(ESR)process is widely used to produce high-quality ingots and billets for high-alloyed steels and alloys.Both the single-phase and three-phase alternating current diagram with bifilar and monofilar connection are in use for heavy ingot manufacturing.The numerical simulation of the three-phase bifilar circuit for the 120 t three-phase bifilar six-electrode ESR furnace at different variants of electric connection was presented and discussed.At the bifilar diagram of power supply,the geometrical location of electrodes in a mould holds critical importance for performances:the close location of bifilar pair electrodes provides the highest heat productivity,but the equidistant location of electrodes gives a much more uniform heat distribution.The monofilar mulit-electrode diagram of three-phase connection without phase shift shows the most uniform distribution of potential and heat generation as well as a favorable magnetic field that makes this kind the most promising for providing a high quality of heavy ingots.

Casting process design and practice for coolant pump impeller in AP1000 nuclear power station

The coolant pump impeller casting is the only rotating component in the nuclear island of an AP1000 nuclear power station, and is required to have a 60-year service time, which requires advanced materials and processing technologies to guarantee. In this paper, the casting process was studied, designed and modified by means of numerical simulation. The gating system was distributed symmetrically and the runner diameter was a little bigger for avoiding sand wash and turbulence;the feeding system focused on the solution of blades feeding, as some parts of which should reach Severity Level 1 radioactive testing standard. Therefore, upper and lower plates cooperating with chillers acted as feeding method besides additional 2-3 times thickness;in addition, lowering sand core strength, decreasing pouring temperature and increasing dimension allowance would be adopted to avoid crack defects. Finally, the pilot impeller was cast. The results show that the casting process design is reasonable, as the liquid rises very smoothly when pouring, and no volume defects are found by means of 100% radioactive testing. Based on this casting process, 16 coolant pump impellers have been successfully produced and delivered to customers.

Effect of Electric Potentials on Microstructure, Corrosion and Wear Characteristic of the Nitrided Layer Prepared on 2Cr13 Stainless Steel by Plasma Nitriding

Plasma nitriding is a widely used technology to enhance the surface performance and extend the service life of alloy parts.The current research mainly focuses on the influences of time,temperature,gas type and pressure parameters on nitriding behavior,while fewer studies have been conducted on the electric potential.This paper mainly reports the effect of the electric potential on nitriding behavior.Test conditions were set using cathodic,anodic and floating potentials in a plasma nitriding furnace.2Cr13 stainless steel was nitrided at 450°C for 5h in an NH3 atmosphere.The experimental results show that the nitriding treatment can be well performed under the different electric potentials,but differences exist in microstructures,morphologies and performance results of the modified layers.The thickness and hardness values of the nitrided layer are ranked as follows:cathodic[anodic[floating potential.The anodic nitrided surface has an obvious particle deposition layer composed of nitrides and oxides.Electrochemical and tribological experiments show that the corrosion resistance and wear resistance were significantly improved after a nitriding treatment using the three electric potentials.Moreover,the floating nitriding treatment resulted in the best tribological performance and corrosion resistance.

Effects of alloying elements on thermal conductivity of pearlitic gray cast iron

A quantitative model is proposed to describe the thermal conductivity of alloyed pearlitic gray cast iron. The model is built by combining the computational thermodynamics and effective medium theory. The volume fractions and concentrations of precipitated phases in as-cast structure are estimated in consideration of partial and para-equilibrium. The conductivity of alloyed ferrite is calculated, taking into account the electronic and vibrational contributions of alloying elements. The model provides a good agreement with microstructure analysis and measured thermal conductivity. The influence of common alloying elements was discussed from the viewpoint of precipitation of phases and scattering of alloying atoms. This model can also be used as a numerical tool for designing the pearlitic gray cast irons with high thermal conductivity and high tensile strength.

Molecular Dynamics Simulations of the Elastic Anisotropy of Pd at Extreme Conditions

It is very interesting to discover the elastic properties of engineering material palladium, especially its elastic anisotropy along Hugoniot states. We here investigate the evolution of its high pressure and temperature(PT) elastic ansotropy along Hugoniot using molecular dynamics simulations based on accurate classical interatomic potential. In order to testify the validity of the interatomic potential of Pd in describing the high PT elastic properties, we calculate its isothermal and adiabatic elastic moduli using molecular dynamics method. The obtained data are in good agreement with experimental data. From the isothermal elastic constants, we deduce the Hugoniot acoustic velocities and find that the resulting data are in good agreement with experimental acoustic velocity data. Based on the reliable elastic constants, we further investigate the spacial elastic ansotropy along Hugoniot PT states. It is found that the spacial elastic anisotropy of Pd increases along Hugoniot states.