Influence of Ni on Cu precipitation in Fe Cu Ni ternary alloy by an atomic study

The early aging Cu precipitations in Fe-3%Cu and Fe-3%Cu-4%Ni ternary alloys are investigated by molecular dynamics （MD） simulations. The results show that the average size of Cu clusters in Fe-3%Cu-4%Ni alloy is larger than that in Fe-3%Cu alloy. The diffusion of Cu is accelerated by Ni according to the mean square displacement （MSD）. Furthermore, the whole formation process of Cu-rich clusters is analyzed in detail, and it is found that the presence of Ni promotes small Cu-rich clusters to be combined into big ones. Ni atoms prefer to stay at the combination positions of small clusters energetically due to a large number of the first nearest neighbor Cu-Ni interactions, which is verified by first-principles calculations based on density functional theory （DFT）.

Study of Cu-rich phase precipitation in austenitic antibacterial stainless steel

Cu precipitation behaviors in two Cu-bearing austenitic antibacterial stainless steels,type 304 and type 317L,were systematically studied by using relatively simple methods for materials analysis,including micro-hardness,electrical resistivity,electrochemical impedance spectroscopy,X-ray diffraction and differential scanning calorimetry.The results indicated that after aging at elevated temperature,the micro-hardness, electrical resistivity,electrochemical impedance and lattice constant of the steel were all varied at different degrees due to the precipitation and growth of Cu-rich phases.The results also showed that the heat evolution during the process of Cu precipitation could be sensitively detected by means of differential scanning calorimetry,obtainning the starting temperature,peak temperature,peak area of the Cu-rich precipitation,and even the activation energy by calculation.The results confirmed that the Cu-rich phased precipitation in the Cu-bearing austenitic antibacterial stainless steel should be a thermal activation process controlled by Cu diffusion.All the materials analysis methods used in this study can be more simple and effective for application in R & D of the Cu-bearing antibacterial stainless steels.

Effect of Ni and vacancy concentration on initial formation of Cu precipitate in Fe–Cu–Ni ternary alloys by molecular dynamics simulation

In the present work, the effects of Ni atoms and vacancy concentrations （0.1%, 0.5%, 1.0%） on the formation process of Cu solute clusters are investigated for Fe-1.24%Cu-0.62%Ni alloys by molecular dynamics （MD） simulations. The presence of Ni is beneficial to the nucleation of Cu precipitates and has little effect on coarsening rate in the later stage of aging. This result is caused by reducing the diffusion coefficient of Cu clusters and the dynamic migration of Ni atoms. Additionally, there are little effects of Ni on Cu precipitates as the vacancy concentration reaches up to 1.0%, thereby explaining the embrittlement for reactor pressure vessel （RPV） steel. As a result, the findings can hopefully provide the important information about the essential mechanism of Cu cluster formation and a better understanding of ageing phenomenon of RPV steel. Furthermore, these original results are analyzed with a simple model of Cu diffusion, which suggests that the same behavior could be observed in Cu-containing alloys.

A supersaturated Cu-Ag nanoalloy joint with ultrahigh shear strength and ultrafine nanoprecipitates for power electronic packaging

Ag-Cu bimetallic nanoalloy,integrating the advantages of reducing migration and cost of nano-Ag and alleviating oxidation of nano-Cu,is a prospective bonding material for power electronic packaging.The Ag-coated Cu nanoparticles(Cu@Ag NPs)paste can execute bonding with high quality at 250℃,and the achieved supersaturated Ag-Cu nanoalloy joint with ultrahigh shear strength(152 MPa)dramatically exceeds most nano-paste joints.The interstitial solid solutions with atomic-level metallurgical bonds at the interface dominantly promoted the shear strength.Besides,the numerous ultrafine nanograin,high proportion of low angle grain boundaries(7.44%)without deformation,and the Cu nanoprecipitates in the joint would improve subordinately.Furthermore,the high content(16.8%)of∑3 twin boundaries would contribute to the electrical and thermal conductivity.Thus,the multiple strengthening mechanisms with the solid solution,the second precipitated phase,and ultrafine nanograin can dramatically enhance shear strength and electro-thermal conductivity of joints for high-temperature device packaging.

Antibacterial Behavior of a Cu-bearing Type 200 Stainless Steel

A preliminary study was made on the antibacterial behavior of a newly designed Cu-bearing type 200 stainless steel by different methods including Vickers hardness measurement, electrochemical test and antibacterial test. The results showed that Cu-rich precipitations caused crucial factor for its excellent antibacterial capability, as the commercial type 200 stainless steel. by the saturated Cu in the steel matrix was not only a but also for its equivalent corrosion resistance as good

Strength-Toughness Improvement of 15-5PH Stainless Steel by Double Aging Treatment

To obtain better strength-toughness balance of 15-5PH stainless steel,a double aging treatment is proposed to investigate the mechanical properties and microstructure evolution.In this study,Cu precipitates and reversed austenite played a determining role to improve strength-toughness combination.The microstructure was observed using electron backscattered diffraction,transmission electron microscopy and scanning transmission electron microscopy.The volume fractions of Cu precipitates and reversed austenite were calculated with Thermo-Calc software and measured by X-ray diffraction.The results showed that the reversed austenite is formed at the martensitic lath boundaries and its volume fraction also increases with the increase of the aging temperature.At the same time,the size of the Cu precipitates gradually increases.Compared with the traditional single aging and double aging treatment,double aging treatment of 15-5PH stainless steel can increase the toughness while retaining the necessary strength.During double aging of 550℃×4 h+580℃×1 h,15-5PH stainless steel has the best strength and low-temperature(-40℃)toughness match.Its yield strength,ultimate tensile strength and the Charpy impact energy are 1.037 GPa,1.086 GPa and 179 J,respectively.

Tempering Process to Improve Hardness Uniformity of Plastic Mould Steel

A new design of copper-bearing non-quenched plastic mold steel is presented and explained. Two kinds of microstrueture can be obtained from this new type copper-bearing steel via cooling with different cooling rates, bain- ite and a mixed microstructure consisting of ferrite and bainite. It is found that, after proper tempering process, the hardness will be increased. Moreover, the hardness difference between different microstructures will be reduced. For further investigation, the samples tempered at different temperatures were examined by XRD and 3PAP （three di- mensional atom probe） analysis. Results show that the improvement is contributed mainly by the precipitation of Cu phase and transformation of residual austenite.

Effect of Aging on Hardening Behavior of 15-5 PH Stainless Steel

Microstructure transformation and aging hardening behavior of 15-5 PH stainless steel were studied by optical microscopy （OM）, X-ray diffraction （XRD） and transmission electron microscopy （TEM）. The results showed that the 15-5 PH stainless steel consists of NbC precipitates and lath matensite with a high dislocation density after solution treatment. With increasing aging temperature and aging time, the martensitic laths were resolved gradually. Meanwhile, the nanometric-sized Cu precipitates gradually coarsened and lost their coherency with＇the martensite matrix, which exhibited an elliptical shape finally. Fine Cu precipitates can lead to significant dispersion hardening effect, while the coarsened Cu precipitates have no contribution to strengthening. The reversed austenite was observed in the speci- mens aged at 550 ℃ and above; moreover, the amount of reversed austenite increased as aging temperature in- creased. The precipitation hardening behavior of 15-5 PH stainless steel may depend on the balance between the softening caused by the formation of reversed austenite and the hardening caused by the precipitation of copper.