Subgrains and boron distribution of low carbon bainitic steels

The structure variation of deformed austenite during the relaxation stage after deformation at various temperatures in an Nb-B ultra low carbon bainitic steel and Fe-Ni alloy was studied by the thermo-simulation. Optical microscope and TEM were applied to analyze the microstructure after RPC （Relaxation-precipitation-controlling phase transformation technique） and the evolution of dislocation configuration. The particle tracking autoradiography （PTA） technique, revealing the distribution of boron, was employed to show the change of boron segregation after different relaxation times. The results indicate that during the relaxation stage the recovery occurs in the deformed austenite, the dislocations rearrange and subgrains form. During the subsequent cooling the boron will segregate at the boundaries of subgrains.

Near net shape casting process for producing high strength 6xxx aluminum alloy automobile suspension parts

The automobile suspension parts of a high strength 6xxx aluminum alloy were produced using a novel technique known as near net shape casting for forging stock preparation. Based on the outline dimension of the forging stock, the shape of the ingot was designed as the near net shape and its casting process was studied by the numerical simulation and experimental investigation. The results show that the shrinkage of the ingot was highly correlated to its shape parameters and could be successfully forecast by the stimulation model. The casting parameters of the near net shape ingot were optimized and the near net shape 6xxx aluminum alloy ingots free of defects were cast in the laboratory. In order to obtain high performance forged suspension parts, the hot compression tests of the ingot were carried out. The results show that the subgrain fraction of the forged ingot was strongly affected by Zener-Hollomon parameters （Z parameters）. The intermediate Z parameters, 1.09×10^16 s^-1, will contribute to the larger number fraction of subgrains inside the forged ingot, which contributes to the high performance of the forged products.

Strengthening mechanisms of reduced activation ferritic/martensitic steels:A review

This review summarizes the strengthening mechanisms of reduced activation ferritic/martensitic(RAFM)steels.High-angle grain boundaries,subgrain boundaries,nano-sized M_(23)C_(6),and MX carbide precipitates effectively hinder dislocation motion and increase high-temperature strength.M23C6 carbides are easily coarsened under high temperatures,thereby weakening their ability to block dislocations.Creep properties are improved through the reduction of M23C6 carbides.Thus,the loss of strength must be compensated by other strengthening mechanisms.This review also outlines the recent progress in the development of RAFM steels.Oxide dispersion-strengthened steels prevent M23C6 precipitation by reducing C content to increase creep life and introduce a high density of nano-sized oxide precipitates to offset the reduced strength.Severe plastic deformation methods can substantially refine subgrains and MX carbides in the steel.The thermal deformation strengthening of RAFM steels mainly relies on thermo-mechanical treatment to increase the MX carbide and subgrain boundaries.This procedure increases the creep life of TMT(thermo-mechanical treatment)9Cr-1W-0.06Ta steel by~20 times compared with those of F82H and Eurofer 97 steels under 550℃/260 MPa.

Microstructure evolution of Al-Zn-Mg-Cu-Zr alloy during hot deformation

Compression tests of the Al-Zn-Mg-Cu-Zr（7055） alloy were performed at various strains and temperatures from 300 to 450℃ under a con- stant strain rate between 10^-2 s^-1and 1 s^-1. Microstructures during hot deformation were studied by transmission electron microscopy （TEM）. Dislocation density, dislocation cells and subgrains of the deformed samples were investigated in detail and compared to make a better understanding of the microstructure evolution. The results showed that stress-strain curves under the experimental conditions belonged to the type of dynamic recovery. When the alloy deformed at various strains and 300℃, the microstructure underwent a process of disordered dislocations to cell structure, subgrain structure and subgrain coarsening. With the temperature increasing, subgrains grew and dislocation density in the interior decreased at a strain rate of 1 s^-1. At the temperature of 350℃, the average diameter of subgrains decreased, sub-boundaries broadened and dislocation density in the interior decreased when the strain rate was increased. The deformed samples of 7055 alloy had smaller subgrains than that of 7005 alloy at the same compression condition because of high alloy content.

Effect of strain rate on plastic deformation bonding behavior of Ni-based superalloys

Plastic deformation bonding(PDB)has emerged as a promising solid state bonding technique with limited risk of phase transformations and residual thermal stresses in the joint.In this study,the PDB behavior of IN718 superalloy was systematically investigated by performing a series of isothermal compression tests at various processing conditions.It was revealed that,with increasing PDB strain rate at 1000?C,different extents of dynamic recrystallization(DRX)occur in the bonding area of IN718 joints.The extent of DRX,average size of DRXed grains,and a newly proposed"interfacial bonding ratio(?Bonding)"parameter(to quantify the bond quality)were initially reduced with increase in the strain rate up to 0.1 s-1 and later increased at further higher strain rates.Electron backscattered diffraction(EBSD)and transmission electron microscopy(TEM)based interfacial microstructure analyses indicated that the quality of the bonded joints is closely related with the development of fine DRXed grains at the bonding interface with the increasing strain,which promotes adiabatic temperature rise.It was revealed that the initial bulging and subsequent migration of the original interfacial grain boundary(IGB)were the main mechanisms promoting DRX in the well bonded IN718 superalloy joints.Moreover,the mechanical properties of the bonded joints were not only controlled by the recrystallized microstructure but also depended upon the Bonding parameter of the joints.

A step deformation method for superplasticity improvement of coarse-grained Ti–15V–3Cr-3Sn-3Al

Ti–15V–3Cr–3Sn–3Al（Ti–15–3）, a kind of metastable beta titanium which has high specific strength and good cold-formability, is highlighted for applications in the aerospace manufacture industry. However, the technique for improving its formability at elevated temperatures is still a challenge at present. In this work, a step deformation method is proposed for superplasticity improvement of coarse grained Ti–15–3 plates at temperatures around its beta transus. The effects of the strain rate and the strain at the first stage on the superplasticity are investigated. The results show an increase of the strain rate sensitivity and a decrease of the flow stress under the step deformation mode compared to those obtained under constant strain rates at 780℃. The maximum strain to failure obtained in the step mode is 93% higher than that deformed in the constant strain rate mode. Strain rates, strains at the first stage, and temperatures have influences on the superplasticity improvement. The deformation mechanism is concluded as subgrain formation accommodated by grain boundary sliding rate-controlled by dislocation climb. The improved m value in the step deformation is accounted to the extra dislocation density produced during the strain rate reduction.

The simultaneous improvements of strength and ductility in additive manufactured Ni-based superalloy via controlling cellular subgrain microstructure

Fine cellular subgrain structure was formed in the Selective Laser Melting(SLM) manu factured IN718 alloy via optimizing the processing parameters.During the subsequent homogenization heat treatment process,the Laves phase dispersed at the subgrain boundaries can be eliminated while the cellular subgrain structure is reserved in the printed samples after holding at 1080℃ for 50 min.With the prolongation of the holding time,the subgrain boundaries undergo low angle rotation via the motion ofdislocation,which leads to the annihilation of the cellular subgrain structure.Moreover,during the subsequent double aging heat treatment process,the reserved cellular subgrain structure in the homogenized samples promotes the precipitation of γ" second phase nanoparticles,and these precipitated γ" phase nanoparticles prefer to distribute at subgrain boundaries.It was found that these unique subgrain boundaries with γ" phase precipitates can hinder but not fully terminate the motion of dislocation during the plastic deformation process,which contributes to increasing the strength as well as holding the stable plastic flow.Hence,the strength and ductility of final prepared IN718 alloy with cellular subgrain microstructure were improved simultaneously compared to the prepared alloy without cellular subgrain structure,which even exceed the mechanical properties standards(AMS 5662) of wrought IN718 alloy.These results in our work suggest that controlling the subgrain structure is a promising effective strategy to improve the mechanical properties of SLM manu factured nickel-based superalloy.

An Investigation of Dislocation in Olivine Phenocrysts from the Hawaiian Basalts

Intracrystalline distortions(like undulose extinction,dislocations,and subgrain boundaries)in olivine from naturally-deformed peridotites are generally taken as signs of dislocation creep.However,similar features in olivine phenocrysts that have been found in basaltic magmas are still not well understood.In particular,whether subgrain boundaries in olivine phenocrysts arise from plastic deformation or grain growth is still debated(in the latter case,they are essentially grain boundaries but not subgrain boundaries.Therefore,we used hereinafter subgrain-boundary-like structures instead of subgrain boundaries to name this kind of intracrystalline distortion).Here we carried out a detailed study on dislocations and subgrainboundary-like(SG-like)structures in olivine phenocrysts from two Hawaiian basaltic lavas by means of petrographic microscopy,scanning electron microscopy,and transmission electron microscopy(TEM).Abundant and complex dislocation substructures(free dislocations,dislocation walls,and dislocation tangles)were observed in the decorated olivine grains,similar to those in olivine from peridotite xenoliths entrained by the Hawaiian basalts.The measured average dislocation density is(2.9±1.3)×1011 m-2,and is three to five orders of magnitude higher than that in laboratory-synthesized,undeformed olivine.TEM observations on samples cut across the SG-like structures by FIB(focused ion beam)demonstrated that this kind of structures is made of an array of dislocations.These observations clearly indicate that these structures are real subgrain boundaries rather than grain boundaries.These facts suggest that the observed high dislocation densities and subgrain boundaries cannot result from crystal crystallization/growth,but can be formed by plastic deformation.These deformation features do not prove that the olivine phenocrysts(and implicitly mantle xenoliths)were deformed after their capture by the basaltic magmas,but can be ascribed to a former deformation event in a dunitic cumulate,which was formed by magmatic fractionation,then plastically deformed,and finally disaggregated and captured by the basaltic magma that brought them to the surface.