查看更多>>摘要：This work reports additive manufactured (Fe, Ni)Al-reinforced nickel aluminum bronze by the electron beam powder bed fusion (EB-PBF) method. All EB-PBF composites show a high relative density of >99% and a homogeneous microstructure with equiaxed grains and homogeneously distributed (Fe, Ni)Al precipitates, enabling their nearly-isotropic mechanical behavior in build and transverse directions. This isotropic mechanical behavior results from a collective effect of high relative density, homogeneously-distributed precipitates, equiaxed grains, and a subsequently sustainable strain-hardening ability from progressively twin-precipitate and dislocation-precipitate interactions.
查看更多>>摘要：Sm_2Co_(17 )type permanent magnets were prepared by a novel processing technique involving pulsed magnetization followed by cold isostatic pressing (CIP). The magnetic properties and microstructure of the CIPed mag-netswere compared with those of the magnets processed through conventional uniaxial compaction in a transverse feld press (TP)and heat treated under identical conditions. Enhanced values of remanence (Br ~ 10.7 kG) and energy product [(BH)max ~ 29 MGOe] were obtained in the CIPed magnetsas compared to those of the TP magnets[Br ~ 10 kG and (BH)max ~ 25 MGOe]. However, the values of intrinsic coercivity (iHc) and knee feld (Hk) of CIPed magnet were found to be lower than those of the TP magnet.The results were explained on the basis of the difference in texture, microtexture and microstructure obtained in these magnets due to different processing methods adopted.
查看更多>>摘要：This article addresses the infuence of stacking fault formation on the deformation and strain hardening behaviour of steels showing the twinning-induced plasticity effect. The role of the twin nucleation process was demonstrated on the results of mechanical testing, microstructure analyses, and constitutive modelling that were performed exemplarily for a high-alloy austenitic steel X2CrMnNi16-7-10. The microstructural evolution of the steel was studied in situ by acoustic emission measurements and post mortem by X-ray diffraction, and by scanning and transmission electron microscopy. The acoustic emission data provided information about the movement of partial and perfect dislocations and about the onset and extent of the deformation-induced twinning. The X-ray diffraction quantifed lattice strains caused by the microstructure defects, and revealed averaged dislocation densities and stacking fault probabilities. The scanning electron microscopy delivered grain sizes and dislocation densities and unravelled the relationship between local grain orientation and the governing deformation mechanism. Transmission electron microscopy confrmed the validity of the microstructure model that was used for evaluation of the X-ray diffraction data, and visualized the interplay between individual deformation mechanisms. For the description of the mechanical response of the TWIP steel, a constitutive model was formulated, and supplied with the entirety of the experimental data. This approach helped to reduce the number of the free parameters of the constitutive model. Even with less free parameters, the model was able to describe the strain hardening of the steel reliably. It was found that, in contrast to dislocation motion, deformation twinning contributes to strain only marginally. The major impact of twinning on the enhanced plasticity arises from the reduction of the dislocation mean free path and the associated dislocation-related strain hardening. Based on the experimental data and model predictions, general guidelines for austenitic steel design were established.
查看更多>>摘要：To facilitate the processing and manufacturing of heat-treatable aluminum alloy components, the initial temper of the billet material is usually adjusted to the as-quenched (AQ) temper by solid solution treatment. However, for most as-quenched aluminum alloys, the obvious natural aging (NA) may occur and cause sensitive time scheduling for the subsequent forming processes. Taking the 2195 Al-Cu-Li alloy as the case material, from the aspects of Portevin-Le-Chatelier (PLC) effect, properties and microstructure evolution, this study deeply and systematically explores the NA behaviors and reveals the underlying mechanism. The combined interaction of Cu and Li solute atoms with mobile dislocations is the major mechanism for the PLC effect in this alloy. The NA induced decreasing of the interaction of solutes with the dislocations makes the number of dislocations in the PLC bands reduce, resulting in its character to change from type B to type A during NA. The Al-Cu-Li alloy exhibits a strong response at the early stage of NA, viz., the tensile strength increases rapidly in the initial 4 h and then reaches the plateau 24 h later. Major sources of strengthening in the NA state are the δ'/β' precipitates, GP zones and solute atoms. The extended age hardening model were used to predict the evolution of the YS and micro-structure variables of the alloy during NA. The results indicate that the contribution of solute atoms (σ_(ss)), δ'/β' precipitates (σδ) and GP zones (σ_(gp)) to strength in NA state is ~161 MPa, ~80 MPa, and ~ 141 MPa, respectively. The electrochemical measurement results indicate that the NA state has a higher corrosion resistance than the AQ state. The pitting pores form associating with the negative potential of the spherical 5' phase in NA state.
查看更多>>摘要：ASTM standards for tensile tests defne specifc sample size requirements regardless of grain size of the material. However, sample size requirements for testing should be considered in conjunction with the number of grains within its reduced cross-sectional area. This is particularly important for in-situ Scanning Electron Microscope (SEM) tensile tests, as they have to be conducted on smaller samples.In this study, a comprehensive experimental and numerical evaluation of the effect of specimen thickness (and the resulting number of grains within the cross-section) were conducted using in-situ SEM tensile test (on sub-millimeter thick samples) and ex-situ tensile tests (on samples of 0.68-5.9 mm thickness) and the results are compared with FEM simulations outcome. All tests were conducted at room temperature and the results are correlated to the number of grains within the thickness of specimens.The tensile test results indicated that even though the 0.2% proof stress and the tensile strength do not vary for the current range of thicknesses (with number of grains in the cross-section varying from 13 to 118), a difference in necking mechanism exists. Post tensile strength, thinner samples undergo more shear failure and diagonal localized necking whereas thicker samples experience more diffuse necking indicated by decreased area of shear failure at the edges and increased area of dimpled tensile failure at the center of the sample. FEM results complement the experimental fndings by showing the formation of conjugated localized shear bands on the upper and lower surface in the 0.68 mm thick sample and shear bands combining to form singular shear bands in thicker samples. These results also confrm the validity of the in-situ SEM tensile tests conducted on thinner samples as long as the required minimum number of grains exist within the cross-section.
查看更多>>摘要：Tristelle 5183 (Fe-21%Cr-10%Ni-7%Nb-5%Si-2%C in wt%) is an alternative material to cobalt-based alloys for wear resistant hardfacing applications. In this work, gas atomised powder was consolidated by hot isostatic pressing (HIPing) at 1120 ± 10 oC which is ~ 100 ℃ below the melting onset point. The microstructure formation in the alloy, following densifcation, was investigated using X-ray diffraction, scanning and transmission electron microscopy and electron backscatter diffraction. The hot isostatically pressed (HIPed) alloy contains principally fcc γ-Fe, NbC, and M7C3 with a small fraction of bcc a/δ-Fe and a π-ferrosilicide phase. This contrasts with the metastable gas atomised powder microstructure in which M7C3 formation is largely suppressed and NbC precipitation is reduced. Following HIPing, a wide distribution of γ-Fe grain sizes is found. The larger grains exhibit sub-grain structures with signifcant intra-grain misorientations as identifed by kernel average misori-entation (KAM) maps. The smaller grains (< 10 μm) contain annealing twins, indicating that recrystallization had occurred only in certain localised regions which underwent suffcient plastic deformation in the early stages of HIPing. The work demonstrates the potential for HIPing hardfacing alloys to achieve a fne scale homogeneous microstructure.
查看更多>>摘要：In this paper, a defected laser welded joint of 304 stainless steel and T2 copper was studied concerning structural arrangement, grain structure and texture evolution. According to SEM and EBSD results, the as-welded fusion zone contained a major fraction of Fe-Cr-Ni rich α structure and minor fraction of Cu-rich ε structure. Furthermore, three types of ε were observed including blocky, curved and spherical forms. Both grain size and crystalline orientation varied largely from 304 side to T2 side. Additionally, epitaxial zone was progressively wider on 304 side than T2. In mixed zone, ε possessed comparably dispersed orientation and fne grains. Based on solidifcation sequence of δ -> Fe-Cr-Ni rich δ/γ -> Cu rich γ in the fusion zone, the dispersed primarily solidifed of Fe-Cr-Ni rich spheres with random orientation led to the formation of fne grains in ε. As for α, the continuous growth of Fe-Cr-Ni rich bulks with uniform orientation promoted the identical growth and orientation of inside Cu rich spheres.
查看更多>>摘要：The present paper has reported a comparative study on the infuence of environmental temperature on the microstructural evolution and tensile mechanical behaviors between squeeze cast (SC) and gravity cast (GC) Mg-10Gd-3Y-0.5Zr (GW103K) alloys. The results revealed that for both alloys, the operative deformation mechanisms at testing temperatures (25-300 ℃) were identifed to be (1012) extension twinning, (1011) compressive twinning and (1013) compressive twinning in addition to basal or non-basal slips. The elongation increased monotonically with increasing temperature within the range of 25 ℃ to 300 ℃. However, the ultimate tensile strength (UTS) abnormally increased (for GC alloys) or keep constant (for SC alloys) with increasing the temperature from 25 ℃ to 250 ℃, and decreased with further increasing the testing temperature to 300 ℃. Moreover, at all testing temperature, both the UTS and elongation of the squeeze cast material were improved relative to those of the gravity cast alloy. The underlying mechanism for the abnormal increase of strength within certain elevated temperature range was attributed to the insuffcient strain hardening at lower temperatures that restrict further plastic deformation and lead to pre-mature fracture, and suffcient plastic deformation and softening at higher temperatures promote plastic deformation. The strengths improvements in squeeze cast GW103K alloy can be mainly ascribed to the preventing or limiting of crack nucleation sites and hindering of crack propagations through the reduction of micropores and increasing the amount of grain boundaries.
查看更多>>摘要：The infuence of Er-doping on the microstructure evolution and corresponding properties of Ti_(82-x)Nb_(16)Al2Er_x high temperature shape memory alloys were systematically investigated in the present study. A small number of Er-rich phases were observed except for the α′′ martensite phase when Er content was above 0.3 at.%. The Er-rich phase accrued and agglomerated with the increasing of Er doping. The Er-rich phase would affect the nucleation and growth of martensite plates. The martensite morphology was featured with the fne martensite plates at the nearby of Er-rich phase. The martensitic transformation temperatures of Ti_(82-x)Nb_(16)Al2Er_x alloys decreased frstly and then increased as the Er contents increased, which was mainly ascribed to the variation of the matrix composition, grain refnement and the formation of the non-transformation Er-rich phase. It was noteworthy that the yield strength and fracture strength raised frstly and then dropped sharply. The yield strength and fracture strength up to 780Mpa and 902 MPa could be obtained in the Ti81Nb16Al2Er1 alloy. The shape memory properties could be optimized by doping the proper Er content. The maximum recoverable strain of 3.1% could be gained with 4% pre-strain.
查看更多>>摘要：The intrinsic partitioning behavior of Al and its infuence on the nanoscale precipitation of Cu-rich nanocluster-strengthened steels were investigated by using atom probe tomography (APT) and frst-principles calculations. The APT results reveal that Al partitions to Cu-rich nanoclusters, which results in a slight decrease in the volume fraction of the nanoclusters. The frst-principles calculations indicate that the Al substitution in body-centered cubic (bcc) Cu is more energetically favorable as compared with that in bcc Fe. In addition, the Al partitioning has no signifcant infuence on the chemical driving force and interfacial energy but slightly increases the strain energy for nucleation, thereby increasing the critical energy for the formation of Cu-rich nanoclusters. As a result, the nanoscale Cu precipitation is slightly inhibited in the Al-containing ferritic steels. In addition, the effects of Al on the precipitation strengthening response were quantitatively evaluated, and the results indicate that the degree of precipitation strengthening depends majorly on the combined effect of cluster size and inter-cluster spacing.
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Elsevier