Fe 10Ni Research Articles

Texture and grain boundary study in high strength Fe–18Ni–Co steel related to hydrogen embrittlement

Crystallographic texture and grain–boundary distribution are introduced as a new method of reducing susceptibility to crack formation and propagation. In this work, slow strain rate tests (SSRT) were performed on fully martensitic ultra-high strength Fe–18Ni–Co steel in 0.6M NaCl atmosphere at −1.2 VSCE to investigate hydrogen embrittlement (HE). Micro and meso-texture in both crack stricken and crack free regions were characterized using electron backscattering diffraction (EBSD) technique, to find a pattern between crystallographic orientation and HE susceptibility. The Taylor factor analysis was used to determine the grains with higher susceptibility to crack initiation and propagation. The results showed that cracks can initiate and propagate through <100>//ND oriented grains, while {111} and {110}//ND fibers indicate higher resistance path for crack propagation. Moreover, the high amount of deformed fraction with high stored energy acts as one of the main reasons for high susceptibility of this steel against SSRT. It is concluded that the susceptibility of high strength Fe–18Ni–Co steel to HE can be reduced through crystallographic texture control and grain boundary engineering.

Change in dislocation networks of deformed austenitic model steel during relaxing process

The thermo-simulation test and transmission electron microscopy were applied to investigate the evolution of dislocation configuration and strain-induced precipitation behaviour during relaxation at 850°C in deformed Fe–40Ni–Ti alloy. The stress relaxation curve can be divided into three stages, namely, the process of incubation, nucleation and growth and the coarsening of strain-induced precipitates. The highly dense and twisted dislocations formed during the deformation develop into dislocation networks and finally, the sub-grains can be observed when relaxing to 1000 seconds. The strain-induced precipitates occur both onto the dispersed dislocations and dislocation networks. The precipitates pin the dislocations, which results in retarding the progress of dislocation evolution. As precipitates start to coarsen, the pinning effect weakens and the dislocations get rid of the pinning though a bypassing mechanism.

The evolution of microbands and their interaction with NbC precipitates during hot deformation of a Fe–30Ni–Nb model austenitic steel

The present work has investigated the evolution of microbands (MBs) and their interaction with strain-induced NbC precipitates during uniaxial compression of a model austenitic Fe–30Ni–Nb steel at 925°C. The 〈110〉 fibre grains, both without and with copious amounts of precipitates, contained up to large strains crystallographic MBs aligned close to the highly stressed {111} slip planes having large Schmid factors. The MBs thus maintained their crystallographic character during straining, through continuously rearranging themselves, and did not follow the macroscopically imposed rigid body rotation. During double-pass deformation, fine NbC particles formed at short inter-pass holding remained strongly pinned at small reloading strains and appeared to be dragged by rearranging MB walls. With increasing reloading strain, the fine precipitates became progressively released from the above walls. During reloading after increased holding time, the coarsened particles tended with their increased size to become increasingly detached from the MB walls already at a small strain. The precipitate-free MB wall segments rearranged during straining to maintain their crystallographic alignment, while the detached precipitates followed the sample shape change and rotated towards the compression plane. The MB wall rearrangement generally occurred through cooperative migration of the corresponding dislocation networks.

Structural and magnetic transition in stainless steel Fe-21Cr-6Ni-9Mn up to 250 GPa**Project supported by the National Natural Science Foundation of China (Grant Nos. U1230201, 11274281, and 11304294), the Industrial Technology Development Program, China (Grant No. 9045140509), and the Funds from the Chinese Academy of Sciences (Grant Nos. KJCX2-SW-N03 and KJCX2-SW-N20).

Stainless steel Fe-21Cr-6Ni-9Mn (SS 21-6-9), with ∼21% Cr, ∼6% Ni, and ∼9% Mn in weight percentage, has wide applications in extensive fields. In the present study, SS 21-6-9 is compressed up to 250 GPa, and its crystal structures and compressive behaviors are investigated simultaneously using the synchrotron angle-dispersive x-ray diffraction technique. The SS 21-6-9 undergoes a structural phase transition from fcc to hcp structure at ∼12.8 GPa with neglectable volume collapse within the determination error under the quasi-hydrostatic environment. The hcp structure remains stable up to the highest pressure of 250 GPa in the present experiments. The antiferromagnetic-to-nonmagnetic state transition of hcp SS 21-6-9 with the changes of inconspicuous density and structure, is discovered at ∼50 GPa, and revealed by the significant change in c/a ratio. The hcp SS-21-6-9 is compressive anisotropic: it is more compressive in the c-axis direction than in the a-axis direction. Both the equations of states (EOSs) of fcc and hcp SS 21-6-9, which are in accordance with those of fcc and hcp pure irons respectively, are also presented. Furthermore, the c/a ratio of hcp SS 21-6-9 at infinite compression, R∞, is consistent with the values of pure iron and Fe–10Ni alloy.

Improvement of Oxidation Resistance of Remelted Zone in an Al2O3-Forming Austenitic Stainless Steel by Annealing

To improve the oxidation resistance of a remelted zone in Al2O3-forming austenitic stainless steels, effects of annealing temperature on microstructural evolution and Al2O3 formation in remelted zone of Fe–25Ni–10Cr–5Al–1Nb–Y steel (wt%) were investigated. The results showed that a continuous Al2O3 film could not form in the remelted zone before annealing. After annealing at 1,100, 1,150 and 1,180 °C, a continuous Al2O3 film formed. The existence of a second phase containing Al is the key for the formation of a continuous Al2O3 film. The thickness of Al2O3 film increased with the rise in Al content in the second phase.

Development of pseudoelasticity in Fe–10Ni–7Mn (wt%) high strength martensitic steel by intercritical heat treatment and subsequent ageing

Abstract In this research, ternary Fe–10Ni–7Mn (wt%) high strength martensitic steel was subjected to study the reverse transformation of martensite to austenite by intercritical annealing at 600 °C in the ferritic–austenitic (α+γ) dual phase region for various holding times. Subsequent isothermal ageing was performed at 480 °C for 3.6 ks. The experimental results revealed that during intercritical annealing above the heating rate of 5 °C/s, reverse martensite transformation occurs by the diffusionless mechanism. Also, the X-ray diffraction (XRD) and electron back scattering diffraction (EBSD) analyses represented that the volume fraction of the retained austenite at room temperature increases with increasing the holding time up to 7.2 ks and then it gradually decreases. Furthermore, the cyclic tensile test results revealed pseudoelastic behavior of the retained austenite after subsequent ageing. The maximum amount of pseudoelasticity was obtained about 40% at the sixth loading–unloading cycle for the specimen intercritically annealed at 600 °C for 7.2 ks after subsequent isothermal ageing at 480 °C for 3.6 ks.

Deformation mechanisms in a precipitation-strengthened ferritic superalloy revealed by in situ neutron diffraction studies at elevated temperatures

The ferritic superalloy Fe–10Ni–6.5Al–10Cr–3.4Mo strengthened by ordered (Ni,Fe)Al B2-type precipitates is a candidate material for ultra-supercritical steam turbine applications above 923K. Despite earlier success in improving its room-temperature ductility, the creep resistance of this material at high temperatures needs to be further improved, which requires a fundamental understanding of the high-temperature deformation mechanisms at the scales of individual phases and grains. In situ neutron diffraction has been utilized to investigate the lattice strain evolution and the microscopic load-sharing mechanisms during tensile deformation of this ferritic superalloy at elevated temperatures. Finite-element simulations based on the crystal plasticity theory are employed and compared with the experimental results, both qualitatively and quantitatively. Based on these interphase and intergranular load-partitioning studies, it is found that the deformation mechanisms change from dislocation slip to those related to dislocation climb, diffusional flow and possibly grain boundary sliding, below and above 873K, respectively. Insights into microstructural design for enhancing creep resistance are also discussed.

Microtension behaviour of lenticular martensite structure of Fe–30 mass% Ni alloy

Abstract Microtension testing of an Fe–30Ni alloy revealed that yielding in the lenticular martensite plates is not necessarily caused by the slip system with the highest Schmid factor. Dislocation gliding likely occurred on the slip system parallel to the plane of nanotwins in the lenticular martensite plate. The martensite plate exhibited a critical resolved shear stress of ~190 MPa. Strain accumulation at the variant interfaces between the primary plate and the aggregated fragments led to a unique faceted fracture. This is attributed to anisotropic plasticity in the lenticular martensite plates owing to the interaction of the mobile dislocations with the nanotwins.

Evolution of strain-induced precipitates in a model austenitic Fe–30Ni–Nb steel and their effect on the flow behaviour

The present work investigated the evolution of strain-induced NbC precipitates in a model austenitic Fe–30Ni–Nb steel deformed at 925°C to a strain of 0.2 during post-deformation holding between 3 and 1000s and their effect on the reloading flow stress. The precipitate particles preferentially nucleated on the nodes of the periodic dislocation networks constituting microband walls. Holding for 10s resulted in the formation of fine, largely coherent NbC particles with a mean diameter of ∼5nm, which displayed a cube-on-cube orientation relationship with austenite and caused the maximum increase in the reloading steady-state flow stress. A further increase in the holding time from 30 to 1000s led to the formation of semi-coherent, gradually coarser and more widely spaced particles with a mean diameter of 8nm and above, which led to a gradual decrease in the reloading steady-state flow stress. The holding time increase resulted in progressive disintegration of the dislocation substructure and dislocation annihilation through static recovery processes, which was also reflected by the measured softening fractions. The precipitate particle shape changed during post-deformation annealing from elliptical to faceted octahedral and subsequently to tetra-kai-decahedral.

Precipitation and austenite reversion behavior of a maraging steel produced by selective laser melting

Abstract

Effect of Fe Content on the Interfacial Reliability of SnAgCu/Fe–Ni Solder Joints

Fe–Ni films with compositions of Fe–75Ni, Fe–50Ni, and Fe–30Ni were used as under bump metallization (UBM) to evaluate the interfacial reliability of SnAgCu/Fe–Ni solder joints through ball shear test, high temperature storage, and temperature cycling. The shear strengths for Fe–75Ni, Fe–50Ni, and Fe–30Ni solder joints after reflow were 42.57, 53.94 and 53.98 MPa, respectively, which were all satisfied the requirement of industrialization (>34.3 MPa). High temperature storage was conducted at 150, 175 and 200 °C. It was found that higher Fe content in Fe–Ni layer had the ability to inhibit the mutual diffusion at interface region below 150 °C, and the growth speed of intermetallic compound (IMC) decreased with increasing Fe concentration. When stored at 200 °C, the IMC thickness reached a limit for all three films after 4 days, and some cracks occurred at the interface between IMC and Fe–Ni layer. The activation energies for the growth of FeSn2 on Fe–30Ni, Fe–50Ni, and Fe–75Ni films were calculated as 246, 185, and 81 kJ/mol, respectively. Temperature cycling tests revealed that SnAgCu/Fe–50Ni solder joint had the lowest failure rate (less than 10%), and had the best interfacial reliability among three compositions.

Synthesis, characterization, and nanoindentation response of single crystal Fe–Cr–Ni alloys with FCC and BCC structures

Fe-based alloys are used extensively in many structural applications including under irradiation conditions in the nuclear industry. In this study, model Fe–Cr, Fe–Ni and Fe–Cr–Ni alloys that are the basis of many structural steels were synthesized as single crystals and characterized. The compositions investigated were Fe–15Cr, Fe–30Cr, Fe–30Ni and Fe–15Cr–15Ni (at%). Several key mechanical properties were determined which will be useful in further studies of irradiation/deformation-induced defects. Incipient plasticity and slip characteristics were investigated by nanoindentation on (001) and (1¯10) surfaces, and hardness, modulus, pop-in behavior and theoretical strength were determined. The slip trace patterns after microindentation were imaged in a microscope. A novel slip trace analysis was developed and the underlying deformation mechanisms identified. The analysis shows that under both (001) and (1¯10) indentations, the activated slip system for the BCC alloys is {112}〈111〉; for the FCC alloys the activated slip plane is {111}. These results were confirmed with finite element simulations using a slip-based crystal-plasticity model. Finally, the effects of heterogeneous pop-in mechanisms are discussed in the context of incipient plasticity in the four different alloys.

Three-Dimensional Approach to Observing Growth of Blocks and Packets in Fe-18Ni Maraging Steel

The present study aims to clarify the development of blocks and packets in lath martensite in Fe–18Ni maraging steel using three-dimensional observations. The specimens were step-quenched in order to clarify the sequential development of the three-dimensional morphology in a prior austenite grain. In a prior austenite grain, we found that five independent packets formed during the early stage of martensitic transformation. Four of the packets exist along the prior austenite grain boundaries and one packet grows from the boundary edge into the prior austenite grain. Each packet consists of a single block, although the fraction of martensitic transformation is 50.6%. The observed rules of the block-selection are as follows: (1) the blocks have near Kurdjumov–Sachs orientation relationship with adjacent austenite grains and elongated directions of the laths are parallel to adjacent grain boundaries and (2) transformation shear directions of the laths are parallel to adjacent grain boundaries.

A criterion for determining the optimum value of twist in the topological model

In previous works, in which the topological model has been applied to martensitic phase transformations, the value of twist angle ω was determined based on the habit plane-(HP) matching method, where the physical realization of the so-predicted interfacial defect networks may require reorientations of defect line directions by short-range diffusion, though no long-range diffusion was needed. In the present work, a novel criterion for determining the optimum value of twist is proposed so that the predicted interface defects are not only able to fulfil the function of fully accommodating the coherency strains arising on the terrace plane, but also capable of reaching the required position at the HP without long- or short-range diffusions. A numerical analysis for an Fe–20Ni–5Mn alloy is demonstrated based on the newly proposed criterion, and the predictions so obtained are in good agreement with the results provided by the phenomenological theory and experimental measurements.

Strain-induced martensite to austenite reverse transformation in an ultrafine-grained Fe–Ni–Mn martensitic steel

Research was conducted to evaluate the effect of heavy cold rolling on microstructural evolution in an Fe–10Ni–7Mn (wt.%) martensitic steel. The chemical driving force for the strain-induced martensite to austenite reverse transformation was calculated using thermodynamic principles and a model was developed for estimating the effect of applied stress on the driving force of the martensite to austenite reverse transformation through heavy cold rolling. These calculations show that, in order to make a reverse transformation feasible, the applied stress on the material should supply the total driving force, both chemical and non-chemical, for the transformation. It is demonstrated that after 60% cold rolling the required driving force for the reverse transformation may be provided. Experimental results, including cold rolling and transmission electron microscopy images, are utilized to verify the thermodynamic calculations.

Interaction of a screw dislocation with Frank loops in Fe–10Ni–20Cr alloy

The interaction of 2 and 5nm Frank loops with a moving screw dislocation is studied in Fe–10Ni–20Cr alloy (a model of austenitic 304 and 316 steels) employing the newly developed Fe–Ni–Cr interatomic potential in molecular dynamics simulations. The applied potential ensures full stability of FCC phase and smooth evolution of stacking fault energy (SFE) as a function of chemical composition, fitted to be in a close agreement with the CALPHAD database. A model of Fe–10Ni–20Cr random alloy closely reproduces elastic properties and SFE of 316-type austenitic steels. The results reveal a number of interaction mechanisms depending on loop orientation and ambient temperature. Half of the observed reactions lead to loop unfaulting despite a low SFE of the alloy. The unfaulting reactions are enhanced with temperature and the critical stress for the unfaulting is regularly higher in comparison with the loop shear interaction. By comparing present results with a recent study done in a low SFE Fe–50Ni alloy, we reveal that a magnitude of local variation of SFE is an important factor controlling the formation of dislocation constrictions. In the Fe–50Ni alloy, characterized by strong variations of local SFE, the constrictions are almost never observed so that the loop shear interaction prevails, while absorption is rare. In the Fe–10Ni–20Cr alloy, characterized by small variations of local SFE, the constrictions are regularly formed resulting in frequent loop unfaulting.

Growth of subscale on Fe–20Ni binary alloy in the presence of magnetic field

The effect of magnetic field on the oxide-scale growth of Fe–20Ni binary alloy was investigated. Results indicated that the oxide scales consisted of an external Fe-oxide layer and an internal oxidation zone (IOZ). The oxidation kinetics, gained by weight loss method, showed that the application of magnetic field retarded the growth of oxide scales. After measuring the penetrated depths of the oxide precipitates in subscale, evidence on the retarding effect of magnetic field on the growth of subscale was observed.

Application of metal injection moulding to soft magnetic materials

Powder metallurgy is an effective way to produce the complex shaped soft magnetic materials, and also to decrease the eddy current loss in high frequency by subdividing the eddy current area due to the small grains. Especially, metal injection moulding (MIM) process allows nearly full dense and net shaping of a variety of engineering materials. The application of MIM process to hard and brittle materials such as ferromagnetic materials demonstrates the potential of this novel process. This study considers the processing of three types of soft magnetic materials such as Fe–6·5Si, Fe–9·5Si–5·5Al and Fe–50Ni alloy compacts through the MIM techniques using different types of powders to obtain high performance of soft magnetic properties.

Manufacture of μ-PIM gear mold by electroforming of Fe–Ni and Fe–Ni–W alloys

The micro gear mold for powder injection molding was made by electroforming process of Fe–Ni and Fe–Ni–W alloys using UV-lithography process. Kinetics and activation energies in electroplating of both alloys were investigated to determine the best process conditions. Fe content within electrodeposited Fe–Ni alloys increased with the increase of rotating disk speed and the decrease of temperature and it is considered from the calculated activation energy of iron content that the rate determining step is controlled by mass transfer. Iron content in Fe–Ni electrodeposit varied from 58.33% to 70.45% by increasing current density from 2 to 6 A/dm2. Also, iron content in Fe–Ni–W electrodeposit increased from 59.32% to 70.15%, nickel content decreased from 27.86% to 17.07% and the content of tungsten was almost consistent in the range of 12.78%–12.82% although the current density increases from 1.5 to 5 A/dm2. For the electroforming of micro gear mold, SU–8 mandrel with 550 μm in diameter and 400 μm in height was prepared by UV-lithography processing. Subsequently, Fe–36Ni and Fe–20Ni–13W alloys micro gear molds were electroformed successfully. Surface hardness values of the electroformed micro molds were measured to be HV490 and HV645, respectively.

Roles of Manganese in the High-temperature Oxidation Resistance of Alumina-forming Austenitic Steels at above 800 °C

A new family of alumina-forming austenitic (AFA) stainless steels is under development for uses in aggressive oxidizing conditions. This paper investigates the effect of manganese additions on the oxidation kinetics and alumina scale formation in two series of AFA steels, i.e., Fe–20Ni–14Cr–2.5Al and Fe–18Cr–25Ni–3Al base. At 800 °C in dry air, the oxidation resistance was moderately degraded with additions of larger than 1 wt% Mn in the AFA steels based on Fe–14Cr–20Ni–2.5Al. At 900 °C in air with 10 % water vapor, however, additions of Mn in these AFA steels based on Fe–18Cr–25Ni–3Al would significantly destabilize the alumina scale formation and degrade the oxidation resistance. Our analysis revealed that additions of Mn stimulated formation of the coarse spinel CrMn1.5O4 and Cr2O3 oxide and destroyed the continuity of the protective alumina scales, thus worsening the oxidation performance. In addition, it was found that there exists an upper limit for the Mn additions which is decreased with the increase of the service temperatures and presence of aggressive oxidizing agents.