Austenite Lattice Research Articles

Microstructure of nitrided and nitrocarburized layers produced on a superaustenitic stainless steel

The expanded austenite γN can be produced in austenitic stainless steels by plasma nitriding, carburizing or nitrocarburizing at low temperatures. This metastable phase presents higher hardness and toughness if compared with traditional nitride layers whilst also maintaining the corrosion resistance. However, the application of plasmas composed by both nitrogen and carbon is technologically recent and the effect of such process on the microstructure and properties of the nitrocarburized layers is still under investigation. In this study, samples of UNS S31254 superaustenitic stainless steel were produced by plasma nitriding and nitrocarburizing at 400°C, 450°C and 500°C for 5h. The plasma treated samples were observed by optical and transmission electron microscopy and also analyzed by X-ray diffraction. The thickness of the layers increased with temperature and the nitrocarburized layers were thicker than nitrided at a given temperature. The presence of expanded austenite was confirmed by X-ray diffraction through its characteristic anomalous shift on the diffracted peaks related to the austenite. Nitride formation on samples produced at 400°C was only identified by transmission electron microscopy where fine rounded particles with 10–15nm size revealed reflections consistent with the CrN cubic chromium nitride. The estimated lattice parameter from the expanded austenite ranged from 0.38 to 0.41nm depending on the employed {hkl} reflection which was found to be 6–11% larger than the untreated austenite lattice parameter.

Biocompatibility studies of low temperature nitrided and collagen-I coated AISI 316L austenitic stainless steel

The biocompatibility of austenitic stainless steels can be improved by means of surface engineering techniques. In the present research it was investigated if low temperature nitrided AISI 316L austenitic stainless steel may be a suitable substrate for bioactive protein coating consisting of collagen-I. The biocompatibility of surface modified alloy was studied using as experimental model endothelial cells (human umbilical vein endothelial cells) in culture. Low temperature nitriding produces modified surface layers consisting mainly of S phase, the supersaturated interstitial solid solution of nitrogen in the austenite lattice, which allows to enhance surface microhardness and corrosion resistance in PBS solution. The nitriding treatment seems to promote the coating with collagen-I, without chemical coupling agents, in respect of the untreated alloy. For biocompatibility studies, proliferation, lactate dehydrogenase levels and secretion of two metalloproteinases (MMP-2 and MMP-9) were determined. Experimental results suggest that the collagen protection may be favourable for endothelial cell proliferation and for the control of MMP-2 release.

Stability of expanded austenite, generated by ion carburizing and ion nitriding of AISI 316L SS, under high temperature and high energy pulsed ion beam irradiation

Expanded austenite can be generated on austenitic stainless steels either by ion carburizing or ion nitriding. In both cases the resulting fcc crystal structure, supersaturated with nitrogen or carbon, is strongly hardened with improved wear-resistance, while maintaining the original resistance to corrosion. In this work, we have studied the stability of expanded austenite, generated by ion nitriding and ion carburizing on AISI 316L SS with N and C, under: a—high temperature (225°C – 504°C), and b—under irradiation with high energy (30keV – 500keV), high fluence (~1015cm−2), short duration (~400ns) light (deuterium and helium) ion beams. It was found that expanded austenite is stable below 325°C. Between 325°C and 504°C expanded austenite lattice parameter presents gradual reduction with increasing temperature. We observed microstructural changes related only to the temperature treatment. We did not observe any microstructure change due to the duration of the heat treatment. Over 504°C, the lattice parameter returns to the material's austenite original parameter. On the other hand, when irradiated with pulsed ion beams, a gradual reduction of the lattice parameter corresponding to the expanded austenite with the number of pulses was observed. This behavior can be explained through the thermal shock induced on the surface by each beam, consisting in fast heating followed by fast cooling that induces the gradual exo-diffusion of N (or C). Nevertheless, after 20 ion pulses, a final lattice parameter slightly higher than the corresponding to the original austenite was found as stable limit. This residual expansion can be attributed to partial amorphization of the first few micrometers that induces stresses on the crystals of austenite which are closer to the surface layers.

Role of Phase Stress in Variations of Cell Behavior on NiTi

Phase stresses that arise from volume mismatch between austenite and martensite crystal lattices were studied at different stages of direct and reverse martensitic transformations in NiTi alloy for medical application. The absolute stress values of tensile and compressive stresses that operated in austenite and martensite respectively were shown to be Gaussian functions of phase composition: they increased from zero in the initial single-phase state, achieved maximum at the equilibrium between the initial and new phases and decreased to zero in the ultimate phase. Phase stress distribution on the surface of simulated two-phase sample and related distinctions in osteoblast-like cells’ behavior were analyzed. Negative effect of compressive stress on cell viability and probable mechanism of cell apoptosis are discussed. Recommendations concerning the choice of permissible size discrepancy between organ and implant as well as NiTi alloy composition to avoid/minimize negative consequences of two-phase state after implant installation are suggested.

Szacowanie wielkości krystalitów i udziału faz w austenitycznej stali nierdzewnej 1.4307 po walcowaniu za pomocą dyfrakcji promieni X

The material under investigation was a commercial austenitic stainless steel SS 1.4307 (EN) equivalent to AISI 304L. The samples of SS 1.4307 were rolled, achieving a 56, 60 and 62% reduction in thickness. The plastic deformation reduced austenite phase at the expense of the newly formed martensite phase. The volume fraction of austenite phase, crystallite size and lattice strain were calculated from X-ray line broadening. The X-ray diffraction measurements were carried out in a Philips X-Pert diffractometer using KαCu radiation.

Updating a software package for the simulation of diffraction patterns and the computation of the parameters of the twinning and self-accommodation of martensite crystals

An updated software package for the simulation of point diffraction pictures and the computation of several crystallographic characteristics of martensite crystals in alloys with the shape-memory effect is presented. Optimization was carried out for the procedure of the analysis of the orientation relationships between the lattices of austenite and martensite from the viewpoint of the possibility of the formation of self-accommodation complexes.

Rate-controlling mechanisms of hot deformation in a medium carbon vanadium microalloy steel

Isothermal compression tests were carried out on a medium carbon vanadium microalloy steel (roughly Fe-0.33C-1.5Mn-0.1V, wt%) by using a Gleeble-1500 simulator. Based on constitutive analysis including an Arrhenius term, activation energy for hot working was calculated and used to evaluate the rate-controlling mechanism of hot deformation. At low strain rates (0.1–1s−1), the activation energy for hot working (287.4kJ/mol) is very close to the austenite lattice self-diffusion activation energy, indicating that the rate-controlling mechanism is dislocation climb. While at high strain rates (10–30s−1), the activation energy becomes very high (500.6kJ/mol), and activation volume is better used under such conditions. Then, activation volume analysis based on both Schöck model and Kocks–Argon–Ashby model demonstrates that the rate-controlling mechanism at high strain rates is cross slip. That is, the rate-controlling mechanisms of hot deformation for the medium carbon vanadium microalloy steel at high and low strain rates are intrinsically different. Inspired by the findings above, processing map analysis based on dynamic materials model was further preceded and different peak domains of power dissipation efficiency in high and low strain rate regimes were found.

Stability of the austenite lattice of a high-nickel iron-based alloy toward the martensitic transformation

First-principles calculations of the variation of the total energy of a ferromagnetic Fe-31.2% Ni alloy upon bainitic transformation are presented. The problem of the loss of stability of the fcc lattice of iron alloys is discussed to show that the fcc lattice is mechanically stable in this alloy.

Effect of electrical pulse treatment on the thermal fatigue resistance of bionic compacted graphite cast iron processed in water

In order to further enhance the thermal fatigue resistance of bionic compacted graphite cast iron (CGI) which is processed by laser in water, the electrical pulse treatment is applied to improve the thermal fatigue resistance of bionic units. The results show that the electrical pulse treatment causes the supersaturated carbon atoms located in the lattice of austenite to react with the iron atoms to form the Fe3C. The microstructures of the bionic units processed in water are refined by the electrical pulse treatment. The cracks on the bionic units are reduced by the electrical pulse treatment before the thermal fatigue tests; and during the tests, the thermal fatigue resistance of bionic units is therefore enhanced by reducing the crack sources. By this way, the thermal fatigue resistance of bionic CGI processed in water is improved.

On measurement of carbon content in retained austenite in a nanostructured bainitic steel

In this study, the carbon content of retained austenite in a nanostructured bainitic steel was measured by atom probe tomography and compared with data derived from the austenite lattice parameter determined by X-ray diffraction. The results provide new evidence about the heterogeneous distribution of carbon in austenite, a fundamental issue controlling ductility in this type of microstructure.

Crystal lattice of martensite and the reserve of recoverable strain of thermally and thermomechanically treated Ti-Ni shape-memory alloys

X-ray diffraction has been used to study shape-memory alloys of composition Ti-(49.73–51.05 at %) Ni subjected to quenching and thermomechanical treatment (TMT) by the scheme “cold deformation (e = 0.3–1.9) + postdeformation annealing (200–500°C) to provide different defectness of the parent B2 austenite. For the quenched alloys, the concentration dependences of the lattice parameters of the B19′ martensite, maximum lattice strain upon martensitic transformation, the crystallographic orientation of the lattice in single crystals, and the reserve of recoverable strain in polycrystals have been determined. The lattice parameters of martensite formed from polygonized, i.e., nanosubgranular, or from nanocrystalline austenite differ from the corresponding parameters of quenched martensite formed from recrystallized austenite, and their difference increases with increasing defectness of the parent-austenite lattice. An increase in the defectness of the austenite lattice is accompanied by a decrease in the reserve of recoverable strain. The deformation of the existing martensite or the formation of stress-assisted martensite under the anisotropic action of external stresses changes the interplanar spacing and the thermal expansion coefficient in different crystallographic directions but does not affect the averaged lattice parameters near the M s-M f interval and the reserve of recoverable strain.

Influence of processing parameters on lattice parameters in laser deposited tool alloy steel

Laser aided direct metal deposition (DMD) has been used to form AISI 4340 steel coating on the AISI 4140 steel substrate. The microstructural property of the DMD coating was analyzed by means of scanning electron microscopy, transmission electron microscopy and X-ray diffractometry. Microhardness of the DMD was measured with a Vickers microhardness tester. Results indicate that DMD can be used to form dense AISI 4340 steel coatings on AISI 4140 steel substrate. The DMD coating is mainly composed of martensite and retained austenite. Consecutive thermal cycles have a remarkable effect on the microstructure of the plan view of the DMD coating and on the corresponding microhardness distribution. Orientation relationships among austenite, martensite and cementite in the DMD coating followed the ones in conventional heat treated steels. As the laser specific energy decreased, cooling rate increased, and martensite peaks broadened and shifted to a lower Bragg's angle. Also martensite lattice parameters increased and austenite lattice parameters decreased due to the above parameter change.

Pseudoelasticity of Shape Memory Alloys ‐ a 1D Material Model and Finite Element Implementation

AbstractThis contribution is concerned with the formulation of a 1D‐constitutive model accounting for the pseudoelastic behavior of shape memory alloys. The stress‐strain‐relationship is idealized by a hysteresis both in the compression as in the tension loading range. It is characterized by an upper loading path, which is to be ascribed to the transformation of the lattice to a martensitic structure. Unloading the material, a lower path is described, because of the reverse transformation into austenitic lattice. The constitutive model is based on a switching criterion which serves as a potential function for the evolution of the internal state variables. The model distinguishes between local and global variables to describe the hysteresis effects for the compression and tension range. A strain driven algorithm which captures the complete nonlinear material behavior is presented. The boundary value problem is solved for a truss element applying the finite element method. A consistent linearization of the nonlinear equations is derived. Simple examples will demonstrate the applicability of the proposed model. For future developments the usage of shape memory alloys within civil engineering structures is aimed. The advantage of the material is the very good damping behavior and the potential to overcome great strains. Both properties are distinguished to be of engineering interest. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation of self-accommodation complexes of martensite crystals during transformations of distortion type in alloys with the shape memory effect

This work discusses the formation of self-accommodation complexes of martensite crystals during transformations of distortion type and experimentally observed shape memory effects in unordered solid solutions. A method for the analysis of orientation relationships between the lattices of austenite and marten-site in terms of the possibility of formation of self-accommodation complexes is given.

TEM studies of plasma nitrided austenitic stainless steel

Cross-sectional transmission electron microscopy and X-ray phase analysis were used to study the structure of a layer formed during nitriding the AISI 316L stainless steel at temperature 440 degrees C. It was found that the applied treatment led to the formation of 6-microm-thick layer of the S-phase. There is no evidence of CrN precipitation. The X-ray diffraction experiments proved that the occurred austenite lattice expansion - due to nitrogen atoms - depended on the crystallographic direction. The cross-sectional transmission electron microscopy studies showed that the layer consisted of a single cubic phase that contained a lot of defects such as dislocations, stacking faults, slip bands and twins. The high-resolution electron microscopy observations were applied to study the defect formation due to the nitriding process. It was shown that the presence of great number of stacking faults leads to formation of nanotwins. Weak, forbidden {100} reflections were still another characteristic feature of the S-phase. These were not detected in the X-ray spectra of the phase. Basing on the high-resolution electron microscopy studies it can be suggested that the short-range ordering of the nitrogen atoms in the octahedral sites inside the f.c.c. matrix lattice takes place and gives rise to appearance of these spots. It is suggested that the cubic lattice undergoes not only expansion but also slight rombohedral distortion that explains differences in the lattice expansion for different crystallographic directions.

Low Temperature Thermochemical Surface Treatment of Austenitic Stainless Steel for Improved Mechanical and Tribological

This paper describes the results of four thermochemical surface treatments of austenitic stainless steels carried out at 450oC in a fluidised bed furnace and they are nitriding, carburizing and the newly developed hybrid process involving the simultaneous and sequential incorporation of nitrogen and carbon to form a dual layer structure in order to achieve much enhanced surface hardness and wear resistance without compromising the corrosion resistance of the steel. In all these treatments there formed alloyed layers with a common feature of being precipitation-free and supersaturated with nitrogen, or carbon or both in the austenite lattice which is known as S Phase or expanded austenite. However the layer thickness was not uniform in any of these treatments and an effective layer was produced after 8h treatment duration. The nitriding treatment produced thicker and harder layer compared to other treatments; the maximum hardness was over 1500 Hv for nitriding and the minimum hardness of 500 Hv for carburizing treatment. The nitriding treatment sample gave high wear resistance which corresponded to high hardness values.

Structure and properties of low temperature plasma carburized austenitic stainless steels

Austenitic stainless steels cannot be conventionally surface treated at temperatures close to 550 °C due to intense precipitation of nitrides or carbides. Plasma carburizing allows introducing carbon in the steel at temperatures below 500 °C without carbide precipitation. Plasma carburizing of AISI 316L was carried out at 480 °C and 400 °C, during 20 h, using CH 4 as carbon carrier gas. The results show that carbon expanded austenite ( γ C), 20 µm in depth, was formed on the surface after the 480 °C treatment. Carbon expanded austenite ( γ C), 8 µm in depth, was formed on the surface after the 400 °C treatment. DRX results showed that the austenitic FCC lattice parameter increases from 0.358 nm to 0.363 nm for the 400 °C treatment and to 0.369 nm for the 480 °C treatment, giving an estimation of circa 10 at.% carbon content for the latter. Lattice distortion, resulting from the expansion and the associated compressive residual stresses increases the surface hardness to 1040 HV 0.025. Micro-scale tensile tests were conducted on specimens prepared with the conditions selected above, which has indicated that the damage imposed to the expanded austenite layer was more easily related to each separated grain than to the overall macro-scale stresses imposed by the tensile test.

Enhancing Deep Drawing Processes by Using a Thermomechanical Tool Design

The nature of phase transformation in metastable austenitic steels due to strain- induced ’-martensite formation is used as new contribution to lightweight construction in crash safety applications. A thermomechanical tool design was developed to enhance the ’-martensite evolution in local areas. Draw and stop beads allow a concerted stretch-out of material by increasing true strain . The local warming of defined regions avoids the phase transformation to retain the original austenitic lattice while a regional cooling enforce this nature by realizing a constant true strain .

Crystallographic patterns of the shape memory effect in non-ordered solid solutions

A method is proposed to estimate the possibility of achieving shape memory effects in martensite alloys with disordered lattices. The analysis of orientational relationships between the lattices of austenite and martensite allows one to detect those which are able to form self-accommodation complexes, an important part of the memory effect mechanism. This method has been applied to the Ti48Zr48Nb4 alloy in which two martensite phases are formed: hexagonal α′ and orthorhombic α″ martensites.

An extended near-coincidence-sites method and the interfacial structure of austenite precipitates in a duplex stainless steel

The faceted morphology and dislocation structures in all the facets of austenite precipitates in a duplex stainless steel were investigated with an extended near-coincidence-sites (NCS) method based on the orientation relationship (OR) generated by the O-line model. The prominent facets of the precipitates are parallel to three sets of principal Moiré planes containing high areal density of NCS, respectively. The Burgers vector and the spacing of the misfit dislocations were determined by a Moiré plane trace analysis and a displacement decomposition method. The observed misfit dislocations were consistently explained by the current approach, but the calculated fine dislocations were not observed experimentally. The terrace-ledge-kink structures in terms of both ferrite and austenite lattices were also graphically illustrated. The present geometric approach can also be applied to other systems containing lath- or rod-shaped precipitates, especially when the interfacial structures cannot be described by periodic O-lattice elements.