Lattice Parameters Of Martensite Research Articles

Hydrogen penetration into the NiTi superelastic alloy investigated in-situ by synchrotron diffraction experiments

Microstructural changes induced by a hydrogen permeation into the NiTi superelastic alloy were investigated in-situ using the X-ray synchrotron diffraction. A new design of an electrochemical cell enabled to uncover time and position dependent processes under a flat alloy surface exposed to the cathodic hydrogen. The diffraction data supported by thermo-elastic FEM calculations helped to quantify an evolution of compressive stresses in the B2 austenitic phase hosting hydrogen atoms. The compressive stress state initiates a formation of martensitic phases starting from the exposed surface layer and advancing into the alloy volume with increasing time of hydrogen charging. We have performed the ab-initio DFT study in order to rationalize volumetric changes associated with variations in the B2 austenite and B19′ martensite lattice parameters. The numerical results also contributed to the identification of a new hydride phase with orthorhombic crystal structure and lattice parameters a = 0.8505 nm, b = 0.7366 nm and c = 0.4722 nm.

An in-situ neutron diffraction investigation of martensitic transformation in a metastable β Ti-10V-2Fe-3Al alloy during uniaxial tension

The deformation behaviour of a metastable β Ti-10V-2Fe-3Al (wt%) alloy containing ~10 vol% α phase was investigated via single-peak fitting and Rietveld refinement of in-situ neutron diffraction data collected during uniaxial tension. The formation of the 021α″ fibre is related to martensite variant selection during β → α″ transformation under applied load. The β lattice parameter increases by ~0.08% during tension whereas α″ accommodates deformation through a decrease and increase in a and b lattice parameters, respectively. The origin of the variation in α″ martensite lattice parameters with strain for different Ti alloys is linked to their different rhombicity values.

Comparative Study of the Tempering Behavior of Different Martensitic Steels by Means of In-Situ Diffractometry and Dilatometry.

Martensitic steels are tempered to increase the toughness of the metastable martensite, which is brittle in the as-quenched state, and to achieve a more stable microstructure. During the tempering of steels, several particular overlapping effects can arise. Classical dilatometric investigations can only detect effects by monitoring the integral length change of the sample. Additional in-situ diffractometry allowed a differentiation of the individual effects such as transformation of retained austenite and formation of cementite during tempering. Additionally, the lattice parameters of martensite and therefrom the tetragonality was analyzed. Two low-alloy steels with carbon contents of 0.4 and 1.0 wt.% and a high-alloy 5Cr-1Mo-steel with 0.4 wt.% carbon were investigated by dilatometry and in-situ diffractometry. In this paper, microstructural effects during tempering of the investigated steels are discussed by a comparative study of dilatometric and diffractometric experiments. The influence of the chemical composition on the tempering behavior is illustrated by comparing the determined effects of the three steels. The kinetics of tempering is similar for the low-alloy steels and shifted to much higher temperatures for the high-alloy steel. During tempering, the tetragonality of martensite in the steel with 1.0 wt% carbon shifts towards a low carbon behavior, as in the steels with 0.4 wt.% carbon.

Molecular-Dynamics Simulation of the Influence of Silicon on the Ordering of Carbon in the Martensite Lattice

The results of computer simulation of the influence of silicon impurities on the degree of tetragonality and on the interaction of carbon atoms in the body-centered cubic (BCC) lattice of iron by the molecular-dynamics method are reported. The influence of silicon on the martensite-lattice parameters has been established, as well as the dependence of the deformation interaction parameter λ0 determining the critical temperature of the BCC–BCT (body-centered tetragonal) transition on the silicon concentration, is calculated on the basis of the Zener–Khachaturyan theory of ordering. It is found that the λ0 parameter decreases by 18% from 5.2 to 4.2 eV/atom upon an increase in the silicon content up to 10 at %.

Молекулярно-динамическое моделирование влияния кремния на упорядочение углерода в решетке мартенсита

AbstractThe results of computer simulation of the influence of silicon impurities on the degree of tetragonality and on the interaction of carbon atoms in the body-centered cubic (BCC) lattice of iron by the molecular-dynamics method are reported. The influence of silicon on the martensite-lattice parameters has been established, as well as the dependence of the deformation interaction parameter λ_0 determining the critical temperature of the BCC–BCT (body-centered tetragonal) transition on the silicon concentration, is calculated on the basis of the Zener–Khachaturyan theory of ordering. It is found that the λ_0 parameter decreases by 18% from 5.2 to 4.2 eV/atom upon an increase in the silicon content up to 10 at %.

Bainite Transformation During Continuous Cooling: Analysis of Dilatation Data

The amount of athermal martensite as a function of undercooling below the martensite start temperature was quantified by analyzing the dilatation data using a novel method, and the results are compared with existing empirical equations. The discrepancy between the two results was attributed to the difference in the concentration ranges of the alloying elements considered. The importance of including the effect of substitutional elements on the lattice parameters of martensite for accurate quantitative interpretation of dilatation data was highlighted. Equations that include the effect of substitutional alloying elements were proposed to calculate martensite lattice parameters. It is further shown that it is possible to calculate the lattice parameter coefficient of a substitutional alloying element directly from the dilatation curve. It was used to estimate, for the first time, the lattice parameter coefficient of aluminum (Al) in ferrite/martensite from the dilatation curves of the two alloy steels studied in the current work. To corroborate the value of the lattice parameter coefficient of Al estimated from the dilatation data, the Bain model was also used to calculate the lattice parameter coefficient of Al independently and a good match was obtained. The lattice parameter coefficient value of Al in ferrite/martensite calculated by both these methods follows the overall trend shown by other substitutional alloying elements. The equations proposed for the lattice parameters of martensite were validated by Rietveld analysis of the X-ray diffraction (XRD) patterns.

The effect of carbon content on the c/a ratio of as-quenched martensite in Fe-C alloys

The as-quenched martensite crystal structure is widely accepted as body-centered tetragonal. The classical Honda & Nishiyama model, c/a = 1 + 0.045wt%C, was obtained based only on the experimental results for steels containing more than 0.6wt% of carbon. This model was used to predict that steels containing less than 0.6wt% of carbon would follow the same relation. However, it was reported by Sherby that the steel with less than 0.6wt% of carbon is body-centered cubic, and c/a ratio equals to 1. This debate is caused by highly overlapped martensite (002) and (200) peaks when the carbon content of steel is less than 0.6wt%. In this paper, the martensite lattice parameters of the as-quenched steels with different carbon contents were measured using high-resolution X-ray diffractometer. Rietveld refinement was used to deconvolute overlapped martensite peaks. For the steel with less than 0.6wt% of carbon, the structure is proved to be body-centered tetragonal. It was found that the relationship between c/a ratio and carbon content follows as c/a = 1 + 0.031wt%C.

Influence of martensitic transformation on the magnetic transition in Ni-Mn-Ga

The magnetic transition with a temperature hysteresis of about 7K was observed in the martensitic phase of Ni51.9Mn27Ga211. The measurements of AC magnetic susceptibility in constant magnetic fields up to 570kA/m have proved its magnetic origin. The transport and caloric measurements were used to gain better understanding of the nature of this phenomenon. The variation of the martensite lattice parameters with temperature is suggested to account for the hysteresis of the magnetic transition.

Carbon ordering in martensite lattice under external stress: MD simulation

Rapid cooling of the Fe-C fcc solution leads to the formation of a phase called martensite, which has high mechanical properties. Martensitic transformation is the basis of quench steel hardening. Experimental investigation have shown that martensite lattice parameters a and c depend linearly on the carbon content. It is well known that under stress a martensite has the ability to plastically deform within a small range, despite the large value of the macroscopic yield strength. This effect is explained in different ways, mainly on the basis of dislocation theory. Professor M.Shtremel suggested an original treatment of the effect, based on the possibility of transferring the tetragonal axis of the martensite crystal. The experimental verification of the hypothesis is hampered by the difficulty in obtaining a crystal with one orientation. We investigated the influence of external stresses on carbon ordering in dilute Fe-C solid solutions and creation of tetrahedral distortion of crystal lattice in martensite. Molecular dynamics with embedded atom (EAM) interatomic potential have been used. According to simulations, interstitial carbon atoms migrate under compressive external stress applied along tetragonality direction from z-sublattice, and tetrahedral distortion of lattice changes its orientation to other ones. The simulation confirmed the validity of Shtremel’s theory. The influence of temperature and carbon concentration on this process was studied. The discrepancy between computer simulation results and theoretical data are discussed.

Microstructure evolution and lubricant wear performance of laser alloyed layers on automobile engine chains

Wear resistant layers on nodular cast iron chains with C–B–W–Cr powders were fabricated by laser surface alloying (LSA). Microstructure, phases and lattice parameters, were investigated by means of optical microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffractometry. Micro-, nano-hardness and elastic modulus were measured with a Vickers microhardness tester and a nano-indendation tester. Lubricant sliding wear performance was performed on a ball-on-disk apparatus in ambient air using the straight line reciprocating wear form. Results indicate that microstructure of the alloyed layers changes from hyper-eutectic to hypo-eutectic, varing with laser specific energy. Nano-grain size and micro-hardness decrease while martensite lattice parameters increase with laser specific energy. Existence of graphite in the substrate increases the carbon content in the retained austenite to 1.59 wt%. Nano-hardness and elastic modulus of the alloyed layers are close. Friction and wear properties of the layers are improved by LSA compared with the substrate. Wear mechanism of them is illustrated.

Residual Stresses and Dimensional Changes Related to the Lattice Parameter Changes of Heat-Treated JIS SKD 11 Tool Steels

The residual stresses and dimensional changes related to the lattice parameter of the JIS SKD11 tool steel have been investigated for both conventional and cryogenic treatments. As trapped carbon atoms are released from the martensite, the martensite c/a ratios and dimensions of the specimens treated conventionally decreased at tempering temperatures above 673K. However, these values increased owing to the increase in the c-axis when tempering was conducted at temperatures higher than 673K. At higher temperatures, the retained austenite is converted into martensite. The martensite c/a ratios and dimension change ratios in the cryogenically treated specimens decreased significantly with the tempering temperature because the amount of retained austenite is reduced by cryogenic treatment. In the conventionally treated specimens, the compressive residual stress decreased with quenching after tempering because of the release of trapped carbon atoms. However, the retained austenite to martensite transformation and the precipitation of alloy carbides leads to increased martensite lattice parameters and dimensional residual stress changes. The residual stress is also changed to generate compressive residual stresses. The dimension change ratio in cryogenically treated and tempered specimens decreased as the tempering temperature increased through a reduction in the amount of trapped carbon. However, the magnitude of the compressive residual stress was reduced to zero. [doi:10.2320/matertrans.M2014031]

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.

The effect of Fe additions on the shape memory properties of Ru-based alloys

Equiatomic RuNb and RuTa alloys exhibit a shape memory effect (SME) and high transformation temperatures (>800 °C). Controlling the transformation temperatures while retaining the SME can be achieved either by changing the alloy composition or by adding a ternary element such as Fe. Eight alloys are investigated. The SME decreases with decreasing Ru content, which is in accordance with the evolution of the lattice parameters of martensite.

Crystal and magnetic structure temperature evolution in Ni–Mn–Ga magnetic shape memory martensite

The current work presents the original results concerning the temperature evolution of the magnetic and crystal structure of Ni–Mn–Ga magnetic shape memory alloys within the entire temperature interval of the martensite phase. The atomic positions and the correct space group of the 5M martensite unit cell has been derived, which is crucial for the electronic structure calculations. Neutron diffraction studies of the Ni–Mn–Ga alloy, possessing a large magnetic shape memory effect (>5% shear strain), revealed that there are several magnetic transitions in the martensite phase due to the smooth strongly anisotropic temperature dependence of the martensite lattice parameters.

Low-temperature X-ray diffraction study of martensite lattice parameters in binary Ti–Ni alloys

Concentration and temperature dependency of the B19′-martensite lattice parameters (MLP) in binary Ti–Ni alloys as well as the effect of the structural state of a parent austenite on these parameters were studied using low-temperature X-ray diffraction analysis. The existence of a concentration dependence of the MLP found for the hyper-equiatomic nickel concentration range is proved for cryogenic temperature range and extended, at least, up to 51.2 at.% of nickel. Temperature dependency of MLP is observed in the studied nickel concentration range, and they are approximately the same for different alloys in the temperature range of stable martensite. During heating in the temperature range of the martensite existence, all parameters of the B19′-martensite monoclinic cell change towards the values of corresponding parameters of the austenite tetragonal cell with which they have “genetic” relations. The maximum transformation lattice strain calculated at the martensite-start temperature of each alloy, and, hence, the resource of the recoverable strain, are higher for pre-equiatomic and equiatomic alloys than that for alloys in hyper-equiatomic nickel concentration range. For Ti–50.7 at.% Ni alloy, the lattice parameters of martensite formed from the austenite containing a well-developed dislocation substructure deviate from the corresponding lattice parameters of the quenched martensite formed from a low-dislocated recrystallized austenite. This distinction is a general feature for the alloys undergoing B2 → B19′ and B2 → R → B19′ martensitic transformations.

Microstructural study of equiatomic PtTi martensite and the discovery of a new long-period structure

The martensitic phase in an equiatomic PtTi alloy was studied by electron microscopy, neutron and X-ray diffraction, and differential scanning calorimetry. The expected orthorhombic B19 structure with Type I twinning on {111} planes was found plus a new 33 long-period microtwin stacking based on this B19 structure, also including plate twinning. The measured microtwin volume fraction for the B19 is found to be larger than the value calculated from crystallographic theory, which can be explained by the fact that no martensite lattice parameters at the transformation temperature are known. An atomic model is suggested for the new long period structure in agreement with the electron diffraction and high resolution electron microscopy data.

Comparative X-ray and time-of-flight neutron diffraction studies of martensite crystal lattice in stressed and unstressed binary Ti–Ni alloys

An X-ray and time-of-flight neutron diffraction study of a Ti–50.0 at.% Ni alloy was performed. The B19′ martensite lattice parameters are different for the quenched martensite and for the martensite formed from the austenite containing a well-developed dislocation substructure due to transformation-induced hardening. A tensile stress increases the “underrecovery” of the d h k l values of planes perpendicular to the axis of tension (and of the “apparent” martensite lattice parameters calculated from these d h k l values) after a cycle of transformation-induced hardening compared with the corresponding values of the initial quenched martensite. A probable cause for the differences between d h k l and lattice parameters of martensite formed from highly dislocated austenite and corresponding parameters of quenched martensite formed from recrystallized austenite are tensile components of stress fields generated by the dislocation substructure and/or residual stresses of different origin which are present in the initial austenite.

A study on lattice parameters of martensite in Ni–Ti–Ta shape memory alloys

The lattice parameters of B19′ martensite in Ni–Ti–Ta shape memory alloys are analyzed by means of the X-ray diffraction (XRD), the selected area diffraction patterns (SADPs) of transmission electron microscopy (TEM) and the multiphase Rietveld methods. Experimental results indicate that when the Ni content is constant, the lattice parameters a and c, monoclinic angle β and unit cell volume V increase, while lattice parameter b decreases with increasing Ta content; when the Ta content is constant, the lattice parameters a and c, monoclinic angle β and unit cell volume V increase, while lattice parameter b decreases with increasing Ni content. With the unit cell volume V increased, the R factor and the aberration of unit cell increase.

A comparative study of the wear behaviour of sintered and laser surface melted AISI M42 high speed steel diluted with iron

Powders of AISI M42 high-speed steel (HSS) were blended with different proportions of water-atomised iron powders. The powders were subsequently submitted to uniaxial pressing and then divided in three lots. The first was submitted to sintering, the second was submitted to sintering plus laser surface melting (LSM) and the third was submitted to sintering plus LSM plus double tempering at the secondary hardening peak temperature of M42 HSS. The objective of this procedure was to evaluate the processing route that leads to reduced porosity in AISI M42 HSS and to higher abrasive wear resistance. Therefore the samples, with different chemical compositions and microstructures, were submitted to a detailed microstructural characterisation followed by microscale hardness and abrasive wear tests. It was observed that LSM leads to almost complete elimination of residual porosity and to the dissolution of large brittle carbides that are present in the as-sintered samples, leading to a homogeneous and extremely fine microstructure. This microstructure is formed of saturated plate martensite and a small proportion of retained austenite. The double tempering treatment, carried out in the laser surface melted samples samples, leads to the elimination of retained austenite and to a decrease of the lattice parameters of martensite due to the precipitation of thin carbides within martensite. As a result, while the hardness of the material in the sintered condition is between 245 and 625 HV (depending on the proportion of dilution with iron), after LSM the hardness is higher than 820 HV in all the samples. Surprisingly, the abrasive wear resistance of the laser melted and of the laser melted and tempered samples is lower than that of the as-sintered ones. Observation of the wear craters by scanning electron microscopy shows that this result is due to the different wear mechanisms acting on the samples processed by different routes.

On the lattice parameters of phases in binary Ti–Ni shape memory alloys

An X-ray diffractometry study of Ti–47.0 to 50.7 at.%Ni alloys was performed. In the 50.0–50.7 at.% range of nickel content, a concentration dependence of B19 ′-martensite lattice parameters (MLP) is observed. MLP are found to be identical for 47.0 and 50.0 at.% of nickel content. The temperature dependence of MLP is observed, and this dependence is enhanced in the reverse transformation temperature range for Ti–50.0 at.%Ni alloy. MLP are different for the quenched martensite and for the martensite formed from the austenite containing a well-developed dislocation substructure. It is proven that the presence of an intermediate R-phase during martensitic transformation is not responsible for the changes in MLP, observed in hyper-equiatomic alloys or in alloys having a highly dislocated austenite substructure. In the 50.0 at.%Ni alloy, no changes in MLP are observed after a 25% cold-deformation of the already formed thermal martensite.