Strength Of Martensite Research Articles

Strain Partitioning and Load Transfer in Constituent Phases in Dual-Phase Steels

In this investigation, a new approach is proposed to calculate in situ true stress and strain in the constituent phases of dual-phase steel during deformation. The model incorporates modified law of mixture and constitutive equations. The model proposes that the deformation of martensite is pseudoelastic, and on little permanent deformation within the martensite allows the locked-in dislocation to release in the ferrite at the interface region of the ferrite and martensite. On application of the model to a set of dual-phase steels with varying microstructural parameters, it is established that the amount of strain partitioning is affected by the strength and amount of the hard phase, as well as grain size, work hardening ability, and yield strength of the softer phase. On the other hand, grain size, carbon content, and fraction and strength of martensite are the main factors controlling the load transfer among the phases.

Electrochemically Induced Surface Annealing of Stable Martensite and Its Influence on Corrosion Resistance

The phenomenon of electrochemically induced surface annealing (EISA) described the phase transformation from austenite to martensite after the treatment of a series of cathodic/anodic electrochemical pulses in an aqueous solution of sodium nitrite. It could promote the pitting resistance of steels. But all the EISA occured in metastable-austensite stainless steel. Most martensite in industry is caused by heat treatments which was much more stable, so EISA treatment for the stable martensite is important for its widely application. The 17-7PH type stainless steel was applied whose composition (wt. %) was C≤0.09, Si≤1.0, Cr 16-18, Ni 6.5-7.75, Mn≤1.0, Al 0.75-1.5, S≤0.030, P≤0.035. Originally, the samples were solution treated and aged. The content of martensite was tested by XRD and estimated through the relative diffraction strength of its {110} crystal plane. The EISA treatmen was carried out in an 8M aqueous solution of NaNO2 at 80±1°C in a three-electrode device. A series of anodic/cathodic electrochemical pulses were applied at the output voltages of -1.44 V (v.s. SCE) for 280 s, and +0.30 V (v.s. SCE) for 60s, alternately. Potentiodynamic scanning was carried out in 3.5% NaCl solution. The corrosion morphology was observed at 400 times of magnification. The corroded samples were mechanically polished, and the pitting numbers were counted at various thicknesses, until no pits could be observed. Figure (A) shows the XRD spectra of a sample before and after the anodic/cathodic electrochemical pulses of 6h. After the treatment, the relative diffraction strength of martensite {110} crystal plane decreased，which indicates the decrease of martensite content. I b and I a were used to represent the relative diffraction strength of martensite {110} crystal plane before and after EISA treatment, respectively. Define R=(Ib-Ia)/Ib to represent the decrease of martensite content after the electrochemical pulse treatment. It shows that R increased with the increase of the treatment time, so the martensite content decreased more when the treatment time increased. Figure (B) shows the hardness almost kept the origin values after the pulse treatment. The constant hardness should only originated from the special martensite phase transformation because the precipitation hardening of the steel could not happen at the treatment temperature of 80°C. The martensite content decreased while the hardness almost kept constan. Such phenomenon is the character of EISA phase transformation. Therefore, the EISA phase transformation did occur during the pulse treatment. The samples were anodic treated at +0.3V (v.s. SCE) for 6h and cathodic treated at -1.44V (v.s. SCE) for 6h, respectively . The XRD spectra showed that the R was 2.63% after the anodic treatment and -11.33% after the cathodic treatment. So both the anodic treatment and the cathodic treatment could not cause the martensite transformation, the only cause of the martensite content decrease should be the anodic/cathodic alternative treatment. So EISA phase transformation did occur in the steels other than metastable-austenite stainless steels. The corrosion morphology of the samples before and after the 6h EISA treatment was observed after the 1.0 V anodic charge at 50°C for 5 min in 3.5% NaCl aqueous solution. The average of pit number on the surface of three untreated samples was 2108 cm-2, whereas that of three EISA treated samples was only 1116 cm-2. Figure (C) shows the deepest pit of untreated samples was almost 180 μm, but that of EISA treated ones was only 140 μm. The pit number of the EISA treated samples was smaller than that of the untreated samples at all depth. Therefore, the EISA treatment promoted the pitting resistance of the steel. Figure (D) Shows that the pitting breakdown potential increased much from 0.087V to 0.523V. So EISA treatment could promote the pitting resistance of the steel. On the whole, the EISA phase transformation was first observed in steels other than metastable austenite stainless steel. The EISA treatment promoted the pitting resistance. So the corrosion protection method of EISA can be extended to different kind of steels with relative stable martensite obtained in heat treatment. Figure caption: (A) XRD test results of a sample before and after EISA treatment of 6h; (B) Hardness test results before and after electrochemical pulse treatment for different time; (C) Pit numbers at various distances to the surface of samples; (D) Dynamic potential scanning results of untreated and 6h EISA treated samples. Figure 1

Multiaxial stress–strain response and displacive transformations in NiTi alloy from first principles

Present ab initio study was focussed on a response of NiTi martensite to a superposition of shear and tensile or compressive stresses acting normally to the shear planes. The theoretically predicted base-centered orthorhombic (BCO) ground-state structure was found unstable under uniaxial compression and two transformations, one from orthorhombic to a monoclinic symmetry and the other back from monoclinic to orthorhombic symmetry, were observed in the computational model. The former transformation shows that the uniaxial compressive stress of about 4 GPa destabilizes the BCO structure by reducing its symmetry to the experimentally observed monoclinic one. However, superposition of small shear stresses remarkably lowers the compressive stress necessary for this destabilization. The latter transformation then draws the crystal lattice to the B19 structure. The theoretical shear strength of NiTi martensite was subsequently computed as a function of the normal stress. The results obtained show that the effect of the normal stress is surprisingly opposite to that calculated for NiTi austenite and other cubic metals, i.e., that the shear strength is lowered by the compressive normal stress and vice versa.

Understanding the factors controlling the hardness in martensitic steels

A unified description for the hardness in martensitic steels for a wide range of carbon contents is presented. It is based on describing the strength contributions of lath and plate martensite, precipitates and retained austenite. Descriptions of the dislocation density in both martensitic structures are obtained in terms of carbon content and tempering conditions. It is shown that a peak in hardness usually observed for carbon contents ranging 0.6–1wt% is a result of a compromise between the strength of martensite, and the increase in retained austenite. A parametric analysis is performed suggesting possible scenarios for hardness improvement.

A model for the microstructure behaviour and strength evolution in lath martensite

A new model describing the microstructure and strength of lath martensite is introduced. The packet and block size were found to linearly depend on the prior-austenite grain size when introducing relevant crystallographic and geometric relationships of their hierarchical arrangements. A mechanism for the lath boundary arrangement within a block is postulated to ensure complete carbon redistribution to the lath boundaries. Accordingly, the dislocation density is obtained by considering the lattice distortion energy within a lath being equal to the strain energy of the dislocation density at the lath boundaries. Tempering effects are introduced by estimating the extent of carbon diffusing away from the lath boundaries; this mechanism relaxes the Cottrell atmospheres of lath dislocations and coarsens the boundaries. The yield stress as well as the microstructure evolution during tempering are successfully predicted by combining these results. The model is further extended to describe the yield stress in dual-phase steel microstructures by employing the iso-work principle. The model predictions are validated against experimental data in seven martensitic and five dual-phase steels, where the prior-austenite grain size, carbon content, tempering conditions and martensite volume fraction are employed as input. These results cover wide composition, initial microstructure and tempering conditions.

Microstructural heterogeneity and its relationship to the strength of martensite

The complex microstructure of ferrous martensite is reflected in its complex mechanical response. In an attempt to highlight how carbon redistribution during quenching and/or low temperature tempering can affect mechanical response, a set of controlled experiments were performed. By rapidly quenching samples it was possible to limit autotempering allowing the evolution of mechanical response and microstructure to be followed with low temperature tempering. This was compared to a situation where the material was more slowly quenched, leading to a highly autotempered state. The gradual transition from elastic to plastic deformation is interpreted based on possible sources of microstructural heterogeneity. Lath-to-lath variations of dislocation density are discussed as a contributor to the development of microstructural heterogeneity during tempering. These results shed light on the possible origins of the continuous-composite like mechanical response of lath martensite proposed in recent work.

Influence of Microstructural Length Scale on the Strength and Annealing Behavior of Pearlite, Bainite, and Martensite

A medium carbon steel containing 0.4 pct C, 2.9 pct Mn, and 1.9 pct Si (wt. pct) was heat treated to produce pearlitic, bainitic, and martensitic microstructures. The three microstructures were cold rolled to large strains in order to investigate the change in strength/hardness as a result of the refinement of the microstructural length scale due to plastic deformation. The results show that the hardness does not saturate in any of the above microstructures, implying that geometric strengthening plays an important role not only in lamellar microstructures such as pearlite, but also in lath-type microstructures such as bainite and martensite. Low temperature annealing of the deformed microstructures revealed that they are very resistant to recrystallization.

Size effects in the superelastic response of Ni54Fe19Ga27 shape memory alloy pillars with a two stage martensitic transformation

The superelastic behavior of Ni54Fe19Ga27 shape memory alloy (SMA) single crystalline pillars was studied under compression as a function of pillar diameter. Multiple pillars with diameters between 10μm and 200nm were cut on a single crystalline bulk sample oriented along the [110] direction as the compression axis and that had undergone fully reversible two stage martensitic transformation, i.e. L21 austenite to 10M/14M modulated martensite and then to L1o martensite. The results revealed an increase in the critical stress for stress-induced martensitic transformation and the yield strength of martensite with decreasing pillar size. The stress hysteresis also increased with the reduction in pillar size and the superelastic response started to diminish below 500nm pillar diameter. Two-stage martensitic transformation was suppressed for pillar sizes of 1μm and below, which were shown to exhibit a direct austenite to L1o transformation. Such a change in the transformation pathway, i.e. from a two stage to one stage transformation, was also observed in bulk single crystals with increasing temperature. We demonstrated the absence of two stage transformation in bulk at high temperatures. This finding suggests that decreasing the sample size and increasing the temperature have similar effects on the superelastic response of NiFeGa SMAs that had undergone two-stage transformation and indicates that a reduction in pillar diameter decreases the transformation temperature due to the difficulty of martensite nucleation on small scales. The damping coefficients of the pillars were also calculated and the results highlighted that damping capacities higher than those of bulk metallic alloys can be achieved using submicron sized pillars.

Effect of tensile deformation of austenite on the morphology and strength of lath martensite

A hot-rolled steel, 22SiMn2TiB, was employed to study the effect of austenite deformation on the microstructure and strength of the subsequently formed lath martensite. It was revealed that the sizes of the martensite packet, block and lath were refined by the tensile deformation of austenite at temperatures above 850 °C. With the increase of the deformation temperature, the packet size increased, whereas the block size decreased. The width of the lath was independent of the prior austenite grain size and the deformation temperature. The refinement of martensite blocks was considered to strengthen the ausformed martensite.

Effect of austenite grain structure on the strength and toughness of direct-quenched martensite

Abstract The effect of prior austenite grain structure on the microstructure and properties of two low alloyed hot-rolled and direct-quenched martensitic steels was investigated. Strength properties were determined using uniaxial tensile testing, while toughness properties were characterized by using Charpy-V and fracture toughness tests. Microstructures were characterized using OM, SEM, EBSD and a novel EBSD-based image quality (IQ) technique. It was found that an increase in the rolling reduction in the non-recrystallization temperature regime of austenite was an effective way to improve the strength, impact and fracture toughness without a significant decrease in uniform elongation. In addition, this approach also decreased the in-plane anisotropy of the tensile and toughness properties. Refinement of the equiaxed austenite grain structure during rolling in the recrystallization temperature regime (roughing) also improved the strength and toughness properties, but the effect was found to be weaker than the effect of rolling in the non-recrystallization regime. In toughness testing, a correlation was found between the Charpy-V 28J transition temperature and the fracture toughness characteristic temperature T 0 . However, the correlation differs significantly from that reported in the literature for lower strength ferritic steels. In all cases, the steel microstructures consisted of mainly auto-tempered martensite and lower bainite. A brief discussion of the microstructural features controlling the strength and toughness properties is given.

Effect of Cu addition on the high temperature shape memory properties of Ti50Ni25Pd25 alloy

Abstract The effects of Cu addition, replacing Ni in a Ti50Ni25Pd25 alloy, on the microstructure, mechanical properties and shape memory characteristics are investigated. The addition of Cu slightly enhances the strength of B19 martensite through solid solution strengthening mechanism, without deteriorating the strain to fracture. The addition of Cu slightly increases the transformation temperatures but does not affect the thermal hysteresis (13 K). Results of thermal cycling tests under various constant stress levels reveal that the addition of Cu significantly increases the resistance against plastic deformation at high temperatures and at high stress levels (i.e. >450 MPa) indicating a relatively better creep resistance and stable cyclic response. TiNiPdCu alloys show better dimensional stability and demonstrate better recovery ratio upon thermal cycling as compared to that of ternary TiNiPd alloys.

Effect of Microstructural Evolution on Mechanical Properties of 55NiCrMoV7 Steel

55NiCrMoV7 steel is extensively used as mould steel. However, how to obtain a product combined with both high strength and good toughness is still a problem in the heat treatment process. In current study, a series of 55NiCrMoV7 steel specimens were quenched with different cooling rates to obtain martensite, lower bainite and upper bainite, then tempered at 600°C, 620°C and 650°C, respectively. The micrographs show the evolution of microstructure with the variation of cooling rates and tempering temperature. Mechanical properties test has revealed that, 1) at the same tempering temperature, the tempered martensite has the highest strength and hardness, while the tempered lower bainite has the best impact toughness; 2) at the same ratio of yield strength to tensile strength (YS/TS), the ductility and toughness of martensite are better than that of bainite. Furthermore, fractographs of impact specimens show the ductile and brittle fracture behavior for martensite and upper bainite respectively.

Effect of thermo-mechanical cycling on the microstructure and strength of lath martensite in the weld CGHAZ of HSLA steel

This work has investigated the effect of thermo-mechanical cycling on the microstructure and strength of lath martensite in the weld Coarse Grained Heat Affected Zone (CGHAZ). The weld CGHAZ was simulated by Gleeble simulator according to the thermal cycle calculated by Rosenthal equation. The results show clearly that deformation promotes the dislocation density during cooling and thus decreases the width of lath martensite in the weld CGHAZ. In addition, the fraction of particle increased in the deformed weld HAZ and this is because the precipitation kinetics is accelerated by deformation. Meanwhile, lath martensite strength in deformed CGHAZ was a higher than undeformed CGHAZ, which is due to the contribution of grain boundary strengthening and precipitation strengthening.

Microstructures and hardness of as-quenched martensites (0.1–0.5%C)

Four commercial steels with carbon contents in the range 0.1–0.5 wt.% have been examined in the as-quenched condition using electron microscopy, X-ray diffraction and atom probe tomography. The austenite had been deformed 0%, 10% and 30% prior to brine quenching. No influence of this deformation was evident on the martensite hardness or in any of the microstructure measurements. Increasing carbon content showed a well-known marked effect on the hardness but resulted in little refinement in the grain structure of the martensite. All crystal structures were cubic; no evidence of tetragonality was seen even at the highest carbon level but some systematic changes in grain boundary misorientations existed. The content of carbon in true interstitial solid solution deduced from X-ray line shifts was small (∼0.02 wt.%), and was independent of the total carbon content in the steel. Atom probe tomography showed that carbon was almost completely segregated to lath boundaries and dislocations but with an increasing density of segregates in the higher carbon steels. Calculations of diffusion distances confirmed that the segregation patterns were compatible with autotempering of the martensite during quenching. Analysis of different possible contributions to strength leads to the conclusion that segregated carbon atoms at defects behave similarly to carbon in true solid solution and that this is the largest single factor controlling the strength of as-quenched martensite.

Effect of Martensite Morphology on Tensile Deformation of Dual-Phase Steel

Three morphologies of martensite in dual-phase microstructure of 0.2% C steel were obtained by different heat treatment cycles. These morphologies consisting of grain boundary growth, scattered laths, and bulk form of martensite have their distinct patterns of distribution in the matrix (ferrite). In tensile testing martensite particles with these distributions behaved differently. A reasonable work hardening was gained initially during plastic deformation of the specimens. The control on ductility was found to depend on the alignment of martensite particles along the tensile axes. The increased surface area contact of martensite particles with ferrite, in grain boundary growth and scattered lath morphologies, facilitated stress transfer from ductile to hard phase. The ductility in the later part of deformation was dependent on the density of microvoids in the necked region. The microvoids are formed mostly by de-cohesion of martensite particles at the interface. The fracture of martensite particles is less prominent in the process of microvoid formation which predicts high strength of martensite.

Modelling of damage and failure in multiphase high strength DP and TRIP steels

Abstract Multiphase high strength steels such as dual phase (DP) and Transformation Induced Plasticity (TRIP) steels show excellent strength and formability due to the coexistence of harder and softer phases in their microstructures. The damage mechanism and failure behaviour of these steels are very complex and strongly affected by microstructural constituents. In experiments, two failure modes-cleavage and dimple fracturing-were observed simultaneously at the microscale. The void nucleation was caused by the de-bonding of martensite from ferritic matrix or martensite cracking. The crack initiation and the contribution of each fracture mode depended on the stress state or triaxiality, the purity degree, the volume fraction of retained austenite, the carbon content of retained austenite and martensite, and the locations of the neighbouring austenite grain and martensitic islands. To describe the effects of the multiphase microstructure, representative volume elements (RVE) were used within the framework of continuum mechanics. The partitioning of carbon in microstructure was taken into account for the flow curve description of each individual phase. The Gurson–Tvergaard–Needleman (GTN) damage model was applied to the RVE simulations to describe the ductile damage occurring mostly in the softer ferritic phase. Additionally, a cohesive zone model (CZM) was used to represent a cracking mechanism as the de-bonding of interfaces. The failure prediction was verified with different sheet forming experiments. The effects of amount and strength of martensite in a DP microstructure were also numerically investigated.

Structure and Hardness of Martensite in Quenched Fe-C Steels

The exceptional high hardness of lath martensite in quenched Fe-C steels is explained by the Engel-Brewer valence electron theory for crystal structures. The theory predicts the transformation sequence FCC-HCP-BCC with FCC iron as Fe3v, HCP iron as Fe2v, BCC iron as Fe1v and carbon as C4v. Electronic compatibility requires transformation from FCC to HCP to form two separate components. Carbon-rich clusters of C4v with 8 Fe3v atoms are distributed uniformly in a carbon-free matrix of HCP Fe2v atoms. The carbon-iron clusters are viewed as particle-like, calculated as 0.63 nm in size, and is responsible for the high strength of martensite. The carbon-free region experiences shear deformation during FCC to HCP transformation leading to work hardened fine grains. Subsequent transformation to BCC iron maintains the same size carbon cluster with additional shearing deformation during HCP to BCC formation in the carbon-free region. Tempering studies of quenched martensite are shown to support the carbon-iron cluster model.

On the plasticity of small-scale nickel–titanium shape memory alloys

Focused ion beam machined compression pillars created from [1 1 1], [0 0 1] and [2 1 0] NiTi demonstrate that orientation plays a dominant role in determining dislocation flow stress in stress-induced martensite. This is in contrast to bulk NiTi in which martensite strength is primarily dictated by precipitate size. Post-mortem transmission electron microscopy and Laue microdiffraction measurements reveal respectively dense dislocation structures and stabilized martensite consistent with bulk observations in heavily deformed NiTi.

Development in high-grade dual phase steels with low C and Si design

Cold rolled dual phase steels with low C and Si addition were investigated in terms of combination of composition and processing in order to improve mechanical properties and workability including welding and galvanizing. Mo and Cr could be used as alloying elements to partially replace C and Si to assure enough hardening ability of the steels and also give solute-hardening. Mo addition is more effective than Cr addition in terms of obtaining the required volume fraction of martensite and mechanical strength. The ferrite grain was effectively refined by addition of Nb microalloying, which gives optimized mechanical properties. The experimental results show that it is possible to obtain the required mechanical properties of high grade 800 MPa dual phase steel, i.e., tensile strength > 780 MPa, elongation > 15%, and yield/tensile strength ratio < 0.6 in the condition of low carbon (C < 0.11 wt.%) and low silicon design (Si < 0.05 wt.%) through adequate combination of composition and processing.

Effect of Annealing Temperature on the Microstructure and Properties of Pure Vanadium

Abstract Effects of annealing temperature on the microstructure and properties of pure vanadium manufactured by hot isostatic pressing(HIP) were studied. The results show that the microstructure of pure vanadium in the annealing process presented three obvious phases of recovery, recrystallization and grain growth. With increasing of annealing temperature, the quantity of band martensite and tensile strength decreased gradually and band martensite was diminished completely over 950 °C; the ductility of pure vanadium increased first, and then decreased, reaching maximum after annealed at 950 °C. The ultimate strength, elongation and reduction in area of pure vanadium annealed at 950 °C reached 380 MPa, 35.2%, 64%, respectively.