Increasing Martensite Volume Fraction Research Articles

Effective phase transformation behavior of NiTi triply periodic minimal surface architectures

The effective behavior of shape memory alloy triply periodic minimal surface (TPMS) structures is investigated using finite element analysis and numerical homogenization. The TPMS unit cells considered are primitive, IW-P, gyroid, and diamond subjected to different loading conditions. Under uniaxial displacement-driven loading, the results show a dramatic increase in effective stress and martensite volume fraction with increased relative density of the TPMS unit cell. Comparison among the four types of TPMS unit cells shows that diamond has superior mechanical performance for the loading cases considered. Based on numerical homogenization results, the onset and subsequent thresholds of phase transformation are determined for all four unit cells considering multiaxial loading. At lower relative density, the loading surfaces corresponding to the onset of phase transformation were reasonably well represented by either von Mises or Hill’s criteria. The observed fit with the von Mises model degenerated with increased effective martensite volume fraction, while a proper fit was maintained with Hill’s criterion. Subsequent loading surfaces, corresponding to monotonically increasing martensite volume fraction, show a nonlinear hardening behavior, which seems to follow a similar trend regardless of geometry. The loading surfaces were found to reach asymptotic states with distinctly different features compared to their initial shapes.

Mesoscopic origin of damage nucleation in dual-phase steels

The damage modes in ferrite-martensite dual-phase (DP) steels include ferrite grain boundary (F/F) decohesion, ferrite/martensite interface (F/M) decohesion and martensite cracking. To explore the mesoscopic origin for each damage nucleation mode, we investigated ferrite-martensite dual-phase (DP) steels with different volume fractions of martensite, and characterized mesoscopic strain and stress distributions using microscopic-digital image correlation in SEM (μ-DIC) and finite element (FE) calculations. We performed in-situ tensile testing in a SEM and observed that the dominant damage nucleation mode changed from ferrite grain boundary (F/F) decohesion to ferrite/martensite interface (F/M) decohesion and finally to martensite cracking, with increasing martensite volume fraction (Vm) and varying martensite distribution. Mesoscale stress and strain analysis based on μ-DIC and FE calculations clearly reveal mesoscale origins of these damage modes: 1) F/F decohesion is caused by high strain which promotes accumulation of regular dislocations near the grain boundary and residual dislocations in the grain boundary; 2) F/M decohesion is attributed to high strain gradient which is associated with the accumulation of geometry necessary dislocations; 3) Martensite cracking stems from high stress that is partitioned to martensite. Our work demonstrated the pivot of microstructure engineering to improve the damage resistance of composite-like alloys consisting of soft and hard constituent phases, such as dual-phase steels.

Fatigue Crack Growth and Fracture Toughness in a Dual Phase Steel: Effect of Increasing Martensite Volume Fraction

Crack growth resistance in dual-phase steel was studied. The dual phase steel microstructure was modified through heat treatments to increase the martensite volume fraction from 10% to 40%. The as-received and heat-treated samples were evaluated using a uniaxial tensile test, fatigue crack growth test, and fracture toughness test. Extended Finite Element Method (XFEM) was used to simulate the crack growth in compact tension test specimens. The results showed that an increase in martensite volume fraction is an effective way to increase the fracture resistance under different load conditions, quasistatics and dynamic, increasing the fracture toughness, tensile strength and fatigue resistance of the heat-treated material. Presence of a highest content of martensite results in formation of an important number of secondary cracks during the fatigue crack growth, which slow down the crack propagation. Moreover, martensite generates a crack closure over the crack tip, making the propagation difficult due to the irregularities caused by the crack growth on the martensite. Finally, the computational load-displacement curves are in good agreement with the experimental data.

Effect of Martensite Volume Fraction on Strain Partitioning Behavior of Dual Phase Steel

Monotonic deformation behavior of ferrite-martensite dual phase steels with martensite volume of 13-43% have been analyzed in the current investigation using micromechanics based finite element simulation on representative volume elements. The effects of martensite volume fraction on the strain partitioning behavior between soft ferrite matrix and hard martensite islands in dual phase steels during tensile deformation have been investigated. As a consequence of strain incompatibility between hard martensite and soft ferrite phases, inhomogeneous deformation and finally deformation localization occur during tensile deformation. Restricted local deformation in ferrite phase caused by the adjacent martensite islands triggers the local stress triaxiality development. As the martensite volume fraction increases, the local deformation restrictions in ferrite phase also increases and which results in higher stress triaxiality development. Similarly the strain partitioning behavior between ferrite matrix and martensite island is also influenced by the volume fraction of martensite. The strain partitioning coefficient increases with increasing martensite volume fraction.

Influence of martensite volume fraction and hardness on the plastic behavior of dual-phase steels: Experiments and micromechanical modeling

Model dual-phase microstructures were developed to decouple the effect of martensite volume fraction and martensite hardness on the plastic behavior of dual-phase steels. The martensite volume fraction ranges from 11% to 37%, involving two levels of martensite hardness. The yield strength and tensile strength increase with increasing martensite volume fraction, while the uniform elongation decreases. The martensite hardness has a weak impact on the initial yield strength, but it significantly affects the flow behavior for sufficiently large martensite volume fraction. Increasing the hardness of the martensite leads to higher tensile strength combined with only a limited impact on uniform elongation, resulting in an improved strength/ductility balance. The experimental results are successfully captured using finite element based micromechanical analysis. Among others, periodic cell calculations show very good predictive capabilities of the overall plastic response when the stage-IV hardening of the ferrite is taken into account. Our numerical analysis reveals that an accurate description of the elasto-plastic behavior of the martensite is a key element to rationalize the mechanical behavior of DP steels. This modeling approach provides a framework for designing dual-phase steels with optimized plastic flow properties.

Influence of Martensite Volume Fraction on Ductile to Brittle Transition of Triple Phase Ferrite-Perlite-Martensite Steels Joined by GMAW and Electrical Arc Welding

In this study, the e ect of martensite volume fraction on ductile to brittle transition of microalloyed steel joined by welding was investigated. Triple phase steel contains ferrite, perlite and martensite phases and was joined by two types of welding method as gas metal arc welding and the electric arc welding. Presence of three phases in adjacent to each other was con rmed by metallographic analysis and scanning electron microscopy studies. All welded samples were exposed to Charpy impact test at the temperature range of −30 ◦C to 30 ◦C in order to measure the fracture energies of samples having di erent amount of martensite volume fractions. The fracture surface micrographs of Charpy specimens, examined by scanning electron microscopy and optical microscopy, con rmed the reduction in ductility of ferrite perlite martensite steels with increasing martensite volume fraction.

Effect of microstructure on mechanical properties of dual phase steels in the presence of stress concentrators

The study investigated the effect of intercritical heat treatment on mechanical properties of three hot rolled steels with carbon contents ranging from 0.06 to 0.18wt%. The steels were heat-treated at temperatures ranging from 740 to 820°C, followed by water quenching (in some cases air cooling). The primary objective of this study was to investigate the relationship between microstructure and the effect of stress concentrator on deformation and fracture behavior of ferrite–martensite dual-phase (DP) steels. Besides the tensile tests with smooth specimens, notched tensile specimens with two notch geometries (flat rectangular specimens with a central hole and circumferentially notched cylindrical specimens) were used in order to modify the stress triaxiality. Moreover, Charpy impact test was carried out on sub-size specimens (10mm×2.5mm×55mm) with notch root radius of 0.25mm and on pre-cracked specimens. It was found that with increasing martensite volume fraction (MVF) the capacity of the steels to absorb energy (expressed as the product of the ultimate tensile strength and uniform elongation) remains roughly stable or decreases in case of smooth tensile specimens while it always increases in case of specimens with stress concentrator. The experimental results presented in this study clearly showed that DP steels with an excellent combination of strength and uniform or total elongation may have poor energy absorption capacity in the presence of stress concentrators.

Effect of martensite volume fraction on stress triaxiality and deformation behavior of dual phase steel

Strain incompatibility among softer ferrite matrix and harder martensite phase arises during tensile straining due to difference in the flow characteristics of two phases. Strain incompatibility between ferrite and martensite phases during tensile straining results strain partitioning, inhomogenous deformation and finally deformation localization. The local deformation in ferrite phase is constrained by adjacent martensite islands which results in local stress triaxiality development. As martensite distribution varies within the microstructure, the stress triaxiality also varies in a range within the microstructure. Effect of martensite volume fraction on stress triaxiality and tensile deformation behavior on dual phase steels are examined in the current investigation by conducting rigorous finite element study on representative volume elements. The investigation reveals that with increasing martensite volume fraction the band of stress triaxiality distribution increases (i.e. locally stress triaxiality approaches towards plane strain condition) and as a consequence uniform elongation reduces.

Influence of martensite volume fraction on tensile properties of triple phase ferrite–bainite–martensite steels

► Triple phase microstructures were obtained by step-quenching heat treatment. ► Higher dislocation density in ferrite, adjacent to martensite, was observed by TEM. ► This evidence is due to higher localized plastic strains of martensitic transformation. This paper presents the effect of martensite volume fraction ( V M ) on mechanical properties of AISI 4340 steel with ferrite–bainite–martensite microstructures. The steel bars were austenitized at 900 °C for 1 h followed by intercritical annealing at 740 °C for 100 min and quenching into a salt bath with temperature of 300 °C and holding at different times to obtain triple phase (TP) microstructures with 34 vol.% fraction ferrite and various martensite (or bainite) contents. Presence of three phases in adjacent to each other was confirmed by metallographic analysis and TEM studies. TEM observation also indicates higher density of dislocations within ferrite near martensite colonies compared to that near the bainitic phases. Results of the tensile and hardness tests showed that with increasing martensite volume fraction, yield strength, ultimate tensile strength and hardness increase which is related to higher martensite volume fraction, higher strength of ferrite and higher residual stresses. The increment of martensite volume fraction increased ductility in terms of uniform and total elongation and tensile fracture energy. This evidence is due to the change of martensite morphology, increase of dislocation density at interphase boundaries and increment in residual stresses of the ternary phase steel with increasing V M . Fracture surface micrographs of tensile specimens, examined by SEM and stereo optical microscopy, confirmed the reduction in ductility of TP steels with increasing V M .

Abrasive wear behaviour of SiCp/Al alloy composite in comparison with ausferritic ductile iron

Abstract The abrasive wear behaviour of SiCp/Al composites (MMCs) prepared by liquid metallurgy method was investigated to find out effects of applied load and weight fraction on a pin-on-disc configuration. The MMC pins containing 20 wt.% particles with sizes of 50 μm and its 2014 Al alloy were tested under different conditions against SiC abrasives. The wear performances of MMCs were also compared with those of ductile iron (DI), partially austenitized and austempered ductile iron (PADI) and conventionally austenitized (full austenitized) and austempered ductile iron (CADI) under similar conditions. Moreover, wear surfaces of tested samples were examined in a scanning electron microscope (SEM). Hardness, density and porosity increased with increasing wt.% of particle for the composite, but for the PADI and CADI sample, hardness increased with increasing martensite volume fraction and ausferrite volume fraction. The experimental results showed that wear rate of the composite decreased slightly with increasing SiCp contents and increased with increasing load. The wear resistance of MMCs was found to be better then those of DI, PADI and CADI materials, when tested against 70 μm sizes of abrasives. Furthermore, SEM examination showed that a few wear craters, combined with re-attachment of debris particles, was dominant for the composite, but the fragmented debris particles, when tested at lower loads, were found the more dominant for MMCs. Adhesion, chipping and abrasion were responsible mechanisms for the alloy, but abrasion was the most effective mechanism for the CADI samples.

Comparison of abrasive wear behavior of ductile iron with different dual matrix structures

Abrasive wear behavior of ductile irons with different dual matrix structures has been investigated. In order to obtain ductile irons with different dual matrix structures an unalloyed ductile iron specimens were austenitized in the two-phase region (α + γ) at various temperature (795 °C and 815 °C) and then rapidly transferred to a salt bath held at the 365 °C for austempering for 30, 90 and 120 min. Some specimens were quenched from same intercritical austenitizing temperatures and tempered at 550 °C for 60 and 300 min. Some specimens were also conventionally austempered and/or quenched from 900 °C for comparison. Experimental results showed that, the tensile strength increased and ductility decreased with increasing martensite volume fraction in the specimen with martensite dual matrix structure. By increasing the tempering time, the yield and UTS decreased and ductility increased. In addition, the specimens with ausferrite dual matrix structures exhibited much greater ductility than conventionally austempered ones. The tensile strength increased while ductility decreased with increasing ausferrite volume fraction. Furthermore in all austenitized specimens, the abrasive weight loss of austempered specimens (A series) was lower than those of quenched specimens (Q series) irrespective of all loads due to increased AFVFs and total elongation. It was shown that wear loss of both tested materials in abrasive wear was proportional to the applied load. However, there was a decreasing trend in the weight loss of the A795 with dual matrix structure austempered for 30 and 90 min with increasing load. The reason was because of the fact that the specimen surface was work hardened with cutting efficiency of the abrasive reduced through clogging, and attrition jointly leading to less weight loss. Moreover, increasing the austempering time caused more ductile ausferritic structure to displace hard martensite. In all austempered samples, the abrasive weight loss increased with increasing the austempering time. As for the case of Q samples, the abrasive weight loss increased more or less linearly with load since an increase in the applied load might increase the contact stress. Among the Q samples, the highest weight loss was obtained for the Q795-300, Q815-300 sample because of lower martensite volume fraction, but the lowest weight loss was observed for the Q900 sample due to the highest martensite volume fraction. For Q900 samples, the amount of fracture of the abrasives was found to be increase with the harder specimen, and it may have contributed somewhat to the increased wear. Furthermore, microchips were dominant wear mechanism by cutting mode for higher ductile materials while micro-ploughing was predominant wear for harder materials, but wear also occurred by combinations of ploughing and embedding particles into the surface for Q samples. Cross-section examination by SEM through the wear surfaces revealed that a more smoother surface was observed for the A795 sample than that of the Q795 sample. However, a more rougher surface was observed for the A900-120 sample than that of the Q900 sample.

Tensile properties of iron-based P/M steels with ferrite + martensite microstructure

The effect of intercritical heat treatments on the tensile properties of iron-based P/M steels was investigated. For this purpose, atomized iron powder (Ancorsteel 1000) was admixed with 0.3 wt.% graphite powder. Tensile test specimens were cold pressed at 700 MPa and sintered at 1120 °C for 30 min under pure argon gas atmosphere. After sintering, ∼20% pearlite volume fraction in a ferrite matrix was obtained. To produce coarse ferrite + martensite microstructures, the sintered specimens were intercritically annealed at 724 and 760 °C and quenched in water. To obtain fine ferrite + martensite microstructures, the sintered specimens were first austenitized at 890 °C and water-quenched to produce a fully martensitic structure. These specimens were then intercritically annealed at 724 and 760 °C and re-quenched. After the intercritical annealing at 724 and 760 °C and quenching, martensite volume fractions were ∼ 18% and 43%, respectively, in both the coarse- and fine-grained specimens. Although the intercritically annealed specimens exhibited higher yield and tensile strength than the as-sintered specimens, their elongation values were lower. Specimens with a fine ferrite + martensite microstructure showed high yield and tensile strength and ductility in comparison to specimens with a coarse ferrite + martensite microstructure. The strength values of specimens increased with increasing martensite volume fraction.

Correlating microstructural features with wear resistance of dual phase steel

In order to explore the tribological potential of the dual phase (DP) steel as a wear resistant material, the wear characteristics of the dual phase (DP) steel have been investigated with varying amounts of martensite from 43 to 81 vol pct, developed by varying holding time at the intercritical annealing temperature of 780 °C. Dry sliding wear tests have been conducted on DP steels using a pin-on-disk machine under different normal loads of 61.3, 68.5, 75.7 and 82.6 N and at a constant sliding speed of 1.20 m/s. At these loads, the mechanism of wear is mainly delamination, which has been confirmed by SEM micrographs of the subsurface and wear debris of the samples. Wear and friction properties have been found to be improved with increasing martensite volume fraction in dual phase steels.

Microstructural characterization and impact toughness of intercritically annealed PM steels

In this study, atomized iron powder (Ancorsteel 1000) was mixed with 0.3 and 0.5 wt.% graphite powder. After sintering at 1120 °C, coarse and fine ferrite and martensite (dual phase) microstructures were produced through intercritical annealing heat treatment at different temperatures. Specimens with various martensite volume fraction and size were impact tested. Impact toughness increased with increasing martensite volume fraction and decreasing martensite size. It was also seen that the impact toughness of specimens with 0.3 wt.% graphite was higher than those specimens with 0.5 wt.% graphite. Furthermore, the impact toughness of intercritically annealed specimens with fine ferrite and martensite microstructure was compared with a specimen that was conventionally quenched and tempered after sintering. The results showed that especially for high intercritical annealing temperatures, the impact toughness of intercritically annealed specimens was higher than that of conventionally heat-treated ones.

Effects of ferrite grain size and martensite volume fraction on dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels

Effects of ferrite grain size and martensite volume fraction on quasistatic and dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens that had different ferrite grain sizes and martensite volume fractions, using a torsional Kolsky bar, and then the test data were compared in terms of microstructures, tensile properties, fracture mode, and adiabatic shear band formation. Under dynamic torsional loading, maximum shear stress and fracture shear strain increased with decreasing ferrite grain size and increasing martensite volume fraction. Observation of the deformed area beneath the fracture surface after the dynamic torsional test indicated that adiabatic shear bands of 5 to 15 μm in width were formed along the shear stress direction, and that voids or microcracks initiated at ferrites or martensite/ferrite interfaces below the shear band. The width of the shear band decreased as the ferrite grain size increased or the martensite volume fraction decreased. These phenomena were then analysed by introducing concepts of theoretical critical shear strain.

Effects of volume fraction of tempered martensite on dynamic deformation properties of a Ti-6Al-4V alloy having a bimodal microstructure

The effects of the volume fraction of tempered martensite on the tensile and dynamic deformation properties of a Ti-6Al-4V alloy having a bimodal microstructure were investigated in this study. Five microstructures having various tempered-martensite volume fractions were obtained by varying heat-treatment conditions. Dynamic torsional tests were conducted on them using a torsional Kolsky bar. The test data were analyzed in relation to microstructures, tensile properties, and adiabatic shear-band formation. Under a dynamic loading condition, the maximum shear stress increased with increasing tempered-martensite volume fraction, whereas the fracture shear strain decreased. Observation of the deformed area after the dynamic torsional test indicated that a number of voids initiated mainly at α-phase/tempered-martensite interfaces, and that the number of voids increased with increasing martensite volume fraction. Adiabatic shear bands of 6 to 10 µm in width were formed in the specimens having lower martensite volume fractions, while they were not formed in those having higher martensite volume fractions. The possibility of adiabatic shear-band formation was explained by concepts of absorbed deformation energy and void initiation.

Effects of volume fraction of tempered martensite on dynamic deformation properties of a Ti-6Al-4V alloy having a bimodal microstructure

The effects of the volume fraction of tempered martensite on the tensile and dynamic deformation properties of a Ti-6Al-4V alloy having a bimodal microstructure were investigated in this study. Five microstructures having various tempered-martensite volume fractions were obtained by varying heat-treatment conditions. Dynamic torsional tests were conducted on them using a torsional Kolsky bar. The test data were analyzed in relation to microstructures, tensile properties, and adiabatic shear-band formation. Under a dynamic loading condition, the maximum shear stress increased with increasing tempered-martensite volume fraction, whereas the fracture shear strain decreased. Observation of the deformed area after the dynamic torsional test indicated that a number of voids initiated mainly at α-phase/tempered-martensite interfaces, and that the number of voids increased with increasing martensite volume fraction. Adiabatic shear bands of 6 to 10 μm in width were formed in the specimens having lower martensite volume fractions, while they were not formed in those having higher martensite volume fractions. The possibility of adiabatic shear-band formation was explained by concepts of absorbed deformation energy and void initiation.

Development of wear resistant medium carbon dual phase steels and their mechanical properties

Dual phase (DP) steels containing four different amounts of martensite ranging from 42 to 72 vol.-% have been developed from 0.42 wt-% carbon normalised steel by intercritical heat treatment at a fixed temperature of 740°C but varying holding times followed by water quenching. Mechanical properties of dual phase steels with increasing volume fraction of martensite have been investigated highlighting the effect of martensite content. The macrohardness has been found to increase with increasing martensite content in dual phase steel. The yield and tensile strengths have been found to increase with increasing amount of martensite whereas the percentage elongation and the percentage area reduction have been found to decrease. This has been attributed to the presence of hard and brittle martensite phase, which increases the strength at the expense of ductility. The mode of fracture has been found to change from purely ductile to mixed (ductile + brittle) as the martensite volume fraction increases from 42 to 72% in dual phase steels. Friction and wear properties under dry sliding conditions have also been found to improve with increasing martensite volume fraction in dual phase steels. The applications of medium carbon DP steels in the field of mineral processing and mining have been discussed.

Effect of martensite content on friction and oxidative wear behavior of 0.42 Pct carbon dual-phase steel

In order to explore the tribological potential of the dual-phase (DP) steel as a wear-resistant material, the friction and wear characteristics have been investigated for this steel with varying amounts of martensite from 42 to 72 vol pct, developed by varying holding time at the intercritical annealing temperature of 740 °C. Dry sliding wear tests have been conducted on DP steels containing 0.42 wt pct carbon using a pin-on-disk machine under different normal loads of 14.7, 19.6, 24.5, 29.4, and 34.3 N and at a constant sliding speed of 1.15 m/s. Weight loss has been measured at different time intervals on the same specimen. The variation of cumulative volume loss with sliding distance has been represented by two linear segments signifying the run-in and the steady state of wear. The mechanism of wear is primarily oxidative in nature, which has been confirmed by X-ray diffraction patterns of the wear debris generated during sliding. The wear rate varies linearly with load in both the run-in and the steady state. The wear rate decreases linearly with increasing volume fraction of martensite in DP steels reflecting the effect of hardness imparted by the increasing amount of martensite, which is a hard and load bearing phase. The average coefficient of friction also decreases linearly with increasing load as well as with increasing martensite volume fraction. In the run-in stage, the wear coefficient does not change significantly between DP1 and DP2 steels containing 42 and 51 vol pct martensite, respectively, but decreases sharply as one moves from DP2 to DP4 containing, respectively, 51 and 72 vol pct martensite. But in the steady state, the wear coefficient decreases almost linearly with increasing volume fraction of the martensite. The decrease in wear coefficient may be attributed to the decreasing wear rate dominating over the decrease in real area of contact due to increasing hardness.

Surface carburised AISI 8620 steel with dual phase core microstructure

In this study, the production of dual phase steel structure in the core of surface carburised AISI 8620 cementation steel and the effect of martensite volume fraction on tensile properties have been investigated. For these purposes, surface carburised (~0·8 wt-%C) specimens were oil quenched from 900°C to obtain a fully martensitic starting microstructure. Then specimens were oil quenched from intercritical annealing temperatures of 731 or 746°C to produce dual phase steel structure in the core of specimens with martensite fractions of ~25 or ~50 vol.-% and nearly wholly martensitic microstructure at the surface. Generally, specimens with dual phase microstructure in the core exhibited slightly lower tensile and yield strengths but superior ductility without sacrificing surface hardness than those specimens with fully martensitic microstructure in the core produced by using conventional heat treatment involving quenching from 850 to 950°C. Also tensile strength increased and ductility decreased with increasing martensite volume fraction.