Partial Martensitic Transformation Research Articles

Исследование усталостных свойств и трибологические испытания стали AISI 316 после кручения под высоким давлением

The effect of high-pressure torsion process in a new die on the fatigue properties of corrosion-resistant steel AISI 316 has been studied. The deformation was carried out at cryogenic and room temperatures. The number of deformation cycles was 8. The initial blank had a ring shape with a diameter of 76 mm, a width of 3.5 mm and a thickness of 3 mm. It is revealed that the fatigue strength of both samples increases due to structure refinement and twinning in austenite during high pressure torsion deformation, partial martensitic transformation and increase of the share of more angular boundaries during cyclic deformation. The main factor of higher fatigue limit of AISI 316 steel after cryogenic deformation in comparison with room temperature is more intensive grinding of microstructure and presence of increased share of large-angle boundaries and more complete martensitic transformation.

Cryogenic thermal cycling induced simultaneous improvement of strength and ductility in a Zr-based bulk metallic glass composite

Preparing bulk metallic glass composites (BMGCs) is an effective way to improve the plasticity of bulk metallic glasses (BMGs). However, the enhancement of plasticity is usually at the cost of deterioration in strength. In this study, a series of BMGCs with the nominal composition of Zr48Cu48Al4 was prepared. Finely dispersed B2 CuZr crystals are embedded in a glassy matrix and subsequently, these composites were cryogenically cycled in liquid nitrogen followed by heating to various temperatures below the glass transition temperature. It was found that the samples after cryogenic thermal cycling (CTC) treated at an intermediate upper temperature shown a simultaneous improvement of yielding strength from 1680 MPa to 1832 MPa and plasticity from 5.6% to 9.1%. The enhancement of yielding strength is ascribed to the existence of the B19′ phase resulted from the partial martensitic transformation during the CTC treatment, while the improvement of plasticity is attributed to the higher free volume content and the larger “blocking effect” from the B19’ after the samples were treated by CTC. The combination of CTC treatment and precipitation of crystals provides a new strategy for improving the mechanical properties of the BMGs.

Optimisation of Solidification Structure and Properties of Hypoeutectic Chromium Cast Iron.

This paper presents a comprehensive approach to optimising the structure and properties of chromium cast iron that is intended for use in the production of castings that operate under abrasive-wear conditions. In the study, chromium cast iron was inoculated to reduce the grain size in the solidification structure. The finer-grained structure of the casting has a positive effect on its mechanical properties. A number of inoculants have been used that allow the elimination of many types of casting defects: hot cracks and porosities that often occur during the production of chromium cast iron castings. Another advantage of the developed inoculation procedure is the resulting increase in the toughness of chromium cast iron. It should be emphasised that this cast iron does not have a high impact strength in its as-cast condition due to the formation of chromium carbides in the structure. This work also proposes a specially designed heat treatment for inoculated cast iron. The parameters of the applied heat treatment were determined on the basis of dilatometric tests. The visible deviation on a dilatogram at a temperature of about 600 °C is the result of a partial martensitic transformation in the area of grain boundaries. Therefore, the increase in abrasion resistance chromium cast iron is mainly due to the appearance of martensite. The microstructure of the investigated cast iron is particularly desirable in the case of alloys that work with lubrication. The microcavities that are formed by the abrasion of the softer phase constitute natural grease, which reduces abrasive wear. Under the influence of heat treatment, only a part of austenite located near the carbides is destabilized and transformed into martensite. Therefore, this phase of composition formation provided much greater resistance to abrasive wear and hardness.

Nanosized Lamellar Structures and Tensile Properties of Intercritical‐Annealed Medium Mn Steels Containing Multiphases

The effect of extended intercritical annealing time on the changes of the microstructure and tensile properties is studied in a medium Mn steel. Nanosized lamellar structures are formed after intercritical annealing at 550 °C for 8 and 16 h; they consist of mainly tempered‐α′ martensite with retained austenite. Many cementite precipitates are observed in the tempered‐α′ martensite matrix and at the high‐angle boundaries. The orientation relationship between θ‐cementite and tempered‐α′ martensite is identified to be ()θ//()tempered‐α′, [011]θ//[011]tempered‐α′, and []θ//[]tempered‐α′. Fresh α′ martensite is detected between the tempered‐α′ martensite and austenite lamellae in the quenched specimens. The fresh α′ martensite is formed due to partial martensitic transformation of austenite upon cooling. The fractions of cementite and fresh α′ martensite increase with extended annealing time, leading to reduced austenite fraction after quenching. Additional baking treatment at 200 °C increases yield stress and elongation due to the stabilization of the austenite. This study provides a detailed understanding of these complex nanosized microstructures and discusses the key factors governing their yield stresses and work hardening behaviors.

Numerical investigation of the transient interfacial material behavior during laser impact welding

During laser impact welding, severe plastic strains and temperature spikes occurring in less than 1 µs make experimental observation impractical and necessitate computational modeling to characterize in-situ behavior. To understand the effects of microstructure and the associated inhomogeneity/anisotropy in laser impact welding, an Eulerian framework featuring aluminum 1100 flyer and stainless steel 304 target foils is applied to simulate cases with and without microstructure modeling. The transient thermomechanical phenomena revealed by the dynamic simulation provide insights into evolution of the in-situ structure-property relationship, including microstructural variation, phase transformation, and material jetting. In contrast to the homogeneous model, the inhomogeneous model suggests a 10 µm-thick zone of grain refinement at the weld interface establishing new grains 0.1-1 µm in diameter in the flyer, and causing partial martensitic phase transformation in the target, attributable to rapidly induced equivalent plastic strains of up to 10.71 in the flyer and 0.98 in the target.

Microstructural evolution and mechanical properties of V-containing medium-Mn steel manufactured via cold rolling and intercritical annealing

V was added into a cold-rolled medium-Mn steel for optimizing comprehensive mechanical properties by the V-carbide precipitation. After annealing, the V-carbide particles in austenite grains easily coarsen than that in ferrite. The contributed yield strength increment of V-carbide precipitation strengthening in ferrite is in a range of 200–250 MPa. With increasing annealing temperature, the morphology of ferrite and austenite gradually transforms from lamellar to equiaxed. The sample after intercritical annealing at 650 °C shows 1450 MPa ultimate tensile strength, 1210 MPa yield strength, and 18% tensile elongation. In addition, samples with lamellar and equiaxed structures display larger Lüders plateaus than samples with a single equiaxed/lamellar structure. Partial martensitic transformation and continuous interactions between dislocation and twin are responsible for the Lüders plateau propagation. Therein, the number fraction of coarse austenite grains should be responsible for the propagation of Lüders plateau.

Effect of Nb contents on microstructure characteristics and yielding behavior of Fe–4Mn–2Al-0.2C steel

The microstructure characteristics and yielding behavior of two Fe–4Mn–2Al-0.2C steels with different Nb contents, i.e. 0.02 wt% and 0.1 wt%, were investigated in the present study. The results show that the increase of Nb content will bring about 50–100 MPa more fine-grain strengthening effect. But on the other hand, Nb promotes the precipitation of cementite and delays recrystallization, so that the stability of austenite decreases and presents a single lathy morphology. The partial martensitic transformation of austenite in the cooling process promotes the diffusion of carbon from martensite to retained austenite, and causes the segregation of carbon at the austenite phase boundaries, leading to the Lüders effect. The steel with 0.02 wt% Nb undergoes a more persistent TRIP effect during the deformation process due to two austenite morphologies with higher volume fractions and exhibits excellent comprehensive mechanical properties with the yield strength of 570 MPa, ultimate tensile strength of 1011 MPa, total elongation of 35.6% and product of strength and elongation of 36 GPa·% after intercritical annealing at 725 °C for 30 min without the appearance of Lüders effect. Thus, this paper is of great importance to the effect of Nb contents on microstructure and yielding behavior of medium Mn steels.

Influence of Filler on the Microstructure, Mechanical Properties and Residual Stresses in TIG Weldments of Dissimilar Titanium Alloys

The influence of titanium alloy (Ti–5Al–2.5Sn) and commercially pure titanium (cpTi) as fillers on dissimilar pulsed tungsten inert gas weldments of Ti–5Al–2.5Sn/cpTi was investigated in terms of microstructure, mechanical/nano-mechanical properties, and residual stresses. A partial martensitic transformation was observed in the weldments for all the welding conditions due to high heat input. The microstructure evolved in the FZ/cpTi interfacial region was observed to be the most sensitive to the proportion of α stabilizer in the filler alloy. Furthermore, the addition of filler alloy improved the tensile properties and nano-mechanical response of the weld joint owing to the increased volume of metal in the weld joint. As compared to the Ti–5Al–2.5Sn wire, the use of cpTi filler wire proved to be better in terms of energy absorbed during tensile and impact tests, tensile strength and ductility of the dissimilar welds. An asymmetrical residual stresses profile was observed close to the weld centerline, with high compressive stresses on the Ti–5Al–2.5Sn side for both the weldments obtained with and without filler wires. This was attributed to mainly the low thermal conductivity of Ti–5Al–2.5Sn. The presence of residual stresses also influenced the nano-hardness profile across the weldments.

Influence of electron beam oscillation patterns on the microstructure, texture, residual stress and mechanical properties of Ti-5Al-2.5Sn alloy weldments

Vacuum electron beam welding is widely employed for the welding of titanium alloys using different beam oscillation patterns. Since these patterns influence the physical phenomenon in the weld pool, its effect on the microstructure, texture, mechanical properties and residual stresses is of prime interest. In order to understand this influence, electron beam welding was used to prepare Ti-5Al-2.5Sn weldments using beam oscillations of triangular and rectangular waveform. It was observed that a change of welding pattern had a strong influence on the residual stresses, impact properties and texture of weld zone while tensile properties were not significantly affected. A partial martensitic transformation was observed in both the triangular and rectangular waveform of oscillations. An increase in alpha lathe width was observed in the fusion zone and similar strength of the rectangular pattern as compared to triangular pattern. Despite of this, the observed higher Vickers hardness of the fusion zone of rectangular pattern as compared to triangular and no-oscillation was attributed to texture strengthening using rectangular waveform.

Martensitic transformation and mechanical behavior of a medium-entropy alloy

Diffusionless martensitic transformation (MT) exerts one of the most significant influences on the mechanical properties of alloys. However, the application of martensitic transformation to improve mechanical performance was seldom involved in the manufacture of high-entropy alloys (HEAs) and medium-entropy (MEAs) alloys. In this work, an innovative non-equiatomic MEA, Fe42Co42Cr16, was proposed with incorporation of martensitic transformation during water quenching and plastic deformation. Water quenching for the alloy in the high-temperature single-phase region produced a partial MT; i.e., transformation of γ–FCC austenite phase into an ε-HCP martensite phase, responsible for the coexistence of γ and ε phases in the dual-phase (DP) alloy. Another triple-phase (TP) alloy, including γ–FCC austenite, ε-HCP martensite and B2-BCC precipitates, was obtained by quenching the alloy in γ+B2 phase region. Owing to the low intrinsic stacking fault energy (γI), both DP (γI=10.9 mJ/m2) and TP (γI=12.2 mJ/m2) alloys involved the complete polymorphic MT process during plastic deformation; i.e., the transformation of the γ–FCC austenite phase into the α-BCT martensite phase with an intermediate ε-HCP martensite phase. Due to the transformation-induced plasticity effect and precipitation strengthening, the produced TP alloy exhibited a yield strength above 1 GPa with a total elongation of as high as 25%.

Microstructure, mechanical properties, residual stresses and texture analysis of Ti-5Al-2.5Sn alloy weldments obtained using electron beam of different oscillation patterns

Electron beam welding was used for welding of Ti-5Al-2.5Sn alloy using three types of beam oscillation patterns, i.e. elliptical, concentric circles and arrow shape waveform. The influence of these patterns on microstructure, mechanical properties and residual stresses was investigated using optical microscopy, electron back scattered diffraction, scanning electron microscopy, microhardness measurement, residual stress measurement, impact testing, smooth and notch tensile testing. A partial martensitic transformation was observed in the fusion zone of all the three welding patterns. The fusion zone of arrow pattern weldment exhibited approximately 16% and 1.2% higher hardness than elliptical and concentric circles patterns, respectively. Owing to an improved texture of the basal and prismatic planes in the cross-weld direction, the arrow pattern showed improved strength, reduced tensile residual stresses than elliptical and concentric circles pattern. Elliptical pattern showed superior elongation characteristics as well as increased impact strength as compared to the arrow and concentric circle patterns.

Boron addition in a non-equiatomic Fe50Mn30Co10Cr10 alloy manufactured by laser cladding: Microstructure and wear abrasive resistance

A non-equiatomic Fe50Mn30Co10Cr10 alloy was prepared by laser cladding, and the effects of boron addition on the microstructure, hardness and abrasive wear-resistance were investigated. Elemental powders were mixed using an attritor mill for 30 h and then applied by laser cladding on a stainless steel 316L substrate. The effect of boron addition (0.1, 0.66 and 5.40 at%) on the alloy microstructure was assessed using optical and electron microscopy, and the phase composition was studied using X-ray diffraction. The laser claddings were exposed to abrasive wear conditions using the dry sand/rubber wheel test. The microstructure of the laser claddings exhibited columnar dendrites formed by two crystalline structures (fcc and hcp) with the same chemical composition. The hcp structure was the result of the partial martensitic transformation of the fcc structure. Boron addition led to the formation of a eutectic phase along the interdendritic regions with a crystalline structure consistent with M2B-type borides (M = Cr, Fe). Hardness and abrasive wear-resistance of the laser claddings were strongly influenced by boron content. When boron content was increased from 0 at% to 5.40 at% the microhardness of the material was from 291 HV to 445 HV. Similarly, the boron content improved the behaviour against abrasive wear due to the increased volume fraction of borides in the microstructure. The high content of the boride phase in the laser cladding with 5.40 at% B allowed reducing the abrasive wear rate by more than 30% when compared with the alloy without boron content.

Microstructure and mechanical properties of stainless steel clad plate joints produced by TIG and MAG hybrid welding

This paper investigated the microstructure and mechanical properties of Q235/304 stainless steel clad plate welding seam produced by hybrid welding of tungsten inert gas welding (TIG) and metal active gas arc welding (MAG). The results showed that the dual phases containing ferrite and austenite appeared in the stainless steel covering welding (SSCW), while the partial martensite phases appeared in the carbon steel backing welding (CSBW), which is attributed to the dilution behavior of Ni and Cr elements from stainless steel to the Q235 steel results into the movement of the CCT curve to the right side and the decrease of critical martensite formation cooling rate. The CSBW possesses the highest microhardness value in the weld metal due to the existence of the martensite zone. The impact tests were carried out and the results showed that the Charpy absorbed energy of weldments (81 J) is almost equal to that of base clad plate (83 J). The SSCW layer possesses the ductile fracture characteristics accompanying many dimples. However, in the CSBW layer, some cleavage fracture characteristics are presented in the radiation zone while many dimples are located in the fibrous zone, revealing a complex combination of brittle and ductile fracture behavior, which is due to the existence of martensite zone, different stress states and crack propagation velocity.HighlightsHybrid (TIG, MAG) welding is suitable for welding stainless steel clad plate;The martensite formation in CSBW is related to dilution and diffusion of Cr and Ni;Partial martensite transformation can strengthen and toughen the welding seam.

The effect of boron content on the microstructure and mechanical properties of Fe50-XMn30Co10Cr10BX (x = 0, 0.3, 0.6 and 1.7 wt%) multi-component alloys prepared by arc-melting

High and Medium- Entropy Alloys (HEAs and MEAs) are a novel type of materials, which due to the reduction in configurational entropy, can offer an excellent combination of properties, such as, high strength, high ductility and moderate hardness. During the study, four alloys based on Fe50-XMn30Co10Cr10BX system (x = 0, 0.3, 0.6 and 1.7 wt%) were manufactured by arc melting. The microstructure of the materials was analyzed by optical microscopy, scanning electron microscopy, and X-ray diffraction. The Vickers microhardness of these alloys were also measured. Detailed characterization revealed the formation of a dual-phase matrix (fcc + hcp) due to the reduction in the stability of the fcc phase that led to a thermal induced partial martensitic transformation into a ε-hcp phase. The addition of boron promoted the formation of M2B-type borides (M = Cr, Fe), whose content increased with the addition of B. Changes in the phase composition, specifically the decrease of the hcp phase, were mainly due to the loss of manganese content. The microstrain analysis showed the absence of residual stresses in the crystal lattice due to a decrease in the dislocation’s density after the martensitic transformation. When boron content was brought from 0 wt% to 1.7 wt%, the microhardness of the material increased from 296HV to 452 HV, which is comparable to the microhardness of high-manganese steel. Finally, the thermodynamic parameter Φ was calculated using a phase prediction package AlloyASAP, allowing to establish that boron addition to Fe50Mn30Co10Cr10 originated medium-entropy alloys (Φ < 1), while the absence of boron give a high-entropy alloy (Φ > 1).

Meta-equilibrium transition microstructure for maximum austenite stability and minimum hardness in a Ti-stabilized supermartensitic stainless steel

The maximization of stable reverted austenite at room temperature through inter-critical tempering is a widely used method to reduce hardness in supermartensitic stainless steels. Nevertheless, partial martensitic transformation might occur due to insufficient compositional stabilization. In this work, we conducted a time-resolved triple-step inter-critical tempering, specially designed to obtain maximum austenite stability and minimum hardness through the progressive suppression of the martensitic transformation. The mechanism behind the progressive increase in stable reverted austenite was the generation of a meta-equilibrium state, which imposed a limit in both high temperature austenite reversion and room temperature austenite stabilization. Such limit corresponded to the high temperature volume fraction of austenite, obtained right before cooling from the first cycle. This effect was associated to the Ni-rich fresh martensite laths acting as local Ni compositional pockets, providing site-specific austenite reversion; and to the suppression of any additional nucleation at the Ni-poor matrix as the T0 temperature for austenite reversion was strongly increased. The softening mechanism was mainly controlled by the carbon arrest effect by the precipitation of Ti (C, N), which was completed after the first tempering cycle. Nevertheless, maximizing reverted austenite and suppressing fresh martensite at room temperature did not result in additional hardness reductions.

Structural changes in metastable austenitic steel during equal channel angular pressing and subsequent cyclic deformation

The paper reports a substantial improvement of the static and cyclic strength of a Cr-Ni-Ti austenitic stainless steel nanostructured by equal channel angular pressing (ECAP). After ECAP at room temperature or 673 K, the mean grain size decreased from 14 µm to 430 nm or 940 nm, respectively; corresponding ultimate tensile strength increased from 610 MPa to 1230 MPa or 940 MPa, and the fatigue limit increased from 275 MPa to 375 MPa or 475 MPa. These enhanced strength properties result from the grain refinement assisted by the intensive twinning in the austenite during ECAP at room temperature and 673 K as well as partial martensitic transformation during ECAP at room temperature. Moreover, the partial martensitic transformation and an increase in the fraction of high angle grain boundaries during subsequent high-cycle fatigue tests were particularly favorable for the improvement of fatigue properties.

Effects of upper cycle temperature on the actuation fatigue response of NiTiHf high temperature shape memory alloys

The present study focuses on the effects of upper cycle temperature (UCT) on the actuation fatigue response of nano-precipitation hardened Ni50.3Ti29.7Hf20 high temperature shape memory alloy (HTSMA). A series of actuation fatigue experiments were conducted under different constant tensile stresses while cycling temperature between two extreme martensitic transformation temperatures. The cyclic evolution of actuation strain and fatigue lives were monitored as a function of UCT and applied stress. The results revealed that the samples subjected to 300 °C UCT exhibit fatigue lives twice that of the samples with 350 °C UCT, and those tested under 300 MPa with 300 °C UCT withstand more than 10,000 cycles with actuation strains of 2–3%. Actuation strains remained constant or increased with thermal cycling in the 350 °C UCT experiments, while those subjected to 300 °C UCT exhibited decreasing actuation strains, at all stress levels. In the 300 °C UCT experiments, partial martensitic transformation becomes operative, resulting in a reduction of actuation strain in each cycle, and postpones damage accumulation. In the 350 °C UCT cases, increase of actuation strain is attributed to the partial recovery of cyclically-induced remnant deformation at higher temperatures and as a result, larger volume of transforming material. This, in turn, accelerates the formation of cracks, that open and close during thermal cycling and reversible martensitic transformation, and manifests itself as additional recoverable strain in each cycle. Overall, the current work constitutes the first study on actuation fatigue response of HTSMAs and an important step towards understanding microstructure - fatigue performance relationship in these alloys.

Effect of Severe Cold or Warm Deformation on Microstructure Evolution and Tensile Behavior of a 316L Stainless Steel

The deformation microstructures and mechanical properties of a 316L austenitic stainless steel subjected to large strain cold or warm plate rolling are studied. The cold or warm rolling was carried out at room temperature or 573 K, respectively, to different total true strains up to 3. The structural changes are characterized by the development of nanocrystalline structures with the transverse grain size of about 80 or 160 nm at room temperature or 573 K, respectively. The evolution of nanocrystalline structure under cold working conditions (room temperature) is assisted by the development of mechanical twinning and partial martensitic transformation that result in rapid grain refinement. The development of nanocrystalline structures during cold/warm working is accompanied by significant strengthening. The yield strength approaches 1 680 or 1 070 MPa after cold or warm rolling to a total strain of 3, respectively.

Effect of prior martensite on bainite transformation in nanobainite steel

Nanobainite transformation behavior was comparably studied using in situ neutron diffraction measurements, scanning electron microscopy and electron backscatter diffraction observations for two heat treatments: with and without partial quenching before isothermal holding at 523 or 573K. Prior martensite transformation was found to accelerate the subsequent nanobainite transformation. Bainitic lathes formed adjacent to a pre-existing martensite plate exhibited an almost identical orientation. Dislocations introduced in austenite due to stress relaxation of transformation strains are believed to assist bainite transformation accompanying variant selection. Diffraction profiles of austenite were found to show symmetric broadening with martensite transformation whereas nonsymmetric broadening occurred with nanobainite transformation, indicating the generation of two populations of austenite. Diffraction line broadening analysis using the convolutional multiple whole profile method provided a dislocation density of 1.51×1015m−2 in austenite after partial martensite transformation.

Microstructure evolution in a 316L stainless steel subjected to multidirectional forging and unidirectional bar rolling

The formation of ultrafine-grained structures was studied in a 316L stainless steel during severe plastic deformation. The steel samples were processed up to a total amount of strain of 4 at ambient temperature using two different methods, i.e., multidirectional forging and unidirectional bar rolling. The large strain developed upon cold working resulted mechanical twinning and partial martensitic transformation. The latter was readily developed during multidirectional forging. After straining to the total amount of strain of 4, the austenite fractions comprised approximately- 0.45 as well as 0.15 in the rolled and forged samples, respectively. Both the multidirectional forging and bar rolling led to extensive grain refinement. The uniform microstructures consisting of austenite and ferrite crystallites with the transverse size of 60 nm and 30 nm were evolved at a total amount of strain of 4 in the rolled and forged samples, respectively. The grain refinement by severe plastic deformation was accompanied by an increase for the microhardness to 5380 and 4970 MPa for the forged and rolled samples, respectively.