Resolving the Martensitic Transformation in Q&P Steels In-Situ at Dynamic Strain Rates Using Synchrotron X-ray Diffraction

The dynamic deformation response of two quenching and partitioning (Q&P) steels was investigated using a high strain rate tension pressure bar and in-situ synchrotron radiography and diffraction. This allowed for concurrent measurements of the martensitic transformation, the elastic strains/stresses on the martensite and ferrite, and the bulk mechanical behavior. The steel with the greater fraction of ferrite exhibited greater ductility and lower strength, suggesting that dislocation slip in ferrite enhanced the deformability. Meanwhile, the kinetics of the martensitic transformation appeared similar for both steels, although the steel with a greater ferrite fraction retained more austenite in the neck after fracture.

     

Thermal Stability in Nanocrystalline Fe-30?Wt?Pct Ni Alloy Induced by Surface Mechanical Attrition Treatment

By means of surface mechanical attrition treatment (SMAT), a nanocrystalline surface layer is produced in Fe-30 wt pct Ni alloy, accompanying the formation of the strain-induced martensite. The thermal stability of nanocrystalline martensite and parent phase austenite in Fe-30 wt pct Ni alloy is studied by X-ray diffraction (XRD) and transmission electron microscope (TEM). The grain growth kinetics parameters, time exponent, n, and activation energy, Q, for both martensite and austenite, are determined, respectively. The TEM observations indicate that abnormal grain growth occurs during annealing at high temperatures.     

Mechanical Behaviors of Ultrafine-Grained 301 Austenitic Stainless Steel Produced by Equal-Channel Angular Pressing

The technique of equal-channel angular pressing (ECAP) was used to refine the microstructure of an AISI 301 austenitic stainless steel (SS). An ultrafine-grained (UFG) microstructure consisting mainly of austenite and a few martensite was achieved in 301 steel after ECAP processing for four passes at 523 K (250 °C). By submitting the as-ECAP rods to annealing treatment in the temperature range from 853 K to 893 K (580 °C to 620 °C) for 60 minutes, fully austenitic microstructures with grain sizes of 210 to 310 nm were obtained. The uniaxial tensile tests indicated that UFG 301 austenitic SS had an excellent combination of high yield strength (>1.0 GPa) and high elongation-to-fracture (>30 pct). The tensile stress–strain curves exhibited distinct yielding peak followed by obvious Lüders deformation. Measurements showed that Lüders elongation increased with an increase in strength as well as a decrease in grain size. The microstructural changes in ultrafine austenite grains during tensile deformation were tracked by X-ray diffraction and transmission electron microscope. It was found that the strain-induced phase transformation from austenite to martensite took place soon after plastic deformation. The transformation rate with strain and the maximum strain-induced martensite were promoted significantly by ultrafine austenite grains. The enhanced martensitic transformation provided extra strain-hardening ability to sustain the propagation of Lüders bands and large uniform plastic deformation. During tensile deformation, the Lüders bands and martensitic transformation interacted with each other and made great contribution to the excellent mechanical properties in UFG austenitic SS.     

Effects of thermal cycling on the kinetics of the γ → ε martensitic transformation in an Fe-17 wt pct Mn alloy

The thermal cycling of an Fe-17 wt pct Mn alloy between 303 and 573 K was performed to investigate the effects of thermal cycling on the kinetics of the γ → ε martensitic transformation in detail and to explain the previous, contrasting results of the change in the amount of ε martensite at room temperature with thermal cycling. It was observed that the shape of the γ → ε martensitic transformation curve (volume fraction vs temperature) changed gradually from a C to an S curve with an increasing number of thermal cycles. The amount of ε martensite of an Fe-17 wt pct Mn alloy at room temperature increased with thermal cycling, in spite of the decrease in the martensitic start (M _s) temperature. This is due to the increase in transformation kinetics of ε martensite at numerous nucleation sites introduced in the austenite during thermal cycling.     

A multi-scale grained microstructure of the surface nanocrystallized 304 stainless steel sheets after warm-rolling

An ultrafine grained microstructure was obtained for 304 stainless steel(304SS)sheets by using surface nanocrystallization and warm-rolling.The microstructure and mechanical properties were determined by X-ray diffraction(XRD),transmission electron microscope(TEM)and a test on microhardness.Experimental results were shown that the microstructure was featured by a continuous distribution from the nanocrystalline on the surface to micro-grains in the center,in which the volume fraction of the micro-sized grains is about 40% in the surface layer.This multi-scale grained microstructure was composed of austenite and martensite phases with a gradient increasing volume fraction of austenite from the surface to the centre.The microhardness of the resultant steel was higher than 150% of that as received,due to the refined grains and strain-induced martensitic transformation.The hardness distribution was consistent with the microstructural variation,suggesting a good combination of high strength and improved ductility.     

Load Partitioning and Strain-Induced Martensite Formation during Tensile Loading of a Metastable Austenitic Stainless Steel

In-situ high-energy X-ray diffraction and material modeling are used to investigate the strain-rate dependence of the strain-induced martensitic transformation and the stress partitioning between austenite and α′ martensite in a metastable austenitic stainless steel during tensile loading. Moderate changes of the strain rate alter the strain-induced martensitic transformation, with a significantly lower α′ martensite fraction observed at fracture for a strain rate of 10^?2 s^?1, as compared to 10^?3 s^?1. This strain-rate sensitivity is attributed to the adiabatic heating of the samples and is found to be well predicted by the combination of an extended Olson–Cohen strain-induced martensite model and finite-element simulations for the evolving temperature distribution in the samples. In addition, the strain-rate sensitivity affects the deformation behavior of the steel. The α′ martensite transformation at high strains provides local strengthening and extends the time to neck formation. This reinforcement is witnessed by a load transfer from austenite to α′ martensite during loading.     

Effect of Cyclic Thermal Process on Ultrafine Grain Formation in AISI 304L Austenitic Stainless Steel

As-received hot-rolled commercial grade AISI 304L austenitic stainless steel plates were solution treated at 1060 °C to achieve chemical homogeneity. Microstructural characterization of the solution-treated material revealed polygonal grains of about 85-μm size along with annealing twins. The solution-treated plates were heavily cold rolled to about 90 pct of reduction in thickness. Cold-rolled specimens were then subjected to thermal cycles at various temperatures between 750 °C and 925 °C. X-ray diffraction showed about 24.2 pct of strain-induced martensite formation due to cold rolling of austenitic stainless steel. Strain-induced martensite formed during cold rolling reverted to austenite by the cyclic thermal process. The microstructural study by transmission electron microscope of the material after the cyclic thermal process showed formation of nanostructure or ultrafine grain austenite. The tensile testing of the ultrafine-grained austenitic stainless steel showed a yield strength 4 to 6 times higher in comparison to its coarse-grained counterpart. However, it demonstrated very poor ductility due to inadequate strain hardenability. The poor strain hardenability was correlated with the formation of strain-induced martensite in this steel grade.     

Determination of heat of transformation in a cold-rolled martensitic tini alloy

We studied the heat of transformation, ΔH, in martensitic transformations in a cold-rolled equiatomic TiNi alloy with differential scanning calorimetry (DSC), X-ray diffraction (XRD), and microhardness measurements. Results of our experiment indicate that the martensite stabilization and stress-induced parent (SIP)B2 phase are introduced when the TiNi martensite is cold rolled at room temperature. The SIP formation seems to be related to the lattice softening phenomenon occurring in the martensite, while the ΔH value of the first reverse martensitic transformation decreases enormously for the cold-rolled equiatomic TiNi alloy. We are proposing possible explanations for these results: (1) the occurrence of SIP, which reduces the transformable martensite volume; (2) the release of accumulated elastic energy induced by the cold rolling; and (3) the recovery of defects induced by cold rolling and release of the heat of recovery. We also found that the retained dislocations can depress the martensitic transformation temperatures and induce the R-phase transformation after the occurrence of the first reverse martensitic transformation. Formerly Graduate Student, Institute of Materials Science and Engineering, National Taiwan University     

Effect of strain rate on stress-strain behavior of alloy 309 and 304L austenitic stainless steel

The effect of strain rate on stress-strain behavior of austenitic stainless steel 309 and 304L was investigated. Tensile tests were conducted at room temperature at strain rates ranging from 1.25×10⁻⁴s⁻¹ to 400 s⁻¹. The evolution of volume fraction martensite that formed during plastic deformation was measured with X-ray diffraction and characterized with light microscopy. Alloy 304L was found to transform readily with strain, with martensite nucleating on slip bands and at slip band intersections. Alloy 309 did not exhibit strain-induced transformation. Variations in ductility and strength with strain rate are explained in terms of the competition between hardening, from the martensitic transformation and a positive strain rate sensitivity, and softening due to deformational heating. Existing models used to predict the increase in volume fraction martensite with strain were examined and modified to fit the experimental data of this study as well as recent data for alloys 304 and 301LN obtained from the literature.     

<Emphasis Type="Italic">In-Situ</Emphasis> Observations of Martensitic Transformation in Blast-Resistant Steel

A hybrid in-situ characterization system, which couples the laser scanning confocal microscopy (LSCM) with the time-resolved X-ray diffraction (TRXRD) measurement with synchrotron radiation, was used to characterize the microstructure evolution during heat-affected zone (HAZ) thermal cycling of high-strength and blast-resistant steel. The combined technique has a time resolution of 0.3 seconds that allows for high-fidelity measurements of transformation kinetics, lattice parameters, and morphological features. The measurements showed a significant reduction in the martensite start transformation temperature with a decrease in the prior austenite grain size. In addition, the LSCM images confirmed the concurrent refinement of martensite packet size with smaller austenite grain sizes. This is consistent with dilatometric observations. The austenite grain size also influenced the rate of transformation (df _m /dT); however, the measurements from the hybrid (surface) and dilatometric (volume) measurements were inconsistent. Challenges and future directions of adopting this technique for comprehensive tracking of microstructure evolution in steels are discussed.     

Aging effects in a Cu-12Al-5Ni-2Mn-1Ti shape memory alloy

The isothermal aging effects in an as-quenched Cu-11.88Al-5.06Ni-1.65Mn-0.96Ti (wt pct) shape memory alloy at temperatures in the range 250 °C to 400 °C were investigated. The changes in the state of atomic order and microstructural evolutions were traced by means of in situ X-ray diffraction and electrical resistivity measurements, as well as transmission electron microscopy (TEM) and optical observations. The kinetics of the aging process, i.e., the temperature and time dependence of the properties including hardness, resistivity, martensitic transformation temperatures, and shape memory capacity were characterized, and at least three temperature-dependent aging stages were distinguished: (1) D0₃ or L2₁ atomic reordering, which causes the martensitic transformation temperatures to shift upward and leads the M18R martensite to tend to be a N18R type structure; (2) formation of solute-depleted bainite which results in a drastic depression in martensitic transformation temperatures and loss of the shape memory capacity, accompanied by the atomic disordering in both the remaining parent phase and bainite; and (3) precipitation of the equilibrium α and γ ₂ phases and destruction of the shape memory capacity.     

Martensitic Transformation During Cold Rolling Deformation of an Austenitic Fe-26Mn-0.14C Alloy

A high-Mn austenitic steel was deformed in cold rolling to study the martensitic transformation and microstructure using X-ray diffraction and electron backscatter diffraction. Despite heavy deformation of 70 pct reduction (1.2 true strain), α′-martensite could not be induced in this alloy, but about 90 pct of the austenite transformed to ε-martensite. However, a small fraction (~4 pct) of α′-martensite could be observed when the same alloy was subjected to low strain compression tests in a Gleeble simulator. The stability of ε-martensite was attributed to the increase in stacking fault energy of the steel, expected to be more than 20 mJ/m² because of the increase in temperature during the cold rolling deformation.     

The effects of cold rolling on the martensitic transformation of an equiatomic TiNi alloy

The effects of cold rolling on the martensitic transformation of an equiatomic TiNi alloy have been studied by internal friction and shear modulus measurements, hardness test and TEM observation. The martensite stabilization can be induced by cold rolling at room temperature. A variety of deformed martensite structures has been observed. Both deformed martensite structures and deform-induced dislocations/vacancies are considered to be related to the martensite stabilization. The hardness test results also support this viewpoint. After the occurrence of the first reverse martensitic transformation of B19′ → B2, the martensite stabilization dies out and the transformation temperatures are depressed by retained dislocation on subsequent thermal cycles. Experimental results indicate that the martensite stabilization can depress the rate of martensitic transformation in the equiatomic TiNi alloy.     

Kinetics of strain-induced martensitic nucleation

Intersections of shear bands in metastable austenites have been shown to be effective sites for strain-induced martensitic nucleation. The shear bands may be in the form of ε’ (hcp) martensite, mechanical twins, or dense bundles of stacking faults. Assuming that shear-band intersection is the dominant mechanism of strain-induced nucleation, an expression for the volume fraction of martensite vs plastic strain is derived by considering the course of shear-band formation, the probability of shear-band intersections, and the probability of an intersection generating a martensitic embryo. The resulting transformation curve has a sigmoidal shape and, in general, approaches saturation below 100 pct. The saturation value and rate of approach to saturation are determined by two temperature-dependent parameters related to the fee-bee chemical driving force and austenite stacking-fault energy. Fitting the expression to available data on 304 stainless steels gives good agreement for the shape of individual transformation curves as well as the temperature dependence of the derived parameters. It is concluded that the temperature dependence of the transformation kinetics (an important problem in the development of TRIP steels) may be minimized by decreasing the fee, bec, and hep entropy differences through proper compositional control.     

Effects of Fine Particle Peening Conditions on the Rotational Bending Fatigue Strength of a Vacuum-Carburized Transformation-Induced Plasticity-Aided Martensitic Steel

The effects of fine particle peening conditions on the rotational bending fatigue strength of a vacuum-carburized transformation-induced plasticity-aided martensitic steel with a chemical composition of 0.20 pct C, 1.49 pct Si, 1.50 pct Mn, 0.99 pct Cr, 0.02 pct Mo, and 0.05 pct Nb were investigated for the fabrication of automotive drivetrain parts. The maximum fatigue limit, resulting from high hardness and compressive residual stress in the surface-hardened layer caused by the severe plastic deformation and the strain-induced martensite transformation of the retained austenite during fine particle peening, was obtained by fine particle peening at an arc height of 0.21 mm (N). The high fatigue limit was also a result of the increased martensite fraction and the active plastic relaxation via the strain-induced martensite transformation during fatigue deformation, as well as preferential crack initiation on the surface or at the subsurface.     

Young’s modulus and mechanical properties of Ti-29Nb-13Ta-4.6Zr in relation to α″ martensite

An investigation on the formation of α″ martensite and its influence on Young’s modulus and mechanical properties of forged Ti-29Nb-13Ta-4.6Zr (wt pct) alloy is reported in this article. For ice-water-quenched specimens after solution treatment at 1023, 1123, and 1223 K in the single β-phase field for 1.8, 3.6, 14.4, and 28.8 ks, X-ray diffraction and internal friction measurements showed that the volume fraction of the α″ martensite changes with both solution temperature and time. This effect has been attributed mainly to the influence of grain size of the β-parent phase on the stability of the β phase and, consequently, on the martensitic start (M _s) temperature. A critical grain size of 40 μm was identified for the β phase, below which the martensitic transformation is largely suppressed because of low M _S temperature. With the β grain size increasing above this critical value, the volume fraction of the α″ martensite increases significantly at first and then decreases gradually with further grain growth. The α″ martensite was shown to possess good ductility and, compared to the β phase, lower strength and hardness but nearly identical Young’s modulus in the studied alloy.     

In-Situ High-Energy X-Ray Diffuse-Scattering Study of the Phase Transition of Ni2MnGa Single Crystal under High Magnetic Field

The defects-related microstructural features connected to the premartensitic and martensitic transition of a Ni₂MnGa single crystal under a high magnetic field of 50 KOe applied along the [ 1[`1]0 ] \left[ {1\bar{1}0} \right] crystallographic direction of the Heusler phase were studied by the in-situ high-energy X-ray diffuse-scattering experiments on the high energy synchrotron beam line 11-ID-C of APS and thermomagnetization measurements. Our experiments show that a magnetic field of 50 KOe applied along the [ 1[`1]0 ] \left[ {1\bar{1}0} \right] direction of the parent Heusler phase can promote the premartensitic transition of Ni₂MnGa single crystal, but puts off martensite transition and the reverse transition. The premartensitic transition temperature (T _PM ) increases from 233 to 250 K (−40 to −23 °C). The martensite transition start temperature (M _s ) decreases from 175 to 172 K (−98 to −101 °C), while the reverse transition start temperature (A _s ) increases from 186 to 189 K (−87 to −84 °C). The high magnetic field leads to a rapid rearrangement of martensite variants below the martensite transition finish temperature (M _f ). The real transition process of Ni₂MnGa single crystal under the high magnetic field was in-situ traced.     

In-Situ Synchrotron Diffraction Studies on Transformation Strain Development in a High-Strength Quenched and Tempered Structural Steel—Part II. Martensitic Transformation

In-situ synchrotron diffraction studies on the kinetics of phase transformation and transformation strain development during bainitic transformation were presented in part I of the current article. In the current article, in-situ phase transformation behavior of a high-strength (830 MPa yield stress) quenched and tempered S690QL1 [Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt. pct)] structural steel, during continuous cooling and under different mechanical loading conditions to promote martensitic transformation, has been studied. Time–temperature–load resolved 2D synchrotron diffraction patterns were recorded and used to calculate the phase fractions and lattice parameters of the phases during heating and cooling cycles under different loading conditions. In addition to the thermal expansion behavior, the effects of the applied stress on the elastic strains during the martensitic transformation were calculated. The results show that small tensile stresses applied at the transformation temperature do not change the kinetics of the phase transformation. The start temperature for the martensitic transformation increases with the increasing applied tensile stress. The elastic strains are not affected significantly with the increasing tensile stress. The variant selection during martensitic transformation under small applied loads (in the elastic region) is weak.     

Strain-Induced Martensitic Transformation Kinetic in Austempered Ductile Iron (ADI)

A model for the strain-induced martensitic transformation in austempered ductile iron (ADI) has been developed based on neutron diffraction studies. Quantitative phase analysis was carried out using the Rietveld method including texture analysis. The key parameters applied in this model that influence the strain-induced martensitic transformation are temperature, strain state, and loading type. An empirical relation was derived for the martensite start temperature M _s in austempered ductile iron, which takes into account the Ni and carbon content. The M _s temperature was used as a scaling parameter for the stability of austenite in the model to describe the strain-induced phase transformation in austempered ductile iron.