The effect of deformation on γ-ε martensitic transformation in an Fe-Mn-Mo alloy

Effect of prior plastic deformation of austenite on the martensite start temperature, volume fraction and strength of martensite have been studied in an Fe-14.3%Mn-3.7%Mo alloy. Mo was chosen to examine the possible effect of the third alloying element in an Fe-Mn based alloy and the obtained results were compared with those of the Fe-Mn binary alloys given in the literature. Predeformation of austenite created considerable changes on the formation characteristics and also the strength of the martensitic phase and the obtained results were discussed in terms of the dislocations formed during the deformation process.     

The morphology of martensite in Fe-C,Fe-Ni-C and Fe-Cr-C alloys

The morphology of martensite in widely varying series of Fe-C, Fe-Ni-C and Fe-Cr-C alloys was investigated using optical microscopy. The effects of formation temperature and alloying elements on the martensite morphology were studied in detail. It was found that in Fe-C alloys, lath martensite forms in alloys with less than 0.8wt% carbon, butterfly martensite forms in alloys with between 0.98 and 1.42wt% carbon and lenticular martensite forms in alloys with more than 1.56wt% carbon. In Fe-Ni-C alloys, four different martensite morphologies form depending upon the formation temperature and composition, and for alloys of a fixed carbon content the martensite morphology changes from lath to butterfly to lenticular to thin plate as the formation temperature is decreased. In Fe-Cr-C alloys, lath martensite forms at high temperature, and below the lath formation temperature mainly {2 2 5}_f plate martensite is formed. Based on the results obtained, the importance of the strength of austenite, and the austenite stacking fault energy to the martensite morphology was discussed.     

Fractal analysis of martensite in an Fe-Ni alloy

In general, microstructures can be described by microstructural parameters such as grain size or particle spacing, e.g. an average value can be obtained by quantitative metallography. In some cases these methods of classical quantitative metallography fail. For example, martensitic microstructures may appear highly disordered at the transition to microstructural chaos. They cannot be described by any average crystal size or particle spacing. A new approach has been applied to characterize a martensitic microstructure of an Fe-Ni alloy by fractal analysis. This includes the amount of martensite as well as the size spectra of martensite crystals and the distribution of residual austenite. In addition, the interface length martensite/austenite and the interface density were determined.     

Improvement of NiTi Shape Memory Actuator Performance Through Ultra‐Fine Grained and Nanocrystalline Microstructures

Ultra‐fine grain sizes have been shown to enhance some key mechanical and functional properties of engineering materials, including shape memory alloys. While the effect of ultra‐fine and nanocrystalline grain sizes on pseudoelastic shape memory materials is well‐appreciated in medical device engineering, the effect of such microstructures on actuators has not been sufficiently characterized. In the present work, it is demonstrated that NiTi spring actuators with ultra‐fine grained microstructures can be obtained by conventional wire drawing in combination with heat treatments and that the final grain size can be controlled by varying the final annealing temperature. Annealing at 400 °C for 600 s allows for the evolution of microstructures with median grain sizes of about 34 nm, while annealing at 600 °C for the same length of time results in median grain sizes of about 5 µm. It is observed that the grain size strongly affects the elementary processes of the martensitic phase transformation. Small austenite grain sizes inhibit twinning accommodation of transformation strains, such that a higher driving force is required to nucleate martensite. This increase in the martensite nucleation barrier decreases the martensite transformation temperatures such that only partial transformation to martensite is possible upon cooling to room temperature. The incomplete martensitic transformation reduces the exploitable actuator stroke; however, a reduction in grain size is shown to improve the functional stability of the material during thermal and thermomechanical cycling by reducing the irreversible effects of dislocation plasticity.     

Structural variations in heat treated low alloy steel forgings

A study has been made of the microstructures of two low alloy Cr–Mo–Ni–V steel forgings in the quenched and tempered condition. Optical metallography of variously etched specimens, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) and X-ray diffraction were used to characterize the tempered martensite microstructures. Variations in prior austenite grain size and martensite lath structure have been related to the observed different carbide distributions. These arise from small differences in composition, mainly vanadium content, and the consequent response to heat treatment. The metallographic results can account for the differences in tensile and fracture toughness properties observed between the two materials as well as between the two ends of the same forging.     

Effect of Compressive Deformation on Lenticular and Thin Plate Martensitic Transformations

The effect of compressive deformation testedabove the M_s temperature on the martensitemorphology in Fe-Ni-C alloys has been studied.Inthe Fe-30Ni-0.12C alloy,the M_s temperature is-50℃ The cylindrical specimens werecompressively deformed at -40℃.The strain rateswere 10,20,30 and 40%.X-ray analysis andmetallographic examination showed that nostrain-induced martensite was found.After quench-ing to -53℃,some thin plates and unusualmorphologies of lenticular martensites with bentand/or broken mid-ribs were observed.In theFe-30Ni-0.34C alloy,the M_s temperature is-120℃.Compressive deformation with differentstrain rates were carried out at room temperature.After quenching to the liquid nitrogen temperature,some bent thin plate matensites(unbroken)occur-red.The transformed twins in bent plate were alsobent and nearly parallel to the γ-α'interfaces.Orientation relationship between austenite and bentmartensite has been examined by means of trans-mission electron microscope.It was proved thatthese unusual morphologies are inherent in thecompressive pre-deformed austenite.     

Effects of high austenitizing temperature and austenite deformation on formation of martensite in Fe-Ni-C alloys

The effects of high austenitizing temperature and the deformation of austenite matrix below the range of strain-induced martensite formation on the morphology, substructure and crystallography of martensite formed in different Fe-Ni-C alloys have been studied by means of transmission electron microscopy. The formation behaviours of both thermal and stress-assisted martensites were examined under various physical conditions and martensite morphology was found to be closely dependent on the high austenitizing temperatures besides the influence of austenite deformation. Although the orientation relationship between austenite and thermally induced martensite was found as the Kurdjumov-Sachs type, it was also observed to change to Nishiyama-Wasserman type in the samples transformed under the stress-assisted conditions.     

A Comparative Study on Crystal Structure and Magnetic Properties of Fe-Mn-Si and Fe-Mn-Si-Cr Alloys

In this study, the crystal structure and magnetic properties of Fe-30Mn-6Si (wt.%) and Fe-30Mn-6Si-5Cr (wt.%) alloys were compared by using X-Ray Diffraction (XRD) and Physical Properties Measurement System (PPMS) measurements. Detailed analysis of diffractograms at room temperature demonstrates that the Cr-free sample contains austenite and martensite phases, but for Cr-added sample the martensite phase disappears. According to micro hardness measurements, the presence of chromium decreased the hardness of the alloy. The magnetic saturation values at room temperature were measured as 11.32 emu/g for Fe-30Mn-6Si (wt.%) alloy and 18.34 emu/g for Fe-30Mn-6Si-5Cr (wt.%) alloy. The addition of Cr increased the magnetic saturation value of FeMnSi alloy while for both systems the hysteresis loop was quite narrow. As a result, both alloys exhibited soft magnetic characteristic.     

Effect of aluminum content on solidification microstructure and properties of Fe‐Cr‐B‐Al alloys

The microstructures and mechanical properties of eight kinds of Fe‐Cr‐B‐Al alloys containing X wt.%Al‐0.35 wt.%C‐10.0 wt.%Cr‐1.4 wt.%B‐0.6 wt.%Si‐0.8 wt.%Mn (X = 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0) were studied by means of optical microscopy (OM), scanning electron microscopy (SEM), X‐ray diffraction (XRD), Rockwell hardness and Vickers micro‐hardness testers. The results indicate that the as‐cast microstructure of aluminium‐free sample consists of the martensite, austenite and eutectic borocarbides, and the eutectic borocarbides are the mixture of (Fe, Cr)₂B and (Cr, Fe)₇(C, B)₃, and its hardness reaches 65 HRC. When a small amount of aluminium element (Al ? 1.0 wt.%) is added, the phase composition has no significant change, and the hardness excels 65 HRC. When the concentration of aluminium reaches 1.5 wt.%, the matrix of Fe‐Cr‐B‐Al alloy becomes pearlite and δ‐ferrite, leading to a sharply decrease of the hardness. The proportion of ferrite goes up along with increasing aluminium concentration, and the hardness of Fe‐Cr‐B‐Al alloy has slight decrease.     

In vitro and in vivo degradability,biocompatibility and antimicrobial characteristics of Cu added iron-manganese alloy

In recent years,iron(Fe)based degradable metal is explored as an alternative to permanent fracture fixation devices.In the present work,copper(Cu)is added in Fe-Mn system to enhance the degradation rate and antimicrobial properties.Fe-Mn-xCu(x=0.9,5 and 10 wt.％)alloys are prepared by the melting-casting-forging route.XRD analysis confirms austenite phase stabilization due to the presence of Mn and Cu.As predicted by Thermo-Calc calculations,Cu rich phase precipitations are noticed along the austen-ite grain boundaries.Degradation behaviours of Cu added Fe-Mn alloys are investigated through static immersion and electrochemical polarization where enhanced degradation is found for higher Cu added alloys.When challenged against E.Coli bacteria,the Fe-Mn-Cu alloy media extract shows a significant bac-tericidal effect compare to the base alloy.In vitro cytocompatibility studies,as determined using MG63 and MC3T3-E1 cell lines,indicate increased cell density as a function of time for all the alloys.When implanted in rabbit femur,the newly developed alloy does not show any kind of tissue necrosis around the implants.Better osteogenesis and higher new bone formation are observed with Fe-Mn-10Cu alloy as evident from micro-computed tomography(μ-CT)and fluorochrome labelling.     

Characterization, Processing, and Alloy Design of NiAl-Based Shape Memory Alloys

The microstructures and phase transformations in binary Ni-al, ternary Ni-Al-Fe, and quaternary Ni-Al-Fe-Mn shape memory alloys (SMAs) were investigated by light and electron microscopy, electron and X-ray diffraction, and differential scanning calorimetry. The effects of alloying additions (B, Fe, and Mn) on martensite stability, shape recovery, and tensile ductility were also studied. NiAl-based SMAs can be made ductile by alloying with B for enhanced grain boundary cohesion and Fe for improved bulk properties. Iron has the undesirable effect that it decreases the martensite → austenite transformation temperatures (A_p). Fortunately, A_p can be increased by decreasing the “equivalent” Al content of the alloy. In this way, a high A_p temperature of 190°C has been obtained without sacrificing ductility. Recoverable strains of 0.7% have been obtained in a Ni-Al-Fe alloy with A_p temperature of 140°C. Manganese additions (2–10%) lower A_p, degrade hot workability, and decrease room temperature ductility. Good-quality, ductile SMA ribbons have been produced by melt spinning. However, additional alloy design is required to suppress the aging-induced embrittlement caused by Ni₅Al₃ formation.     

Thermal decomposition study of electrodeposited Fe-C and Fe-Ni-C alloys by differential scanning calorimetry

Fe-0.96mass%C and Fe-15.4mass%Ni-0.70mass%C alloys with hardness of 810 and 750 HV respectively have been electrodeposited at 50°C from sulphate based baths containing a small amount of citric acid and L-ascorbic acid. Differential scanning calorimetry of the electrodeposited samples has been carried out in the temperature range of 293–725 K in argon atmosphere. Electrodeposited pure Fe is also investigated for comparison purposes. The DSC curves of both alloys contain two exothermic peaks: at about 411 K and 646 K for the Fe-C alloy, and 388 K and 639 K for the Fe-Ni-C alloy. These peaks are irreversible and do not appear during a second thermal cycling. The lower temperature peaks (designated as I) have been attributed mainly to the formation of /-Fe₂C (first stage of tempering), while the higher temperature peaks (designated as III) are ascribed predominantly to -Fe₃C formation (third stage of tempering). The presence of these peaks in the DSC curves confirms that electrodeposited Fe-C and Fe-Ni-C alloys are in a metastable state, where carbon atoms are entrapped in the iron lattice. The decomposition sequence of electrodeposited Fe-C and Fe-Ni-C alloys is found to follow the same general pattern as that of thermally prepared martensite. Attempt has been made to estimate the activation energy values for the reactions associated with the DSC peaks of the electrodeposited alloys and these values are compared with the available data on thermally prepared martensite.     

Relations between the Lattice Parameter and the Stability of Austenite against ε Martensite for the Fe-Mn Based Alloys

The influences of lattice parameter of austenite, the electron concentration, the yield strength of parent phase on γ→ε emartensite start temperature Ms in the Fe-Mn alloys containing C, Al, Ge and Si have been experimentally investigated. The results show that the lattice parameter of austenite is more important than the electron concentration and the yield strength of parent phase in governing the γ→ε martensitic transformation in Fe-Mn based alloys. A relation between the Ms and lattice parameter of austenite in Fe-Mn based alloys is suggested. The elements Mn, C, Al, Ge, which increase the lattice parameter of austenite lower the Ms; while the element Si, which decreases the lattice parameter increases the Ms. The depressing effect of antiferromagnetic transition on the γ→ε martensitic transformation may be related to the increase of lattice parameter due to the positive magnetostriction during the antiferromagnetic transition.     

Microstructures and mechanical properties of dual phase steel produced by laboratory simulated strip casting

Conventional dual phase (DP) steel (0.08C–0.81Si–1.47Mn–0.03Al wt.%) was manufactured using simulated strip casting schedule in laboratory. The average grain size of prior austenite was 117 ± 44 μm. The continuous cooling transformation diagram was obtained. The microstructures having polygonal ferrite in the range of 40–90%, martensite with small amount of bainite and Widmanstätten ferrite were observed, leading to an ultimate tensile strength in the range of 461–623 MPa and a corresponding total elongation in the range of 0.31–0.10. All samples exhibited three strain hardening stages. The predominant fracture mode of the studied steel was ductile, with the presence of some isolated cleavage facets, the number of which increased with an increase in martensite fraction. Compared to those of hot rolled DP steels, yield strength and ultimate tensile strength are lower due to large ferrite grain size, coarse martensite area and Widmanstätten ferrite.     

Role of strain-induced martensite on microstructural evolution during annealing of metastable austenitic stainless steel

Metastable austenitic stainless steel of type AISI 304L was cold rolled to 90% with and without inter-pass cooling. Inter-pass cooling produced 89% of strain-induced martensite whereas no inter-pass cooling resulted in the formation of 43% of martensite in the austenite matrix. The cold-rolled specimens were annealed at various temperatures in the range of 750–1000 °C. The microstructures of the cold-rolled and annealed specimens were studied by the electron microscope. The grain size and low angle boundaries were determined from the orientation maps recorded by the scanning electron microscope-based electron backscattered diffraction technique. The observed microstructural changes were correlated with the reversion mechanism of martensite to austenite and volume fraction of martensite. It was noted that large volume fractions of martensite at low annealing temperatures, below 900 °C, were most suitable for the formation of fine grains. On the contrary, reversion of small volume fractions of martensite at critical annealing temperature of 950 °C resulted in grain refinement.     

Microstructure and mechanical property of sintered Fe-Cr-Mo steels due to phase transformations with fast cooling rates

The influence of carbon content in the range of 0.01–0.3 wt.% on microstructure, hardness and tensile property of sintered Fe-Cr-Mo steels was investigated. The sintered Fe–3.0 wt.%Cr–0.5 wt.%Mo–(0.1, 0.2, 0.3) wt.% C steels were prepared by using powder metallurgical process. After sintering, the specimens were rapidly cooled by nitrogen at the rate of 5.4 °C/s. It was found that in the sintered steels with a lower carbon content of 0.01 and 0.1 wt.%, the allotriomorphic ferrite and Widmanstӓtten ferrite formed at austenite grain boundaries and grew to occupy the whole prior austenite grains. With higher carbon contents of 0.2 and 0.3 wt.%, the microstructures consist of bainite, martensite and some retained austenite. These steels exhibited increases of hardness, tensile strength and elongation at break with increasing carbon content. Increase of strength is due to the transformations from austenite, formed during sintering, to hard bainite and martensite structures.     

Size distribution of martensite plates in an Fe-Ni-Mn alloy

In this paper, the size distribution of the martensite plates in an Fe-23.2 Ni-2.81 Mn (wt%) alloy, which transforms isothermally at subzero temperatures, is reported. The distribution of the martensite plates has been determined as a function of the reaction temperature, volume fraction of martensite, the austenitic grain size, a superimposed elastic stress and prior plastic strain (at room temperature) of austenite. Increasing the driving force either by decreasing the reaction temperature or by a superimposed elastic stress changes the size distribution by enhancing the extent of radial growth of the martensite plates. Pre-straining of austenite does not allow the martensite plates to grow to the full extent. The present results show that the radial growth of the martensite plates increases with increasing driving force and decreases due to work-hardening of austenite. The transformation is found to progress through a combination of the spreading-out of clusters and filling-in of pockets, both occurring simultaneously. However, the extent of filling-in, i.e. compartmentalization of austenite grains, is more in the coarse-grained (0.09 mm) and medium-grained (0.048 mm) specimens compared to that in the fine-grained (0.019 mm) specimens.     

Mössbauer study of martensitic transformations in an Fe-29.6% Ni alloy

The application of an external stress may form band shaped strain-induced martensites in the austenite structure of Fe alloys. Mössbauer spectroscopy and transmission electron microscopy techniques were used to clarify certain properties of strain-induced martensite in an Fe-29.6% Ni alloy. The reverse transformation mechanism between thermal plate martensite and the matrix austenite was also studied. Mössbauer spectroscopy made it possible to examine the same area of the austenitic thin foils during the thermal cycles, and the volume fraction changes were determined. The habit plane and orientation relationship of strain-induced martensite were measured from the electron diffraction patterns and the latter parameter was found to be K-S type as with thermal plate martensites of the Fe-Ni alloys. The isomery shifts caused by the deformation and cycling procedures were also calculated for both austenite and martensite structures and the hyperfine magnetic field parameter of Fe-29.6% Ni strain-induced martensite was found to be equal to that of Fe-Ni-C alloys reported earlier.     

Structure,phase composition,and microhardness of carbon steels after high-pressure torsion

The phase composition, mechanical properties, and microstructure of binary Fe–C alloys with various carbon concentrations (0.25, 0.45, 0.6, 1.3, 1.5, and 1.7 wt.%) were studied by transmission electron microscopy, X-ray diffraction analysis, and microhardness measurements. The investigations were carried out for three states of the material, namely for as-cast, annealed (725 °C) and deformed by high-pressure torsion (HPT) samples. The grain size after HPT is in the nanometer range. Only Fe₃C (cementite) and α-Fe remain in the alloys after HPT. The residual austenite disappears and phase composition closely approaches the equilibrium corresponding to the temperature and pressure of HPT. Analysis of the microhardness behavior revealed that hardening of the deformed alloys takes place due to the grain refinement and dispersoid mechanism.