Hcp Martensite Research Articles

A thermo-mechanical correlation with driving forces for hcp martensite and twin formations in the Fe–Mn–C system exhibiting multicomposition sets

The thermodynamic properties of the Fe–Mn–C system were investigated by using an analytical model constructed by a CALPHAD approach. The stacking fault energy (SFE) of the fcc structure with respect to the hcp phase was always constant at T0, independent of the composition and temperature when other related parameters were assumed to be constant. Experimental limits for the thermal hcp formation and the mechanical (deformation-induced) hcp formation were separated by the SFE at T0. The driving force for the fcc to hcp transition, defined as a dimensionless value –dGm/(RT), was determined in the presence of Fe-rich and Mn-rich composition sets in each phase. Carbon tended to partition to the Mn-rich phase rather than to the Fe-rich phase for the compositions studied. The results obtained revealed a thermo-mechanical correlation with empirical yield strength, maximum true stress and maximum true strain. The proportionality between thermodynamics and mechanical properties is discussed.

Strain-controlled fatigue properties of dissimilar welded joints between Ti–6Al–4V and Ti17 alloys

The aim of this study was to evaluate the microstructure, hardness and cyclic deformation behavior of electron beam welded dissimilar joints of Ti–6Al–4V and Ti17 (Ti–5Al–4Mo–4Cr–2Sn–2Zr) titanium alloys. The welding resulted in a significant microstructural change across the joint, with hexagonal close-packed (hcp) martensite α′ and orthorhombic martensite α″ in the fusion zone (FZ), α′ in the heat-affected zone (HAZ) of Ti–6Al–4V side, and coarse β in the HAZ of Ti17 side. A characteristic asymmetrical hardness profile across the dissimilar joint was observed with the highest hardness in the FZ and a lower hardness on the Ti–6Al–4V side than on the Ti17 side, where a soft zone was observed. The dissimilar joint exhibited a lower Young′s modulus and higher cyclic strain hardening exponent than both Ti–6Al–4V and Ti17 base metals (BMs), and had the monotonic and cyclic yield strengths lying in-between those of two BMs with higher values for Ti17 alloy. Both BMs and joint showed essentially symmetrical hysteresis loops and equivalent fatigue life, and exhibited cyclic stabilization at lower strain amplitudes up to 0.6%, while cyclic softening occurred after initial cyclic stabilization at higher strain amplitudes. The initial cyclic stabilization was shortened with increasing strain amplitude. In the Ti–6Al–4V BM fatigued at a high strain amplitude of 1.2%, a short initial cyclic hardening emerged, corresponding to the presence of twinning and its resistance to the dislocation movement. Fatigue failure of the dissimilar joint occurred in the HAZ of Ti17 side where the soft zone was present, with crack initiation from the specimen surface or near-surface defect and crack propagation characterized by typical fatigue striations.

Phase transformation temperatures of Zr alloy Excel

The transformation temperatures of Zr alloy Excel (Zr–3.5%Sn–0.8%Nb–0.8%Mo) pressure tube material were studied using electron microscopy and a quantitative metallography technique. The αZr+βZr→βZr transus temperature was found to be 968°C. Also the volume fraction of stabilized βZr starts to increase in the temperature range of 600–690°C. Furthermore, it was observed that βZr does not transform to α′-hcp martensite upon quenching from up to a temperature between 846°C and 864°C in the αZr+βZr region. Instead, the microstructure of βZr transforms into a mixture of retained βZr with a fine dispersion of athermal ω particles and few α″ orthorhombic martensite laths inside the βZr grains. βZr was first seen to fully transform to α’-hcp martensite after quenching from 864°C; a drastic increase in the volume fraction of βZr occurs above this temperature.

Comparative topographic study of surface micro‐relief of primary martensite plates in shape memory alloys with different crystalline structures

AbstractThe surface micro‐reliefs of primary martensite plates, representative for two shape memory alloys (SMAs) with different crystalline structures were compared from qualitative and quantitative point of view by scanning electron microscopy and atomic force microscopy, respectively. Qualitative evaluations revealed larger widths and heights of the primary plates of ε hexagonal close packed (hcp) martensite, in an Fe‐Mn‐Si‐Cr‐Ni SMA than those of β2′ orthorhombic (9R) martensite, in a Cu‐Zn‐Al SMA. Quantitative evaluations were based on systematic dimensional measurements of the width and height of primary plate profiles. The measurements were performed on one hundred and twenty five profiles, five on each martensite plate belonging to five typical groups of primary plates, with length above 50 micrometers, of both ε hcp and β2′ 9R martensites. In order to compare the topographies of the two types of plates a statistical evaluation of the dimensional intervals of width and height of measured plates was performed.

Crystallographic Behaviors of Uni-Axial Deformed High Manganese Steels

High manganese TRIP/TWIP steels with different Mn contents were subjected to uniaxial deformation. The textures, misorientations and orientation relationships were determined in austenite, HCP/BCC martensites and ferrite. It is observed that the mechanically stabilized austenite possesses mainly stable deformation texture, the intermediate HCP martensite possesses mainly unstable tilted basal texture and the BCC martensite possesses stable deformation texture which was actually mixed with transformation texture. Special misorientations due to either the inherence from austenite or due to twins or variant selection were main components in each phase. K-S relationship became much scattered due to slip-induced misorientations both in martensite and in parent austenite.

FCC to HCP transformation kinetics in a Co–27Cr–5Mo–0.23C alloy

In this work the kinetic aspects associated with the FCC → HCP martensitic transformation in a Co–27Cr–5Mo–0.23C alloy processed by powder metallurgy were investigated. In situ X-ray diffraction during isochronous heat treatments in a hot stage indicated that a fully metastable FCC matrix transforms rather fast at temperatures above 725 °C and reaches a maximum transformation into the HCP phase at 940 °C. Alternatively, when the matrix is HCP, some HCP martensite reverts to metastable FCC. Apparently, at low temperatures carbon excess in the HCP martensite promotes the reversal to metastable FCC. In addition, the volume percent of e-martensite precipitated from stable FCC was determined as a function of time and temperature during isothermal aging between 675 and 900 °C. From these results, TTT diagrams were plotted for a 1% HCP transformed martensite. Maximum transformation rates were found to occur between 825 and 850 °C and activation energies, Q s of 41–52 kcal/mol were estimated from the experimental outcome. The aged microstructures indicated that below 800 °C, the isothermal transformation was dominated by a lamellar morphology. Nevertheless, aging above 800 °C promoted carbide nucleation and coarsening along the grain boundaries independently of the FCC → HCP martensitic transformation.

Phase Constitution and Heat Treatment Behavior of Ti-7mass% Mn-Al Alloys

Titanium exhibits many attractive properties. It is considered to be ubiquitous since it has the 9th-highest Clarke number of all the elements. However, the principal beta-stabilizing elements for titanium can be very expensive, making many titanium alloys expensive. Manganese is a beta stabilizer for titanium alloys and it is also considered to be ubiquitous since it has the 11th-highest Clarke number of all the elements. The behavior of Ti-Mn alloys during heat treatment has been investigated and it was found that in some alloys the isothermal omega phase is precipitated. Because this phase can lead to brittleness, it is very important to suppress its precipitation. Since it is well-known that aluminum suppresses isothermal omega precipitation, we investigated the effect of adding aluminum using Ti-7mass% Mn-0, 1.5, 3.0 and 4.5mass% Al alloys by performing electrical resistivity, Vickers hardness, and X-ray diffraction measurements. In solution-treated and water-quenched 0 and 1.5 alloys, only beta phase was identified, while hcp martensite and bate phase were identified in 3.0 and 4.5Al alloys. The resistivities at room and liquid-nitrogen temperatures were found to increase monotonically with increasing Al content. Isothermal  precipitation was suppressed by aluminum addition, while alpha precipitation was accelerated by Al addition.

Complex martensitic nanostructure in Zr nanowires: A molecular dynamics study

We use molecular dynamics to characterize the martensitic nanostructure that develops when bcc Zr nanowires are cooled down and transform to hcp (martensite). We find that size has a strong effect on nanostructure and even very small wires (a few nanometers in diameter) exhibit complex, multidomain structures with large internal strains (up to $\ensuremath{\sim}6\mathrm{%}$). Long and thin wires result in domains coexisting along their axes while those with small aspect ratios exhibit coexistence of domains within their cross-section. We also that find regions of fcc Zr that develop to bridge neighboring hcp domains.

Thermomechanical behavior of Fe–Mn–Si–Cr–Ni shape memory alloys modified with samarium

The deformation and training behavior of Fe–14Mn–3Si–10Cr–5Ni (wt.%) shape memory alloys containing samarium addition has been studied in the iron-based shape memory alloys. It is noticed that thermomechanical treatment (training) has significant influence on proof stress, critical stress and shape memory behavior of the alloys. The improvement in shape memory behavior can be attributed to the decrease in the proof stress and critical stress which facilitates the formation of ɛ (hcp martensite). It is also observed that alloy 2 containing samarium undergoes less softening as compared to alloy 1 with training which inhibits the formation of ά (bcc martensite) and thus enhances the shape memory behavior. The excessive thermomechanical treatment with increase in the training cycle has led to the formation of ά (bcc martensite) along with ɛ (hcp martensite) in the alloy 1 which appeared to have decline in the shape memory effect. This has been demonstrated by the examination of microstructure and identification of ά (bcc martensite) martensite in the alloy 1 as compared to alloy 2.

AFM Observation of Microstructural Changes in Fe-Mn-Si-Al Shape Memory Alloy

We analyzed the surface relief caused by stress-induced hcp martensitic transformation in Fe-30Mn-5Si-lAl shape memory alloy by atomic force microscopy. The alloy exhibits a good shape memory effect and an improved ductility due to a small addition of Al to a conventional Fe-30Mn-6Si shape memory alloy. The orientation of an austenite matrix was determined with surface traces of four {111} f planes, which enabled us to determine the surface tilt angles for all twelve variants of hcp martensites and deformation twins. On the basis of these values, stress-induced martensite and deformation twin coexisting in the same grain were identified by studying the surface tilt angles. The surface relieves caused by the stress-induced martensite recovered after heating above the reversed transformation temperature, however some relieves originating from the deformation twin remained.

Ｔｉ‐Ｃｒ系合金の相構成と等温時効挙動

By virtue of their high strength and excellent cold workability, β titanium alloys have been used for various applications, e.g. parts of airplanes and eyeglass frames. Generally, β titanium alloys contain vanadium and molybdenum as alloying elements. However, the official quotations of vanadium and molybdenum are unstable. Therefore, use of those alloying elements is limited. We adopted chromium and iron, whose official quotations are more stable than those of vanadium and molybdenum, and reported some parts of the research of Ti-13Cr-1Fe-3Al alloy. Cr is one of the attractive elements to develop the cost affordable titanium alloys. Thus, it is very important to investigate the Ti-Cr binary alloys as basic study. It is also important to study the heat treatment behavior of the Ti-Cr alloys. In this study, phase constitution and isothermal aging behavior of Ti-Cr alloys was investigated by measurements of electrical resistivity and Vickers hardness, X-ray diffraction, optical and scanning electron microscope observations and tensile tests. The obtained results are as follows. In solution treated and quenched stated (STQed) state, hcp martensite, α’ was only identified at 3Cr alloy. β phase and α’ were identified at 5Cr alloys. Above 5Cr, β phase was identified. In 7Cr and 10Cr, athermal ω was also identified. Maximum of HV appeared at 7Cr and then HV decreased with increase of Cr content up to 15Cr. Above 15Cr, HV slightly increased due to solution hardening by Cr addition. On isothermal aging, precipitation of the α phase was delayed by Cr addition. In STQed state, tensile strength is 961 MPa in 10Cr, 988 MPa in 13Cr and 967 MPa in 15Cr, respectively. Elongation is 27.1 % in 10Cr, 26.8 % in 13Cr and 23.9 % in 15Cr, respectively. In 15Cr alloy, nominal stress-nominal strain curve showed work-softening phenomenon after yield stress, whereas S-S curve of 10Cr alloy did not show that phenomenon. In 773 K-12 ks aged state, tensile test specimens of 10Cr and 13Cr were broken with no elongation, while the specimen of 15Cr alloy was broken after about 5 % in elongation. It is considered that difference of tensile properties between the 15Cr and others is due to differences of shape and volume fraction of precipitated α.

Anisotropy of Young's modulus and tensile properties in cold rolled α′ martensite Ti–V–Sn alloys

Young's modulus and tensile properties of cold rolled Ti–8 mass% V and (Ti–8 mass% V)–4 mass% Sn alloy plates consisting of α′ martensite were investigated as a function of tensile axis orientation in this work. A single phase of α′ (hcp) martensite is obtained in Ti–8 mass% V and (Ti–8 mass% V)–4 mass% Sn alloys by quenching after solution treatment. By 86% cold rolling, acicular α′ martensite microstructures change into extremely refined dislocation cell-like structure with an average size of 60 nm, accompanied with the development of cold rolling texture in which the basal plane normal is tilted from the plate normal direction (ND) toward transverse direction (TD) at angles of ±49° for Ti–8% V alloy and ±46° for (Ti–8 mass% V)–4 mass% Sn alloy. No apparent anisotropy of Young's modulus ( E) is observed for as-quenched Ti–8% V ( E = 76–83 GPa) and (Ti–8% V)-4%Sn ( E = 69–79 GPa). In contrast, Young's modulus increases with increasing angle from the rolling direction (RD) to TD for cold rolled Ti–8% V ( E = 72–94 GPa) and (Ti–8% V)–4%Sn ( E = 63–85 GPa). The observed anisotropy of Young's modulus can be reasonably explained in terms of the cold rolling α′ texture. 0.2% proof stress and tensile strength are independent of tensile orientation for cold rolled Ti–8% V and (Ti–8% V)–4%Sn alloys. In contrast, larger elongation to fracture is obtained in specimens deviated by 30°, 45° and 60° from RD than by 0°, 75° and 90°. Scanning electron microscopy (SEM) fractographs reveal that quasi-cleavage-like fracture plane appears in 0° specimen of cold rolled Ti–8% V which shows brittle fracture and other specimens of cold rolled Ti–8% V and (Ti–8% V)–4%Sn alloys are fractured accompanied with necking and dimple formation. It is suggested from these results that brittle fracture is related to the activation of limited number of slip system and Sn addition leads to the activation of multiple slip systems.

Martensitic Transformation in a Cast Co-Cr-Mo-C Alloy

The face-centered-cubic (fcc) to hexagonal close-packed (hcp) martensitic transformation exhibited by an as-cast Co-Cr-Mo-C alloy was investigated in this work. The alloy was annealed at 1150 °C, water quenched, and then isothermally aged at 700 °C to 900 °C. Quantitative measurements of transformed hcp martensite (also known as e-martensite) as a function of time and temperature were used in plotting C curves describing the transformation kinetics. Moreover, microstructural characterization indicated that the transformation exhibited two distinctive e-martensite morphologies. In the neighborhood of coarse carbides, multiple nucleation events were linked to the formation of e-martensite, while in the bulk of the dendrite matrix, the dominant morphology was as straight plates. Kinetic determinations of activation energies, Q, in either region indicated that in platelike e-martensite, Q = 57.24 kcal/mol, but in “pearlite”-like morphologies, Q = 37.88 kcal/mol. Apparently, as the activation energy was reduced, multiple nucleation events were favored leading to the “pearlitic” appearance.

Effect of Magnetic Field on Isothermal Martensitic Transformation in SUS304L Stainless Steel

We have found that solution-treated or sensitized SUS304L stainless steel transforms isothermally from the γ (fcc)-phase to the α' (bcc) martensite via the ε' (hcp) martensite, that is, the amount of the α'-phase increases with increasing holding time. By applying magnetic field, the α' martensite can be induced in the ε'-plate when the steel has a sufficiently large ε' plate formed beforehand by isothermal holding. However, the α' martensite cannot be induced by magnetic field in the ε'-plate formed beforehand by a deformation-induced transformation.

Determination of stacking fault probability in fcc Fe–Mn–Si–Al alloy by electron diffraction

The stacking disorder in the fcc structure of Fe–23Mn–2Si–2Al alloy after tensile testing at −75°C was investigated by electron diffraction, using a relationship between the stacking fault probability and the shift of diffraction spots deduced in the present work (and based on Kakinoki’s theory [Acta Crystallogr. 23, 875 (1967)] and Kajiwara’s previous work [Jpn. J. Appl. Phys. 9, 385 (1970); J. Phys. Soc. Jpn. 22, 795 (1967)]). Shifts of diffraction spots along the [111] direction in the fcc structure were observed, and the stacking fault probabilities in two selected areas with different densities of stacking faults were determined as α=0.15 and α=0.35. The stacking fault probabilities measured by electron diffraction are much larger than average values determined by x-ray diffraction, indicating that the distribution of stacking faults is localized. A mechanism for the γfcc→εhcp transformation is suggested, whereby during deformation the following sequences take place: dissociation of perfect dislocation→localization of stacking faults→evolution from stacking disorder to stacking order→the formation of the perfect hcp martensite (α=1) or the hcp martensite with stacking faults (α→1). This proposed mechanism for strain-induced hcp martensite formation should be also applicable to the thermally induced hcp martensite.

Fe-30Mn-5Si-1Al 形状記憶合金の原子間力顕微鏡組織観察

The surface relief caused by stress-induced fcc/hcp martensitic transformation in an Fe-30Mn-5Si-1Al shape memory alloy, which exhibited a good shape memory effect and an improved ductility due to a small addition of aluminum to a conventional Fe-30Mn-6Si shape memory alloy, was quantitatively analyzed by atomic force microscopy. The orientation of an austenite grain was determined with surface traces of four {111}f planes, which made it possible to determine the surface tilt angles for all 12 variants of hcp martensite and deformation twins. On the basis of these values, the stress-induced martensite and deformation twin, coexisting in a grain, were identified by investigating the surface tilt angles. The surface relief caused by the stress-induced martensite recovered after heating above the reverse transformation temperature, but that due to the deformation twin remained unaltered.

Factors influencing the stress-induced fcc ↔ hcp martensitic transformation in Co–32Ni single crystal

This study investigated the mechanical reversibility of the stress-induced face-centred cubic (fcc) ↔ hexagonal close-packed (hcp) martensitic transformation in Co–32Ni single crystal. The as-fabricated single crystal, which was predominantly in the fcc γ-phase, contained a large number of stacking faults at room temperature, effectively containing a small portion of the hcp phase. The transformation to hcp martensite upon cooling was incomplete, resulting in a mixed structure of fcc and hcp regions. Consequently, the alloy showed stress-induced fcc → hcp martensitic transformation on cooling and stress-induced hcp → fcc reverse transformation on heating from room temperature. The stress-induced transformations were found to stabilise quickly, i.e. the transformation volume diminished rapidly after a small number of cycles. This is attributed to the trapping of mobile partial dislocations at internal pinning sites and the interlocking of boundaries of hcp martensite variants. It is also unique that the alloy exhibited transformation-induced strains in the same direction of the applied stress for both the forward and the reverse transformations, in contrast to conventional shape memory alloys. This is attributed to the multiplicity of the return path of the hcp martensite to the fcc austenite. Such condition jeopardises the transformation system to serve as a mechanism for shape memory effect and related properties. This conclusion should be universal for all alloy systems exhibiting fcc ↔ hcp martensitic transformations.

Internal friction study on fcc/hcp martensitic transformations in thermomechanically treated Fe–28Mn–6Si–5Cr–0.53Nb–0.06C (mass%) alloys

We have studied the temperature dependence of the internal friction and Young's modulus of the Fe–28Mn–6Si–5Cr–0.53Nb–0.06C (mass%) alloys with or without NbC precipitates produced by three thermomechanical treatments: (i) solution treatment, (ii) rolling at 870 K and subsequent aging at 1070 K, and (iii) rolling at 1070 K, in the vicinity of the forward ( M s) and the reverse ( A s) martensitic transformation temperatures using a dynamic mechanical analyzer (DMA). The specimen subjected to rolling at 870 K and subsequent aging showed the highest M s temperature and the largest amount of the hcp martensite. The solution treated specimen showed the lowest M s temperature and the minimum amount of the martensite. The magnetic susceptibility measurements made by means of a superconducting quantum interference device (SQUID) revealed that the martensitic transformation is hindered by the preceding magnetic transition from the paramagnetic (cubic) phase to the antiferromagnetic (tetragonal) phase.

Internal friction of an Fe–28Mn–6Si–5Cr–0.5NbC shape memory alloy

An internal friction study was done on an Fe–28Mn–6Si–5Cr–0.5NbC (mass%) shape memory alloy using a dynamic mechanical analyzer. Internal friction and elastic modulus were concurrently measured as a function of temperature, strain amplitude and vibration frequency. Large internal friction peaks are observed during the fcc/hcp martensitic transformations. These high values of internal friction are mainly due to transient contribution, which appears only during cooling and heating. In isothermal conditions, no significant damping capacity can be seen when applied strain is less than 10 −4. Internal friction sharply increases with increasing applied strain amplitude. On the other hand, the internal friction is independent of vibration frequency. The internal friction behavior is considered to be due to the reversible motion of the fcc/hcp interfaces, which is achieved by the reversible motion of the Shockley partial dislocations at the tip of the hcp martensite plates. It was also found that the thermomechanical treatment consisting of pre-rolling and subsequent aging enhances the damping capacity of the alloy.

Simulation of martensitic microstructure

The defects appearing during the process of martensitic transformation from bcc austenite to a microtextured hcp martensite are examined, with reference to molecular dynamics simulations in zirconium. The simulations involve cooling from the high temperature austenitic bcc phase. A variety of geometric defects are identified, with the structure and dynamics being understood in terms of vicinal twin boundaries and dislocations. These defects are classified as either geometrically necessary or nanoscale effects, and the evolution of the material in response to annealing and external stress is described. The observed mechanism for twin boundary motion under applied strees involves a combination of stress concentration by preexisting sessile dislocation, causing nucleation, motion and absorption of defects in the boundary plane.