Martensitic Reaction Research Articles

Do intermetallics interfere with the martensitic reaction in Cu-rich alloys?

Do intermetallics interfere with the martensitic reaction in Cu-rich alloys?

Critical Diameters of Grains or Individual Particles for the Start of a Martensitic Reaction from the Positions of Dynamic Theory

Critical Diameters of Grains or Individual Particles for the Start of a Martensitic Reaction from the Positions of Dynamic Theory

Martensitic transformations in Fe-Mn alloy with low stacking fault energy

Object of investigation is highly alloyed binary Fe-Mn alloys which in austenitic state are characterized with different stacking fault energy (SFE). Experimental results using the internal friction and X-Ray methods are obtained for the martensitic non isothermal transformation upon cooling and heating. It was determined that mainly the structural characteristics of the alloys define the type of the possible transformation mechanisms and the products of the martensitic reaction. The role of the SFE for the mechanisms of formation and growth of the martensite phase and the relation between the micro-structural parameters of the solid solution and the mechanism of non isothermal martensitic transformations is shown. On the basis of the existing models of dynamic relaxation associated with phase transformations, here are pivotal conclusions about the characteristics of the non-diffusion mechanism of transformation into a low SFE.

Interfacial Stresses and Anomalous Shape of Pseudoelastic Deformation Curves in Ni49Fe18Ga27Co6 Alloy Crystals Compressed along the [011]A Axis

We have performed experimental and theoretical investigation of the anomalous form of the compression diagrams and shape memory restoration curves in Ni49Fe18Ga27Co6 alloy crystals deformed by uniaxial compression along the [011]A crystallographic direction (A-austenite) in the temperature range of 200–350 K. It is found that in the investigated temperature range, all compression diagrams contain anomalous segments of smooth and sharp decrease in deforming stresses. It is shown that the segments of a smooth decrease in stress are associated with peculiarities in martensite reaction L12 → 14M, while segments of a sharp drop are due to instability of martensite reactions 14M → L10 and L12 → L10. A possible source of reaction instability is associated with interfacial stresses at the interfaces between the martensite and austenite phases (lamellas) due to different elastic moduli of contacting phases. The magnitude of these stresses is significant in the case of 14M → L10 and L12 → L10 transformations, which induces a sharp drop of the deforming stress, while the restoration of the shape memory effect is of a burst nature. It is established that the contribution of interfacial stresses to the free energy of martensite transformation is smaller than the dissipative (entropy) contribution to this energy; however, interfacial stresses higher than a certain threshold strongly affect transformation kinetics and, hence, determine the strongly anomalous shape of pseudoelastic deformation curves and burst restoration of the shape memory effect.

Межфазные напряжения и аномальный вид кривых псевдоупругой деформации в кристаллах сплава Ni-=SUB=-49-=/SUB=-Fe-=SUB=-18-=/SUB=-Ga-=SUB=-27-=/SUB=-Co-=SUB=-6-=/SUB=-, деформированных сжатием в направлении оси [011]-=SUB=-A-=/SUB=-

AbstractWe have performed experimental and theoretical investigation of the anomalous form of the compression diagrams and shape memory restoration curves in Ni_49Fe_18Ga_27Co_6 alloy crystals deformed by uniaxial compression along the [011]_ A crystallographic direction ( A -austenite) in the temperature range of 200–350 K. It is found that in the investigated temperature range, all compression diagrams contain anomalous segments of smooth and sharp decrease in deforming stresses. It is shown that the segments of a smooth decrease in stress are associated with peculiarities in martensite reaction L1_2 → 14M, while segments of a sharp drop are due to instability of martensite reactions 14M → L1_0 and L1_2 → L1_0. A possible source of reaction instability is associated with interfacial stresses at the interfaces between the martensite and austenite phases (lamellas) due to different elastic moduli of contacting phases. The magnitude of these stresses is significant in the case of 14M → L1_0 and L1_2 → L1_0 transformations, which induces a sharp drop of the deforming stress, while the restoration of the shape memory effect is of a burst nature. It is established that the contribution of interfacial stresses to the free energy of martensite transformation is smaller than the dissipative (entropy) contribution to this energy; however, interfacial stresses higher than a certain threshold strongly affect transformation kinetics and, hence, determine the strongly anomalous shape of pseudoelastic deformation curves and burst restoration of the shape memory effect.

EBSD investigation of microstructure evolution during cryogenic rolling of type 321 metastable austenitic steel

Electron backscatter diffraction (EBSD) was employed to establish microstructure evolution in type 321 metastable austenitic stainless steel during rolling at a near-liquid-nitrogen temperature. A particular emphasis was given to evaluation of microstructure-strength relationship.As expected, cryogenic rolling promoted strain-induced martensite transformation. The transformation was dominated by the γ→α′ sequence but clear evidence of the γ→ε→α′ transformation path was also found. The martensitic reactions were found to occur almost exclusively within deformation bands, i.e., the most-highly strained areas in the austenite. This prevented a progressive development of deformation-induced boundaries and thus suppressed the normal grain-subdivision process in this phase. On the other hand, the preferential nucleation of martensite within the deformation bands implied a close relationship between the transformation process and slip activity in parent austenite grains. Indeed, the martensite reactions were found to occur preferentially in austenite grains with crystallographic orientations close to Goss {110}<100> and Brass {110}<112>. Moreover, the martensitic transformations were governed by preferential variant selection which was most noticeable in ε-martensite. The sensitivity of the martensitic reactions to the crystallographic orientation of the austenite grains resulted in re-activation of the transformation process after development of a deformation-induced texture in the austenitic phase at high strains. Both martensitic phases were concluded to experience plastic strain which resulted in measurable changes in misorientation distributions. Cryogenic rolling imparted dramatic strengthening resulting in a more-than-sixfold increase in yield strength. The main source of hardening was the martensitic transformation with lesser contributions from dislocations and subboundary strengthening of the austenite.

Development of methods of structural-analytical mesomechanics that take into account the statistical properties of martensitic transformations in materials with shape memory effect

Methods for structural-analytical mesomechanics for materials with shape memory effect are developing, in these methods a pronounced synergetic effect is manifested, which is characterized by the appearance of self-accomodating groups of martensitic crystals, forming during the initiation of martensitic reactions extended domains oriented towards acting effective stresses.

Anomalous stress-strain behaviour in Ni49Fe18Ga27Co6 crystals compressed along [110

Compressive stress-strain properties of Ni49Fe18Ga27Co6 alloy single crystals in the temperature range of 220-320 K have been studied. Anomalous loading diagrams with regions of abrupt and gradual stress decrease were recorded. This anomalous behavior can be attributed to the existence of the interphase elastic stresses. These stresses have a strong impact on the martensitic reactions which are typical for that alloy. Experimental results were analysed using the theory of diffused martensitic transformations.

Anomalous behavior of curves of pseudo-elastic deformation of Ni–Fe–Ga–Co alloy crystals as a result of interphase stresses

The compression diagram of Ni49Fe18Ga27Co6 alloy crystals in the [011] direction was studied until full shape memory strain at various temperatures in the range of 259–340 K. It is found that all load curves are anomalously shaped and contain portions of sharp and gradual decreases in deformation stresses. Simulation of pseudo-elastic stress–strain curves within the theory of diffuse martensitic transitions, describing not only equilibrium of phases, but also the kinetics of the transition between them, shows that elastic interphase stresses during martensitic reactions Ll2 → 14M and 14M → Ll0 characteristic of this alloy can be responsible for the extraordinary shape of compression diagrams.

Effect of chemical banding on the local hardenability in AISI 4340 steel bar

The microstructure of a 240mm diameter AISI 4340 mill shaft, quenched and tempered, was analyzed through its longitudinal cross section. Normally, higher cooling rates promote martensitic structure while the diffusion assisted austenite decomposition products are expected in the lower cooling rate condition. The martensitic reaction takes place inside the grains while the products diffusion starts from the prior austenite grain boundaries, nucleation and grow processes. An anomalous hardenability behavior was reported with a mixed structure containing martensite and bainite, in different proportion from the surface (higher cooling rate) to the core (lower cooling rate). This behavior was attributed to the wide banded structure. Banding due to the as cast structure segregation, is directly related to the solidification rate as well as the deformation degree due (during) to forging and rolling operations. When the deformation degree is high, the banding thickness is thin and approaches the austenite grain size. In this study the thickness of the bands embedded about four to five austenite grain sizes, in such a way that every band behaves like a particular steel composition. A semi quantitative chemical analysis was carried out through the banded structure to understand the differential hardenability behavior. The results were discussed using classical hardenability formula as well as calculated TTT diagrams for each composition.

Relationship between austenite stability and martensite formation in modified 9Cr-1Mo steel

Abstract Aimed at investigating the relationship between austenite stability and martensite formation in modified 9Cr-1Mo steel, the fully austenitized steel was cooled to a temperature above or below the start temperature of martensite transformation (Ms o, 406 °C), which occurs upon continuous cooling from the austenitizing temperature to room temperature, and held for different times (10 min, 25 min, and 45 min) followed by cooling to room temperature. The concept of primary and secondary martensite is introduced to indicate that two different, sequential, martensites formed in this processing. When cooled to 650 °C or 450 °C for different times, the austenite stability decreased with prolonged holding time, and during the subsequent cooling the primary martensite start temperature (M s) was raised to a value higher than Ms o. In contrast, M s was reduced to a level lower than Ms o when cooled to 420 °C or 410 °C for 45 min. On the other hand, as the steel was isothermally quenched at temperatures below Ms o, during the subsequent cooling a secondary martensite reaction could be observed, and the secondary martensite start temperature (Ms ′) decreased with holding time. Retained austenite in the form of thin films along martensite lath boundaries was obtained when cooled to 380 °C or below for different times.

Anisotropy of Retained Austenite Stability during Transformation to Martensite in a TRIP‐Assisted Steel

AbstractRetained austenite as a key constituent in final microstructure plays an important role in TRansformation Induced Plasticity (TRIP) steels. The volume fraction, carbon concentration, size, and morphology of this phase are the well‐known parameters which effects on the rate of transformation of retained austenite to martensite and the properties of steel, are studied by many researchers. Of the transformation of retained austenite to martensite under strain in a TRIP steel is studied in this paper. The experimental results show that the transformation rate of retained, austenite with similar characteristics, to martensite in differently processed TRIP steel samples, exhibits an anisotropic behavior. This phenomenon implies a kind of variant selection of martensitic reaction of retained austenite under strain and is explained by ferrite texture developed in steel.

The Kinetics of Phase Transformations During Continuous Cooling of the Ti6Al4V Alloy from the Single-Phase β Range

The Kinetics of Phase Transformations During Continuous Cooling of the Ti6Al4V Alloy from the Single-Phase β RangeThe paper reports on a study of the kinetics of phase transformations during continuous cooling of the alpha-beta Ti6Al4V titanium alloy from the β range. After cooling from 1020°C at a rate higher than critical the martensitic β ← α' transformation occurred. At rates lower than critical in addition to the martensitic reaction also the diffusional β ← α transformation took place. Metallographic examination of samples cooled at various rates within the range of diffusional transformations, revealed variations in the α phase morphology changing from lamellar, after cooling at between 7.3 and 0.015°C/s, to granular, as observed after cooling at 0.012°C/s. The phase transformations were also clearly seen in dilatometric curves.

Microstructure and properties of Ni–Mn–Ga alloys produced by rapid solidification and pulsed electric current sintering

Ni–Mn–Ga alloys were compacted using pulsed electric current sintering (PECS) at 850–875 °C (50 MPa, 8 min) of flake-like powders made from the rapidly quenched melt-spun ribbons. Two kinds of ribbons were used: one made with a relatively slow wheel speed (6 m/s; average grain size ∼14 μm), and another with a faster wheel speed (23 m/s; average grain size ∼5 μm). Both sets of flake-like powders consisted of a mixture of non-modulated martensite (NM) and seven-layered modulated martensite (7M) structure. The amount of NM was greater in the slower speed material, while the other one exhibited mostly the 7M structure. These crystal structures were inherited by the sintered samples. In the compacts having the NM structure the multi-step martensitic reaction overlapped with the magnetic transition, and the Curie temperatures during heating and cooling differed from each other. In the compacts having mainly 7M structure the Curie point was about 100 °C and the martensitic transition took place in the paramagnetic state, while the intermartensitic one occurred in the region of 60–85 °C. This material demonstrated good magnetic properties and saturation magnetization, at best ∼50 emu/g. Mechanical properties of the compacts were good, and comparable to those of the polycrystalline Ni–Mn–Ga samples in compression.

Structural transformations in Cu-24% Ga alloy

The transformation kinetics and the morphology of structures formed in the Cu-24 at % Ga alloy at a cooling rate of 1 to 4 × 105 K/s are studied. A massive transformation shows up in the Cu-24 at % Ga alloy at slow cooling rates, and this transformation can be replaced, upon sharp quenching, by a martensitic reaction. The transformation kinetics and the morphology of structures formed in the Cu-24 at % Ga alloy at cooling rates of up to 400 kK/s are analyzed in the study.

Microstructure and martensitic transformation of cast TiNiSi shape memory alloys

The martensitic transformation behavior, second phases and hardness of Ti51Ni49−xSix shape memory alloys (SMAs) with x=0, 1 and 2at.% are investigated. The transformation temperature of one stage martensitic reaction B2↔B19′ is associated with the forward (Ms) and reverse (As) martensitic transformations, respectively. All experimental DSC results such as martensitic transformation peaks (M*) and reverse martensitic transformation peaks (A*) are increased and became sharper with increasing Si-content. The microstructure investigation of the studied SMAs (Ti51Ni49−xSix) showed that there are two types of precipitated second phase particles. The first one is Ti2Ni which mainly located at grain boundaries and intermetallic compound of Ti2(Ni+Si) phase distributed inside the matrix. The volume fraction of these two phases is increased with Si content. Additionally, a small amount of Si remained in solid solution of the matrix of Ti51Ni49−xSix SMAs. Moreover, hardness of Ti51Ni49−xSix SMAs is increased as the Si-content increases.

Microstructural path analysis of martensite burst

Modeling the martensite reaction requires reckoning with spatial aspects of the reaction. For that, we used formal kinetics, more specifically, the microstructural path method (MPM) to analyze the microstructure observed in a burst. The microstructural path analysis revealed that the size of the spread cluster in extended space, characterized by the Vandermeer and Juul-Jensen's impingement compensated mean intercept length, λG, remained constant, independently of the parent austenite grain size. Moreover, current analysis introduced a purely formal description of the reaction progress by taking the parent austenite grain size as the progress variable. This description worked very well and resulted in a relationship between the volume fraction of partially transformed austenite, V VG, and austenite grain size, λG. The significance of these findings in the light of the advantages and disadvantages of formal kinetics is discussed.

Fine structure of martensitic phases in the Cu-24% Ga alloy

The transformation kinetics and the morphology of structures formed in the Cu-24 at % Ga alloy at cooling rates of 1 to 4 × 105 K/s have been studied. At low cooling rates, a massive transformation takes place in the Cu-24 at % Ga alloy; upon rapid quenching, this massive transformation can be replaced by the martensitic reaction.

Ag-rich phase precipitation in the Cu-9 mass% Al alloy with Ag additions

The Ag-rich phase precipitation in the Cu-9 mass% Al was studied using differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The results indicated that Ag additions did not interfere on the metastable transitions sequence of the Cu-mass% Al alloy but Ag precipitation disturbs the β phase formation reaction and the martensitic phase decomposition reaction.

Unified description of martensite microstructure and kinetics

A quantitative metallography method is described to obtain size and number per unit volume of martensite units from linear intercept measurements. The entailed relationship between the number per unit volume of martensite plates and the volume fraction transformed is consistent with the autocatalytic nature of martensite. Application to the athermal and the isothermal martensite reactions allowed development of a unified microstructure-kinetic model. Validation of the model equations was achieved with data pertaining to FeNiC and FeNiMn alloys found in the literature. The apparent activation energy for propagation of isothermal martensite yielded by the transformation curve is compatible with the value obtained from the initial transformation rate. The defect redistribution process austenite/martensite established during the thickening of the plates has a crucial role in autocatalysis.