The effect of composition on the pressure-induced HCP (∈) transformation in iron

The pressure-induced phase transformations in iron-rich Fe?Mn and Fe?Ni?Cr alloys were studied using an opposed diamond anvil high-pressure X-ray diffraction unit and a liquidmedium hydrostatic pressure apparatus. Transformations occurring with both increasing and decreasing pressure were studied. It was found that alloy additions of manganese and of nickel plus chromium significantly reduce the formation pressure of the hcp phase and can in some cases stabilize the phase enough to prevent it from transforming into some other phase during pressure release. All of the transformations are shown to be martensitic. Pressurization of prepolished surfaces, a large transformation pressure hysteresis, and the “abaric” formation of the ∈ phase establish the transformation as martensitic.     

Thermodynamic Basis of Non-equilibrium Phase Transformations of bcc β-Phase in Ti–Mo System

This study reports the experimental identification of transformation products of high temperature bcc β phase in Ti–xMo (x = 1, 7, 15, 25 wt%) alloys and aims to understand the transformations by thermodynamic modelling. The high temperature bcc β phase had undergone martensitic transformation in Ti–1Mo and Ti–7Mo alloys, resulting in acicular martensitic structure within large β grains. X-ray diffraction (XRD) analysis confirmed the martensite to be hcp (α′) and orthorhombic (α″) in Ti–1Mo and Ti–7Mo alloys respectively. Combined analysis of XRD and transmission electron microscopy (TEM) suggested the formation of fine plates of α″, omega (ω) and bcc β phases in Ti-15 and Ti-25 Mo alloys. Calculation of enthalpy of formation supported the stability of solid solution phase over the amorphous phase in the entire concentration range of Mo.     

Development and characterization of high strength impact resistant Fe-Mn-(Al-, Si) TRIP/TWIP steels

Iron manganese steels with Mn mass contents of 15 to 30 % exhibit microstructural related superior ductility and extraordinary strengthening behaviour during plastic deformation, which strongly depends on the Mn content. This influences the austenite stability and stacking fault energy γ_fcc and shows a great impact on the microstructure to be developed under certain stress state or during severe plastic deformation. At medium Mn mass contents (15 to 20 %) the martensitic γ-ε-ά phase transformation plays an important role in the deformation mechanisms of the TRIP effect in addition to dislocation glide. With Increasing Mn mass content large elongation is favoured by intensive twinning formation. The mechanical properties of plain iron manganese alloys are strongly influenced by the alloying elements, Al and Si. Alloying with Al Increases the stacking fault energy and therefore strongly suppresses the martensitic γ-ε transformation, while Si sustains the γ-ε transformation by decreasing the stacking fault energy γ_fcc. The γ-ε phase transformation takes place in Fe-Mn-X alloys with γ_fcc ≤ 20 mJm⁻². The developed light weight high manganese TRIP and TWIP (twinning induced plasticity) steels exhibit high ultimate tensile strength (600 to 1100 MPa) and extremely large elongation of 60 to 95 % even at high strain rates of έ = 10³ s⁻¹. Particularly due to the advanced specific energy absorption of TRIP and TWIP steels compared to conventional deep drawing steels high dynamic tensile and compression tests were carried out in order to investigate the change in the microstructure under near crash conditions. Tensile and compression tests of iron manganese alloys with varying Mn content were performed at different temperatures and strain rates. The resulting formation of γ twins, ά- and ε martensite by plastic deformation was analysed by optical microscopy and X-ray diffraction. The deep drawing and stretch forming behaviour at varying deformation rates were determined by performing cupping tests and digitalised stress-strain-analysis.     

Structural and Magnetic Transformations in the Fe‐Mn Binary System

The phase transformations of five binary iron‐manganese (Fe‐Mn) alloys with manganese contents ranging from 1 to 21 weight percent have been characterized in the temperature range between room temperature and 1250 °C. Differential scanning calorimetry and dilatometry were used to experimentally characterize both the phases and magnetic transformation temperatures. X‐ray diffraction and light optical microscopy were employed for the room temperature microstructure characterization. Depending on the manganese content of the alloy, three different crystal structures can be found: body centered cubic (bcc) (α/α'), face centered cubic (fcc) (γ), and hexagonal compact (hcp) (?). At manganese contents lower than 10% the phases present are the α/α’ (bcc) and γ (fcc). Above ~10 weight percent manganese increasing amounts of ? (hcp) is formed at the expense of the body centered cubic structures, and no α/α’ (bcc) is observed for the 21 weight‐percent manganese alloy.     

Phase transformations in the Zr-rich part of the Zr-Fe system resulting from heat treatment and plastic deformation

Phases and phase transformations occurring in the Zr-rich part of the Zr-Fe system during heat treatment and plastic deformation were identified by means of Mössbauer spectroscopy and X-ray diffraction analysis. Low iron alloys (<0.02 wt pct Fe) undergo a complete β → α_m reaction (martensitic type) on quenching. For higher iron content alloys (0.02 to 0.25 wt pct Fe) the β → α_m ransformation is accompanied by formation of metastable intermediate phase designated θ. The iron concentration of θ-phase is much higher than that of αZr(Fe). During the aging process, at the outset of the equilibrium state, the θ-phase disappears by transforming to Zr₂Fe intermetallic. Cold rolling of quenched(α _m + θ) specimens leads to formation of the athermal ω-phase. Presence of the intermediate θ-phase seems to be a prerequisite for the athermal ω-phase formation. A decrease in specific volume (ΔV_θ < 0) accompanying the α→ θ transition was suggested as a possible mechanism of the α →θ → ω transformation. Mössbauer parameters for the thermal and athermal ω -phase were determined. Presence of θ and athermal ω -phases were identified by Mössbauer spectroscopy only, being undetectable by X-ray diffraction, because of their minute quantities. Solubility of iron in the α Zr(Fe) solid solution was determined in the range of temperatures 713 to 943 K (440 to 670 °C).     

Co-Al二元系中的fcc/hcp马氏体相变和形状记忆效应

It is known that pure Co undergoes martensitic transformation from γ phase (fcc) to ε phase (hcp) by the movement of a/6<112> Shockley partial dislocations at around 400 ℃, however, there have been few systematic works on the SM effect in Co and Co-based alloys. In this study, the fcc/hcp martensitic transformation and the SM effect were investigated in Co-Al binary alloys(mole fraction of Al=0～16%).The γ/ε martensitic transformation temperatures were found from the DSC measurements to decrease with increasing Al content, while the transformation temperature hystereses were observed to increase from 60 ℃ at x(Al)=0 to 150 ℃at x(Al)= 16%. The SM effect evaluated by a conventional bending test was enhanced by the addition of Al over 4%(mole fraction) and Co-Al alloys containing over 10%(mole fraction) exhibit a good SM effect associated with the hcp →fcc reverse transformation above 200 ℃. The SM effect was significantly improved by precipitation ofβ (B2) phase and the maximal shape recovery strain of 2. 2% was obtained, which can be explained by precipitation hardening. The crystallographic orientations between theβ, ε and γ phases were also determined. Finally, the magnetic properties were investigated and it was found that the Curie temperature and saturation magnetization of Co-14% Al(mole fraction) are 690 ℃and 120 emu/g, respectively. It is concluded that the Co-Al alloys hold promise as new high-temperature and ferromagnetic SM alloys.     

钛合金固态相变的归纳与讨论（Ⅵ）——阿尔法

钛合金中,涉及到钛元素的相成百上千,这些相中,大部分无需了解,但其中有7个相尤其需要关注,他们是6个同素异构相α、β、α′、α″ 、ω、β′和一个共析相α2。这7个重要的相的表达中以阿尔法为主体表达式的相有4个：α、α′、α″和α2相。根据相关资料和作者在钛合金方面的工作经验,对这4个以阿尔法为主体表达式的相的结构、形态、相转变特点以及他们之间的联系进行了介绍,旨在为初涉钛合金领域的工程技术人员研究钛合金组织和相变提供一定的参考。     

Transformation behavior of nearly stoichiometric Ni-Mn alloys

The transformation behavior of Ni-Mn alloys in the vicinity of the stoichiometric composition has been studied using transmission electron microscopy, X-ray diffraction, optical microscopy, and electrical resistivity measurements. The transformation behavior was found to be markedly different in Mn-rich alloys and Ni-rich alloys. In Mn-rich alloys a martensitic transformation between L2₀ (B2) and L1₀ structures takes place, which possesses many features common to alloys exhibiting a thermoelastic martensitic transformation. On the other hand, in Ni-rich alloys an order-disorder transformation between A2 and L1₀ structures occurs. The martensitic transformation features {111} transformation twins as the transformation substructure while the ordering reaction involves {101} order twins. In the Mn-rich alloys, the martensitic phase, if either slowly cooled or annealed at intermediate temperatures, becomes “tempered”, resulting in a noncrystallographic, essentially featureless microstructure apart from the presence of occasional {111} twins. Formerly with University of Illinois.     

Effect of cryoforming of austenitic Cr Ni-steels at 77 K on martensitic transformation and work-hardening characteristics

The effect of cryoforming at 77 K on the flow and work-hardening characteristics was investigated considering the martensitic transformation behaviour in austenitic Cr Ni steels with different nickel contents. The test steels can be divided into two groups relating to the flow and work-hardening characteristics and martensitic transformation behaviour at 77 K. The first group comprises steels with less than 16 % nickel, the second group those with more than 16 % nickel. The flow curves of the first-group steels show two inflection points on the basis of γ → α'-transformation. α_γ'-martensite is observed and ?- and α_?'-martensite too. The flow curves of the second-group steels do not show any inflection points. The γ → α'-martensitic transformation is not induced, ?- and α_?'-martensite are provable by light and scanning microscopy. The stress-strain intervals were determined for the individual martensite transformations at 77 K in the test steels. They are dependent from the nickel content. The stress which specifies the first inflection point on the flow curve and the minimum of the work-hardening rate, respectively, characterizes the stress for initiating the deformation-induced α_γ'-martensite formation. Transformation of the austenite to α' martensite will end in achieving a stress of 1200 to 1400 MPa, i.e. in achieving the second inflection point of the flow curve and the maximum of the work-hardening rate, respectively. The stress interval is not dependent from the nickel content.     

Effect of Nitrogen on the Deformation Behaviour of High‐Nitrogen Austenitic Stainless Steels

The deformation behaviour of high‐nitrogen austenitic steels with the base composition of Fe‐18Cr‐10Mn containing various contents of nitrogen was investigated. Two deformation modes including deformation‐induced martensitic transformation (DIMT) and deformation twinning (DT) were observed depending on the nitrogen content. In the alloys with lower nitrogen contents, γ→?→α' martensitic transformation sequentially occurred, whereas DT acted as a main deformation mode and DIMT was suppressed in the alloys with increasing nitrogen content. Both DIMT and DT showed strong crystallographic orientation dependence. The competing mechanism between them was discussed in terms of the variation of stacking fault energy with nitrogen content.     

Transformation of austenite into bainitic ferrite and martensite

The stress induced martensitic transformation in the upper metastable intermediate state of γ-α transformation in ferrous materials, structured as ferritic bainite, is discussed. The fibrous structured ferritic bainite consists of retained austenite and ferrite platelets growing in the [111]α//[101]γ direction. The ferrite growth Induces carbon enrichment of the adjacent austenite at the phase boundaries. Strengthening at high stress levels up to the yield point causes dislocation tangles in the ferrite fibre and the formation of shear bands crossing each other in the retained austenite. At lower carbon contents of the austenite, lath martensite precipitates at the shear band intersections and at high shear band densities martensite blocks are observed. In carbon enriched austenite martensite lenses formed by shear processes have been observed. At alternating loading conditions, exceeding the stress level for athermic martensite formation, various shear planes are activated forming characteristic patterns of plate martensite.     

Effect of strain-induced martensitic phase transformation on the formability of nitrogen-alloyed metastable austenitic stainless steels

Strain-induced martensitic phase transformation and its influence on the formability of newly developed nitrogen-alloyed metastable austenitic stainless steels were systematically investigated. Yield strength for the asreceived steels bearing lownickel content was around 300 MPa and their elongation ratios varied from 55. 2% to61. 7%. Erichsen numbers of these samples differed from 13. 82 to 14. 57 mm. Although its Cu content was lower than that of other samples,steel D2 exhibited better plasticity and formability,which was attributed to γ→α'martensitic phase transformation. EBSD,XRD,and magnetism tests showed that increases in deformation ratio gradually increased the α' martensite phase of a sample,thereby contributing to its strain and inducing the optimal transformation-induced plasticity effect. An M_(d30/50) temperature of around 20 ℃,which is close to the deformation temperature,provided the austenite with adequate stability and gradually transformed it into martensite,thereby endowing lean ASS with better formability.     

Ni55Mn25Ga18Ti2高温形状记忆合金的热循环稳定性

选择双相韧化的Ni?Mn?Ga?Ti高温形状记忆合金为研究对象。制备了淬火态Ni₅₅Mn₂₅Ga₁₈Ti₂高温形状记忆合金,并对其在室温至480 ℃之间进行高达500次的相变热循环,获得了5, 10, 50, 100和500次热循环态样品。采用X射线衍射、扫描电镜、能谱仪、同步热分析仪及室温压缩等实验方法,研究了淬火态和热循环态合金样品的微观组织、相变行为、力学及记忆性能,进而分析其热循环稳定性。研究结果表明:经500次循环后,Ni₅₅Mn₂₅Ga₁₈Ti₂合金相结构和显微组织未发生明显变化,均为由非调制四方结构的板条马氏体相和面心立方富Ni的γ相组成的双相结构;随着循环次数增加,马氏体相变温度几乎不变,逆马氏体相变温度和相变滞后在循环5次后趋于稳定;抗压强度及压缩变形率波动幅度较小;形状记忆性能下降,但形状记忆应变仍保持在1.4%以上;Ni₅₅Mn₂₅Ga₁₈Ti₂高温形状记忆合金显示出良好的热循环稳定性。      

Effect of Cobalt on Phase Formation, Microstructure, and Mechanical Properties of Cu50?xCoxZr50 (x?=?2, 5, 10, 20?at.?pct) Alloys

In this work, the effect of cobalt on the phase formation and mechanical properties of rapidly solidified Cu_50?xCo_xZr₅₀ (x?=?2, 5, 10, and 20?at.?pct) alloys was investigated. CuZr martensite forms in the case of low Co contents (x?=?2 and 5?at.?pct), while in the alloys with 10 and 20?at.?pct Co, the B2 phase is stable even at room temperature. The deformation behavior of the rods under compressive loading depends strongly on the microstructure and, thus, on the alloy composition. Cobalt affects the fracture strength of the as-cast samples, and deformation is accompanied by two yield stresses for high Co-content alloys, which undergo deformation-induced martensitic transformation.     

Microstructures of rapidly-solidified binary TiAl alloys

The microstructures of rapidly-solidified binary TiAl alloys containing 46–70 at.% Al have been studied using optical and analytical transmission electron microscopy (AEM). The phases present in the alloys and their distribution were found to be a sensitive function of composition. Essentially single-phase microstructures were seen for alloys with 46 at.% Al, 50–52 at.% Al and 60–65 at.% A. The primary solidification phases present in these alloys were α-Ti, ordered γ-TiAl and disordered cubic TiAl, respectively. The 60–65 at.% Al alloys showed indications of the solid-state formation of long-period superlattice structures based upon γ-TiAl, due to the excess Al. In other composition ranges, two-phase microstructures were seen. The 48 at.% Al alloy contained α₂-Ti₃Al + γ-TiAl, with α₂-Ti₃Al as the primary solidification phase. Alloys from 53 to 55 at.% Al were also α₂-Ti₃Al + γ-TiAl, but with γ-TiAl as the primary solidification phase. The 70 at.% Al alloy was two phase TiAl₂ + TiAl₃. A strong effect of interstitial oxygen content on the α₂-Ti₃Al + γ-TiAl phase relations was also seen. Comparison of these results with the equilibrium phase diagram and with ingot studies of the same alloys showed that most of the microstructures produced by rapid solidification were metastable. A possible metastable phase diagram for TiAl which is consistent with the results is proposed.     

Deformation Induced Martensite Formation and its Effect on Transformation Induced Plasticity (TRIP)

The knowledge of the stress‐ and deformation‐induced martensite formation in metastable austenitic steels including the formation temperatures and amounts formed is of considerable importance for the understanding of the transformation induced plasticity. For this purpose a stress‐temperature‐transformation (STT) and a deformation‐temperature‐transformation (DTT) diagram have been developed for the steel X5CrNi 18 10 (1.4301, AISI 304). It is shown that the M_d‐temperature for γ→?, ?→α', γ→?→α’ and γ→α’ martensite formation is defined by two stress‐temperature curves which show a different temperature dependence. They specify the beginning and the end of the deformation‐induced martensite formation in the range of uniform elongation. The intersection point defines the corresponding M_d‐temperature. The stress difference which results from the stresses for the end and the beginning of the martensite formation shows positive values below the M_d‐temperature. It defines the amount of martensite being formed. When the M_d^γ→? temperature is reached and the formation of the first deformation‐induced amount of ?‐martensite appears, an anomalous temperature dependence of the maximum uniform elongation starts. The highest values of the maximum uniform elongation are registered for the tested steel in the immediate vicinity of the M_d^γ→α' or the M_d^γ→?→α' temperature ‐ similar as in other metastable austenitic CrNi steels. At this temperature the highest amount of deformation‐induced ?‐phase exists. The transformation plasticity in the test steel is considerably caused by the deformation‐induced ? and α’ martensite formation. Using the new evaluation method, the increase of plasticity ΔA (TRIP‐effect) and strength ΔR can be quantified.     

Fractality and reversibility of ferrous martensite

Martensite formation is characterized by a diffusionless structural phase transformation from austenite to martensite, associated with a considerable amount of lattice variant shear γ_γα ? 0.2. Ferrous martensite shows all possible features connected with the transformation. Different modes of initiation of the martensite formation are possible. The reasons for the burst phenomenon can be considered as an analogy to discontinuous yielding. The transformation only procedes if further thermodynamical driving force is provided by cooling or shear stress. In some cases fractal microstructures are formed in which several fragmentations can be recognized. In contrast to shape memory alloys, steels usually do not show reversibility of the reverse α → γ transformation. The factors which favour reversibility have been defined. Knowledge of these is necessary for the development of iron-base shape-memory alloys.     

Identification of fine precipitate particles in unidirectionally solidified NiAlMo eutectic alloys by means of edx and sad analyses

An attempt is made to identify the fine precipitate particles formed in the α-Mo fibers in the unidirectionally solidified NiAlMo eutectic alloys. The energy dispersive X-ray (EDX) microanalysis and the selected area diffraction (SAD) analysis are used for this identification. Since all particles in the fibers are smaller than the electron probe size, the method proposed by Cliff et al. [Developments in Electron Microscopy and Analysis, p. 63. Inst. Phys., London (1983)] is adopted in the EDX analysis in order to exclude effects from the surrounding α-Mo phase. In the SAD analysis, the diffraction patterns are taken at the particles in the transverse (001)_α and the longitudinal (110)_α sections. These analyses show that the fine particles in the α-Mo fibers are the γ' phase having the fixed composition identical to the γ' matrix around the α-Mo fibers, the crystal structure of L1₂ type and the NW orientation relationship with the α-Mo phase. With reference to the recent phase diagram of Miracle et al. [Metall. Trans. 15A, 481 (1984)], this γ' formation in the α-Mo fibers is understood as the stable phase formation and hence precludes any possibility of carbides, metastable phases or the detrimental δ-NiMo phase. Discussion is extended for observing well-defined diffraction patterns in examining the orientation relationship between the f.c.c. γ' phase and the b.c.c. a phase.     

On the constitutive relations for δ → α and α → δ martensitic transformation plasticity in plutonium alloys

A constitutive model for transformation plasticity based on isothermal martensitic kinetics is applied to δ → α and α → δ transformation in Pu alloys. The model is in good agreement with available data for the α → α transformation plasticity behavior of PuGa alloys in uniaxial compression as a function of composition, temperature and strain rate. Application to α → δ transformation in uniaxial tension and crack-tip stress states indicates that general transformation in uniaxial tension is precluded by premature brittle fracture of the α phase, but localized transformation will occur at crack tips as supported by metallographic observations.     

The Effect of V/C Ratio on the Phase Formation Behavior in Vanadium Carbide-Based Tool Steel Fabricated through Powder Metallurgy

This study investigates phase formation behavior by considering the vanadium (V) to carbon (C) ratio in vanadium carbide-based tool steel. The V/C ratios are controlled by changing the V content. The samples with the V/C ratio of 4.78 and 6.12 are first prepared through gas atomization with a form of powder and then turned into bulk forms through hot isostatic pressing. The bulk samples are annealed at 1070 °C and then quenched. The hardness values of the 4.78V/C and 6.12V/C samples are 50.7 and 17.3 HRC, respectively. Phases of each sample are analyzed to confirm the reason for the hardness difference. The α′-martensite phase is formed in the matrix of the 4.78V/C sample, whereas the matrix of the 6.12V/C sample is the α-ferrite phase. Thermodynamic calculations and high-temperature phase analysis are performed to examine the difference in the phase formation. In the 4.78V/C sample, the γ-austenite phase is formed at 1070 °C, which transforms to the α′-martensite phase during cooling in the heat treatment process. Meanwhile, the 6.12V/C sample has no austenite phase at 1070 °C, so martensitic transformation does not occur. These analyses confirm that it is crucial to control the phase formation by controlling the contents of V and C with an appropriate ratio.