Mechanism on the Effect of Al upon the γ→ε Martensite Transformation in the Fe-Mn Alloys

The structure of the solid solution treated and deformed Fe-25Mn-(0-3.11)Al-0.15C alloys was examined by means of metallograph and X-ray diffraction, which has confirmed that Al restrains the γ→ε transformation in the Fe-Mn alloys. The mechanism on the restraining effect of Al has been discussed on the basis of the efFect of Al upon the stacking fault energy in the Fe-Mn alloys.Aluminium increases thermodynamically the stability of the Fe-Mn austenite and the driving force limit necessitated to be overcome for the γ→ε transformation, because the thermodynamicalcalculation indicates that Al increases the stacking fault energy in the Fe-Mn alloys at 300 K.Al will decrease the number and the size for the ε-martensite nucleus according to the stacking fault mechanism of the ε-martensite nucleation

Electron Microscopy Analysis of Deformation Induced ε Martensite Transformation in an Fe-Mn-Si-Cr-Ni Alloy

The configurations of stacking faults and morphologies of strain induced ε martensite plates in an FeMnSiCrNi alloy were investigated through electron microscopy analysis. The Shockley partial dislocation structures. sensitive to external stress. determine the configurations of stacking faults in γphase Partial dislocations at the front sides of stacking faults are usetul for the nucleation of εmartensite plates. The growth of ε martensite plates is accompanied with the disappearance of local pre-existing stacking faults, The ε martensite vanants behave in three morphologies of respective stopping. continuous penetrating and intersections with the formation of secondary ε martensite plates

Reverse Transformation Behavior of Martensite Formed during Cooling Proce under Constant Stress in TiNi Alloys

The reverse transformation temperature and recovery strain ratio of the martensite formed during the cooling process under a constant stress in TiNi shape memory alloy wires are studied in this paper. Results show that a higher level of the applied constant stress during the cooling process will induce martensite with a higher reverse martensitic transformation start temperature As and a smaller recovery strain ratio. Similarly, a prestrain at the room temperature elevates the As temperature and decreases the recovery strain ratio. However, the As temperature and the recovery strain ratio of the martensite formed during the cooling process under a constant stress are lower than those of the martensite formed by prestrain at the room temperature.

Antiphase boundary-like structure in α′′ martensite of TC21 titanium alloy

The morphology and formation mechanism of the substructure of martensite in TC21 alloy was investigated by XRD and TEM. The results showed that the martensitic transformation from β to α＂ occurs upon quenching after solution treatment between 960-1000 ℃. The antiphase boundary （APB）-like structure was observed clearly in the α＂ martensite plates. The APB-like contrasts exist along the （001） and （020） planes of α＂ martensite. This APB-like structure of α＂ martensite was identified as a kind of stacking fault with an APB-like morphology induced by martensitic transformation and not by order/disorder transition. During martensitic transformation, martensitic domains nucleate and grow, eventually encounter each other, resulting in the formation of the APBdike contrast.

Bar martensite性能与强化途径

按其组织形态马氏体(martensite)组织可以分为条状马氏体(bar martensite)、片状马氏体和蝶状马氏体等。其中条状马氏体又再分为扁条状板条马氏体和薄板状板条马氏体。Wc∠0.2%的钢淬火处理后形成以条状马氏体为主的马氏体混合组织。条状马氏体组织具有强韧结合的综合机械性能。强化工艺为淬火后低温回火。

Precipitation behavior and martensite lath coarsening during tempering of T/P92 ferritic heat-resistant steel

Tempering is an important process for T/P92 ferritic heat-resistant steel from the viewpoint of microstructure control, as it facili- tates the formation of final tempered martensite under serving conditions. In this study, we have gained deeper insights on the mechanism underlying the microstructural evolution during tempering treatment, including the precipitation of carbides and the coarsening of martensite laths, as systematically analyzed by optical microscopy, transmission electron microscopy, and high-resolution transmission electron mi- croscopy. The chemical composition of the precipitates was analyzed using energy dispersive X-ray spectroscopy. Results indicate the for- mation of M3C （cementite） precipitates under normalized conditions. However, they tend to dissolve within a short time of tempering, owing to their low thermal stability. This phenomenon was substantiated by X-ray diffraction analysis. Besides, we could observe the precipitation of fine carbonitrides （MX） along the dislocations. The mechanism of carbon diffusion controlled growth of M23C6 can be expressed by the Zener＇s equation. The movement of Y-junctions was determined to be the fundamental mechanism underlying the martensite lath coarsening process. Vickers hardness was estimated to determine their mechanical properties. Based on the comprehensive analysis of both the micro- structural evolution and hardness variation, the process of tempering can be separated into three steps.

Interface Migration between Martensite and Austenite during Quenching and Partitioning (Q&P) Process

An Fe-0.2C-1.5Si-1.67Mn steel was subjected to quenching and partitioning （Q＆P） process, and the interface migration between martensite and austenite at an elevated partitioning temperature was observed. The interface migration is excluded in constrained paraequilibrium （CPE） model. Based on ＂endpoint＂ predicted by CPE model the thermodynamic condition of interface migration is analyzed, that is, the difference in the chemical potential of iron in both ferrite （martenisite） and austenite produces the driving force of the iron atoms to migrate from one phase to the other phase. In addition, the interface migration can change the austenite fraction; as a result, the austenite fraction at partitioning temperature may be higher than that at quenching temperature through the interface migration, but this phenomenon cannot be explained by CPE model.

Apparent Morphologies and Nature of Packet Martensite in High Carbon Steels

The apparent morphologies of packet martensite in eight high carbon steels were researched by using optical microscope, scanning electron microscope, and transmission electron microscope. It was found that the apparent morphologies, substructures, and habit plane of packet martensite in high carbon steels are entirely different from that in low carbon steels; the substructures of packet martensite in high carbon steels possess fully twinned structure, while the substructures of individual coarse martensite plates in these steels bear both fully and partially twinned structures. The formation reason for apparent morphologies, substructures and two habit planes （i. e, { 111 }, and { 225}r） of high carbon martensite were discussed in detail.

STABILIZATION OF MARTENSITE IN Cu-Zn-Al ALLOY AND ITS REVERSE SHAPE MEMORY EFFECT

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ORDER STATE AND STABILITY OF MARTENSITE IN AN AIR-QUENCHED Cu-Zn-Al ALLOY

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Role of thermal martensite in shape memory effect of CoAl and CoNi alloys

To address the role of the HCP martensite in CoAl and CoNi shape memory alloys, the relationship between the shape memory effect （SME） and the content of the thermal and stress-induced HCP martensite was investigated in the solution-treated CoAl and CoNi alloys. In-situ optical observations were employed to investigate the contents of thermal HCP martensite before and after deep cooling and its influence on the stress-induced HCP martensite transformation and SME. The results show that the SME in both the CoAl and the CoNi alloys results from the stress-induced HCP martensite. The role of the thermal HCP martensite in both of them is the strengthening of the matrix. The much higher yield strength in the solution-treated CoAl alloy due to solution strengthening of Al is responsible for its better SME compared with the CoNi alloy.

Ultra-high cycle fatigue behavior of high strength steel with carbide-free bainite/martensite complex microstructure

The ultra-high cycle fatigue behavior of a novel high strength steel with carbide-free bainite/martensite （CFB/M） complex microstructure was studied. The ultra-high cycle fatigue properties were measured by ultrasonic fatigue testing equipment at a frequency of 20 kHz. It is found that there is no horizontal part in the S-N curve and fatigue fracture occurs when the life of specimens exceeds 10^7 cycles. In addition, the origination of fatigue cracks tends to transfer from the surface to interior of specimens as the fatigue cycle exceeds 10^7, and the fatigue crack originations of many specimens are not induced by inclusions, but by some kind of ＂soft structure＂. It is shown that the studied high strength steel performs good ultra-high cycle fatigue properties. The ultra-high fatigue mechanism was discussed and it is suggested that specific CFB/M complex microstructure of the studied steel contributes to its superior properties.

Martensite transformation induced by deformation and its phase electrochemical behavior for stainless steels AISI 304 and 316L

The martensite transformation induced by tensile elongation and its effect onthe behavior of phase electrochemistry of AISI 304 and 316L in 3.5% NaCl solution were studied. Theresults show that the content of α′-martensite in stainless steel 304 increases with the truestrain. As α′-martensite content increased, free corrosion potential and pitting potential ofstainless steel 304 in 3.5% NaCl solution appeared the change trend of a minimum. It was also foundthat pitting nucleated preferentially at the phase interfaces between martensite and austenite.There existed apparent difference between electrochemical properties of austenite and of martensitefor stainless steel 304 and 316L in 3.5% NaCl solution.

New Space Morphology and Habit Plane of Low Carbon Martensite

The as-quenched microstructures of low carbon steels were observed by scanning electron microscope, and the thin foil specimen was examined by transmission electron microscopy. It is found that the space morphology of low carbon martensite is not lath-like but thin sheet-like, which is designated as sheet-like martensite or packet thin sheet martensite. A three-dimensional model was presented. The reason for exhibiting two apparent morphologies, i.e. double contrast packet and simple contrast packet, in as-quenched low carbon steels was analyzed in detail. It is suggested that the data of martensitic habit plane measured by other procedures should be further inspected using optical metallographic method. The apparent morphologies of low carbon martensite confirm that its habit plane should be {557}_r, rather than {111}_r, {345}_r, nor {213}_r.

Influence of rapid heating process on the microstructure and tensile properties of high-strength ferrite–martensite dual-phase steel

Three low-carbon dual-phase （DP） steels with almost constant martensite contents of 20vo1% were produced by intercritical annealing at different heating rates and soaking temperatures. Microstructures prepared at low temperature （1043 K, FH1） with fast-heating （300 K/s） show banded ferrite/martensite structure, whereas those soaked at high temperature （1103 K, FH2） with fast heating reveal blocky martensite uniformly distributed in the fine-grained ferrite matrix. Their mechanical properties were tested under tensile conditions and compared to a slow-heated （5 K/s） reference material （SH0）. The tensile tests indicate that for a given martensite volume fraction, the yield strength and total elongation values are noticeably affected by the refinement of ferrite grains and the martensite morphology. Metallographic observations reveal the formation of microvoids at the ferrite/martensite interface in the SH0 and FH2 samples, whereas microvoids nucleate via the fracture of banded martensite particles in the FH1 specimen. In addition, analyses of the work-hardening behaviors of the DP microstructures using the differential Crussard-Jaoul technique demonstrate two stages of work hardening for all samples.

Separation of the Martensite in TiNi Fiber Reinforced Aluminum Matrix Composite

The reverse martensitic transformation of TiNi shape memory alloy fibers embedded in a pure aluminum matrix was studied in this paper. Results showed that the phase composition of the TiNi alloy fibers prior to prestraining at the room temperature had a significant influence on the differential scanning calorimetry (DSC) results of the composites. By a comparison to the high temperature X-ray diffraction (XRD) results, it was confirmed that the martensite was divided into two groups: the self-accommodating martensite (SAM) and the preferentially oriented martensite (POM). The evolving process of the separation of martensite was discussed.

MARTENSITE TRANSFORMATION OF A TiNiCu ALLOY

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THE LIMITATION OF MATANSITIC INVARIANT HABIT PLANE AND MARTENSITE MORPHOLOGY

The strain tensor and displacement vector associated with lattice deformation were introduced to discuss the effect of self-accommodation and plastic accommodation in martensitic transformation,and the models for {557}_f and {3,10,15}_f martensite fomation were proposed, along with the experimental verification. It is considered that the invariant habit plane may result from the self-accommodation between different martensitic variants causing the internal twin structure. However, as the strength of parent phase is low enough for the plastic accommodation to occur the dislocation substructure in martensite be produced, in which case the invariant habit plane is difficult to form so the{225}_f and {557}_f habit planes are difficult to explain by the phenomenological crystallographic theory. So,there are some limitations for the martensitic invariant habit plane.In the present work the authors also suggest that the martensite morphology depends on the competition between self-accommodation and plastic accommodation including factors not only of strength but also of structure of the parent phase However, when the density of crystal defects is high dislocations rather than twinned martensite will form in spite of the higher strength of the parent phase.

EFFECT OF SILICON ON PRODUCING DUCTILE IRON WITH COMPLEX STRUCTURE OF BAINITE AND MARTENSITE

This paper deals with an important role of silicon in producing ductile iron with quenched complex structure of bainite and martensite. The samples are cast in permanent mold and quenched in solution of sodium silicate. The result of thc experiments shows that the austenizing temperature should rise with increasing silicon content, otherwise much undissolved ferrite is present in the matrix after quenching. However the undissolvec ferrite can be decreased greatly or even eliminated by adding appropriate amount of ooron. On this condition, the amount of bainite gets increasing and the amount of residual austenite decreasing with the silicon cortent increasing. An approach has also been made to the mechanism of the effect of silicon on the transformation of bainite in ductile iron. The T.T.T. curves measured show that the increase of sllicon content causes the curve to shift to the left. This is quite different from the fact in steel.

ON THE STABILIZATION OF THERMOELASTIC MARTENSITE IN A Cu-Zn-Al ALLOY

The stabilization of thermoelastic martensite in a rapidly solidified Cu-Zn-A1 alloy is be- lieved to be the process of disordering in atomic configuration during which the structure of martensite gradually transforms into N9R(b/a=1/3^(1/2))from M18R.This is dependent upon the intrinsic decomposition tendency of the martensite.