Dislocation Model and Morphology Simulation of bcc fcc Martensitic Transformation

By using molecular dynamics computer simulation at atomic level, the effects of single dislocation and dipole dislocations on nucleation and growth of martensitic transformation have been studied. It was found that only the location of tension or compression stress fields of the dislocations are favorable for martensite nucleation in NiAl alloy and the dislocations can move to accommodate partly the transformation strain during the nucleation and growth of martensite. Combined with the molecular dynamics simulation, a two dimensional simulation for martensite morphology based on a dislocation model has been performed. Many factors related to martensitic transformation were considered, such as supercooling, interface energy, shear strain, normal strain and hydrostatic pressure. Different morphologies of martensites, similar to lath, lenticular, thin plate, couple-plate and lenticular couple-plate martensites observed in Fe-C and Fe-Ni-C alloys, were obtained.     

Regular Pattern of Morphology Transition in Mixed Martensite Structures

The morphology and substructure of mixedmartensites in ferrous alloys have been examined byusing optical and transmission electron microscope.The results indicated that the main formation se-quence of martensitic morphology was butterfly→plate→lath,with decreasing forming temperatureswhen the plastic accommodation takes place in theparent phase,which is affected by the transforma-tion strain fiélds.It was shown that the martensitemorphology is not only decided by the formingtemperature alone,but also by the dislocation struc-ture in austenite before the transformation.     

Morphology and Effect of Pre-deformation on Martensites by Colored Metaliography August 24,1990

The colored metallography was used to inves-tigate the morphology of martensite in Fe-Ni-C al-loys.By compression deformation martensite trans-forms from lenticular to thin-plate.It is provedthat the bended and broken martensites are inher-ent from compressive predeformation of austenite.     

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.     

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.     

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.     

The formation of martensite in splat-quenched Fe-Mn and Fe-Ni-C alloys

A two-piston splat-quenching technique has been used to prepare splat-quenched Fe-Mn alloys with 0 to 20 wt % Mn, and splat-quenched Fe-Ni-C alloys with a nominal carbon content of 0.1 wt % and 0 to 40 wt % Ni. The resulting alloy microstructures have been investigated by a combination of optical and scanning electron microscopy, X-ray diffractometry, and microhardness testing; and the splat-quenched structures have been compared with the microstructures of similar alloys prepared by conventional solid-state quenching. In both alloy systems, splat-quenching produces a very small as-solidified austenite grain size, and a depression of the martensite transformation temperature as shown by an increased tendency to retain austenite to low temperatures. Because of the combination of a small austenite grain size and, therefore, fine scale martensite structure, splat-quenched martensitic alloys of Fe-Mn and Fe-Ni-C exhibit very high microhardness values.     

Martensite/austenite interfaces in ultrafine grained Fe–Ni–C alloy

In this article, orientation relationships (ORs) at martensite/austenite interface (M/A interface) transformed from coarse-grained and ultrafine-grained austenites were investigated. The results showed that the OR at the M/A interfaces of lenticular martensite transformed from coarse-grained austenite was close to Kurdjumov–Sachs (K–S). The OR of butterfly martensite was close to K–S in the outer side of M/A interface and it deviated to Nishiyama–Wasserman (N–W) at the inner side of M/A interface. In contrast, the OR of lenticular martensite transformed from ultrafine-grained austenite was close to Greninger–Troiano (G–T), and the OR of butterfly martensite was close to K–S at outer side of M/A interface and it deviated to G–T at the inner side of M/A interface. The significantly small size of martensite plate transformed from ultrafine-grained austenite resulted in the different ORs from coarse-grained austenite.     

论钢中的枣核状马氏体

枣核状马氏体的形态既不同于板条型马氏体又不同于片状马氏体,它的形成取决于两个基本条件‘即钢的含碳量应高于0.5%及淬火温度不能太高。本文提出枣核状马氏体的概念将作为现今马氏体形态研究内容的一个重要补充。     

Formation of the reversed austenite during intercritical tempering in a Fe-13%Cr-4%Ni-Mo martensitic stainless steel

Formation of the reversed austenite obtained by intercritical tempering has been studied via transmission electron microscopy (TEM) in a Fe-13%Cr-4%Ni-Mo low carbon martensitic stainless steel. It is found that the precipitation of M₂₃C₆ carbides along the martensite lath boundaries will result in Ni-enrichment in the adjacent region. The reversed austenite forms with the Ni-enrichment region as the nucleation sites, keeps a cube-cube orientation relationship with the M₂₃C₆ carbides and bears the Kurdjumov-Sachs (K-S) relationship with the martensite. Moreover, the reversed austenite formed inside the martensite laths is also confirmed. The mechanism for formation of the reversed austenite is discussed in detail.     

30CrNi3MoV低合金超高强钢中的马氏体相变

通过对马氏体的显微组织进行分析,并结合线膨胀试验得到的相变动力学信息研究了30CrNi3MoV低合金超高强钢中的马氏体相变特征.结果表明:淬火冷却30CrNi3MoV钢的相变产物包括低碳板条状和高碳针状两种马氏体形态,两者的形成在动力学曲线中截然分开.板条马氏体形成于Ms以下的较高温(310℃～260℃),相变过程中发生了碳的重新分配,造成富碳奥氏体微区的形成;高碳针状马氏体形成于Ms以下的较低温(260℃～170℃),由富碳奥氏体微区转变而成.板条马氏体形成速率远高于针状马氏体.     

Pressure induced martensite transformation in plain carbon steel

Abstract

In the present study, plain low carbon steel with 0·033 wt-% carbon content was subjected to severe pressure during continuous cooling from austenite region. The pressure increased gradually and then suddenly released by the breakdown of ram under pressure. As a result, a microstructure composed of 80% lath martensite and 20% ferrite was produced. Results showed that the martensite formation is not due to the effect of cooling rate but the effect of hydrostatic pressure on the austenite to ferrite transformation start temperature Ar₃.     

An approximate approach for the calculation ofM s in iron-base alloys

Having estimated the critical driving force associated with martensitic transformation,ΔG ^α→M, as $$\Delta G^{\alpha \to M} = 2.1 \sigma + 900$$ whereσ is the yield strength of austenite atM _s, in MN m^?2, we can directly deduce theM _s by the following equation: $$\Delta G^{\gamma \to {\rm M}} |_{M_S } = \Delta G^{\gamma \to \alpha } + \Delta G^{\alpha \to M} = 0.$$ The calculatedM _s are in good agreement with the experimental results in Fe-C, Fe-Ni-C and Fe-Cr-C, and are consistent with part of the data in Fe-Ni, Fe-Cr and Fe-Mn alloys. Some higher “M _s” determined in previous works may be identified asM _a,M _s of surface martensite or bainitic temperature. TheM _s of pure iron is about 800 K. TheM _s in Fe-C can be approximately expressed as $$M_S (^\circ {\text{C}}) = 520 {\text{--- }}\left[ {{\text{\% C}}} \right]{\text{ }}x 320.$$ In Fe-X, the effect of the alloying element onM _s depends on its effect onT ₀ and on the strengthening of austenite. An approach for calculation of ΔG ^γ→α in Fe-X-C is suggested. Thus dM _s/dx _c in Fe-X-C is found to increase with the decrease of the activity coefficient of carbon in austenite.     

Anomalous kinetics of lath martensite formation in stainless steel

The kinetics of lath martensite formation in Fe–17·3 wt-%Cr–7·1 wt-%Ni–1·1 wt-%Al–0·08 wt-%C stainless steel was investigated with magnetometry and microscopy. Lath martensite forms during cooling, heating and isothermally. For the first time, it is shown by magnetometry during extremely slow isochronal cooling that transformation rate maxima occur, which are interrupted by virtually transformation free temperature regions. Microscopy confirms martensite formation after athermal nucleation of clusters followed by their time dependent growth. The observations are interpreted in terms of time dependent autocatalytic lath martensite formation followed by mechanical stabilisation of austenite during the transformation process.     

Structure and hardness of laser-processed Fe-0.2%C-5%Cr and Fe-0.2%C-10%Cr alloys

The effects of high quench rates achieved through a laser surface-alloying process on the martensitic transformations of Fe-0.2%C-Cr steels (up to 10wt% chromium) were investigated. The microstructural variables: martensitic morphology and its substructure; amount of retained austenite; and carbide precipitation were characterized by optical metallography and thin foil transmission electron microscopy (TEM). The microstructures exhibited were fully lath martensitic type, the substructure of which consisted of dislocations. The morphology and substructure of martensite were influenced neither by the chromium content of the alloy nor by the laser parameters (or melt depth) employed. Thin films of retained austenite were observed at packet and lath boundaries of martensite and at prior austenite grain boundaries. The amount of retained austenite was found to decrease with decrease in melt depth. TEM studies also revealed the presence of more or less continuous cementite films both at the lath boundaries and within the laths. Microhardness measurements had shown that the hardness increased with increase in chromium content of the alloy but appeared to be independent of melt depth.     

Effects of carbon content on the formation of nano/ultrafine grained low-carbon steel treated by martensite process

Martensite process consists of simple cold rolling and annealing of martensite structure. This process can produce a nano/ultrafine grained structure in carbon steels. The key to the process is to start from martensite structure. Carbon can hardly affect the morphology of lath martensite. In this study, the effects of carbon content on required rolling reduction and formation mechanisms of nano/ultrafine ferrite grains are investigated. For this purpose, two low-carbon steels containing 0.04 and 0.08 wt% carbon are processed. Rolling reduction ranging from 40 to 80% is used. Specimens are annealed at various temperatures from 450 to 700 °C. After annealing at 500 °C, ultrafine ferrite grains are obtained in the 80% cold-rolled specimen containing 0.08 wt% C. The formation of these ultrafine ferrite grains is interpreted here in terms of the relationship between the size of the laths and the blocks of the lath martensite and required rolling reduction for dividing them into sub-micron scale sections. The required rolling reduction increases with decreasing the carbon content of steel.     

Effects of solution treatment temperature on grain growth and mechanical properties of high strength 18%Ni cobalt free maraging steel

Abstract

The effects of solution treatment (ST) temperature (1073–1473 K) on the prior austenite grain size, microstructure, and mechanical properties of a 2000 MPa grade 18%Ni Co free maraging steel have been investigated. The results show that prior austenite grain size normally increases with increase of ST temperature. Strength and ductility in the solution treated condition are independent of both ST temperature and prior austenite grain size due to constant martensite lath spacing and dislocation tangles. In the solution treated + aged condition, the relationship between yield strength and prior austenite grain size follows the Hall- Petch equation, and ductility improves until the ST temperature used is >1373 K. Accordingly, the fracture mode transforms from intergranular to transgranular at a critical prior austenite grain size of ~ 150 μ m, because of severe segregation of Ni₃(Mo,Ti) and reverted austenite at prior austenite grain boundaries and martensite lath boundaries. The variation of Charpy V notch impact energy with increase of ST temperature in both the solution treated and solution treated + aged conditions is similar to that of the tensile ductility. The fracture toughness K_IC, however, increases with increase of ST temperature. No thermal embrittlement resulted from the Ti(C,N,S) inclusion segregation at prior austenite grain boundaries and martensite lath boundaries in the high temperature solution treatment.     

Secondary carbide precipitation in an 18 wt%Cr-1 wt% Mo white iron

High chromium (18%) white irons solidify with a substantially austenitic matrix supersaturated with chromium and carbon. The austenite is destabilized by a hightemperature heat treatment which precipitates chromium-rich secondary carbides. In the as-cast condition the eutectic M₇Ca₃ carbides are surrounded by a thin layer of martensite and in some instances an adjacent thicker layer of lath martensite. The initial secondary carbide precipitation occurs on sub-grain boundaries during cooling of the as-cast alloy. After a short time (0.25 h) at the destabilization temperature of 1273 K, cuboidal M₂₃C₆ precipitates within the austenite matrix with the cube-cube orientation relationship. After the normal period of 4 h at 1273 K, there is a mixture of M₂₃C₆ and M₇C₃ secondary carbides and the austenite is sufficiently depleted in chromium and carbon to transform substantially to martensite on cooling to room temperature.     

Splat cooling of iron-molybdenum-carbon alloys

Two molybdenum alloy steels, which normally undergo the austenite martensite phase transformation during solid state quenching, have been rapidly cooled from the melt in a controlled atmosphere gun splat cooling device. The matrix phases produced were-ferrite, martensite, and austenite; the carbide Mo₂C was also present in the as-quenched condition in the higher alloy composition studied. The amount of austenite retained to room temperature was found to be inversely related to the cooling rate. The morphology of the martensite in the splat-cooled alloys exhibited a marked change compared with its characteristic appearance in the conventionally solid-state quenched material. This was attributed to the dual effect of increased cooling rate on carbon segregation in the parent austenite and of decreased section thickness in which the martensite forms. The degree of solute segregation observed in the microstructures of the matrix phases was shown to depend on the extent of the equilibrium liquidus-solidus temperature range. The precipitation of Mo₂C during ageing in the range 600 to 700° C paralleled the behaviour of conventionally quenched and tempered alloys, although local inhomogeneities did produce precipitation phenomena not encountered in solid-state quenched material.     

Isothermal decomposition of supercooled austenite in steels

Abstract

A brief rationalisation is made of the isothermal decomposition kinetics of supercooled austenite in steels. From the experimental evidence of the past several decades, it is concluded that each type of isothermal decomposition product, i.e. grain boundary allotriomorphs, Widmanstätten ferrite (and cementite), pearlite, upper bainite, lower bainite, lath martensite, and twinned martensite, has its own independent C-curve in time–temperature–transformation (TTT) diagrams. Special emphasis is placed on the isothermal transformation kinetics of martensite. It is demonstrated that martensite transformation follows C-curve kinetics in isothermal conditions; this is a general rule and holds for all steels. The reason why most experimental TTT diagrams fail to display separate C-curves for different products is briefly explained. A temperature–composition–product (TCP) diagram is constructed for Fe–C alloys (plain carbon steels) to depict the general pattern of the decomposition process and to display the conditions for the formation of the various decomposition products.

MST/1623