Butterfly Martensite Research Articles

Conditions of warm ausforming of low-alloy medium-carbon steel for making fully martensitic microstructure and its refining effect of martensite

This study elucidates the process conditions of ausforming low-alloy medium-carbon steel that can be used to make a fully martensitic microstructure. In the process, the austenitized steel at a high temperature is cooled to warm temperature kept in the supercooled austenite state, then deformed and quenched. The result of this study indicates that the duration time needs to be shortened sufficiently, because supercooled austenite of low-alloy steel only exists for a few seconds. The change of martensitic microstructure is also investigated. Originally, microstructure of low-alloy medium-carbon steel martensite consists of fine lath and coarse butterfly martensite. The frequency of coarse butterfly martensite is decreased by ausforming but is not zero for lower strain levels; coarse butterfly martensite does not form at higher strain levels. This change is presumed to be caused by lattice defects introduced into the austenite matrix by ausforming. In this research, changes in the martensite block width and hardness are investigated. The hardness increases as the block width decreases.

Investigation of Three-Dimensional Geometry of Butterfly Martensite and Analysis of Relationship between Hardness of Lath/Butterfly Two-Phase Martensite and its Grain Thickness in Medium-Carbon Steel

Medium-carbon steel with approximately 0.6 wt% carbon is widely used for mechanical applications, because the martensite contained in it is harder than that of low-carbon and high-carbon steel. The microstructure of steel martensite varies depending on the carbon content, and the microstructure of medium-carbon steel martensite are a mixture of lath martensite (LM) and butterfly martensite (BM). The geometry of LM grains are thin plates having thicknesses of 0.2–0.5 μm. As for BM, some researches demonstrated that the region of martensite spreads in the depth direction of the observation surface. However, the three-dimensional geometry of BM grains remains unclear. Through cross-sectional observation using focused ion beam (FIB), the present study demonstrates that one BM grain is formed via collision of two plates. Previous research shows that the hardness of LM is inversely correlated with the thickness of the grains. The FIB observation result indicates that geometry of LM and BM grains are both plate-like. In the present study, whether the same relationship is valid for the mixture of LM and BM grains is investigated. The results show that the hardness of medium-carbon steel martensite increases according to the decrease in thickness of constituent grains of the two-phase structure of LM and BM.

Classification of Twin Arrangements in Butterfly Martensite Grains and Analysis of Relationship between Twin Arrangement and Butterfly Wing Angle in Medium-Carbon Steel

Classification of Twin Arrangements in Butterfly Martensite Grains and Analysis of Relationship between Twin Arrangement and Butterfly Wing Angle in Medium-Carbon Steel

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.

A growth manner of butterfly martensite

The growth of butterfly martensite in an Fe-Ni-V-C alloy was investigated using an optical microscope and transmission electron microscope through observing its morphology. The present butterfly martensite is dislocation-type, with a few fine twins. One wing of a butterfly martensite is layered more heavily than the other and the concave part is layered more obviously than other regions. Most butterfly martensites have a lath plate outside of and next to one wing. The outside martensite plates grow first, and then two wings of butterfly martensite. The smooth parts of a butterfly martensite grow earlier than the layered regions. A wing of a butterfly martensite grows like a group of lath martensites.

Catalytical kinetics of butterfly martensite

The stress fields around the first butterfly martensite in an Fe-Ni based alloy were calculated. The calculation results inducate that there are tensile stress fields near the convexity and the two ends of a butterfly martensite. There is a compressive stress field near the concavity. New butterfly martensite is likely induced not only near the convexity but also probably near the concavity. The butterfly martensites induced in tensile stress fields are bigger than those in compressive stress fields. The second butterfly martensite is likely induced near the convexity within 2πm region. All these results agree with the experimental observation.

Morphology transition of deformation-induced lenticular martensite in FeNiC alloys

The morphology and habit planes of deformation-induced lenticular martensite were investigated by optical and transmission electron microscopy in Fe30Ni and Fe30Ni0.11C alloys. Transitions in morphology were observed with progressive deformation levels going from lenticular to butterfly and to compact martensite for the Fe30Ni alloy and lenticular to butterfly and to small butterfly martensite for the Fe30Ni0.11C alloy. The habit planes changed from {225} f or {259} f for the thermal lenticular martensite to {111} f for the strain-induced martensite. The morphology and crystallography of the small butterfly martensites was also investigated. A change in the orientation relationships from K- S to N- W relations was also observed. These changes were attributed to the contribution of mobile dislocations which modified the shear mode from twinning to slip, and to a plastic accomodation of transformation strains.

The butterfly martensite nucleation in an iron and nickel alloy

Some butterfly martensite nuclei were observed in an Fe-27.6Ni-0.89V-0.05C alloy. The alloy was austenitized at 1200°C for 1 hour. Some samples were aged at 850° C for 40 minutes and quenched in 10% brine at room temperature. All the samples were cooled in ethyl alcohol for martensite transformation.A nucleus in an unaged specimen is shown in Fig.1. The nucleus has certain contrast different from the matrix and is shaped like one wing of a butter fly martensite. The SADP of the circled region is measured to be: da=dh, and approximate to dγ(111) and dm(110) with ∠AOB = 55° . It is similar to [011]f.c.c and b patterns in the anglez ∠AOB and the ratio ra/rb, respectively. The SADP shows that the structure of the nucleus is between f.c.c and b.c.c. The dislocation structure within the nucleus is shown in Fig.2. Their Burgers vectors and line directions are also given in it. There are many long dislocations near it without dislocations piled up as shown in Fig.3.Long dislocations are closed at one end as an envelope.

The compound formation mechanism of butterfly martensite

The butterfly martensites formed in Fe-1.20% C, CrMn, GCr18Mo, and 25Cr2Ni4WA steels have been investigated by optical and transmission electron microscopy. It has been found that the mid-rib of a butterfly martensite is a thin-plate martensite and that the formation of butterfly martensite is a multi-stage process. In the initial stage, crossed or kinked thin-plate martensites are formed. Subsequently, these become mid-ribs, grow more, then create butterfly martensite in various forms. Crossed thin-plate martensite thickens into butterfly martensite with tails, while the kinked form transforms to tailless butterfly martensite by thickening on only one side of the wings. The formation of butterfly martensite may not be a single continuous process, but rather a compound one comprised of separate stages.

Further investigations on martensites in Fe-0.5 wt% C and Fe-0.5wt% C-24 wt% Ni melt spun ribbons

The types of martensites occurring in Fe−C and Fe−Ni−C melt spun ribbons in the as-solidified and after heat treatment conditions has been investigated using transmission electron microscopy. It has been found that melt spinning of Fe−0.5 wt% C induces a dislocated martensitic structure at room temperature even for grains as fine as 3μm. The martensite laths are separated by thin layers of retained austenite. The martensite/austenite orientations are controlled by both the K-S and N-W relationships. Some of the martensite laths exhibit a twin relation. The dislocations within the laths are mainly screw. The counterpart alloy, Fe−0.5 wt% C−24 wt% Ni exhibits 100% austenite under similar conditions. Increasing the austenite grain size upon annealing enhances martensitic transformation. The present work is concentrated on the details of butterfly martensite obtained in these ribbons.

Butterfly martensite in industrial steels

The morphology and the conditions of formation of butterfly martensite observed in carbon tool steels, low-alloy tool steels and bearing steels have been investigated. The results indicated that the butterfly martensite in various steels in three dimensional space can be classified into two types: i.e., the butterfly martensite with tails and the butterfly martensite without tails. But in the two dimensional metallographic morphology they may be classified as three groups: i.e., the butterfly martensite with double tails, the butterfly martensite with a single tail, and the tailless butterfly martensite. The two wings of butterfly martensite comprise a whole entity and their mid-rib planes can exhibit crossing, kinking, or branching. There are ten possible stretched angles of space between the two wings of butterfly martensite. They are: 41°, 43°, 59°, 61°, 83°, 97°, 119°, 121°, 137°, and 139°. Among them, 137°, 121°, 139° and 119° appear the most frequently. The quantity of butterfly martensite increases with the increase in carbon content and in austenitizing temperature. The morphological features and the quantity of butterfly martensite are little affected by variations in alloying elements at low concentrations.

Electron microscopy studies of butterfly martensite

Crystallographic and substructural features of butterfly martensite formed in Fe-Ni-Cr-C alloys were investigated using transmission electron microscopy.{112} b twins, lengthy dislocations and two types of {101} b planar defects were observed as substructures. The {112} b twins were not distributed uniformly and the twin edges inside a martensite plate were irregular in contrast to those in Fe-Ni lenticular martensitcs. The length dislocations were parallel to the 〈 111〉 b and the dominant set was almost parallel to the expected shape strain direction. In the surrounding of martensite plate austenite slips on (111) f plane which may be associated with the accommodation of the transformation shape strain were observed. The austenite-martensite orientation relationship was (111)ƒ/(011) and [101]y 2.0 deg from [TT1] b . In general, the complex features of the substructures of butterfly martensite and the characteristics of {112} twins and {101} planar defects were found to be quite similar to those observed in nonpaired {225} plate martensites.

THE MORPHOLOGY AND SUBSTRUCTURE OF BUTTERFLY MARTENSITE IN FERROUS ALLOYS

A transmission electron microscopy and diffraction study of butterfly martensite in Fe-Ni-Cr-C alloys was carried out with particular attention to details of the martensite substructure. It was confirmed that a butterfly martensite is consisted with two plates with (252)f and (252)f habit plane variants having the junction plane near {112Ib twins, lengthy dislocations and {lO1>b planar defects were observed as substructures of butterfly martensite. Observed {112Ib twins were not shaped as parallelograms, and twin edges Anside a martensite plate were not parallel to the twinning shear direction [111]b in contrast to those in Fe-Ni lenticular m art en site. The majority of the lengthy dislocations were seen parallel to which is close to the shape strain direction of the (252)f variant. (O1l)b planar defects considered to be produced by the coalescence of martensite platelets were often observed. In the surrounding austenite slip on planes probably produced by the accommodation of the transformation shape strain were observed. In general, although butterfly martensite exhibits a unique paired morphology, its substructures were found to be quite similar to those observed in other morphology of martensite with the same {225lf habit.

Fe-Ni-C合金におけるバタフライマルテンサイトの生成温度域とそれに対するCr元素添加の影響

Fe-Ni-C合金におけるバタフライマルテンサイトの生成温度域とそれに対するCr元素添加の影響

Morphology and crystallography of martensite in Fe-23% Ni-0·38% C alloy induced by plastic deformation of austenite

A tensile deformation at the temperatures between Ms and Md induces in Fe-23% Ni-0·38% C alloy the so-called butterfly martensite, whose morphology is different from that of thermally transformed martensite. The amount of these butterflies increases gradually with deformation and contributes to the transformation — induced plasticity. The difference in morphology corresponds with a different structure, substructure and crystallography and it is caused by the change of transformation mechanism.