Cavitation erosion resistance of Fe-26Mn-6Si-7Cr-1Cu shape memory alloy

Abstract

The cavitation erosion of low stacking fault energy Fe-26Mn-6Si-7Cr-1Cu shape memory alloy has been investigated in water using an ultrasonic vibratory apparatus, and compared with the behaviour of 0Cr13Ni5Mo stainless steel. It is shown that Fe-26Mn-6Si-7Cr-1Cu alloy has higher cavitation erosion resistance than 0Cr13Ni5Mo stainless steel. The cavitation erosion mechanism of Fe-26Mn-6Si-7Cr-1Cu was studied by examining the eroded surface using X-ray diffraction (XRD) and scanning electron microscopy (SEM). During early stages of cavitation erosion, Fe-26Mn-6Si-7Cr-1Cu alloy undergoes strain induced martensitic transformation. Exposure to further cavitation results in the deformation of ? martensite. The boundaries of ? martensite impede plastic deformation, leading to strain accumulation and subsequent material removal. On the basis of an XRD study and indentation tests, the better cavitaton erosion resistance of Fe-26Mn-6Si-7Cr-1Cu alloy is mainly ascribed to strain induced martensitic transformation, which can absorb impact energy without damage.     

The low temperature magnetic properties of austentic FeCrNi alloys: 1. The effect of nickel concentration in FeNi-20 wt% Cr alloys

The magnetic properties of ternary FeCrNi alloys containing 20 wt% Cr and 11 – 32 wt% Ni have been studied over a range of temperatures from 10 – 300 K by both ac and dc techniques, in fields from 5 A m^?1 to 6 MA m^?1. Apart from small amounts of δ ferrite in some alloys, the matrix constituent at room temperature is the paramagnetic fcc austenite phase. On cooling to low temperatures all alloys exhibit peaks in the temperature dependence of ac susceptibility, but in the 20 Cr/11 Ni alloy this is superimposed on the effects of an irreversible martensitic transformation below 195 K. Although a quantitative analysis of the results enables appropriate Neel and Curie temperatures to be determined for the alloys, it also reveals that the changeover from antiferromagnet behaviour at 0 K, with increasing nickel content, is a very gradual process involving the formation of increasing concentrations of superparamagnetic clusters, the moments, numbers, and sizes of which have been determined in alloys containing 16, 18, and 20 wt% Ni. The practical consequences of these findings are discussed in terms of the application of stainless steels in cryogenic engineering and appropriate susceptibility values are tabulated for use as design data.     

Cryogenic stability of retained austenite in Fe–Cr–Ni weld metals obtained by laser welding

Fe–Cr–Ni alloy is the most potential substitute for Ni-based alloys as consumable in low-temperature nickel steel welding. In this study, six groups of Fe–Cr–Ni weld metals with different chemical composition were fabricated by single-pass laser welding. The volume fraction of retained austenite (RA) in the weld metals in as-welded condition increased from 0 to 30.2% with the increment of amounts of alloy elements (Cr, Ni and Mn). The thermal stability of RA was investigated by deep cryogenic treatment (DCT) to see whether sufficient RA can be maintained at low temperature. The results revealed that if the initial content of RA was?<??~?18%, RA would not transform into martensite after DCT. The surrounding martensite can hinder the transformation of RA, which plays a dominant role in the cryogenic stability of RA. The higher the strength/hardness of the surrounding martensite, the stronger the resistance to the transformation of RA. The carbon content of the surrounding martensite is the crucial factor affecting its strength/hardness. However, if the alloy elements amounts were too large, the thermal stability of RA would decrease and some of RA would transform to martensite after DCT. The newly formed martensite (fresh martensite) increased strain concentration of the weld metal, which enhanced the strain energy of martensite transformation and therefore restrained the further transformation of RA. The content of RA in the Fe-13.497Cr-7.249Ni-0.93Mn and Fe-15.548Cr-7.622Ni-0.961Mn weld metals after DCT dropped to?~?18%. The optimum of initial content of RA in the weld metals for low-temperature toughness is?~?18%.

     

Transformation-induced plasticity in Fe-17.20% Cr-7.34% Ni steel

The characteristics of transformation-induced plasticity of Fe-17.20% Cr-7.34% Ni steel were studied. The maximum value of fracture elongation occurred at 20° C in the temperature rangeMs (–196° C) toMd (75° C), and this maximum elongation was brought about by the delay of necking. The percentage of martensite per unit tensile strain after the martensite transformation was then 2.20     

Nickel-free austenitic steels for cryogenic applications: The Fe-23% Mn-5% Al-0.2% C alloys

A study has been made of the influence of carbon and aluminium additions on the mechanical properties under uniaxial tensile loading and by Charpy V notch impact tests at room temperature and ?196°C on the Fe-24 % Mn alloys. It is concluded that the Fe-24 % Mn-5 % Al-0.2 % C appears as a new nickel-free iron-based alloy which is particularly interesting for cryogenic applications. In these alloys, both additions of carbon and aluminium contribute to the stability of the austenitic phase by suppressing the γ → ? martensitic transformation of the binary Fe-24 % Mn and to the solution hardening of the manganese-rich austenitic alloy.     

The structure of splat-cooled Fe-20% Cr-25% Ni austenitic steel

A controlled atmosphere splat quenching gun has been used to produce splats of Fe-20% Cr-25% Ni. Three types of structure were observed by thin foil electron microscopy, namely high-angle cellular, low-angle cellular, and linear arrays of dislocation loops, which were determined primarily by the heat transfer conditions. In the thin, most rapidly cooled areas (lift-off regions) high-angle cellular structures were observed which were largely free of defects. As the cooling rate decreases there was a greater tendency for low-angle cellular structures to form, but at intermediate cooling rates bands of dislocation loops were observed. These are explained in terms of solute segregation and vacancy coalescence along 〈100〉_gg directions in the austenite.     

Fe-25Cr-35Ni系合金中的第二相，

采用光学显微镜和扫描电子显微镜以及能谱仪研究了Fe-25Cr-35Ni系合金中的第二相种类和分布。结果表明：Fe-25Cr-35Ni系合金中的第二相主要有3种,即富铌相、氮化物相和碳化物相;其中,富铌相主要沿晶界分布,并且富铌相之间存在成分差异;氮化物相主要为TiN,一般与富铌相共生,其分布主要与氮化物偏析有关;碳化物相数量较多,分布较广泛,晶内、晶界、孪晶界和孪晶内均有分布,经高温固溶处理后可固溶到奥氏体基体中。     

A Metallographic Study of the High-Temperature Decomposition of Austenite in Alloy Steels Containing Mo and Cr

Optical and electron microscopy have been used to study the complex microstructures developed during the isothermal decomposition of austenite above 550°C in Fe-4Mo-0.2C and Fe-10Cr-0.4C alloy steels. As the transformation temperature is decreased, the decomposition products change from the disordered growth of nodular alloy pearlites to blocky ferrite structures containing fine dispersions of alloy carbide, and finally to acicular ferrite structures also containing alloy carbide. The branched M₆C and M₂₃C₆ of the high-temperature pearlite is replaced by Mo₂C and M₇C₃ with a fibrous or lath morphology in the lower temperature structures. The decomposition microstructures are explained in terms of a model which takes account of the growth of particular alloy carbides at the interfaces of ferrite allotriomorphs, where the growth mechanism, and hence the morphology, is sensitive to transformation temperature.     

Strain induced martensitic transformation of Fe-20Cr-5Mn-0.2Ni duplex stainless steel during cold rolling: Effects of nitrogen addition

The effects of nitrogen addition on the strain induced martensitic transformation (SIMT) behavior of duplex stainless steel (D-STS) with the low nickel content were examined in a wide range of strain by means of cold rolling. Nitrogen of 0.1, 0.2 and 0.3 wt.% was added into Fe-20Cr-5Mn-0.2Ni D-STS (in wt.%) and cold rolling was conducted up to the effective strain of ∼1.45 after annealing at 1000 °C for 30 min. In the as-annealed state, the austenite fraction increased with increasing the N content. Regardless of the N content, the ferrite grain size was coarser than that of austenite. The stacking fault energy of austenite of the present D-STSs inferred by the element partitioning analysis was low enough so that SIM transformation is available. Accordingly, during cold rolling, SIMT occurred in austenite with a sequential manner of austenite → ? martensite → α′ martensite with increasing strain. By contrast, the deformed microstructure of ferrite was dominated by dislocation cells. The SIM fraction, which was normalized with reference to the initial austenite fraction in order to eliminate its effect, increased with increasing the N content. Along with the present results, the factors influencing the SIMT kinetics in the present D-STSs with the different N content were discussed.     

Effect of heat treatment on martensitic transformation in Fe-12.5%Mn-5.5%Si-9%Cr-3.5% Ni alloy

In this study, thermally-induced martensitic transformation (γ(fcc) → ε(hcp)) in Fe-12.5%Mn-5.5% Si-9%Cr-3.5% Ni (weight) alloy was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effect of cooling rate was investigated. It was observed that fast cooled sample exhibited regular overlapping of stacking faults and ε martensite plates were formed parallel to each other. TEM investigations showed that the orientation relationship between γ-ε phases corresponds to Shoji-Nishiyama type orientation relationship.     

形变时效对FeMnSiCrNi合金形状回复率和相变温度的影响

为了进一步提高Fe-14Mn-6Si-8Cr-5Ni合金的形状记忆效应,对固溶态合金采用了形变时效的方法处理,并利用光学显微分析、X射线衍射分析和透射电子显微分析的测试手段分析了时效温度和时效时间对合金形状回复率和相变温度的影响.结果表明,固溶态合金经10%拉伸和600℃时效10 min时,形状回复率提高幅度最大,由固溶态的48%提高到84.7%,并且合金γ→ε马氏体转变的起始温度Ms由固溶态的34℃降低到13.2℃.合金的形状回复率得到提高的主要原因是合金中热诱发ε马氏体已经消失,组织为奥氏体和大量定向α’马氏体,这样的组织特征有利于应力诱发γ→ε马氏体相变以及它们的逆相变.     

Fe-15Mn-5Si-9Cr-5Ni形状记忆合金回复性能及微观结构演变

制备了含不同Al量的2种Fe-15Mn-5Si-9Cr-5Ni形状记忆合金,测定了多次训练之后的记忆效应,并用金相显微镜和透射电镜对材料的微观组织结构进行了观察,结果表明,训练之后的合金记忆效应明显提高,尤其是第二次训练会产生显著效果.含过量的铝对形状记忆效应不利.训练后的合金内存在宽大的层错,层错增多,有利于形状记忆效应的提高.     

Tensile behaviour of two DO3-ordered alloys: Fe3Si and Fe-20 at % Al-5 at % Si

The tensile behaviour, including fracture modes and deformation substructures, of two powder-produced DO₃-ordered alloys having compositions Fe-25 at % Si (Fe₃Si) and Fe-20 at % Al-5 at % Si, has been investigated from room temperature to 800° C. The brittle-to-ductile transition temperature for the Fe₃Si alloy occurred at a temperature between 500 and 550° C, while that of the Fe-20 at % Al-5 at % Si alloy was approximately room temperature. In both alloys fracture occurred by transgranular cleavage at room temperature, with the occurrence of an increasing proportion of intergranular cavitation with increasing temperature. At low strains plastic deformation occurred chiefly by movement of perfect superlattice dislocations which, with increasing strain, dissociated to produce next-nearest-neighbour antiphase boundary trails.     

Effects of Heat Treatment on Hardness and Dry Wear Properties of a Semi-Solid Processed Fe-27 wt pct Cr-2.9 wt pct C Cast Iron

EfFects of heat treatments on hardness and dry wear properties of a semi-solid processed Fe-26.96 wt pct Cr- 2.91 wt pct C cast iron were studied. Heat treatments included tempering at 500℃, destabilisation at 1075℃ and destabilisation at 1075℃ plus tempering at 500℃, all followed by air cooling. Electron microscopy revealed that, in the as-cast condition, the primary proeutectic austenite was round in shape while the eutectic M7C3 carbide was found as radiating clusters mixed with directional clusters. Tempering did not change the microstructure significantly when observed by scanning or transmission electron microscopy. Destabilisation followed by air cooling led to a precipitation of secondary M23C6 carbide and a transformation of the primary austenite to martensite. Precipitation behaviour is comparable to that observed in the conventionally cast iron. Tempering after destabilisation resulted in a higher amount of secondary carbide precipitation within the tempered martensite in the eutectic structure. Vickers macrohardness and microhardness in the proeutectic zones were measured. Dry wear properties were tested by using a pin-on-disc method. The maximum hardness and the lowest dry wear rate were obtained from the destabilisation-plus-tempering heat treatment due to the precipitation of secondary carbides within the martensite matrix and a possible reduction in the retained austenite.     

A Comparative Study on Crystal Structure and Magnetic Properties of Fe-Mn-Si and Fe-Mn-Si-Cr Alloys

In this study, the crystal structure and magnetic properties of Fe-30Mn-6Si (wt.%) and Fe-30Mn-6Si-5Cr (wt.%) alloys were compared by using X-Ray Diffraction (XRD) and Physical Properties Measurement System (PPMS) measurements. Detailed analysis of diffractograms at room temperature demonstrates that the Cr-free sample contains austenite and martensite phases, but for Cr-added sample the martensite phase disappears. According to micro hardness measurements, the presence of chromium decreased the hardness of the alloy. The magnetic saturation values at room temperature were measured as 11.32 emu/g for Fe-30Mn-6Si (wt.%) alloy and 18.34 emu/g for Fe-30Mn-6Si-5Cr (wt.%) alloy. The addition of Cr increased the magnetic saturation value of FeMnSi alloy while for both systems the hysteresis loop was quite narrow. As a result, both alloys exhibited soft magnetic characteristic.     

Effect of carbon content on solidification behaviors and morphological characteristics of the constituent phases in Cr-Fe-C alloys

A combination of transmission electron microscopy, electron backscatter diffraction and wavelength dispersive spectrum has been used to identify crystal structure, grain boundary characteristic and chemical composition of the constituent phases in Cr-Fe-C alloys with three different carbon concentrations. Depending on the three different carbon concentrations, the solidification structures are found to consist of primary α-phase and [α + (Cr,Fe)₂₃ C₆] eutectic in Cr-18.4Fe-2.3 C alloy; primary (Cr,Fe)₂₃ C₆ and [α + (Cr,Fe)₂₃ C₆] eutectic in Cr-24.5Fe-3.8 C alloy and primary (Cr,Fe)₇ C₃ and [α + (Cr,Fe)₇ C₃] eutectic in Cr-21.1Fe-5.9 C alloy, respectively. The grain boundary analysis is useful to understand growth mechanism of the primary phase. The morphologies of primary (Cr,Fe)₂₃ C₆ and (Cr,Fe)₇ C₃ carbides are faceted structures with polygonal shapes, different from primary α-phase with dendritic shape. The primary (Cr,Fe)₂₃ C₆ and (Cr,Fe)₇ C₃ carbides with strong texture exist a single crystal structure and contain a slight low angle boundary, resulting in the polygonal growth mechanism. Nevertheless, the primary α-phase with relative random orientation exhibits a polycrystalline structure and comprises a massive high-angle boundary, caused by the dendritic growth mechanism.     

The low temperature magnetic properties of austenitic Fe---Cr---Ni alloys. 3. The effect of chromium concentration in Fe---Cr-17wt% Ni alloys

The magnetic properties of ternary Fe---Cr---Ni alloys containing 17 wt% Ni and 18–24% Cr have been studied over a range of temperature from 4.2–35 K by both ac and dc techniques in fields from 5 Am⁻¹ − 6 MA m⁻¹. At room temperature, the matrix of all the alloys was the paramagnetic fcc austenite phase (with an unavoidable 0.1 – 0.5 wt% δ ferrite) and on cooling all the alloys showed distinct peaks in ac susceptibility, though equivalent peaks in dc susceptibility were only observed at low fields in the alloy containing 18 wt% Cr. An analysis of the magnetization/field/temperature results showed that all the alloys were superparamagnetic at low temperature, due to the formation of ferromagnetic clusters in the disordered antiferromagnetic matrix, and that the chromium atoms make no measureable contribution to the magnetization of these clusters. Appropriate susceptibility values are tabulated for use as design data.     

Oxydation a haute temperature d'alliages Fe-20Cr-xAl

The oxidation of Fe-20Cr-xAl (x=2, 4, 6) and Fe-20Cr-6Al-0.025 Ce alloys was studied in pure oxygen (1 bar), the temperature ranging between 800 and 1100°C. The parabolic law is obeyed when alloys containing 4 or 6 % Al are oxidised. An α-Al₂O₃ scale is formed. The oxidation rate does not depend on the composition. The reaction is more complex with the Fe-20Cr-2Al alloy. An iron and chromium oxide layer is formed at the beginning. Alumina precipitates in the metal. After a few hours a continuous α-Al₂O₃ barrier is formed. The lower oxidation rate (when compared with other alloys) is then justified and the reason why the parabolic law is not obeyed can be explained.     

Effect of Surface Modification on Room Temperature Tensile Properties for Fe-18A1 and Fe-18AI-5Cr Alloys

The influence of surface modifications on the room-temperature ductility of the Fe-18A1 and Fe-18Al-5Cr alloys was investigated. A variety of surface modification techniques including electroplating, magnetron sputtering and pack treatment were applied to the alloys to produce various protective coatings. From the results of tensile tests in air (70% humidity) at room temperature for the specimens in different surface conditions, several observations can be summarized.

1. The nickel-coating layer by using either electroplating or sputtering deposition had a limited effect on the ductility of alloys.

2. The chromium-coating layer by the electroplating technique had a negative effect on the ductility of alloys,

3. The specimen of Fe-18A1 alloy conducted by packing treatment at 1000°C for 8 hours had the highest elongation (19.6%) among those specimens studied.

The significant improvement in the tensile ductility of Fe-18A1 and Fe-18Al-5Cr alloys by pack treatment are related to the forming of aluminum oxide layer and Al-depleted alloy layer on the specimen surface.     

Carbide Effects on Creep Crack Growth of Ni-Cr Austenitic Steels

By controlling the carbon content ofFe-15Cr-25Ni alloys,three types ofmicrostructures were obtained:single phaseaustenite (γ),γ+intergranular carbides,γ+intergranular carbides + intragranular carbides.Creep crack growth behaviour of the three alloyshas been compared at 973 K and 1123 K.Intergranular carbides show higher creep crackgrowth resistance than intragranular carbides.Cav-ities nucleate at the triple junctions of grain bound-aries for single phase alloy,but at intergranularcarbides for two-phase alloys.The precipitation ofintergranular carbide not only changes thenucleation mechanism of cavities,but also inhibitsthe growth and coalescence of cavities.The precipi-tation of intragranular carbide obstructs thenucleation and growth of cavities furthermore.