The role of ε-martensite in the impact toughness of an Fe-17Mn alloy

The influence of -martensite on the cryogenic toughness of an Fe-17 wt% Mn alloy was studied in this work. Alloys were tempered at various temperatures in order to systematically increase the volume fraction of -martensite. This was followed by Charpy impact testing conducted at room temperature and at–196°C. The experimental results indicated that although room-temperature toughness was not influenced by the -martensite content, the cryogenic toughness was strongly dependent on the volume fraction of -martensite. In particular, with the exception of the alloys tempered at 400 and 450°C, the impact toughness consistently increased with -martensite content. Microstructural and fractographic evaluations using SEM and TEM suggested that the toughness improvements were attributed to the stress-induced martensite transformation. No microstructural evidence was found which could be ascribed to an effect of -martensite on the low-temperature embrittlement exhibited by Fe-Mn alloys tempered at 400–450°C.     

The deformation characteristics of metastable β -phase in a Ti-15 wt % Mo alloy

An investigation has been made of the deformation characteristics of metastable-phase in a commercial Ti-15 wt% Mo alloy within the temperature range –196 to + 146° C. Deformation occurred by slip, and by mechanical twinning on the {332} 113 system. Twinning occurred preferentially with decrease in deformation temperature. Enhanced ductility in tension, observed in the range – 94 to + 20° C, was attributed to the inhibition of necking resulting from strengthening by twin formation. A thinninginduced transformation occurred during foil preparation for electron microscopy.Formerly Imperial College.     

Mössbauer study of martensitic transformations in an Fe-29.6% Ni alloy

The application of an external stress may form band shaped strain-induced martensites in the austenite structure of Fe alloys. Mössbauer spectroscopy and transmission electron microscopy techniques were used to clarify certain properties of strain-induced martensite in an Fe-29.6% Ni alloy. The reverse transformation mechanism between thermal plate martensite and the matrix austenite was also studied. Mössbauer spectroscopy made it possible to examine the same area of the austenitic thin foils during the thermal cycles, and the volume fraction changes were determined. The habit plane and orientation relationship of strain-induced martensite were measured from the electron diffraction patterns and the latter parameter was found to be K-S type as with thermal plate martensites of the Fe-Ni alloys. The isomery shifts caused by the deformation and cycling procedures were also calculated for both austenite and martensite structures and the hyperfine magnetic field parameter of Fe-29.6% Ni strain-induced martensite was found to be equal to that of Fe-Ni-C alloys reported earlier.     

Effect of Austenite Pre-deformation on Isothermal Martensite Transformation in an Fe-20.5Ni-4.8Mn Alloy

With electron microscopy the investigation on isothermal martensite transformation in an Fe20.5Ni-4.8Mn alloy has been carried out to clarify the effect of austenite state on the transformation, by applying pre-deformation to austenite before isothermal holding. Under the condition without pre-deformation, the isothermal martensite products are lath martensite with {111}fhabit planes. Dislocations in austenite seem to contribute to nucleation of martensite, and in this nascent Stage austenite substructure has no obvious effect on martensite growth. The consequent thickening of martensite laths is apparently influenced by local austenite states, resulting in the changes in orientation, morphology as well as substructure of martensite lath. The kinetics of isothermal martensite transformation is controlled by intedece dislocation determined nucleation of martensite in primary stage, but to a larger extent, by the austenite accommodation for the shape strain of martensite in the thickening Stage     

Influence of alloy element partitioning on strength of primary α phase in Ti-6Al-4V alloy

The partitioning effect of Al(α-phase stabilizer) and V elements(β-phase stabilizer) on strength of the primary α phases in the α/β Ti-6 Al-4 V alloy with the bimodal microstructure was investigated.It was found that partitioning of Al and V elements took place in the Ti-6 Al-4 V alloy during the recrystallization process,leading to the variation of the content of Al and V elements in the primary α phases with changing the volume fraction of the primary α phase.Nanoindentation tests reveal a general trend that the strength of the primary α phases increases with decreasing the volume fraction of the primary α phases,and such trend is independent on the loading direction relative to the c-axis of the α phase.The enhanced strength is attributed to the increase of the content of Al element in the primary α phase,but it is not dominated evidently by the change of the V content.The solid solution strengthening contributed from both the elastic strain introduced by the solute atoms and the variation of the density of states was estimated theoretically.     

Identification of the metastable phase α′m in a Zn/Al alloy containing Cu and Mg

Samples of the commercial pressure-diecast alloy ZA8 containing 8.1% Al, 1.1% Cu and 0.024% Mg have been examined by transmission electron microscopy (TEM). During decomposition of the high temperature f c c phase, on cooling after casting, a transitional phase was formed within the phase in both dendrites and eutectic. This phase was identified as the metastable phase _m containing 11% Al or 23% Al, with a f c c crystal structure and lattice parameter (at 11%) of about 0.394 nm. It had adopted a symmetrical cube/cube orientation relationship with the surrounding f c c phase. The stability of this metastable phase at ambient temperatures was greatly increased by the presence of copper.     

The effect of deformation on γ-ε martensitic transformation in an Fe-Mn-Mo alloy

Effect of prior plastic deformation of austenite on the martensite start temperature, volume fraction and strength of martensite have been studied in an Fe-14.3%Mn-3.7%Mo alloy. Mo was chosen to examine the possible effect of the third alloying element in an Fe-Mn based alloy and the obtained results were compared with those of the Fe-Mn binary alloys given in the literature. Predeformation of austenite created considerable changes on the formation characteristics and also the strength of the martensitic phase and the obtained results were discussed in terms of the dislocations formed during the deformation process.     

Stereological characterization of γ′ phase precipitation in CMSX-6 monocrystalline nickel-base superalloy

The purpose of this investigation was to study in detail the means to quantitatively evaluate γ′ phase precipitation. Many of the mechanical properties of superalloys are directly influenced by the presence of the γ′ (gamma prime) precipitate phase dispersed in a γ matrix phase. The γ′ precipitates act as effective barriers to dislocation motion and restrict plastic deformation, particularly at high temperatures. Due to this, it is essential to accurately quantify the γ′ precipitate size, volume fraction and distribution. Investigations based on quantitative metallography and image analysis were performed on a monocrystalline nickel-base superalloy taking into consideration various γ′ precipitate sizes present in that alloy microstructure. The authors of the present paper propose a new method of quantifying the total volume fraction of the γ′ phase applying images of the microstructure with γ′ phase precipitates registered using light microscopy, scanning electron microscopy (at two different magnifications) and scanning transmission electron microscopy.     

Effect of cooling rate on phosphorus segregation behavior and the corresponding precipitation of γ″ and γ′ phases in IN718 alloy

Effect of segregation behaviors of P at different cooling rates on the precipitation of γ″ and γ′ phases and the corresponding strength are investigated. The precipitation of γ″ and γ′ phases during cooling is sensitive to P concentration. With increasing the concentration of P, the amount of γ″ and γ′ particles increases after air cooling. With decreasing the cooling rate, the accelerating effect of P on the precipitation of γ″ and γ′ phases decreased first and then increased, which demonstrates the concentration of P dissolved in the grain interior decreases first and then increases. The different effects of P on γ″ and γ′ phases with different cooling rates were analyzed by the kinetic characteristic of nonequilibrium grain-boundary segregation. The characteristic of nonequilibrium grain-boundary segregation of P in superalloy is further confirmed, and the phenomenon caused by critical cooling rate is captured.     

TEM study of metastable β-phase decomposition in rapidly solidified Ti-6Al-4V alloy

A new rationale is presented for various decomposition products obtained from the metastable -phase found in Ti-6A1-4V alloy produced by hot isostatic pressing comminuted melt-spun fibres and cooled to room temperature by furnace cooling. This alloy has an -matrix with about 8 vol% retained -phase, which is supersaturated with -stabilizers to such an extent that the martensitic transformation has been suppressed. The metastable -phase decomposes by different modes during continuous cooling, depending on the actual composition of individual -grains. Less enrichment of vanadium and iron favours the direct formation of the equilibrium -phase from the -matrix, while greater enrichment of vanadium and iron leads to a spinodal decomposition of the metastable -phase, resulting in a + two-phase structure. During further continuous cooling, the -phase which is lean in -stabilizers will transform into isothermal -phase. In addition, an unknown phase has also been observed in the -phase, which is typified by the appearance of 1/2{112} reflections in the SAD patterns.     

Phase field simulation of morphology evolution and coarsening of γ′ intermetallic phase in Ni–Al alloy

Abstract

The morphology evolution and coarsening behaviour of γ′ (Ni₃Al) phase in Ni–Al alloys were investigated by using the Cahn–Hilliard and Ginzberg–Laudan phase field equations, and the composition, order parameter and average particles size evolution coupling with the internal elastic strain were studied. Cubic γ′ phase was presented firstly and then the rectangle particles were formed by means of coarsening; the mechanisms of γ′ phase coarsening are Ostwald ripening or coalescence of neighbouring particles. The average particle size of γ′ phase increases rapidly at the initial stage, and it slows down at the coarsening dominated stage. The average particle size follows a cubic law of ‘Lifshitz–Slyozov’ encounter modified theory in the coarsening regime.     

The growth mechanisms of θ’ precipitate phase in an Al-Cu alloy during aging treatment

The plate-shaped θ’(Al2 Cu) precipitate acts as one of the primary strengthening phases in Al-Cu alloys.The interface,especially the semicoherent interface,between Al-Cu solid solution(αAl) and θ’ phase contains a lot of clues about phase transformations.Thus,these interfacial structures in an Al-Cu alloy after high-temperature and longtime aging have been analyzed in detail using atomic-scale high-angle annular dark-field scanning transmission electron microscopy and first-principles calculations in this work.It was found that the lateral growth of θ’ precipitates is subjected to a combination of several major mechanisms under this aging condition.Except for some common intermediate phases,two novel and striking structures were observed on the interface,which implies two alternative atomic diffusion mechanisms for θ’ precipitate growth.For one condition,the atomic diffusion from αAl to θ’ phase transformation adopts an interstitialcy mechanism based on additional Al atoms.For the other condition,the diffusion is carried out through Al atoms.Both mechanisms are distinctly different from the previous theory based on direct diffusion of Cu atoms.The first-principle calculations also confirm that these newfound diffusion processes are ene rgetically favored.     

Influence of α′ martensite on shape memory effect in Fe-14Mn-5Si-9Cr-5Ni alloy

Abstract

The strain induced γ → ε and γ → α′ martensite transformations were examined by X-ray diffraction in an Fe-14Mn-5Si-9Cr-5Ni alloy. From the original microstructures of single phase austenite and two phase austenite and α′ martensite, the alloy was examined after prestraining at room temperature and recovery heating at 673 K, and finally, its shape memory effect was determined. It was shown that the alloy with the original two phase microstructure of autensite and α′ martensite exhibits a higher degree of shape recovery. The presence of α′ martensite in the original microstructure before prestraining can considerably improve the extent of shape recovery.     

Kinetics of growth of γ′- precipitates in Ni–6·75AI alloy

Abstract

The size distributions of γ precipitates in a Ni-based alloy containing 6·75 wt–% aluminium have been measured, using dark-field electron microscopy, at five temperatures ranging from 600 to 825°C and the corresponding volume fractions have been assessed. From these distributions, the average particle sizes were obtained and plotted against time according to the kinetics laws proposed by theory for diffusion–controlled growth, interface-controlled growth, and growth in the transition region where neither process dominates. The particle-size distributions were reconstructed in terms of the ratio of the actual particle size/average size, and the resulting distributions were compared with those predicted by theory for the three controlling conditions. Although the values of activation energy obtained from the plots of particle size against annealing time are equally consistent for the three processes, the agreement between the predicted distributions and the experimental distribution is distinctly better for growth in the transition region than for the two alternative processes.

MST/167