Diffusion Of Alloying Elements Research Articles

The effect of cryogenic burnishing on the formation mechanism of corrosion product film of Ti-6Al-4V titanium alloy in 0.9% NaCl solution

Effect of cryogenic burnishing on the corrosion product film formation mechanism of Ti-6Al-4V titanium alloy has been investigated in this work. The corrosion product films of both burnished and unburnished specimens exhibited three distinct oxide layers. Compared with un-burnished specimens, burnished specimens possessed a thicker subsurface layer with high-valent oxides and additional Al2O3 in the inner layer. The chemical activity, diffusion, and distribution of alloy elements were changed by high-density dislocations, nano-grains, twins and compressive residual stress induced by cryogenic burnishing, resulting in the enhancement of the compactness, thickness, growth rate and protection of corrosion product film on the Ti-6Al-4V alloy.

The tensile behaviors and fracture characteristics of stainless steel clad plates with different interfacial status

The mechanical properties and fracture characteristics of stainless steel clad plates by vacuum hot rolling at different rolling temperature of 1100°C (S1), 1200°C (S2) and 1300°C (S3) are investigated in detail. The carburized layer thickness and grain size is gradually increased with the increasing rolling temperature. The sufficient alloy element diffusion, recovery and recrystallization at the bonding temperature of 1300°C lead to the highest interfacial shear strength and the lowest longitudinal tensile strength among the three clad plates. However, the tensile ductility is increased with the increasing rolling temperature, which is attributed to the increased interfacial bonding strength. The strong interface can effectively delay the formation of interfacial delamination crack and premature localized necking, resulting into a prolong uniform plastic deformation stage with a low stress triaxiality. In addition, there are many intergranular tunnel cracks with the length of 50–150µm presented in the carburized layer due to Cr23C6 carbides on the grain boundary, which can effectively toughen the stainless steel clad plates.

Transition between alloy–element partitioned and non-partitioned growth of austenite from a ferrite and cementite mixture in a high-carbon low-alloy steel

The growth of austenite from a mixture of ferrite and cementite in low-alloy steels can be classified into three temperature ranges above the eutectoid temperature. In the first range, the austenite growth and cementite dissolution are controlled by alloy element diffusion from the beginning. In the second, they are controlled by carbon diffusion and switch to alloy element diffusion control at a later stage. In the third, the cementite dissolution, if the ferrite matrix has transformed to austenite upon heating, is controlled by carbon diffusion until completion. The transition temperatures between these ranges are evaluated in a quaternary alloy containing Mn and Cr by Thermo-calc and DICTRA simulation, and are in essential agreement with earlier experimental results. The proposed simple approach of calculating the transition temperatures may facilitate our understanding of austenitization kinetics and the design of heat treatment, for example, homogenization and soaking, of high-carbon steels.

Microstructure and properties at bonding interface of AA4045/AA3003 aluminum alloy cladding billet prepared by semi-continuous casting

AA4045/AA3003 cladding billet was prepared by direct chill semi-continuous casting process. The macrostructures, microstructures, temperature distribution, compositions distribution and the mechanical properties at the bonding interface were investigated in detail. The results show that the cladding billet with few defects could be obtained by semi-continuous casting process. At the interface, diffusion layer of about 10 μm on average formed between the two alloys due to the diffusion of alloy elements in the temperature range from 596 to 632 °C. From the side of AA4045 to the side of AA3003, the Si content has a trend to decrease, while the Mn content has a trend to increase gradually. Tensile strength of the cladding billet reaches 103.7 MPa, the fractured position is located on the AA3003 side, and the shearing strength is 91.1 MPa, revealing that the two alloys were combined metallurgically by mutual diffusion of alloy elements.

The Effect of Interfacial Element Partitioning on Ferrite and Bainite Formation

The formation of bainitic ferrite and that of grain boundary ferrite in low alloy steels have been two of the most important and interesting research topics in the field of solid state ferrous phase transformation for several decades, and various aspects of these two transformations have been discussed extensively in the literature. Recently, a so-called Gibbs energy balance (GEB) model was proposed by the authors to evaluate alloying element effects on the growth of bainitic ferrite and grain boundary ferrite. The model predicts a growth mode transition from paraequilibrium, negligible partitioning to partitioning during the isothermal formation of bainitic ferrite and grain boundary ferrite. Transformation stasis and bay phenomenon are well explained by the GEB model and both of them are found to be due to alloying element diffusion at the interface. This overview gives a summary of the authors’ recent progress in the understanding of the growth of bainitic ferrite and grain boundary ferrite, with particular focus on the growth mode transition, the transformation stasis phenomenon and the bay phenomenon.

Effect of Alloying Element Partition in Pearlite on the Growth of Austenite in High-Carbon Low Alloy Steel

The growth of austenite from pearlite in high-carbon low alloy steel occurs with and without alloy element redistribution depending on the amount of superheating above the eutectoid temperature. The transition temperature of austenite growth (denoted PNTT) is calculated as a function of pearlite transformation temperature and subsequent holding time, which affect the degree of partitioning in pearlite, using experimental partition coefficients k θ/α of Mn, Cr, Co, Si, and Ni reported in the literature. PNTT is the highest in Cr-containing alloys which have the largest k θ/α in pearlite. Post-transformation aging, usually accompanied by cementite spheroidization, leads to a marked increase of PNTT in Mn and Cr alloys. PNTT of Ni alloy does not depend on pearlite transformation temperature because practically the formation of partitioned pearlite is severely limited in this alloy for kinetic reasons. Above PNTT, austenite growth occurs fast initially, but slows down in the order of ten seconds when the ferrite disappears, and the remaining small carbide particles dissolve very slowly under the control of alloy element diffusion.

On the transition between grain boundary ferrite and bainitic ferrite in Fe–C–Mo and Fe–C–Mn alloys: The bay formation explained

The transition between grain boundary ferrite (GBF) and bainitic ferrite (BF) formation in Fe–C–Mo and Fe–C–Mn alloys has been investigated from a diffusional point of view by a so called Gibbs energy balance approach based on solute drag theory, in which the chemical driving force is assumed to be equal to energy dissipation due to alloying elements diffusion inside the interface. The GEB approach predicted that with decreasing temperature the transformation is dominated by GBF and BF formation, respectively, which provided a rationale for the transition between GBF and BF formation. The presence of transformation bay phenomenon in Fe–C–Mo alloys was well captured by the GEB approach, and the bay phenomenon was found to be due to strong solute drag effect of Mo. No bay phenomenon was predicted in Fe–C–Mn alloys. The effect of Mo and Mn on bainite start temperature has also been well predicted by the GEB approach.

Effects of V Addition on Microstructure and Hardness of Fe-C-B-Ni-V Hardfacing Alloys Cast on Steel Substrates

Fe-based hardfacing alloys containing high volume fraction of hard phases are a suitable material to be deposited as wear resistant thick coatings. In the case of alloys containing high amount of interstitial alloying elements, a key factor affecting the performance is dilution with the substrate induced by the coating process. The present research was focused on the analysis of V-bearing Fe-based alloys after calibrated carbon and vanadium additions (in the range from 3 to 5 wt.%) to a commercial Fe-C-B-Ni hardfacing alloy. Vanadium carbides with a petal-like morphology were observed in the high-V hypereutectic alloys allowing to reach hardness values above 700 HV. The solidification range shifted to higher temperatures with increasing amount of vanadium addition and in the case of hypereutectic alloys, the gap remains close to that of the original alloy. In the last step of the research, the microstructural evolution after dilution was analyzed by casting the V-rich alloys on a steel substrate. The dilution, caused by the alloying element diffusion and the local melting of the substrate, modified the microstructure and the hardness for a relevant volume fraction of the hardfacing alloys. In particular, the drop of interstitial elements induced the transition from the hypereutectic to the hypoeutectic microstructure and the formation of near-spherical V-rich carbides. Even after dilution, the hardness of the new alloys remained higher than that measured in the original Fe-C-B-Ni alloy.

Improve oxidation resistance at high temperature by nanocrystalline surface layer.

An interesting change of scale sequence occurred during oxidation of nanocrystalline surface layer by means of a surface mechanical attrition treatment. The three-layer oxide structure from the surface towards the matrix is Fe3O4, spinel FeCr2O4 and corundum (Fe,Cr)2O3, which is different from the typical two-layer scale consisted of an Fe3O4 outer layer and an FeCr2O4 inner layer in conventional P91 steel. The diffusivity of Cr, Fe and O is enhanced concurrently in the nanocrystalline surface layer, which causes the fast oxidation in the initial oxidation stage. The formation of (Fe,Cr)2O3 inner layer would inhabit fast diffusion of alloy elements in the nanocrystalline surface layer of P91 steel in the later oxidation stage, and it causes a decrease in the parabolic oxidation rate compared with conventional specimens. This study provides a novel approach to improve the oxidation resistance of heat resistant steel without changing its Cr content.

Double-coating and porous treatments and evaluation of rolled copper foil surface

Brush plating technology was employed to directly achieve double coatings of Zn–Ni alloy and Sn–Zn alloy on the surface of rolled copper foil. Dealloying technology was applied to implement porous surface. Surface morphology, roughness, chemical composition, corrosion resistance and interfacial bonding strength of copper foil and double coatings were characterized. For the Zn–Ni alloy coating formed by brush plating at a voltage of 10–14V, the corrosion resistance was favorable and the surface roughness was about 200nm. For the brush plating Sn–Zn alloy coating, the surface roughness was about 500nm. Little diffusion of alloy elements occurred at the interface of two coatings. The brush plating Zn–Ni alloy coating is an effective barrier to copper element diffusion and the brush plating Sn–Zn alloy coating is in capacity of anti-oxidation passivation. The tensile bonding strength between the above two coatings surpassed 32MPa. The surface roughness of the dealloyed Sn–Zn alloy coating was uniform and could maximally reach to about 3μm. The peel strength of rolled copper foil combined with resin board was about 1.1N/mm, which exceeded the current standard of 0.8N/mm. It is feasible to introduce brush plating and dealloying technologies into the surface treatment of rolled copper foil. A new compact and efficient surface treatment technology for the high performance rolled copper foil is excepted to be developed.

Water Analog Experimental Method for the Diffusion and Distribution of Alloy Elements in Liquid Steel during Ingot Filling Process

To physically simulate the alloy elements transportation and diffusion in the liquid steel during the making of heavy ingots, a water analogy experimental method is presented. In this method, methylene blue dye is used as solute to simulate the alloy element carbon in the melt steel. And a measurement method is proposed to measure the concentration of the solute, in which the laser reflection intensity detection method is used. The water analogy experiment setup is constructed. The multi-concentration pouring of a 438 t ingot is investigated by this method. The concentration of the ladles decrease with the pouring consequence. The concentration variation curves with time at the tundish outlet and in the mold are acquired and analyzed. The tundish outlet concentration decreases gradually and no fluctuation occurs during the change of ladles. Negative concentration gradient is achieved at the end of pouring, which can be helpful for controlling of the macrosegration usually occurring in the heavy ingot.

Research on Interlayer Alloys for Transient Liquid Phase Diffusion Bonding of Single Crystal Nickel Base Superalloy DD6

Transient Liquid Phase Diffusion bonding (TLP bonding) is an effective method to achieve excellent joint of DD6, which is a new generation single crystal superalloy to manufacture aero-engine turbine blades. In this paper, the interlayer alloys for DD6 TLP bonding were designed. The alloy foils with thickness 40 μm ~ 60 μm, width 4 mm were prepared by using a single roller rapid solidification apparatus and the TLP bonding of DD6 was conducted. Then the joint microstructure and alloying elements diffusion behaviors were analyzed. The results indicate that microstructures of interlayer alloys prepared are fine and homogeneous, the melting point range of alloys from 1070°C to 1074°C and their melting temperature interval is merely 20°C, when the chemical composition of alloys are 1.5 ~ 2.0Cr, 3.2 ~ 4.0W, 3.7 ~ 4.5Co, 2.2 ~ 3.0Al, 0.7 ~ 1.0Mo, 3.2B, remain Ni (wt%). When the welding parameters are bonding temperature 1200?C, holding time 8.0 hour and welding pressure 0.3 MPa, the compacted joints obtained and the microstructure of TLP bonding seams were similar to base metal. The bonding joint is composed of weld center zone, isothermal solidification zone and diffusion-affected zone. Within joint, the elements diffusion is sufficient and borides in the diffusion zone are fewer.

Application of the cyclic phase transformation concept for determining the effective austenite/ferrite interface mobility

A series of cyclic partial austenite–ferrite phase transformation computer simulation experiments have been performed to elucidate the rate controlling dissipative processes during austenite-to-ferrite and the ferrite-to-austenite transformation in lean C–Mn steels. The transformation kinetics is analyzed by comparing the results of two complementary sharp interface models – one is based on the assumption of local equilibrium at the migrating interface − in the other model diffusion in the interface and the interfacial reaction is implemented by an effective interface mobility but substitutional diffusion in the bulk phases is neglected.Values for effective interface mobilities have been obtained for both the austenite-to-ferrite transformation and vice versa. By means of effective mobilities which depend only on initial composition and temperature, the transformation kinetics has been studied for other heat treatments than used to determine the effective interfacial mobility values. Although substitutional diffusion in the bulk is not taken into account, for the low Mn alloys it is possible to obtain similar trends by the effective mobility model as provided by the local equilibrium model. At modest to high interface velocities long range diffusion of the substitutional alloying elements can be ignored but then the effects of local diffusion processes near the interface need to be taken into account via an effective interface mobility. The effective mobility for the austenite-to-ferrite transformation differs from the effective mobility during the ferrite-to-austenite transformation in a rather essential manner.

Change of precipitate free zone during long-term creep in 2.25Cr–1Mo steel

This work presents a residual life assessment method based on the quantitative analysis of changes in the fraction and width of a precipitate free zone (PFZ) around grain boundaries during long-term creep. Such changes resulted from precipitation and coarsening of carbides on grain boundaries and within the ferrite matrix during long-term creep exposure. Testing was conducted at creep temperatures ranging from 450 to 650°C, for a maximum time of 142415.1h. A relationship between the fraction of grain boundary with PFZ, PFZ width, and the Larson-Miller Parameter (LMP) was established, and the mechanism of PFZ formation was discussed. The increase in width, governed by the diffusion of alloy elements to grain boundaries, was mathematically modeled. The relationship between the width and creep exposure time was fitted to a square root law, and the PFZ broadening rate constant and activation energy for width growth were calculated. The proposed model will be applicable to determine the operating parameters of 2.25Cr–1Mo steel components in high-temperature equipment.

Flexural Strength and Toughness of Austenitic Stainless Steel Reinforced High-Cr White Cast Iron Composite

Flexural behavior of high-Cr white cast iron (WCI) reinforced with different shapes, i.e., I- and T-sections, and volume fractions of austenitic stainless steel (310 SS) were examined under three-point bending test. The dimensions of casted beams used for bending test were (50 × 100 × 500 mm3). Carbon and alloying elements diffusion enhanced the metallurgical bond across the interface of casted beams. Carbon diffusion from high-Cr WCI into 310 SS resulted in the formation of Cr-carbides in 310 SS near the interface and Ni diffusion from 310 SS into high-Cr WCI led to the formation of austenite within a network of M7C3 eutectic carbides in high-Cr WCI near the interface. Inserting 310 SS plates into high-Cr WCI beams resulted in a significant improvement in their toughness. All specimens of this metal matrix composite failed in a ductile mode with higher plastic deformation prior to failure. The high-Cr WCI specimen reinforced with I-section of 310 SS revealed higher toughness compared to that with T-section at the same volume fraction. The presence of the upper flange increased the reinforcement efficiency for delaying the crack growth.

Structure Evolving at Bonding Interface of Dual-Alloys Jointed with Different Method under Coupling Action of Heat and Force

During near isothermal forging and heat treatment structure change of bonding interface in Ti3Al/TC11 and Ti2AlNb/TC11 dual alloys jointed with different method has been investigated. The results show that the solidification structure at dual-alloy joint welded by electron beam in vacuum has evolved into forging structure, columnar grains have been changed into equiaxed grains through breaking, crystal lattatice rebuild and re-crystallizing, and the mechanical bonding plus metallurgical bonding structure at joint welded by linear friction weld has transformed into metallurgy structure altogether, because constant high temperature during near isothermal forging can cause the diffusion of alloy elements and reconstruction of lattice structure.

First principles calculations of alloying element diffusion coefficients in Ni using the five-frequency model

The diffusion coefficients of several alloying elements (Al, Mo, Co, Ta, Ru, W, Cr, Re) in Ni are directly calculated using the five-frequency model and the first principles density functional theory. The correlation factors provided by the five-frequency model are explicitly calculated. The calculated diffusion coefficients show their excellent agreement with the available experimental data. Both the diffusion pre-factor (D0) and the activation energy (Q) of impurity diffusion are obtained. The diffusion coefficients above 700 K are sorted in the following order: DAl > DCr > DCo > DTa > DMo > DRu > DW > DRe. It is found that there is a positive correlation between the atomic radius of the solute and the jump energy of Ni that results in the rotation of the solute-vacancy pair (E1). The value of E2−E1 (E2 is the solute diffusion energy) and the correlation factor each also show a positive correlation. The larger atoms in the same series have lower diffusion activation energies and faster diffusion coefficients.

Failure assessment of Nimonic 80A gas turbine blade

Nickel base superalloys are widely used in applications requiring strength at high temperature. During the operation of gas turbines, the blades and other elements of hot gas path suffer service induced degradation which may be natural or accelerated due to different causes. The failure analysis of the 3MW gas turbine first stage blades made of nickel-base alloy Nimonic 80A is presented. The blade experienced base alloy degradation due to operation at high temperature. Initially the blade was sectioned for metallographic and microhardness test. The microstructure and microhardness was compared in four different blade zones i.e. root, 30%, 60%, and 90% of the total height of the hot region of the blade. On the basis of the observed microstructures and the phases present in the alloy, the main cause of failure was found to be creep damage. Finite element method (FEM) analysis is conducted considering peak loading of centrifugal force and surface (aerodynamic) loads. Based on FEM simulation results, the life of turbine blades is predicted using the Larson–Miller method. Finally to improve the blade life two heat treatment cycles were suggested and applied. The effect of heat treatments on grain size, volume friction of γ′ primary phase, and micro hardness were investigated. A detailed analysis of the coating showed that, the loss of coatings resistant to high temperature is due to oxidation, corrosion, erosion and inters diffusion of coating-substrate, which results in diffusion of alloy elements into the coating.

Microstructure Development and Alloying Elements Diffusion during Sintering of Near-β Titanium Alloys

Two near-β alloys, Ti-5Al-5Mo-5V-2Cr-1Fe and Ti-10V-3Fe-3Al, were produced by the blended element powder metallurgy technique. The use of (i) elemental powders with the Al-V master alloy in the case of Ti-5Al-5Mo-5V-2Cr-1Fe and, (ii) the complex Al-Fe-V master alloy in Ti-10V-3Fe-3Al has highlighted the influence of different alloying elements and their combination on microstructure evolution and chemical homogenisation. While Fe has the fastest diffusivity in Ti and its addition improves the density of both sintered alloys it also results in accelerated rates of grain coarsening. The combination of Al and V in the master alloy powder inhibits the diffusion of V into the Ti matrix. The unexpectedly slow diffusion of Cr at the early sintering stage in Ti-5Al-5Mo-5V-2Cr-1Fe was attributed to the formation of surface oxides on the Cr powders.

Mechanisms of microstructure formations in M50 steel melted layer by high current pulsed electron beam

In the present paper, surface melting of the M50 steel was carried out by high current pulsed electron beam (HCPEB). The microstructure evolution in the melted layer was observed using TEM. It is confirmed that the dissolution of the carbides and the diffusion of alloy elements play a determining role on the microstructure evolution. After one pulse irradiation, a mixture of twinned martensite and irregular cellular domains of austenite is observed in the melted layer due to the insufficient diffusion of alloy elements around initial carbides. The zone around initial carbides with high alloy elements content keeps as residual austenite, the zone with low alloy elements content transform into twinned martensite. When the irradiation number increases to 30pulses, the alloy elements will diffuse into the whole melted layer. And the melted layer consists completely of cellular austenite grains with a diameter of about 150nm. The boundary between austenite grains is amorphous structure with little higher alloy elements content.