Thermodynamic calculations for precipitation in maraging steels

Abstract

In the present work, thermodynamic calculations for several maraging systems have been carried out, and the results are compared with experimental data. The calculations were conducted using ThermoCalc. Excellent agreement is obtained between calculation and experimental measurements using mainly the atom probe. As a highlight, calculated equilibrium phases and their mole fractions in 1RK91 steel recently developed by Sandvik compare extremely well with atom probe microchemistry data, which showed the presence of copper rich particles, mixed Ni₃Al and Ni₃Ti, and molybdenum rich precipitates. Calculations also indicate the thermodynamic stability of μphase in the Fe–Ni–Mo and Fe–Ni–Co–Mo systems, Ni₃Al and Ni₃Ti in a chromium containing steel, and NiMn in a Fe–Ni–Mn system. However, it should be noted that thermodynamic calculations may only be used as a guideline for systems not in equilibrium.     

Continuous cooling transformation behaviour of Si–Mn and Al–Mn transformation induced plasticity steels

Abstract

The continuous cooling transformation (CCT) behaviour of two transformation induced plasticity (TRIP) steels was investigated using quench dilatometry. One was an established steel grade with a composition (wt-%) of Fe–0·2C–2Si–1·5Mn while the other steel was a novel composition where 2 wt-% Al replaced the silicon in the former grade. Characteristics of the α→γ transformation during reheating and the subsequent decomposition of austenite during continuous cooling were studied by dilatometry, and CCT diagrams were constructed for both steels. The effects of accelerated cooling and steel composition on γ transformation start temperature Ar ₃, phase transformation kinetics, and microhardness were investigated. The results showed that the Al–Mn steel had a much wider α→γ transformation range during reheating, compared with the Si–Mn steel. Furthermore, the Al–Mn steel exhibited no significant change in the rate of expansion during α→γ transformation. On the other hand, during continuous cooling, the Al–Mn steel exhibited higher Ar ₃, faster transformation kinetics, a higher volume fraction of polygonal ferrite in the microstructure, and lower hardness, compared with the Si–Mn steel. The addition of aluminium was found to have a significant effect on the products of phase transformation, kinetics, and form of the CCT diagram. For both steels, an increase in cooling rate lowered the Ar ₃ temperature, decreased the time of transformation, and increased the hardness.     

Solid state phase transformations in Ti–Al–C and Ti–Al–Nb–C alloys

Abstract

Results are reported of an investigation of solid state transformations in a series of α₂ based alloys having an aluminium content of 26 at.-% with carbon up to 3 at.-%; two α₂ basedquaternary Ti–Al–Nb–C alloys with 5 and 12 at.-%Nb and 3 at.-%C were also studied. Ordering occurs in the ternary Ti–Al–C alloys and also in the 23Al–5Nb–3C alloy on quenchingfrom 1250°C. Additional carbide precipitation was not observed in the ternary Ti–Al–C alloys on reheating to 750°C. Additions of niobium resulted in the presence of the β phase at 1050°C in the 5%Nb alloy and at 1050 and 750°C in the 12%Nb alloy. In the quaternary Ti–Al–Nb–C alloys, (Ti, Nb)₃AlC was found to be the primary phase and was present in the microstructure over the temperature range studied. In the 21Al–12Nb–3C alloy, the ordered β phase transformed to α″₂ martensite on quenching from 1250;amp;#x00B0;C.

MST/1306     

Relationships of diboride phases in Al–Ti(Zr)–B alloys

Abstract

The relationships of diboride phases in Al–Ti(Zr)–B alloys with a variable Ti/B ratio close to the stoichiometry of TiB₂ were studied. The formation of diboride solid solutions was confirmed. A grain refinement mechanism is proposed as that diboride particles in the Al–Ti–B master alloys reacting with aluminium upon adding into an aluminium melt and release titanium into the melt through forming a (Ti,Al)B₂ solid solution and maintain a thin dynamic Ti rich layer on the surfaces of the (Ti,Al)B₂ particles, which nucleates α-Al grains in solidification. The poisoning effect of zirconium on grain refinement of aluminium by Al–Ti–B master alloys is also discussed.     

Hydrogen-induced cracking susceptibility and hydrogen trapping efficiency of different microstructure X80 pipeline steel

The hydrogen-induced cracking (HIC) susceptibility of the X80 steel in H₂S environment and its heated-treatment microstructure was evaluated. The field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM) and energy dispersive spectroscopy (EDS) were employed to study the morphology and chemical composition of the inclusions, precipitates and HIC cracks in the X80 steel. The hydrogen trapping efficiency was investigated by measuring the permeability (J _∞L) and the apparent diffusivity (D _app). The results showed that heat-treated specimens had lower trapping efficiency, but were more susceptible to HIC. Most of the HIC cracks initiated from the inclusions rich in Mn, Al, Ca, and Ti, and propagated transgranularly in the original and air cooled specimens, but mainly intergranularly in water quenched specimens.     

Constitution of Ti–Al–C alloys in temperature range 1250–750°C

Abstract

An investigation has been made of the solid state constitution of the titanium rich portion of the Ti–Al–C system; partial isothermal sections have been established at 1250, 1050, and 750°C by means of electron microscopy (including energy dispersive X-ray analysis) and X-ray diffraction. In addition, a schematic liquidus projection has been deduced based on the solid state and as cast structures. The carbide phases present in the range studied are TiC, Ti₃AlC, and Ti₂AlC.

MST/1305     

Influence of calcium on hot ductility of steels

Abstract

The hot ductility of C–Mn–Al and C–Mn–Nb–Al steels with and without calcium additions have been examined over the temperature range 700–1000°C both after solution treating at 1330°C followed by cooling to the test temperature and directly after casting. Calcium additions invariably improved hot ductility. For hot rolled plate reheated to 1330°C and cooled to the test temperature, calcium is beneficial to hot ductility because it reduces the amount of sulphur able to redissolve and precipitate in a fine form at the new γ-grain boundaries produced on solution treating. For the C–Mn–Al steels, strain concentration occurred in the thin films of softer ferrite surrounding the γ-grains causing voiding around the sulphide inclusions that link up to cause intergranular failure. The removal of the sulphides by calcium addition therefore accounts for the improvement in hot ductility. Similar behaviour was observed for the C–Mn–Nb–Al steels, but for temperatures above the Ae₃ temperature calcium containing steels continued to give improved hot ductility over calcium free steels and this is believed to be due to the fewer sulphides present at the boundaries allowing an earlier onset of dynamic recrystallisation. For steels subjected to direct casting, interdendritic failure as well as intergranular failure by microvoid coalescence occurs. Calcium additions reduce the total amount of sulphur in the steel so that the volume fraction of sulphides precipitated at the interdendritic and γ-boundaries is low.

MST/962     

Systematic investigation of acicular ferrite formation on laboratory scale

By increasing the amount of acicular ferrite (AF) in the microstructure, steel toughness can be improved significantly. The steel composition, cooling rate, non-metallic inclusions and austenite grain size have a strong influence on the formation of AF. The present paper describes and compares two approaches to study AF formation in a titanium-deoxidised high-strength low-alloyed steel and its influencing factors on laboratory scale: route A simulates the formation of AF after heat treatment; route B simulates the formation directly after solidification of the melt. The formation of AF is essentially influenced by the former processing, which also changes the optimum cooling parameters substantially. (Ti,Mn)_xO_y and (Ti,Al,Mn)_xO_yS_z are the predominant active inclusion types in the investigated steel.     

Selective oxidation and segregation during annealing of steels at low oxygen partial pressures

Abstract

Industrial annealing of automotive steel grades is carried out in order to improve the mechanical properties and to adjust the grain size. However, the surface chemistry changes drastically due to selective oxidation and segregation phenomena, which significantly influence the further surface treatment, i.e. hot dip galvanizing.

The paper discusses results of laboratory experiments on selective oxidation and segregation of minor alloying elements Mn, Al, Cr and Si and non-metallic elements B, P and S during annealing of steel sheets in an N₂–5%H₂ atmosphere as a function of dew point. It will be shown that at lower dew points mostly external oxidation of Al and Mn occurs, where B shows a high tendency to segregate to the free surface and to form a BN film, especially observed on ferritic/austenitic steel. With increasing dew point, the oxidation of Al becomes internal and Mn, Si and Cr are oxidized externally. Boron also segregates to the surface by forming mixed Mn–B oxides and in the case of ferritic steels suppresses Si segregation and oxidation. The formation of phosphates by segregation of P becomes important at a D.P. of 0°C.     

Creep behaviour of cast TiAl based intermetallics

Abstract

Constant load tensile creep tests were carried out on the cast TiAl based intermetallics Ti–47 Al–2Mn, Ti–47 Al–2Zr, and Ti–48Al (at.-%), prepared by plasma arc melting. Two microstructural conditions dependent on heat treatment were evaluated as follows: a fully lamellar (FL) scheme consisting of a fully transformed coarse lamellar structure with α₂ laths plus γ laths within the grain interiors; and a duplex scheme consisting of fine equiaxed grains of γ with α₂/γ lamellae. The steady state creep behaviour of both microstructural conditions, for each composition, was studied under stresses of 70–300 MN m^?2 in the temperature range 700–900°C. The microstructure was found to have a pronounced influence on the creep resistance. The FL microstructure exhibited superior creep resistance to the duplex microstructure. At temperatures and stress levels at which direct comparisons can be made, the steady state creep rates of the FL structures are an order of magnitude lower than those of the duplex structure. The apparent creep activation energies and stress exponents were measured for both microstructural conditions for each composition. The temperature and stress dependence of the steady state creep rate of both microstructures can be described by the power law creep equation, suggesting dislocation motion as the operative deformation mechanism.

MST/1962     

Composition and lattice parameters of silicide and matrix in cast Ti–Si–Al–Zr alloys

Abstract

The silicide chemistry, i.e. the type, composition, and lattice parameters of the silicide in as cast titanium based Ti–Si–Al–Zr alloys, has been studied. It has been shown that the stoichiometry of the silicide in the alloys can be expressed as (Ti_1?x , Zr_x)₅(Si_l?y, Al_y) _2·76?3·04(0≤X< 0·2, 0≤y<0·1). The presence of Al and Zr in the silicide increases its lattice parameters. Addition of Al coarsens the eutectic silicide and slows the formation of secondary silicide precipitates by solid state reaction. Addition of zirconium refines the eutectic silicide and promotes secondary silicide precipitation. The silirides are low in Al and rich in Zr, whereas the Ti matrix is rich in Al and low in Zr. The lattice parameters of the Ti matrix are decreased by Al and increased by Zr.

MST/1427     

Enhanced mechanical properties of Al–Si–Cu–Mn–Fe alloys at elevated temperatures through grain refinement and dispersoid strengthening

The effect of Ti content on the microstructure and mechanical properties of heat-treated Al–Si–Cu–Mn–Fe alloys was investigated. It was found that the mechanical properties increased with the increase of Ti content. This was attributed to the refinement of grain size, the increased amount of T (Al₂₀Cu₂Mn₃), the α-Fe (Al₁₅(FeMn)₃(CuSi)₂) precipitated particles, and the decrease in Al₂Cu. At an elevated temperature of 300°C, the heat-treated Al–Si–Cu–Mn–Fe alloy with 0.5% Ti demonstrated the best mechanical properties, which are superior to those of commercial aluminium alloys. The yield strength contribution at 300°C was quantitatively evaluated based on the dispersoid, solid solution, and matrix contributions. It was confirmed that the main strengthening mechanism in the experimental alloys was the dispersoid strengthening.     

Interface evolution of SiO2 glass ceramic and Ti–6Al–4V alloy joint brazed with Ag–Cu–Ti alloy

Abstract

In the present paper, SiO₂ glass ceramic and Ti–6Al–4V alloy were successfully brazed with Ag–21Cu–4·5Ti active braze alloy. The interfacial microstructure and evolution course of SiO₂ glass ceramic/Ti–6Al–4V joint were studied in detail. According to the experimental results, active element Ti plays a quite important role in the formation of reaction layers on the joint interface. The reaction products of the joint are TiSi₂, Ti₄O₇, TiCu, Cu₂Ti₄O and Ti₂Cu respectively. The interface evolution can be generally described by four stages, which are solution and diffusion of atoms, reaction among atoms, formation of reaction layers and precipitation of solid solution layers respectively.     

Transient liquid phase diffusion bonding of continuous SiC fibre reinforced Ti–6AI–4V composite to Ti–6AI–4V alloy

Abstract

A continuous SiC fibre reinforced Ti–6Al–4V composite was diffusion bonded in transient liquid phase to Ti–6Al–4V alloy plate using Ti–Cu–Zr amorphous filler metal. Joint strength increased with bonding time up to 1·8 ks and reached the maximum value of 850 MN m^?2 which corresponded to 90% of the tensile strength of Ti–6Al–4V. The extent of deformation of Ti–6Al–4V in the vicinity of the bonding interface was small compared with that of solid diffusion bonding because of the low bonding pressure. The bonding layer had an acicular microstructure which was composed of Ti₂Cu and α titanium with dissolved zirconium. Brittle products such as (Ti, Zr )₅ Si₃ or (Ti, Zr )₅ Si₄ were formed at the interface between the SiC fibres and the filler metal. These products existed only at the end of fibres, in very small amounts, therefore joint strength was not significantly affected by the products.

MST/1989     

Interface microstructure analysis of SiO2 glass ceramic and Ti–6Al–4V alloy joint brazed with Ti–Zr–Ni–Cu alloy

Abstract

In the present paper, SiO₂ glass ceramic was joined to Ti–6Al–4V alloy with 35Ti–35Zr–10Ni–15Cu (wt-%) filler foil. The whole brazing process was performed under vacuum circumstances at different temperatures (850–1000°C) for several holding times (1–30 min). According to results of scanning electron microscopy, energy dispersive spectrometry, electron probe X-ray microanalysis and X-ray diffraction analysis, the reaction products of the interface are Ti₂O, Zr₃Si₂, Ti₅Si₃, Ti based solid solution and Ti₂(Cu,Ni). There is residual TiZrNiCu braze alloy on the SiO₂ glass ceramic/Ti–6Al–4V alloy interface after brazing. Besides, the interface evolution model of the joint was described by four stages: diffusion and solution among atoms, formation of reaction products, precipitation and growth of reaction layers respectively.     

Hot ductility of directly cast C–Mn–Nb–Al steel

Abstract

Tensile samples of a C–Mn–Nb–Al steel (BS 4360: 50D grade) have been cast in situ and either directly tested in the temperature range 850–1200°C, or were allowed to cool through the transformation, re–solution treated, and then tested in the same temperature range. The hot ductility of the directly tested cast material was found to be superior to that of the reheated material. Carbon extraction replicas taken close to the fracture surfaces showed large differences in the distribution of sulphide inclusions and NbCN precipitates along the γ boundaries. The directly cast material had sulphide inclusions and NbCN precipitates present in the form of coarse particles situated close to the interdendritic boundaries. A significant proportion of these coarse sulphide inclusions and NbCN eutectics, produced during solidification, redissolved on reheating at 1330°C, and subsequently precipitated in a much finer form at the γ grain boundaries, reducing hot ductility. It appears likely that the very marked segregation which occurred during solidification enhanced the interdendritic regions with sulphur to such an extent that the sulphideformed was (Mn, Fe)S, which in gradually changing to the equilibrium precipitate, depleted the surrounding matrix of manganese. The low manganese level accompanying these inclusions allowed a greater degree of solution of the sulphides to occur on reheating and accounted for the subsequent fine precipitation at the boundaries.

MST/361     

In vivo evaluation of Zr-based bulk metallic glass alloy intramedullary nails in rat femora

Zr-based bulk metallic glasses (BMG) show high corrosion resistance in vitro and higher strength and lower Young’s modulus than crystalline alloys with the similar composition. This study aimed to perform an in vivo evaluation of Zr₆₅Al_7.5Ni₁₀Cu_17.5 BMG. Osteotomy of the femur was done in rats and stabilized with intramedullary nails made of Zr₆₅Al_7.5Ni₁₀Cu_17.5 BMG, Ti–6Al–4V alloy, or 316L stainless steel. Systemic and local effects of each type of nail were evaluated by measuring the levels of Cu and Ni in the blood and the surrounding soft tissue. Changes of the surface of each nail were examined by scanning electron microscopy (SEM). Healing of the osteotomy was evaluated by peripheral quantitative computed tomography and mechanical testing. No increase of Cu and Ni levels was recognized. Surface of the BMG showed no noticeable change, while Ti–6Al–4V alloy showed Ca and P deposition and 316L stainless steel showed surface irregularities and pitting by SEM observation. The stress strain index, maximum torque, torsional stiffness, and energy absorption values were larger for the BMG than those for Ti–6Al–4V alloy, although there was no significant difference. The Zr-based BMG can promote osteotomy healing as fast as Ti–6Al–4V alloy, with the possible advantage of the Zr-based BMG that bone bonding is less likely, allowing easier nail removal compared with Ti–6Al–4V alloy. The Zr-based BMG is promising for the use in osteosynthetic devices that are eventually removed.     

Laser-tungsten inert gas hybrid welding of dissimilar metals AZ31B Mg alloys to Zn coated steel

Laser-tungsten inert gas (TIG) hybrid welding has been developed for joining Mg alloys to Zn coated steel in a lap joint configuration. The joint could not be produced in laser or arc welding only, while acceptable joints without obvious defects were obtained with a relatively wide processing window in the hybrid process. Two reaction layers were observed to form at the interface and were identified as Mg–Zn eutectic structure (α-Mg + MgZn) and Fe₃Al phase by TEM analysis. In some cases, Al₆Mn phase also formed adjacent to the Fe–Al reaction layer. The tensile-shear strength attained the maximum value of 68 MPa, representing 52.3% joint efficiency relative to Mg base metal. The element Al from AZ31B Mg alloys diffused to the liquid/solid interface and then reacted with the elements from steel, such as Fe and Mn, contributing to the metallurgical bonding at the interface. The weak bonding between Mg–Zn reaction layer and newly formed Fe–Al layer resulted in the interfacial failure.     

Eutectic alloys based on the Al–Zn–Mg–Ca system: microstructure,phase composition and hardening

The experimental study of aluminium alloys based on the Al–Ca–Zn–Mg system (3.5% Mg and 2.5% Mg, 0–10% Ca, 0–14% Zn (wt-%)) was combined with Thermo-Calc simulations for the optimisation of the alloy composition. Zinc is distributed between aluminium solid solution and phase (Al,Zn)₄Ca. Magnesium was not observed in the intermetallic phase. The eutectic (Al,Zn)₄Ca had fine structure and particles of (Al,Zn)₄Ca were capable of spheroidisation during the heat treatment at 520°С. The maximum level of hardness observed in calcium-containing alloys was higher than 200 HB, suggesting good strength properties. With an example of the Al–Zn(9%)–Mg(3.5%)–Ca(3%) alloy, the possibility of manufacturing thin rolled sheets based on the (Al)–Ca eutectic was demonstrated.

This article is part of a Themed Issue on Aluminium-based materials: processing, microstructure, properties, and recycling.     

Effects of Mn and Ti on microstructure and inclusions in weld metal of high strength low alloy steel

Abstract

The influences of alloying elements on chemical composition of non-metallic inclusions, impact toughness and microstructure in weld metals of high strength low alloy steels have been studied. Results indicated that microstructure had changed from a mixture of acicular ferrite, proeutectoid ferrite, ferrite side plates and microphases to a mixture of acicular ferrite, bainite and microphases due to the addition of Mn and Ti. The impact toughness of weld metal was improved correspondingly. The volume fraction and composition of inclusions both influenced the proportion of acicular ferrite. Mn and Si based oxide globular inclusions located at the boundary of acicular ferrite plates in the weld metal produced using C–Mn–Si–Cu wire. When Mn and Ti were added to welding wires, the inclusions within acicular ferrite plates permitted fewer primary acicular ferrite plates to grow into relatively larger dimensions. Secondary acicular ferrites nucleating on pre-existing ferrite plates refined microstructure effectively.