Microstructure characterisation of biphasic titanium alloy Ti–10V–2Fe–3Al and effects induced by heterogeneities on X-ray diffraction peak’s broadening

Abstract

The present study deals with a microstructural characterisation of a biphasic titanium alloy Ti–10V–2Fe–3Al. The near β titanium alloy is primarily used for high strength applications, such as components for aeronautical industry. The forging process and the heat treatments of Ti–10V–2Fe–3Al induce a complex microstructure, which consists of an equiaxial primary phase α_p (hcp) and a secondary phase α_sec (hcp) precipitating in the cubic phase β matrix. The forged material exhibits some structures hundreds of micrometres in size. Microstructure evolutions of forged and milled samples are analysed using a scanning electron microscope. Superficial treatments on forged and aged components induce a microstructure flow on the surface. Crystallites’ sizes and strain between phases are heterogeneous, and thermomechanical treatments may induce preferred crystallographic orientations. These phenomena broaden peaks strongly and generate difficulties in analysing residual stresses by X-ray diffraction. The present paper presents the microstructure’s evolution, linked to diffraction peaks broadening and crystallographic textures analyses.     

On anisotropy of ferritic ODS alloys

Abstract

Ferritic oxide dispersion strengthened (ODS) alloys are candidate materials to be used as cladding for long term fast reactors, due to their high strength at high temperature and good swelling and irradiation resistances. The fabrication of cladding tubes is usually made by a succession of cold deformation steps where a deformation induced anisotropic microstructure could take place, which would affect the mechanical behaviour of the tube. The characterisation of this microstructural anisotropy is one of the key issues in the development of cladding ODS tubes. In this paper, the microstructural anisotropy of a Fe–14Cr–ODS extruded bar and a Fe–12Cr–ODS plate is characterised and its effect on the mechanical properties is analysed by tensile, impact and small punch testing. In both materials, a reduction of the ductility is observed in the transverse specimens. In addition, the fracture behaviour seems to be strongly dependent on the location of the crack plane regarding the elongated grained microstructure.     

Research on variation and stress status of graphite in laser cladding process of grey cast iron

Abstract

Grey cast iron codenamed HT250 was processed by laser cladding with Fe based alloy to investigate the graphite variation in laser cladding process. To study the phenomenon that microcracks appeared at some graphite tips in the bonding zone, the macro laser cladding model and micro graphite model were established. Thermal–mechanical characteristics of the area around graphite were investigated using the models. The results showed that graphite in the bonding zone had complicated variation and microcracks appeared on a portion of graphite tips. Tensile stress concentration emerged at graphite tips in the cooling period which was a hazardous area of microcracks initiation. Microcracks were caused by coactions of microstructure characteristics of graphite area and stress concentration at graphite tips. The tensile stress was closely related to graphite dimensions. Graphite with large radius and length caused larger tensile stress which resulted in microcracks more easily.     

Precipitation,microstructure and mechanical properties of low nickel maraging steel

Abstract

A maraging steel with a composition of Fe–12·94Ni–1·61Al–1·01Mo–0·23Nb (wt-%) was investigated. Optical, scanning electron and transmission electron microscopy and X-ray diffraction analysis were employed to study the microstructure of the steel after different aging periods at temperatures of 450–600°C. Hardness and Charpy impact toughness of the steel were measured. The study of microstructure and mechanical properties showed that nanosized precipitates were formed homogeneously during the aging process, which resulted in high hardness. As the aging time is prolonged, precipitates grow and hardness increases. Fractography of the as forged steel has shown mixed ductile and brittle fracture and has indicated that the steel has good toughness. Relationships among heat treatment, microstructure and mechanical properties are discussed. Further experiments using tensile testing and impact testing for aged steel were carried out.     

High strain rate superplasticity of ultrafined Ti–10V–2Fe–3Al compact prepared by sintering and isothermal forging

Abstract

High strain rate superplasticity was obtained for powder Ti–10V–2Fe–3Al (Ti-1023) alloy prepared by powder sintering and isothermal forging technology. The selected powder was cold isostatic pressed, sintered and isothermal forged to prepare this powder alloy. Tensile testing was conducted at optimum superplastic temperaure of 1023 K with different initial strain rate, and the elongation to failure, the flow stress and the microstructure were analysed. The experiment results exhibited that the microstructure of this powder alloy is extraordinary uniform and fine, resulted in considerable enhancement of optimum initial strain rate increased from 3·3×10^?4 s^?1 of conventional cast and wrought Ti-1023 alloy to 3·3×10^?3 s^?1 of this powder alloy. The elongation to failure increased first and then decreased with initial strain rate from 3·3×10^?4 to 3·3×10^?2 s^?1. The strain rate sensitivity m is about 0·46 near initial strain rate of 3·3×10^?3 s^?1, larger than conventional cast and wrought Ti-1023 alloy. Microstructure observations showed that dynamic recrystallisation and grain growth were present during superplastic deforming.     

Cold roll bonding of bimetallic sheets and strips

Abstract

The successful production of wide bimetallic sheets and strips by cold roll cladding depends on the technical solution of reduction of rolling load while still guaranteeing good bond quality. In the present paper, systematic experimental data on the cold roll cladding of aluminium–stainless steel, copper–stainless steel, and mild steel–stainless steel are correlated and the basic mechanism is discussed, aided by a scanning electron microscopy study of the separated interface. The effects of conventional rolling and cross-shear rolling (i.e. between rolls of differing peripheral speeds) on roll force and bonding strength are critically compared. It is found that use of the cross-shear cold rolling technique results in a significant reduction in rolling load for equal primary bonding strength and that the incorporation of an optimum final heat treatment considerably decreases the requirement on rolling.

MST/971     

Graphitization in hot forged white cast iron

Abstract

The mechanism of accelerated graphitization in hot forged white cast iron has been analysed and a mathematical relationship between graphite content (vol.-%) and the process variables has been determined. The power law variation of graphitization with annealing time formulated in the present investigation suggests that the first stage graphitization of hot forged white iron is a diffusion controlled precipitation process.

MST/219     

Microstructure and properties of alloy HP50–Nb: comparison of as cast and service exposed materials

Abstract

Spun cast alloy HP50–Nb tubing retired from service in a steam reformer after approximately 8 years of service had stress rupture properties suggesting very short remaining life. Microstructural examination, however, indicated little or no creep damage (cavitation) but did reveal extensive microstructural modification. The microstructure, mechanical properties, and stress rupture properties of the ex service material were determined and compared with virgin as cast material. The microstructural changes that occurred during service included the formation of phases rich in alloying elements such as chromium and niobium that would otherwise be expected to contribute to high creep strength. The creep life of the service exposed material was correspondingly short. The creep ductility of the service exposed material, however, was significantly higher than that of the as cast material. The implications of these findings for remaining life assessments of reformer furnaces using these materials are discussed.

MST/3207     

Mechanical properties of Nb based multiphase alloy studied by nanoindentation and atomic force microscopy

Abstract

The mechanical properties and deformation behaviours of as cast and heat treated Nb–21Ti–4C–xAl (x: 0, 5, 10 and 15 at-%) alloys are comprehensively investigated using nanoindentation and atomic force microscopy. For the Nb–21Ti–4C alloy, nanoindentation tests are performed for the Nb solid solution (Nbss) matrix, carbide and the interface between them. The results show that the hard carbide, which has a strong bonding with the Nbss matrix, can enhance the alloy before and after heat treatment, and the eutectoid transformation of the large sized carbide after heat treatment leads to less possibility for the forming of cracks on the carbide surface, which in turn improves the toughness. For the Nb–21Ti–4C–(0, 5, 10, 15)Al alloys, the hardness of the Nbss matrix increases significantly with increasing Al fraction for both as cast and heat treated alloys. However, deviations of the elastic modulus are inconspicuous with the Al fraction for the as cast and heat treated alloys.     

Evaluation of properties and microstructure of non-heat treatable Al–Mg–Li–C–O alloys with variable Li concentration

Abstract

Fine grained Al–Mg–Li–C alloys, with lithium concentrations from 0.7 to 1.5 wt-%, have been produced by a mechanical alloying–powder metallurgy route. An initial range of compositions was chosen for manufacture into 10 kg billets which were uniaxially forged into plate; subsequently two compositions, alloy A (Al–3.7Mg–0.7Li–1.0C (wt-%) and alloy B (Al–4.4Mg–1.4Li–1.0C), were down-selected for a 20 kg scale-up exercise. Billets were forged at 300°C, using an 8:1 reduction ratio, which provided a sufficient level of work to develop properties, while avoiding excessive grain growth. Alloy B exhibited tensile properties (0.2% proof stress 450 MPa; ultimate tensile strength 510 MPa; strain to failure 6%) that exceeded the AECMA specification for AA 5091. Both alloys were confirmed as non-heat treatable and therefore exploitable in the as forged T1 condition. Microstructural analysis has confirmed that a fine grain size (<1 µm) and nanoscale Al₂O₃/Al₄C₃ and MgO dispersoids provided significant Hall–Petch and Orowan strengthening, respectively, capable of increasing the 0.2% proof stress to 450 MPa. Although optimisation of thermomechanical practice is still required, these Al–Mg–Li–C alloys show considerable potential for aerospace, land, and space applications.     

Effect of thermal treatments on microstructure and impact toughness of die cast Mg–Al–Mn alloys

Abstract

A study of the effects of heat treatment on an Mg–Al–Mn alloy was carried out. Die cast AM60B alloy (Mg–6.0Al–>0.25Mn–<0.010Cu–<0.002Ni–<0.005Fe (wt-%)) specimens for microstructural investigation, tensile testing, and impact toughness testing, were produced using a multispecimen die in a high pressure, cold chamber apparatus. As cast specimens were studied either in their original condition or after they had been subjected to a direct aging treatment at 175°C. Solution treatment was also carried out, producing a T4 temper and a T6 temper by subsequent additional aging, and the resulting specimens examined. The investigation allowed evaluation of modifications to microstructural and mechanical properties produced by thermal treatments. In particular, the analysis of structure and solidification defect evolution showed that, despite an increase in void volume fraction and size induced by thermal treatments, a significant improvement of toughness during the crack growth process could be achieved with the appropriate tempers. Marked modifications to high strain rate loading conditions were detected, with improvements of total absorbed impact energy of up to 40% with respect to the as cast condition. This was associated with changes in fracture mechanisms, promoting a transition to a completely ductile mode in solution treated specimens.     

Effect of aging on high strain rate and high temperature properties of 7075 aluminium alloy

Abstract

The high strain rate and high temperature properties of as cast and aged 7075 aluminium alloy were examined by metallographic observation and by means of a split Hopkinson bar test at temperatures between 25 and 300°C and at strain rates of 1·3 × 10³ and 3·1 × 10³ s^-1. The effect of aging, as well as strain rate and temperature, on the dynamic mechanical response, microstructure evolution, and fracture characteristics are presented. The compressive stress–strain response of as cast and aged 7075 alloy is found to depend strongly on both the applied strain rate and the test temperature. However, the aged material is generally found to be stronger than the as cast material. The work hardening rate is seen to decrease with increasing strain, strain rate, and temperature, and its value is higher in the aged material than in the as cast material. Microscopic observation shows that aging, strain rate, and temperature have a significant influence on the microstructural evolution and the changes in grain morphologies. The average grain size can be expressed by a Hall–Petch type relationship after impact deformation. Fracture surface examination revealed that a high strain rate favours the formation of deformed shear bands that are precursors to crack formation and fracture. The aged material has a better ductility owing to the higher percentage of transgranular fracture and an increased density of microdimples.     

New Al–Mg–Li–C dispersion strengthened alloy Part 2 – Optimisation of material for forging applications

Abstract

This paper describes work aimed at optimising the forging process and mechanical properties of a new powder route Al–Mg–Li–C dispersion strengthened alloy, specifically for forging applications. The alloy, of the AA 5091 type, has a nominal composition of Al–1.2Li–5Mg–0.35C (wt-%), and is non-heat treatable. Materials were supplied by AMC Ltd, and evaluated in laboratory based forging trials to determine the influence of forging parameters on mechanical properties, including an investigation of using extruded bar as forging stock. The process was scaled up to produce an industrial forging, using an EH101 helicopter part, which was cut up for detailed mechanical property assessment. It was found that adequate hot working of the material is needed for good toughness, and that the balance of strength to toughness can be controlled by the forging temperature. The new alloy, in the as forged condition, meets the minimum tensile and fracture toughness levels set in the draft AECMA Specification for AA 5091 forgings.     

Effect of environment on mechanical properties of coated superalloys and gas turbine blades

Abstract

Several investigations of the effects of environment on the mechanical properties of coated superalloys are examined, and the results that have been obtained are discussed. The short–time mechanical properties of superalloys (especially cast alloys, which have their optimum mechanical properties in the as–cast condition) may deteriorate during the coating process (e.g. on thermal cycling during coating, during post–coating heat treatment, or by coating surface effects). The degradation of the properties of the base material leads to an increase in creep rate, so the elongation that can be tolerated before the coating starts to crack will be reached in a shorter time. In a corrosive environment, stresses may be enhanced by general corrosion, with consumption of the coating and a measurable reduction of the cross–section, or by selective corrosion attack after a crack is initiated in the coating and propagates rapidly along grain boundaries, or by both general and selective corrosion. The effects of general corrosion may be calculated, but the effects of selective corrosion cannot.

MST/286     

Microstructural evolution and tensile behaviour of medium carbon microalloyed steel processed through two thermomechanical routes

Abstract

A multiphase microstructure was obtained in a medium carbon microalloyed steel using two step cooling (TSC) from a lower than usual finish forging/rolling temperature (800–850°C). A low temperature anneal was then used to optimise the tensile properties. A multiphase microstructure (ferrite–bainite–martensite) resulted from forging as well as rolling. These were characterised using optical and scanning and transmission electron microscopy. X-ray diffraction, transmission electron microscopy and hardness measurements were used for phase identification. Tensile properties and work hardening curves were obtained for both the forged and the rolled multiphase variants. A Jaoul–Crussard (J–C) analysis was carried out on the tensile data to understand the basic mode of deformation behaviour. Rolling followed by the TSC process produced a uniform microstructure with a very fine grain boundary allotriomorphic ferrite, in contrast to the forged variety, which contained in addition coarse idiomorphic ferrite. The volume fraction of ferrite and its contiguity ratio in the rolled microstructure were greater than in the forged grade. The rolled microstructure exhibited a better combination of strength and toughness than that of the forged material. The rolled steel work hardened more than the forged variety owing to its fine, uniform (bainite–martensite and ferrite) microstructure. Retained austenite present in these steels underwent a strain induced transformation to martensite during tensile deformation. The J–C analysis of the work hardening rates revealed typical three stage behaviour in both varieties during tensile deformation.     

Comparative assessment of fatigue and fracture behaviour of cast and forged railway wheels

A comparative evaluation of fatigue and fracture behaviour of commercially produced cast and forged rail wheels has been made using specimens extracted from various locations of the wheel quadrant. A systematic investigation in the web and rim regions of the wheel quadrant with various notch orientations showed that the forged material exhibited a better intrinsic resistance to fatigue crack growth than the cast material. Since linear elastic fracture mechanics (LEFM) based fracture toughness could not be validated for both the cast and forged wheel material, elastic plastic fracture mechanics (EPFM) based characteristic fracture toughness was used. Results showed that fracture resistance of the forged material is superior to that of the cast material. Cast wheel specimens exhibited unstable crack extension in comparison to substantial stable tearing in forged specimens. Microstructural and fractographic analyses showed that the cast wheel material contained large amounts of inclusions. The poor fracture resistance of cast wheel material is therefore attributed to the inferior material quality.     

Precipitation of iron-rich intermetallics and mechanical properties of Al–Si–Mg–Fe alloys with Al–5Ti–B

Effects of added Al–5Ti–B master alloys on precipitation of iron-rich intermetallics and mechanical properties of A356 cast alloys with high Fe content (1.5?wt-%) were investigated using image analysis, scanning electron microscopy, and tensile testing. Results show that added Al–5Ti–B has apparent refinement on α (Al) grain size of A356 alloys that have high Fe content. 12?wt-% Al–5Ti–B is beneficial for improving mechanical properties of A356 cast alloys with high Fe content. Improved mechanical properties can be attributed to refined microstructure, the proper amounts of TiB₂ and Ti(AlSi)₃, and decreased porosity. An excessive amount of Al–5Ti–B deteriorates mechanical properties of alloys because it leads to the formation of large secondary intermetallics and increased porosity.     

Fabrication of thin film TiO2 nanotube arrays on Co–28Cr–6Mo alloy by anodization

Titanium oxide (TiO₂) nanotube arrays were prepared by anodization of Ti/Au/Ti trilayer thin film DC sputtered onto forged and cast Co–28Cr–6Mo alloy substrate at 400 °C. Two different types of deposited film structures (Ti/Au/Ti trilayer and Ti monolayer), and two deposition temperatures (room temperature and 400 °C) were compared in this work. The concentrations of ammonium fluoride (NH₄F) and H₂O in glycerol electrolyte were varied to study their effect on the formation of TiO₂ nanotube arrays on a forged and cast Co–28Cr–6Mo alloy. The results show that Ti/Au/Ti trilayer thin film and elevated temperature sputtered films are favorable for the formation of well-ordered nanotube arrays. The optimized electrolyte concentration for the growth of TiO₂ nanotube arrays on forged and cast Co–28Cr–6Mo alloy was obtained. This work contains meaningful results for the application of a TiO₂ nanotube coating to a CoCr alloy implant for potential next-generation orthopedic implant surface coatings with improved osseointegrative capabilities.     

Effects of neodymium rich rare earth elements on microstructure and mechanical properties of as cast AZ31 magnesium alloy

Abstract

The effects of neodymium rich rare earth elements [RE(Nd)] on microstructure and mechanical properties of as cast AZ31 magnesium alloy were investigated. The microstructures of as cast AZ31–xRE(Nd) alloys display a dendrite configuration, and the secondary dendrite spacing of the α-Mg phase was decreased with the increasing Nd content. The addition of RE(Nd) resulted in the formation of Al₂Nd and Mg₁₂Nd phases. Mechanical properties were improved significantly due to grain refinement and precipitation of intermetallic phases. When the amount of RE is 1·0 wt-%,The as cast AZ31 alloy reached its maximum tensile strength of 249 MPa at room temperature, yield strength of 169 MPa and elongation of 9·0%.     

Slab analysis of roll bonding of silver clad phosphor bronze sheets

Abstract

A numerical analysis program, based on slab analysis, was developed to calculate the stresses and strains that occur in the component layers of a composite sheet in the roll bite during the fabrication of clad metal (e.g. silver clad phosphor bronze) by roll bonding. Results calculated using the perfect plastic and strain hardening models were compared with experimental results: the results obtained using the strain hardening model were found to be in better agreement with values of rolling force and thickness of component layers measured after roll bonding. The program requires input data such as roll radius, initial thickness of the specimen, initial cladding thickness fraction, reduction ratio, coefficients of friction between rolls and material and between component layers, and front and back tensions. The coefficients of friction were evaluated indirectly by comparison of the measured rolling force with that calculated using slab analysis. Measured coefficients of friction varied with reduction ratio and initial thickness, but were independent of rolling speed.

MST/1334