Influence of retrogression and reaging on fracture toughness of 7010 aluminium alloy

Abstract

Compact tension fracture toughness specimens of high strength aluminium alloy 7010 have been tested after the application of retrogression and reaging (RRA) heat treatments. Short transverse orientation specimens, 25 mm in thickness were cut from a large rectilinear open die forging. Two retrogression temperatures were investigated, 200 and 240 ° C, and the RRA treatments were applied to material in the solution treated and cold compressed (W52) condition. Varying the retrogression time and temperature strongly influences the fracture toughness and does so to a degree that is greater than would be expected from the change in tensile properties alone. The fracture toughness, tensile properties and work hardening characteristics of alloy 7010 are reported and their dependence on retrogression time and temperature is described.     

Microstructures and mechanical properties of an investment cast gamma titanium aluminide

Abstract

Investment castings have been produced in γ-TiAl of composition Ti–48Al–2Nb–2Mn (at.-%) using induction skull melting. The microstructures of the bars were studied in the as cast condition and after hipping and heat treatment. Heat treatment at 1200°C led to a near γ structure whereas treatment at 1350°C resulted in a nearly lamellar structure. However, a duplex structure was retained after treatment at 1300°C. Tensile, fracture toughness, and fatigue crack growth resistance tests have been carried out on specimens machined from different sized bars. The tensile properties increased with decreasing bar diameter but, conversely, both the fracture toughness and fatigue crack growth resistance improved as the bar diameter increased. It has been found that the fracture toughness and fatigue crack growth resistance in nearly lamellar structures were better than those in near γ structures, whereas duplex structures had intermediate properties. However, the tensile properties of duplex structures were better than both near γ and nearly α₂ /γ lamellar structures, with optimum values at 35 ± 5% α₂ /γ lamellae of ～400 MPa 0·2% proof strength, 470 MPa tensile strength, and 0·9% elongation.     

Plane strain fracture toughness of modern ferritic structural steels

Abstract

The temperature dependence of the plane strain fracture toughness of a low carbon, fine grain, ferritic steel for structural applications is investigated. The ductile–brittle transition is found to occur in the interval between 160 and 184 K. The experimental results are interpreted by an analytical model which permits calculation of the plane strain fracture toughness K _1c in the brittle domain as a function of the tensile properties and the cleavage fracture stress, making use of a piecewise approximation for the distribution of tensile stress on the crack axis and applying a deterministic fracture criterion at the stress peak. A similar criterion, which consists of equating the severest strain on the crack axis to a critical strain for cavity nucleation, provides the upper shelf fracture toughness. A relatively simple figure for predicting the transition temperature of steels in this family as a function of material properties can be obtained in this way.     

Microstructure and mechanical properties of hot rolled Fe–40 at-%Al intermetallic alloys with Zr and B addition

Abstract

The microstructure and mechanical properties of hot rolled Fe–40 at-%Al based intermetallic alloys, with 0·1 at-%Zr and different additions of B (varying from 0·01 to 0·1 at-%), are characterised. The additions of Zr and B improve tensile properties at room and elevated temperatures. Increasing B content is also associated with a number of other effects. First, the fracture mode changes from intergranular decohesion to cleavage, which correlates with significant increases in the fracture toughness. Second, there is a certain stabilisation of dislocations arranged in parallel systems of slip bands, as shown by transmission electron microscopy. Numerous complex stacking faults on {100} planes are also observed in the alloy with the highest B content. Third, B is found to modify the formation of second phase particles; such particles (coarse and fine) are analysed by energy dispersive X-ray spectroscopy and electron energy loss spectrometry to obtain compositional information.     

Effect of post-weld heat treatment on microstructure and properties of electron beam welded joint of Ti2AlNb/TC11

Abstract

The present paper reports the influence of post-weld heat treatment (PWHT) on microstructure and properties of electron beam welded dissimilar joint. Ti₂AlNb and TC11 alloys were used to fabricate the joints. Three PWHTs were applied to the welded joints. The structures were analysed using optical microscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy techniques. The results show that weld metal of the fusion zone is mainly composed of α₂ and β phases. As the energy input increases under different PWHTs, the decomposition degree of metastable phases (α′/β) rises, but the tensile strength and impact toughness of the joint reduce. Under each condition, the tensile strength of the joint is higher than that of the TC11 base metal.     

Effect of texture evolution on tensile properties of Al–Ni eutectic alloy fabricated by equal channel angular pressing

Abstract

The present study investigated in detail the effect of texture evolution on the mechanical properties of an Al–5·7 wt-%Ni eutectic alloy, which was subjected to severe plastic deformation by the equal channel angular pressing (ECAP) technique. The ECAP procedure was carried out using two strain introduction methods, route B_C and route A, at a temperature of 298 K and a pressing rate of 0·33 mm s^?1. The as pressed microstructures were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results indicated that the Al–Ni eutectic alloy specimens after ECAP processing by route B_C and route A methods had very different microstructures, which strongly affected the tensile properties of the specimens. It was demonstrated that after ECAP processing by route B_C, fine Al₃Ni particles of ～300 nm were homogeneously dispersed in the aluminium matrix, and the specimens showed no clear anisotropy in tensile properties. After ECAP processing by route A, however, eutectic textures containing α-Al and Al₃Ni fibrous dispersoids had a highly anisotropic distribution and were demonstrated to have significantly anisotropic tensile properties. Based on the experimental results, the fracture mechanism during tensile testing of the Al–Ni eutectic alloy using different strain induction methods is discussed.     

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.     

Short transverse fracture of Al–Li–Cu–Mg–Zr extrudate

Abstract

The short transverse fracture toughness of an Al–Li–Cu–Mg–Zr extrudate was determined as a function of aging condition and testing temperature. To elucidate the underlying micromechanisms, the short transverse fracture surfaces of the extrudate were characterised via scanning electron microscopy, grain boundary precipitates and precipitation free zones were identified via transmission electron microscopy, and segregation of elements to grain boundaries was analysed using secondary ion mass spectrometry. Three principal observations were made as follows. First, with increasing aging time, the short transverse toughness of the extrudate increased when tested at room temperature, but decreased at liquid N₂ temperature, whereas with decreasing testing temperature, it remained essentially constant for the underaged condition, and decreased sharply for the peak aged and overaged tempers. Second, in addition to regions exhibiting shallow dimples, smooth ‘featureless’ zones were revealed on the short transverse fracture surfaces, which are intergranular in nature for all the specimens tested. The area fraction of the featureless regions decreased noticeably with increasing aging time when tested at room temperature, and increased markedly with decreasing testing temperature for the peak aged and overaged conditions. Third, segregation of Li, Si, Na, and H was detected for both the underaged and overaged specimens, and also of K for the underaged specimens only. In general, the enhancement of the room temperature short transverse toughness with aging and the negative effect of cryogenic temperature on fracture toughness are in obvious contrast to the in plane toughness behaviour reported in the literature, the featureless character of the short transverse fracture and its connection with poor toughness seldom having been emphasised. Based upon the present study, segregation induced brittleness is proposed as the critical micromechanism responsible for grain boundary weakness, and thus for the poor short transverse fracture toughness.

MST/1829     

Influence of aging condition on tensile and fatigue fracture behaviour of aluminium alloy 6063

Abstract

An Al–Mg–Si alloy 6063 was heat treated in the underaged, peak aged, and overaged conditions. The microstructures of the alloy in the different aging conditions were examined using transmission electron microscopy. Tensile and fatigue tests were carried out and the resulting fractures were studied using scanning electron microscopy. It has been found that the alloy shows different tensile and fatigue properties and different modes of fracture under different aging conditions. The results have been explained in terms of slip distribution and grain boundary strength.

MST/1188     

Analysis of change in microstructure and properties during high temperature processing of ultralow C and high Nb microalloyed steel

Abstract

To further improve the strength and toughness, the advanced thermomechanical controlled processing has been applied in the development of an ultralow C and high Nb bearing steel. In the present investigation, the effects of processing parameters, consisting of the coiling and starting temperatures in non-recrystallisation region, on the final microstructure and mechanical properties of this steel have been studied by tensile, Charpy impact tests, optical microscopy and transmission electron microscopy. Results indicate that the acicular ferrite dominated microstructure can be greatly refined in grain size with decreasing the starting temperature of finishing rolling. However, for high Nb steels, the too low starting temperature would promote the formation of high temperature transformation products and consequently make against the improvement of mechanical properties. In addition, the optimum temperature window of finishing rolling is found to be also related to alloying levels of austenite stabilising elements. At the high starting temperature of finishing rolling, the precipitation strength contribution increases with increasing coiling temperature. However, the increase in strain accumulation associated with low temperature processing greatly reduces the sensitivity of the precipitation strength contribution to coiling temperature.     

Statistical analysis of phosphorus segregation on intergranular fracture facets in pressure vessel steels

Abstract

A533B and C–Mn steels, widely used as nuclear pressure vessel steels, have been aged at 520°C after tempering at 650°C for various periods of time to produce different levels of embrittlement resulting from the segregation of P to grain boundaries. Metallographic observation and tensile test results showed that the embrittlement heat treatment did not have significant influence on the microstructures or tensile properties of the steels. P segregation at grain boundaries and on intergranular facets was investigated using field emission gun transmission electron microscopy and Auger electron spectroscopy. After such treatment, enhanced segregation was found to be a linear function of the square root of embrittling time. Statistical analysis of the AES measurements indicated that there is a minimum segregation level for intergranular fracture to occur.     

Fracture toughness comparison of five indirect resin composites under the effect of thermal cycling

ABSTRACT

This study investigated the microhardness and fracture toughness values of five dental indirect resin composites under the effect of thermal cycling by single-edge notched beam method. Highest microhardness and fracture toughness were reached in AP-X and Filtek P60 groups (microhybrid resin composite) (P < 0.05). After thermal cycling, significant changes of fracture toughness were found for the tested composites except AP-X. Microdifferences between scanning electron microscope images with and without thermal cycling were observed. It is concluded that irregular-shaped fillers and higher filler content contribute to higher mechanical properties, microhardness, and fracture toughness and result in superior fracture toughness under the effect of thermal cycling.     

Development of mechanical properties during secondary aging in aluminium alloys

Abstract

Earlier work has shown that, if the artificial aging of aluminium alloys is interrupted by a dwell period at lower temperature, higher values of tensile properties and fracture toughness may be achieved than are possible with single stage T6 tempers. A second interrupted aging cycle has now been developed that involves underaging at the elevated temperature, quenching, and then allowing secondary precipitation to occur at, or just above, room temperature. Designated a T6I4 (I=interrupted) temper by the authors, this simpler aging cycle may reduce heat treatment costs. Tests on some 30 cast and wrought alloys have resulted in tensile properties close to those for a T6 temper, with higher values of fracture toughness being recorded for some cases. Such an aging treatment can be incorporated into a paint bake cycle to simplify the heat treatment of coated automotive components.     

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.     

Fracture toughness of M2 and H13 alloy tool steels

Abstract

The influence of microstructural variations on the fracture toughness of two tool steels having compositions (wt-%) lC–4Cr–5Mo–2V–6W (AISI M2 high-speed steel) and 0·35C–5Cr–1·5Mo;amp;#x2013;1V (AISI H13 hot-work steel) was investigated. In the as-hardened condition, the H13 steel has a higher fracture toughness than M2 steel, and the latter steel is harder. In the tempered condition, the H13 steel is again softer and has a higher fracture toughness than M2. There is a decrease in fracture toughness and an increase in hardness when the austenitizing temperature is above I050°C for M2 steel and above 1100°C for H13 steel, in both the as hardened and hardened and tempered conditions. The fracture toughness of both steels was enhanced by reducing the grain size and increasing the overall carbide volume in the matrix. The steel samples of average grain diameter ≥40μm exhibit 2–3 MN m ^?3/2 lower fracture toughness than samples of average grain diameter ≤15 μm. A high content of retained austenite appears to raise the fracture toughness of as-hardened M2 steel. Tempering improved the fracture toughness of M2 and H13 steels. The present results are explained using observations of changes in the microstructure and the modes of fracture.

MST/468     

Microstructure and mechanical properties of spray deposited and extruded 7000 series aluminium alloys

Abstract

The microstructure and mechanical properties of spray deposited 7000 series aluminium alloys were investigated. The 7000 type alloys were produced by the spray atomisation deposition method. These alloys were hot extruded and subsequently heat treated in the T6 and T7 temper conditions. Microstructural characterisation of the alloys was carried out by transmission electron microscopy (TEM). TEM studies revealed the presence of η′ and η(MgZn₂) hardening precipitates in both temper conditions. The mechanical properties were assessed through tensile and notched tensile tests using an Instron machine. It was observed that the 0.2% proof stress of these alloys after T7 temper decreased with increased elongation to fracture values.     

Formability of two aluminium alloys

Abstract

The suitability of two recently developed aluminium alloys (an Al–Mg–Mn alloy and an Al–Li–Cu alloy) for press forming applications has been examined. The characterisation involved the experimental determination of microstructural aspects, tensile properties, and formability parameters such as average plastic strain ratio and planar anisotropy. The forming limit diagram has been experimentally evaluated. A detailed analysis of the strain distribution profiles obtained from punch stretching experiments has been attempted. An attempt has been made to correlate the crystallographic texture with the formability parameters. The fracture surfaces of the punch stretched samples were observed using scanning electron microscopy with a view to obtaining a correlation between fracture behaviour and formability. The alloys, in particular the Al–Mg–Mn alloy, have been found to possess good stretchability but both show very limited drawability. Texture analysis indicated negligible earing during deep drawing. These alloys are suitable for stamping applications where stretching constitutes the major proportion of the deformation.     

Microstructure and mechanical properties of SiC whisker reinforced ZrO2–6 mol.-%Y2O3 composites

Abstract

Microstructure, mechanical properties, fracture behaviour, and toughening mechanisms of hot pressed SiC whisker (SiC_w)

reinforced ZrO₂–6 mol.-%Y₂O₃ composites were investigated via transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and mechanical testing. The experimental results show that there is a continuous increase in the Vickers hardness, elastic modulus, and fracture toughness of the composites with increasing SiC_w content, and an addition of 30 vol.-%SiC_w increases the fracture toughness from 3·42 MN m^?3/2 for the unreinforced matrix to 5·83 MN m^?3/2. The flexural strength is increased from 293 MN m^?2 for the unreinforced matrix to a maximum of 372 MN m^?2 by an addition of 10 vol.-%SiC_w, then it is significantly decreased by further increasing the SiC_w content. Observations via transmission electron microscopy show that no distinct second phase or intermediate layers form at the SiC_w/ZrO₂ interface. Diffusional separation of tetragonal phase from the cubic matrix occurred during cooling after hot pressing. Whisker bridging and crack deflection are the main toughening mechanisms, but whisker pull-out, crack branching, and refinement of the matrix particles also contribute to the improvement in the fracture toughness.

MST/1747     

Development of an ultrahigh strength low alloy steel for armour applications

Abstract

The present paper describes a steel with yield strength exceeding 1900 MPa and fracture toughness in the range of 40–50 MPa?m^1/2, in its optimum heat treated condition. Its strength is similar to that of 18 Ni (300) grade of maraging steel with good fracture toughness. When tempered at 300°C, it shows tempered martensite along with a small amount of retained austenite phase. The steel shows nearly 25% reduction in weight over typical rolled homogeneous armour (RHA) steel against high velocity hard steel core projectiles. The processing, microstructure, mechanical and ballistic properties of the steel are demonstrated.     

Effect of heat treatment on microstructure and mechanical properties of Cr–Ni–Mo–Nb steel

Abstract

The microstructure and mechanical properties of a medium carbon Cr–Ni–Mo–Nb steel in quenched and tempered conditions were investigated using transmission electron microscopy (TEM), X-ray analysis, and tensile and impact tests. Results showed that increasing austenitisation temperature gave rise to an increase in the tensile strength due to more complete dissolution of primary carbides during austenitisation at high temperatures. The austenite grains were fine when the austenitisation temperature was <1373 K owing to the pinning effect of undissolved Nb(C,N) particles. A tensile strength of 1600 MPa was kept at tempering temperatures up to 848 K, while the peak impact toughness was attained at 913 K tempering, as a result of the replacement of coarse Fe rich M₃C carbides by fine Mo rich M₂C carbides. Austenitisation at 1323 K followed by 913 K tempering could result in a combination of high strength and good toughness for the Cr–Ni–Mo–Nb steel.