Effects of pre-strain on plane stress ductile fracture in α-brass

The effects of pre-strain on plane stress ductile fracture in a 70/30 alpha brass Austral 207 have been studied using the deep-edge-notched tension (DENT) specimens. The amount of pre-strain varies between 5 and 35%. It is found that both the specific essential work of fracture (w _e) and the critical crack opening displacement (_c) decrease with increasing pre-strain. A simple theory for estimating the specific essential work of fracture in the presence of pre-strain is suggested and it gives good agreement with experimental results. Elongations to fracture in the DENT specimens are also predictable from a simple deformation analysis which considers the plastic elongations due to crack initiation, crack propagation and final stretch of a ligament that has reached a necking strain equal to that in a simple plain tension test. Micro-hardness measurements show that the strain localization is more intense near the fracture surface as the pre-strain level is increased and this is suggested to be an explanation for the low _c values obtained in pre-strained specimens.     

Effect of addition of tin on the microstructure and machinability of α-brass

This study was aimed at developing lead-free brass alloys with the goal of substituting lead element with tin. For this purpose, lead-free alloys with tin were developed and the microstructure, hardness and machining behaviour of the Cu–30%Zn alloy was compared with Cu–30%Zn–x%Sn (x?= 1.2, 3.2, 5.4, 8, 11.4, 13.9, 17.4). The results showed that the addition of Sn to single-α phase brass led to the formation of duplex (α?+?β′) brass and then the formation of (β′?+?? ) brass both with increased hardness. In addition, the addition of Sn to Cu–30%Zn alloy led to the decrement of equivalent machining forces (F_m), surface roughness and also the promotion of chip fragmentation due to the formation of the β′ phase, which is an improvement in machinability.     

Flow behaviour and microstructural evolution in cold worked 70∶30 α-brass

Abstract

Deformation behaviour and microstructures at failure were investigated in a mill cold worked 70∶30 α-brass over the test temperature range of 298–973 K and strain rate range of 10^?5–5×10^?3 s^?1. Tensile properties as a function of temperature revealed three distinct regions, with their temperature sensitivity being maximum at intermediate temperatures (553–673 K) and much less towards the lower and higher temperature ranges. Two values of activation energy for high temperature deformation Q were obtained to be 117·5 kJ mol^?1 below 623 K and 196·4 kJ mol^?1 above this critical temperature. In the respective temperature range the values of stress exponent n were 5·6 and 3·8. Based on the values of Q and n, the deformation mechanism was suggested to be dislocation climb creep with a probable contribution from dislocation pipe diffusion on lowering the temperature. Both grain size and cavity size were found to increase with increasing test temperature, suggesting them to be interrelated and act as an alternative steps for accommodating grain boundary sliding. Static grain growth study, over the temperature range of 773 to 1073 K, led to activation energy for grain growth to be 71 kJ mol^?1, with the time exponent of 0·37.     

Unusual W-shaped Galvanic Cell Model of ISCC of α-brass in Neutral Mattsson''''s Solution

Intergranular stress corrosion cracking(ISCC) of α-brass in neutral Mattsson's solutionwas found to be controlled by an unusual"W"-shaped galvanic cell whose cathode is thegrain boundary oxide film (G.B.0. film) andsurface film and the anode is fresh metal atthe cracked tip on both sides of the G.B.0.film. Redox reactions involved in the cellhave been proposed here. According to thismdel, initidtion of ISCC is caused by therupturing of surface film along grain boundaries,thus forming a galvanic cell. Propagation ofISCC resulted from alternate advances of G.B.0.film and dissolution on both sides of G.B.0.film caused by the effect of electrochemicalreaction. This work developed an effective approachto investigate the embrittlement process atthe tip of the crack, by increasing the lengthof the embrittlement region through constantstrain test and distinguishing the morphologyand the nature of the corrosion products byoptical microscopy and scanning electronmicroscopy (SEH).     

Studies of the changes in the geometry of grain boundaries and grains during recovery/continuous recrystallization in α-Fe

Samples of -Fe (Armco) have been deformed by 50% in compression. These have then been annealed at 400 °C, considerably below the conventional recrystallization temperature, and the evolution of grain size and shape quantified. Initially the grain size is found to decrease whilst, at the same time, the grain-shape anisotropy also decreases. It is suggested that a continuous recrystallization process which favours the generation of higher angle grain boundaries is the underlying mechanism. Annealing for longer times gives rise to an increase in grain size with the development of undulations on the grain surfaces. The mechanism suggested for this behaviour involves the annihilation of segments of sub-boundaries on to a pre-existing boundary from either side of the boundary.     

Dynamic recrystallization during hot compression of α-Fe

Specimens of high purity -Fe were deformed in the GLEEBLE-1500 at temperatures of 550°C, 700°C, 800°C and 900°C at strain rates ranging from 0.001 to 10 s^–1. The microstructural changes, which occur during the hot compression, have been investigated by optical microscopy and related to the true stress-true strain curves. The experimental results show that the dynamic recrystallization is accelerated with increase of deformation temperature and decrease of strain rate. The relation between the dynamic recrystallization and Z-parameter has been investigated. Dynamic recrystallization takes place approximately in a certain range of Z parameter, i.e., 25 < lnZ < 37.     

High temperature stress relief cavitation of an Al-bearing α-brass

High temperature stress relief intergranular cavitation and subsequent room temperature embrittlement of an aluminium-bearing-brass has been studied metallographically. The behaviour of a cast susceptible to cavitation has been compared to one which does not exhibit intergranular cavitation during stress relief, and which is subsequently more ductile at room temperature. A number of micro-analytical techniques (SIMS and EDX) failed to reveal any difference in the grain boundary chemistry between a cast susceptible to intergranular cavitation and one which was not, but it is suspected that the combined action of dissolved gases (e.g. hydrogen) and trace element impurities plays a major role in cavitation. The cavities formed during stress relief were often polyhedral in shape and it is considered that this occurs by the diffusion of matter around the cavity surface to attain a lower energy surface configuration. Second phase particles were found to play only a minor role in the nucleation of cavities. Room temperature intergranular fracture surfaces of material, in which cavities had formed during stress relief, were interpreted in terms of high temperature cavity formation and coalescence combined with low temperature plastic void growth and interlinkage.     

Role of Edge and Screw Dislocations in Deformation of Metals

It was widely accepted that the screw dislocation is responsible for the strong temperature dependence of the yield stresses observed in bcc metals. In this paper, we show the role of edge dislocations in the deformation of metals and point out that in some cases, its main contribution to the plastic flow behaviour cannot be ignored.     

Size-dependent hardness in annealed and work hardened α-brass and aluminum polycrystalline materials using activation volume analysis

A study of the size-dependent hardness in aluminum and α-brass is presented. The study employs rate-effects to examine the fundamental mechanisms responsible for the indentation hardness size dependence (effect), or (ISE). These rate effects are characterized in terms of the rate sensitivity of the hardness, ∂H/∂lnε_eff, where H is the hardness and ε_eff is an effective strain rate in the plastic zone beneath the indenter. ∂H/∂lnε_eff is measured using indentation creep, load relaxation, and rate change experiments. ∂H/∂lnε_eff is used to calculate the activation volume, V*; activation volume data measured using conventional uniaxial testing are compared with activation volume data measured using nanoindentation. The data for α-brass when plotted V* vs. H (hardness) or σ (flow stress), extrapolated into literature data from conventional uniaxial testing, while the aluminum data suffered an offset. We propose some mechanisms for this offset. Using V* formalism, we demonstrate using materials with different stacking fault energy (SFE) and specimens with different levels of work hardening how increasing the dislocation density affects V*; these effects may be taken as a kinetic signature of dislocation strengthening mechanisms. We depicted an ISE in both H and ∂H/∂lnε_eff(V*). The trend of V*-vs.-H as a result of the ISE is consistent with the trend of testing specimens with different levels of work hardening. This indicates that a dislocation mechanism drives the ISE.     

Characteristics of Deformation and Recovery in Shape Memory Alloy

The mechanical behavior and the effect of pre-strain on recovery behavior of Ti50Ni47Fe3 (at. pct) alloy were investigated systematically by tensile and recovered tests accompanied by electrical resistance measurement. Ti50Ni47Fe3 alloy has different deformation behaviors at different temperature ranges, the deformation curves in different temperature range can be classified into four kinds. The start temperature of recovery increases with the increase of pre-strain. There exists an optimal deformation condition, at which the specimen exhibits maximum free recovery strain. With increasing pre-strain the recovery stress increases and reaches the maximum at 8% pre-strain. R-phase to parent transition offered about 0.2% recovery strain. With pre-strain increasing the recovery stress increases and reaches to the maximum at 8% pre-strain. The recovery stress is corresponding with the critical stress of stress-induced martensitic transformation.     

Corrosion-Facilitating Local Plastic Deformation for α-Ti in Methanol

The effect of corrosion process on facilitating local plastic deformation for α-Ti in methanol has been investigated with the micro-multiplicative moire interferometer technique. The size of plastic zone and the plastic strain ahead of loaded notch have been measured. The results show that the anodic dissolution, or corrosion process itself can enhance the plastic zone and the plastic strain ahead of a loaded notch during stress corrosion of α-Ti in methanol solution     

Structure and Deformation of Intermetallic Beryllides

Intermetallic beryllides are potential light-weight, high-temperature structural materials. In this paper. the processing techniques, microstructure. deformation, and oxidation properties of intermetallic beryllides are described. In addition to nickel beryllides (NiBe). which is treated as a model system.other high beryllium-containing refractory beryllides, such as Nb2 Be17. VBe12. are also studied.The room temperature deformation and high-temperature creep properties of these beryllides are repor4ed. At room temperature. NiBe exhibits certain tensile ductility (~ 1 .3%). but all other beryllides are essentially brittle. Nonetheless, these beryllides become ductile at temperatures above approximately 1000℃. Their creep properties are presented. The creep properties are compared with those of intermetallic aluminides. Also. a comparison is made between the ductile-to-brittle transition behaviour of intermetallic beryllides and that of aluminides. Although beryllides are generally oxidation resistant at high temperatures, some beryllides, e.g., ZrBe13, suffer the pest reaction during oxidation at intermediate tem peratures. The pest mechanisms are proposed     

Deformation mechanism in αCu-Al single crystals and bicrystals under scratching with pyramidal indentor

In order to elucidate the deformation mechanism of materials in abrasive wear process, scratching tests were carried out on the (111) face of Cu-14.7 at % Al single crystals and 13 b bicrystals with pyramidal indentor. In the scratching on the single crystals three kinds of scratching directions, [11 ¯2], [¯1¯12] and [¯110], were chosen. In the case of bicrystals, the [1 ¯21] and [1¯10] directions were adopted. After scratching the dislocation structure, slip trace patterns and surface profiles across the scratched track were examined. In addition, the dislocation distributions inside the crystals were revealed by successively removing thin layers and developing etch pits on the exposed surface.The slip traces on either side of the scratched track are produced more extensively than those around the indented point. These slip traces are observed only in the surface of about 100 m deep, and are not observed in the deeper area. It is found that the microscopic deformations produced due to the scratching consist of three kinds of deformation: formed by indentation, formed by both normal and frictional stresses in the surface layer and formed by stress which is caused in certain limited depth.The swells of the material were produced in the front of the indentor due to the slips on the {111} crystal faces which are arranged so as to be diverging into the inside. The azimuths of formation of the swells are [11¯2] and [¯211] in the [11¯2] and [¯110] scratching respectively and [1¯21], [¯211] in the [¯1¯12] scratching.In the scratching of the bicrystal, the propagation of dislocation in the surface layer of the one side crystal is obstructed by the grain boundary. The microscopic deformation range on the dislocation order is affected by the distribution density of the grain boundaries.     

The role of local strains from prior cold work on stress corrosion cracking of α-brass in Mattsson's solution

The dynamic strain rate ahead of a crack tip formed during stress corrosion cracking (SCC) under a static load is assumed to arise from the crack propagation. The strain surrounding the crack tip would be redistributed as the crack grows, thereby having the effect of dynamic strain. Recently, several studies have shown cold work to cause accelerated crack growth rates during SCC, and the slip-dissolution mechanism has been widely applied to account for this via a supposedly increased crack-tip strain rate in cold worked material. While these interpretations consider cold work as a homogeneous effect, dislocations are generated inhomogeneously within the microstructure during cold work. The presence of grain boundaries results in dislocation pile-ups that cause local strain concentrations. The local strains generated from cold working α-brass by tensile elongation were characterized using electron backscatter diffraction (EBSD). The role of these local strains in SCC was studied by measuring the strain distributions from the same regions of the sample before cold work, after cold work, and after SCC. Though, the cracks did not always initiate or propagate along boundaries with pre-existing local strains from the applied cold work, the local strains surrounding the cracked boundaries had contributions from both the crack propagation and the prior cold work.     

Deformation of sintered copper and 50Cu–50Fe mixture to large strains by cyclic extrusion and compression

Abstract

Sintered compacts of copper and a 50Cu–50Fe mixture have been plastically deformed to large strains (total strain ?_t=13·8) by cyclic extrusion and compression. The hardness changes after deformation indicate that no further work hardening occurs with either material when ?_t>4·6. With copper, strain accommodation at large strains would appear to occur solely by dynamic recovery and recrystallisation. With the Cu–Fe mixture, shear banding is still found at the highest strains used.

MST/1609     

Plane section and spatial characteristics of equiaxed β-brass grains

Abstract

A study was undertaken to define better the concept of the statistical grain. A cube having edges of 52·1 mm was cut from a slowly cooled β-brass ingot which had equiaxed grains. The distributions of sizes and shapes of grain sections on the faces of the cube were determined. The grains were then separated in a solution of mercurous nitrate and nitric acid. Observations were made of the relationship between grain diameter and the number and shapes of faces. For the 941 separated whole grains, the frequency of distribution of grain diameters obeyed the log–normal law. A plane distribution curve was derived for the statistical grain which enables spatial distributions to be calculated from measurements made on a plane section.

MST/825