Low Tensile Ductility Research Articles

The Importance of Microstructural Instability in Determining the Mechanical Behaviour of Cubic Titanium Trialuminides

Cubic trialuminides deform by the movement of dislocations which are clearly dissociated as APB superdislocations at high temperatures, with debate about whether these are dissociated as APB or SISF superdislocations at low temperatures. These materials are characterized by the following mechanical behaviour: (i) strength variable with ternary element addition or titanium content; (ii) mild strength anomaly at high temperature, sometimes; (iii) serrations in stress-strain curve at intermediate temperatures; (iv) low tensile ductility (≈0) at room temperature, increasing at higher temperatures, with an intermediate temperature minimum. These properties are explained by the fine microstructure and its variation locally and with temperature: (a) a tetragonal component of order in ternary alloys in addition to the basic L12 order, varying with ternary element and content; (b) precipitation (sometimes on dislocations) during high temperature testing - accounting for the strength anomaly; (c) extra-ordinarily rapid solute collection at dislocations and APB's - accounting for strain aging and minimum ductility phenomena; (d) strain relaxation at crack tips restricted to single slip planes by the structural modification produced by shear - making a major contribution to brittleness. All these processes are particularly acute in the titanium trialuminides because of the structural instabilities involved, and the fast kinetics of atom rearrangement; thus low temperatures during testing or during cooling after prior heat treatment play a major role. Similar effects may be of importance in other intermetallics such as FeAl, TiAl.

Measurement of the Critical Level of Moisture for Initiation of Water-Vapor-Induced Environmental Embrittlement in an Fe3Al-Based Alloy

Iron aluminides based on Fe[sub 3]Al are of interest as structural materials because of their excellent corrosion resistance in many environments. However, studies have shown that one of the major causes of low room temperature tensile ductility, which so far has limited the use of these alloys, is an environmental reaction involving aluminum in the presence of water vapor. During this reaction, atomic hydrogen is released, moves into the sample ahead of the crack tip during stressing, and causes failure before the true ultimate tensile strength of the material is reached. This reaction is reduced significantly by testing in oxygen or vacuum. In the present study, an Fe[sub 3]Al-based alloy was tensile tested as a function of the level of water vapor in the test environment, from a vacuum of 10[sup [minus]4] Pa to a water vapor partial pressure of 1330 Pa. The results show that a water vapor level of as low as 133 Pa (1 torr) can result in significant embrittlement. The fracture mode remains transgranular cleavage, but the scale of the cleavage facets changes with the water vapor level.

The Fatigue Behavior of γ/α2 Titanium Aluminides

ABSTRACTAxial load controlled high cycle fatigue experiments were conducted on the γ/α2 alloy, Ti-48A1-1V-0.2C (at%), at 23 and 815°C. Four different microstructures, produced through thermomechanical processing, were evaluated to examine the influence of grain size and α2 content on fatigue behavior. The load controlled fatigue life was significantly reduced by increasing grain size and unaffected by α2 content at both 23 and 815°C. Although, α2 content did not greatly influence high cycle fatigue life, the room temperature crack initiation and fast fracture was changed from transgranular to partially intergranular as the volume fraction of α2 was reduced in the fine grain size material. The fatigue strength at 107 cycles (FS) to ultimate tensile strength (UTS) ratio was 0.8 to 0.9 at 23°C and 0.5 to 0.6 at 815°C for all microstructures examined. Low tensile ductility, high work hardening rate and the difficulty in forming strain local-izations all aided the high FS/UTS ratio. The dislocation microstructures produced by fatigue at room temperature were examined in the fine grained high α2 (ductile) microstructure. They consisted of loop patches of all <110] regular dislocations without any <101] or <011] super dislocations because of the large difference in CRSS for these dislocation. The inability to nucleate and move superdislocations inhibited the formation of persistent slip bands as is often found in high and intermediate stacking fault FCC metals.

Microstructure and mechanical properties of mechanically alloyed, ingot metallurgy and powder metallurgy AlLiCuMg alloys

AlLiCuMg alloys with similar chemical compositions were prepared by mechanical alloying, ingot metallurgy and powder metallurgy. Their microstructures and their monotonic and fatigue properties were investigated. All three types of alloy exhibited low ductility, a problem typical of the high modulus, low density AlLi-base alloys. In the assolution-treated condition, uniformly distributed fine particles of oxide and carbides gave a reasonable strength to the alloy prepared by mechanical alloying. Although these particles homogenized the deformation, the presence of larger second-phase particles caused premature crack initiation and low tensile ductility. When alloys prepared by ingot metallurgy and powder metallurgy were deformed in the underaged and slightly overaged conditions, the shearable coherent Al 3Li (δ′) particles that were present promoted planar slip, strain localization and low ductility. A low cycle fatigue study showed that no break was observed in the Coffin-Manson plot, suggesting that no change in the deformation mode occurred in the high and low strain amplitude tests. Also, because of the more homogeneous deformation the alloy prepared by mechanical alloying exhibited a higher fatigue resistance than the alloys prepared either by ingot metallurgy or by powder metallurgy of similar tensile strength.

Effect of Sulfides and Sulfide Morphology on Anisotropy of Tensile Ductility and Toughness of Hot-Rolled C-Mn Steels

The effects of sulfur content (0.004 or about 0.013 pct) and sulfide morphology (stringered or globular) on anisotropy of tensile ductility and Charpy V-notch (CVN) shelf energy were investigated in a series of 0.1 and 0.2 pct carbon, 1.0 pct manganese steels. The effect of sulfide inclusions on fracture strain or CVN shelf energy correlated with a single parameter,P, regardless of inclusion shape, stringered or globular, or test direction, longitudinal, transverse, or through-thickness. The parameterP was defined as the total projected length of inclusions per unit area on a plane parallel to the fracture plane. The fracture strain and CVN shelf energy decreased with an increase inP. The magnitude ofP was directly proportional to the volume fraction of inclusions and inversely proportional to the inclusion dimension perpendicular to the fracture plane. The lower tensile ductility and CVN shelf energy in the 0.2 as compared with the 0.1 pct carbon steels was a consequence of the greater pearlite content in the former steels. This greater pearlite content had no apparent effect on the work-hardening rate,H, but decreased the strain-rate sensitivity of the flow stress,M, and the strain-rate sensitivity of the work-hardening rate,B. The decrease inM andB with increasing pearlite content is in accord with a decrease in tensile ductility according to recent models of neck development in tension tests. It appears that the decrease in tensile ductility with increasing pearlite content is a result of enhanced localized shearing, which promotes the coalescence of voids nucleated at second phases.

Al-Mg-Si系合金の延性と破壊特性におよぼす加工熱処理の影響

High purity Al-Mg-Si base alloys often exhibit low tensile ductility and typical grain boundary fracture occurs when aged to maximum strength levels. The influence of cold work and aging on the ductility and the fracture behaviour of high purity Al-Mg-Si base alloys has been investigated to prevent the grain boundary fracture and to improve the low ductility of the alloys. Marked improvement in the ductility of coarse grained alloys can be attained by proper cold work and aging, and the fracture mode changes from brittle grain boundary fracture to ductile transgrannular fracture. These phenomena are observed only in the coarse grained alloys and not in the fine grained alloy containing Cr. Preaging treatment after quenching is undesirable because of a decrease of the ductility. Marked improvement in the ductility is attributed to the absence of PFZ along grain boundaries, the suppression of coarse grain boundary precipitates and the precipitation of coarse intermediate precipitates in the matrix.

Microstructure-property relations in a machine thixocast Co-base superalloy

Machine thixocasting is a new casting process wherein a semi-solid alloy slurry of spheroidal primary solid particles and liquid is injected into a die. The resulting thixocast microstructure is rather unusual. It consists of spheroidal grains in a fine dendritic matrix. The tensile, stress rupture and high cycle fatigue (HCF) properties of Haynes 31 fabricated by the machine thixocasting technique have been determined. Relative to the conventional counterpart the thixocast material has a significantly higher yield strength and a lower tensile ductility. The stress rupture life and the HCF capability of the thixocasting are also lower than the conventionally cast material. The mechanical property deficiencies are discussed in terms of the thixocasting process and the various microstructural features such as the scale of the dendritic matrix, the morphology of the primary solid particle/matrix interface, and the volume fraction of primary solid particles and their average size. Methods for improving the mechanical properties of the thixocasting are outlined.

The anisotropy of deformation and fracture in a directionally solidified Ni/Ni3Al-Ni3Cb lamellar eutectic alloy

The orientation dependence of deformation and fracture modes was investigated for a directionally-solidified Ni−Ni3Al−Ni3Cb lamellar eutectic alloy (Ni-20 wt pct Cb-2.5 wt pct al-6.0 wt pct Cr) using optical and transmission microscopy to examine tensile and compression specimens tested at temperatures below the softening point of the δ (Ni3Cb) reinforcing phase (∼1050 K). In this temperature range there is a large difference between longitudinal and transverse tensile ductibility (>5 pct longitudinalvs<1 pct transverse). No single preferred fracture path (such as interfacial delamination) could be found to account for the low transverse tensile ductility. Analysis of the δ twinning geometry, however, indicated that the twinning strains for twins of the type {211}, which operate copiously in longitudinal tension, are negative in most transverse orientations, with Schmid factors being very low (<0.013) in the limited range of transverse orientations where {211} twin strains are positive. Examination of transverse tension test specimens broken at 1033 K confirm the absence of {211} twins, with only limited {011} twinning being found in selected grains, leading to the conclusion that the relatively low transverse tensile ductility of the eutectic results from the very limited number of deformation systems which operate in the δ reinforcing phase below the softening temperature.

Hydride orientation and tensile properties of Zr-2.5 wt% Nb pressure tubing hydrided while internally pressurized

AbstractAn investigation was carried out to determine the orientation and the effect on tensile properties of hydrides precipitated in heat-treated Zr-2.5 wt% Nb tubing during the corrosion hydriding of internally pressurized samples. It was found that as the stress during hydriding was increased the hydrides gradually assumed a characteristic radial orientation nearly perpendicular to the hoop stress. The observed radial hydride orientation was influenced by prior β grain size, residual quenching stresses, cold-work strains and stresses applied while the pressure tube was being hydrided. Hoop stresses in excess of 20,000 psi were required to produce complete radial orientation of the hydrides.Transverse tensile tests on the hydrided tubes were conducted using a reduced section ring technique. These tests revealed a low ductility at low temperatures with a transition in the range 60°–100°C for material with radially oriented hydrides. Low tensile ductility was observed at significantly higher temperatures...

The interrelationship of fracture toughness and microstructure in a Ti-5.25Al-5.5V-0.9Fe-0.5Cu alloy

Fracture toughness of anα-Β titanium alloy heat treated to a constant yield strength has been found to depend upon the morphology of α produced or remaining after the initial solution treatment. In equiaxed α structures, fracture toughness depends linearly upon the grain boundary area per unit volume,Sv, and is independent of equiaxed α particle size or spacing. In a grain boundary α structure fracture toughness depends both onSv and, within limits, linearly on the thickness of the α. Explanations are offered for the observed propagation of cracks at α-@#@ Β interfaces and for the observation that high fracture toughness can accompany low tensile ductility.