Void formation,void growth and tensile fracture in Ti-6AI-4V

Metallurgical and Materials Transactions A - The influence of microstructure on void formation, void growth and tensile fracture was investigated for the Ti-6A1-4V alloy, aged to yield strengths of...     

The effect of void arrays on void linking during ductile fracture

As a physical basis for understanding void linking during ductile microvoid fracture, the contrasting behavior of tensile specimens containing random and regular arrays of holes is examined. The results indicate that specimens containing random arrays are less ductile than their regular-array counterparts. The magnitude of this effect depends on the minimum spacing between holes, hole size, and the strain hardening of the material. The experimental results may be understood in terms of the importance of hole/void distribution on a ductile fracture process which is a consequence of both micro- and macroscale shear instabilities.     

Dynamic growth of tensile cracks by ductile and brittle fracture mechanisms in a viscoplastic material

In this paper, a finite element analysis of steady-state dynamic crack growth under Mode I, plane strain, small-scale yielding conditions is performed in a rate dependent plastic material characterized by the over-stress model. The main objective of the paper is to obtain theoretically the dependence of dynamic fracture toughness on crack speed. Crack propagation due to a ductile (micro-void) mechanism or a brittle (cleavage) mechanism, as well as transition from one mode to another are considered. The conversion from ductile to brittle has been observed experimentally but has received very little attention using analytical methods. Local fracture criteria based on strains and stresses are used to describe ductile and brittle fracture mechanisms. The results obtained in this paper are in general agreement with micro-structural observations of mode conversion during fracture initiation. Finally, the particular roles played by material rate sensitivity and inertia are examined in some detail.     

The effect of matrix strength on void nucleation and growth in an alpha-beta titanium alloy,CORONA-5

This work was undertaken to examine the effect of increasing matrix strength at constant equiaxed microstructure on void nucleation and growth in the titanium alloy, CORONA-5, Ti-4.5Al-5Mo-1.5Cr. A martensite and a beta matrix were used in the as-quenched and the heat treated conditions. For each matrix, fine and coarse alpha sizes were produced and a third size of alpha was used for the as-quenched condition of the martensite series. The processing procedures produced an aligned alpha structure which was most pronounced in the fine structure. Void nucleation occurred in an aligned fashion and took place predominantly atα /martensite orα/β interfaces. An explanation is offered for the aligned nucleation in terms of nonuniform deformation of the banded structure which appeared most prominently after heat treatment to produce the coarser microstructure. An incubation strain was found for both types of matrices. The incubation strain increased for the interface in the following order: martensite/martensite,α /martensite, andα/ β. The incubation strain for martensite/martensite interfaces was relatively independent of the matrix strength. Void growth as a function of true strain was generally found to occur in two stages, a slow stage I and a more rapid stage II. Stage II growth occurred as a result of coalescence of voids growing toward one another from nearby particles. Stage II growth was more rapid for the martensite matrix than for theβ matrix. For the martensite matrix void growth rates could not be accounted for either on the basis of strength or strain hardening rates. However, the longest void growth rate was found to increase as the function λ ^N /d _α ^L increased. λ^N is the interparticle spacing normal to the tensile axis and _α ^L is the alpha particle size parallel to the tensile axis. For the beta matrix void growth rates increased with increasing yield s trength and decreased with increasing strain hardening. It was not possible to relate fracture strength to an extrapolated longest void at fracture as was done in earlier studies. This is explained in terms of the nonuniformity of fracture paths observed in the alloy.     

Strength,fracture, and fatigue behavior of advanced high-temperature intermetallics reinforced with ductile phases

The results of recent studies on the fatigue and fracture behavior of extruded Ti-48A1 + 20 vol pct TiNb and hot-isostatically pressed (“hipped”) MoSi₂ + 20 vol pct Nb are presented (compositions in atomic percent unless stated otherwise). The effects of ductile phase reinforcement of Ti-48A1 and MoSi₂ on the micromechanisms of fracture under monotonie and cyclic loading are elucidated. Micromechanics models are applied to the prediction of crack-tip shielding components, and the effects of temperature on tensile/compressive/flexure strengths are discussed. Ductile phase toughening under monotonie loading conditions is shown to be associated with lower fatigue crack growth resistance. The lower fatigue resistance is attributed to the absence of crack-tip shielding, higher crack opening displacements, and the effects of inelastic strains that are developed in ductile phase-reinforced composites under cyclic loading conditions. S.M.L. SASTRY, formerly Program Manager and Fellow, McDonnell Douglas Research Laboratories. This article is based on a presentation made in the symposium “Quasi-Brittle Fracture” presented during the TMS fall meeting, Cincinnati, OH, October 21–24, 1991, under the auspices of the TMS Mechanical Metallurgy Committee and the ASM/MSD Flow and Fracture Committee.     

Structure,tensile deformation,and fracture of a Ti3Al-Nb alloy

The development of Ti₃Al-Nb alloys is an excellent example of the recent resurgence of interest in the use of intermetallics for high-temperature applications. We examine, in this contribution, the structure of a typical alloy Ti-24A1-11Nb and show it to consist primarily of the ordered α₂ phase (based on Ti₃Al, DO₁₉) and β_o, (based on Ti₂NbAl, B2) phases, with small amounts of a third phase, which is distorted slightly to an orthorhombic symmetry from the D0₁₉ (hexagonal) structure. Tensile properties have been examined on samples heat-treated to vary the size, shape, and volume fraction of α₂ phase and the deformation and fracture behavior of the ordered, two-phase mixture established. The tensile ductility is seen to maximize at intermediate volume fractions of the α₂ and β_o phases (∼30 pct) at values of 6 to 10 pct elongation to fracture, depending on the grain size of the β_o phase. A rationale incorporating the failure modes of the two phases—cleavage of α₂ and slipband decohesion of β_o—has been evolved to explain the trends in ductility with heat treatment.     

Influence of β grain size on tensile behavior and ductile fracture toughness of titanium alloy Ti-10V-2Fe-3Al

The β grain size of the alloy Ti-10V-2Fe-3Al was varied by heat treatment, and the tensile behavior and fracture toughness were evaluated as a function of β grain size at room temperature. The alloy showed stress-induced martensitic transformation, and the triggering stress for this transformation varied with grain size. The 0.2 pct yield stress exhibited a Hall-Petch relationship with grain size. The ductile fracture toughness was found to increase with decrease in grain size, and it was also shown to follow a Hall-Petch kind of relationship. The grain boundary and the stress-induced martensitic contribution to fracture toughness were separated out.     

Mechanism of brittle fracture in a ductile 316 alloy during stress corrosion

The ductile f.c.c. 316 alloy is shown to exhibit brittle transgranular (and intergranular) stress corrosion cracking in a 153°C MgCl₂ solution at free corrosion potential. Tests on smooth and pre-cracked specimens are performed to identify the mechanisms of fracture. Transgranular cracking is related to both a discontinuous microcleavage mainly on {100} planes and a microshearing on {111} planes. A new physical modelization is proposed to explain the brittle transgranular cracking. It is based on the influence of the localized anodic dissolution on the enhancement of the plasticity at the crack tip. The formation of dislocation pile-ups and the conditions of restricted slip induce a brittle microcracking. The crack propagation is then limited and arrested by the strong effect of relaxation in the ductile 316 alloy. Such a model is discussed as a function of the main factors governing the transgranular stress corrosion cracking sensitivity of ductile f.c.c. single-phase materials.     

Fatigue crack growth and fracture toughness behavior of an Al-Li-Cu alloy

Slip behavior, fracture toughness, and fatigue thresholds of a high purity Al-Li-Cu alloy with Zr as a dispersoid forming element have been studied as a function of aging time. The fracture toughness variation with aging time has been related to the changes in slip planarity,i.e., slip band spacing and width. Although the current alloy exhibits planar slip for all aging conditions examined, the crack initiation toughness,Klc, compares favorably with those of 2XXX and 7XXX aluminum alloys. Near threshold fatigue crack growth results in air and vacuum suggest that irregularities in the crack profile and the fracture surfaces and slip reversibility are some of the major contributing factors to the crack growth resistance of this alloy.     

Cavity formation from inclusions in ductile fracture of A508 steel

Experiments were performed on A508 class 3 steel in order to determine the conditions for cavity formation from elongated MnS inclusions. Circumferentially notched tensile specimens were employed in order to investigate the effect of negative hydrostatic pressure. The results of finite element calculations and metallographic observations on polished sections were used to evaluate the conditions required for cavity initiation. Tests were performed at different temperatures in both longitudinal and short transverse direction. The results can be explained in terms of a local critical stress independent of temperature. This local stress is tentatively calculated using an extension of Eshelby’s theory for inclusions proposed by Berveiller and Zaoui.     

Effect of porosity on the fracture stress of powder materials in the ductile fracture mechanism

Translated from Poroshkovaya Metallurgiya, No. 5(353), pp. 95–99, May, 1992.     

Unconstrained and constrained tensile flow and fracture behavior of an Nb-1.24 At. Pct Si alloy

The unnotched and notched tensile behavior of the β-phase constituent (Nb with Si in solid solution) of the (Nb)/Nb₅Si₃ composite has been investigated at room temperature and -196 °C. At room temperature, the unnotched tensile behavior comprises significant strengthening due to Si, low strain-rate sensitivity, low strain hardening, extensive ductility, and ductile microvoid coalescence fracture, even at strain rates as high as 1.1 s⁻¹. At −196 °C, the unnotched alloy exhibited much higher strength, good ductility, and cleavage fracture. At room temperature, the notched specimens exhibited cleavagelike fracture with significant plasticity, and at −-196 °C, they exhibited cleavagelike fracture with much lower plasticity at the notch. A finite-element analysis (FEA) of stress and strain fields in the vicinity of the notch root, together with un-notched tensile behavior, indicates that plasticity plays an important role in nucleating cracks, while the high-axial tensile stress component governs crack propagation. These results are used to rationalize the observed toughening and fracture behavior of a (Nb)/Nb₅Si₃ composite.     

Structure/property/continuum synthesis of ductile fracture in inconel alloy 718

Two microscopic ductile fracture processes have been established in a fracture tough superalloy, Inconel 718, aged to five strength levels. At yield strengths less than 800 MPa, the mechanism is a slow tearing process within large pockets of inhomogeneous carbides and nitrides, giving rise to plane strain fracture toughness (K _IC)values greater than 120 MPa-m^1/2. At yield strengths greater than 900 MPa, the mechanism involves fracture initiation at carbides and nitrides followed by off crack plane void sheet growth nucleated at the Laves (σ) phases. Here, the fracture toughness drops to about 80 MPa-m^1/2. A Mode I normal strain growth model for low yield strength conditions and a shear strain void sheet model for high yield strength ones are shown to model K_IC data obtained from a J-integral evaluation of compact tension results.     

The influence of strength and phosphorus segregation on the ductile fracture mechanism in a Ni-Cr steel

Upper-shelf toughness and its degradation through ageing at 500°C have been investigated in quenched-and-tempered 300 M steel as a function of tempering treatment at 650°C. Material having a 0.2% proof stress in the range 550–1000 MPa was examined. Toughness was assessed using the parameters J_Ic and reduction-in-area, and was related to the ductile fracture mechanism through measurements of the strain required for void nucleation and void growth to coalescence. Changes in matrix rheology, void-nucleating precipitate morphology, bulk chemistry and interfacial chemistry were monitored during tempering and ageing, and the associated fractography was quantitatively assessed. The ductile fracture process in unaged material was dominated by the strain required for void nucleation on carbide precipitates. Nucleation strain increased with tempering time at 650°C causing a rise in ductility. Ageing at 500°C produced a loss of ductility for all temper conditions, and the sole cause of this effect was the segregation of phosphorus to carbide-matrix interfaces, identified by high resolution Auger spectroscopy. Both the strain required for void nucleation at carbides and that for void growth to coalescence were suppressed by ageing, through a reduction in interfacial cohesion consistent with the embrittling effect of segregated phosphorus.     

The effects of hydrostatic pressure on the mechanism of tensile fracture of aluminum

The effects of hydrostatic pressures up to 600 MPa on the tensile fracture mechanisms of commercial aluminum and a free machining aluminum-copper alloy have been investigated. Tensile fracture of the commercial aluminum was found to occur by a mechanism leading to double-cup fracture at hydrostatic pressures up to 125 MPa with the pressure causing a progressive delay in the onset of void development. Above this pressure, void development was suppressed and all fractures occurred at chisel points. The chisel point fracture was found to involve the development of two opposing diagonal shear zones crossing through the neck center. Fracture of the aluminum alloy occurred by a cup-cone mechanism at pressures up to 300 MPa. Within this pressure range the natural strain to fracture increased linearly and the amount of void coalescence decreased. At higher pressures fracture of the aluminum alloy occurred entirely by a shear mechanism involving the development of void-sheets.     

The effects of hydrostatic pressure on the mechanism of tensile fracture of aluminum

The effects of hydrostatic pressures up to 600 MPa on the tensile fracture mechanisms of commercial aluminum and a free machining aluminum-copper alloy have been investigated. Tensile fracture of the commercial aluminum was found to occur by a mechanism leading to double-cup fracture at hydrostatic pressures up to 125 MPa with the pressure causing a progressive delay in the onset of void development. Above this pressure, void development was suppressed and all fractures occurred at chisel points. The chisel point fracture was found to involve the development of two opposing diagonal shear zones crossing through the neck center. Fracture of the aluminum alloy occurred by a cup-cone mechanism at pressures up to 300 MPa. Within this pressure range the natural strain to fracture increased linearly and the amount of void coalescence decreased. At higher pressures fracture of the aluminum alloy occurred entirely by a shear mechanism involving the development of void-sheets.     

Evidence of void nucleation and growth on planar slip bands in a Nb-Cr-Ti alloy

Dimpled fracture via void nucleatïon, growth, and coalescence is common in alloys containing hard particles or inclusions, which act as void nucleation sites. Void-covered fracture surfaces have also been observed in planar slip materials. However, the process by which dimples from in the planar slip materials is not understood because these materials generally do not contain hard particles or inclusions. An intersecting slip process was recently proposed as a possible mechanism for the formation of dimpled fracture in the planar slip materials. In this article, experimental evidence of void nucleation and growth on planar slip bands in a Nb-Cr-Ti solid-solution alloy (Nb-13Cr-37Ti, in at. pct) is presented. The experimental observations are compared against the intersecting slip model and a peierls stress calculation to elucidate the mechanism of dimpled fracture in planar slip materials.