An Investigation of the fracture behavior of gamma-based titanium aluminides: Effects of annealing in the α + γ and α2 + γ phase fields

The effects of annealing in the α + γ and α₂ + γ phase fields on the microstructures and fracture properties of Ti-48A1 and Ti-49Al-3.4Nb are discussed in this article. Annealing of the niobium-containing alloy in the α₂ + γ phase field results in the precipitation of ⇌₂ and Nb₅Si₃ predominantly at the grain boundaries. The precipitation decrease the grain boundary cohesion, thereby promoting intergranular separation. Precipitation also decreases the tensile strength and ductility of Ti-49Al-3.4Nb compared to that of the binary alloy. The possible role of interfaces in the transmission of slip is also discussed, and micromechanical models are applied to the prediction of tensile behavior and fracture toughness. Formerly Scientist, McDonnell Douglas Research Laboratories Formerly Director and MDC Fellow, McDonnell Douglas Research Laboratories     

On the mechanisms of high-temperature intergranular embrittlements of Ni3Al-Zr Alloys

Recent studies on the room-temperature fracture behavior of Ni₃Al-Zr alloys after preexposure at elevated temperatures show various types of intergranular failure. In the presently studied Ni₇₈Al₂₁Zr₁B_0.2 alloy, a strong intergranular fracture tendency at room temperature has been found after preexposure at 750 °C, which is caused by the grain boundary precipitation in this alloy. After short-term exposure above 1200 °C and bending fracture at room temperature, the alloy also suffers intergranular embrittlement due to grain boundary melting. The intergranular fracture appearance is quite different from that observed in a previous study for a Ni_77.4Al₂₂Zr_0.6B_0.2 alloy after air exposure for 100 hours at 1200 °C. In that case, the intergranular fracture was accompanied by grain boundary diffusion (invasion) and segregation of oxygen. The mechanisms of these types of grain boundary failure are discussed. Formerly Doctoral Candidate, Institute of Materials Science and Engineering, National Taiwan University.     

The mechanism of formation of a fine duplex microstructure in Ti-48Al-2Mn-2Nb alloys

The mechanism of formation of the fine duplex microstructure resulting from the α → γ transformation in water-quenched Ti-48Al-2Mn-2Nb alloys was studied using transmission and analytical electron microscopy. As-cast Ti-48Al-2Mn-2Nb alloys were heat treated in the α phase field and water quenched to room temperature. The resulting microstructure (referred to as a fine duplex microstructure) consisted of equiaxed grains and abutting lath colonies. Both the colonies and the grains were composed of the γ phase, twinned γ laths, and α₂ laths. It was found that the transformation from α to γ in the fine duplex microstructure took place through long range diffusional processes, and compctitive growth between the equiaxed and lath morphology occurred. Nucleation of they phase from the α matrix can occur through nucleation on stacking faults, followed by growth through the sympathetic nucleation and growth of new γ laths on a substrate lath. The observed misorientations and the interfacial structures between the laths were found to be consistent with such a mechanism. Compctition between such nucleation and growth mechanisms for the equiaxed and lath morphologies of γ leads to the formation of lath colonies (of γ and α₂) interspersed with equiaxed grains in these alloys. Formerly Visiting Scientist, Metals and Ceramics Division, Oak Ridge National Laboratory This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

Aqueous environmental crack propagation in high-strength beta titanium alloys

The aqueous environment-assisted cracking (EAC) behavior of two peak-aged beta-titanium alloys was characterized with a fracture mechanics method. Beta-21S is susceptible to EAC under rising load in neutral 3.5 pct NaCl at 25 °C and −600 mV_SCE, as indicated by a reduced threshold for subcritical crack growth (K _TH ), an average crack growth rate of up to 10 μms, and intergranular fracture compared to microvoid rupture in air. In contrast, the initiation fracture toughness (K _ICi ) of Ti-15-3 in moist air is lower than that of Beta-21S at similar high σ_YS (1300 MPa) but is not degraded by chloride, and cracking is by transgranular microvoid formation. The intergranular EAC susceptibility of Beta-21S correlates with both α-colonies precipitated at β grain boundaries and intense slip localization; however, the causal factor is not defined. Data suggest that both features, and EAC, are promoted by prolonged solution treatment at high temperature. In a hydrogen environment embrittlement (HEE) scenario, crack-tip H could be transported by planar slip bands to strongly binding trap sites and stress/strain concentrations at α colony or β grain boundaries. The EAC in Beta-21S is eliminated by cathodic polarization (to −1000 mV_SCE), as well as by static loading for times that otherwise produce rising-load EAC. These beneficial effects could relate to reduced H production at the occluded crack tip during cathodic polarization and to increased crack-tip passive film stability or reduced dislocation transport during deformation at slow crack-tip strain rates. High-strength β-titanium alloys are resistant, but not intrinsically immune to chloride EAC, with processing condition possibly governing fracture. Formerly Graduate Research Associate, University of Virginia Formerly Graduate Research Associate, University of Virginia     

The effect of deformation-induced transformation on the fracture toughness of commercial titanium alloys

The effect of deformation-induced transformation of metastableβ phase on the ductility and toughness of four commercial titanium alloys was investigated. Tensile tests, Charpy impact tests, and both static and dynamic fracture toughness tests were carried out at temperatures between 77 and 473 K on four titanium alloys containing metastableβ phase. Deformation-inducedα″ (orthorhombic martensite) was observed in an (α + β)-type Ti-6Al-2Sn-4Zr-6Mo alloy. The dynamic fracture toughness of this alloy increased considerably at 223 K compared to those at other temperatures. In another (α + β)-type Ti-6A1-4V alloy, the static fracture toughness at 123 K and the dynamic fracture toughness at 223 K were increased considerably by the presence of deformation-induced martensite compared to those at other temperatures. The strength increased as the temperature decreased in this alloy. An abnormal elongation of aβ-type alloy, Ti-15V-3Al-3Sn-3Cr, at 123 K was attributed to the mechanical twinning of theβ phase. However, the effect of deformation-induced transformation on the fracture toughness of Ti-3Al-8V-6Cr-4Mo-4Zr alloy was not observed. Formerly Visiting Associate Professor, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA. Formerly with the Department of Production Systems Engineering, Toyohashi University of Technology.     

Mechanical properties of Fe-Cr-Co ductile permanent magnet alloys

The structure and mechanical properties of ternary Fe-Cr-Co magnet alloys containing 9 to 11 wt pct cobalt have been investigated. Fine scale spinodal decomposition of the iron rich bcc α-phase into (α + α₂) structure increased the alloy strength and reduced the ductility. The degree of changes in the mechanical properties depended on the cobalt content and the final aging temperature and time which primarily determines the compositional amplitude. As a result of decomposition, the dislocation movement by slip became more difficult, and the mode of deformation changed from predominantly slip to predominantly twinning. The embrittlement during aging and the fracture behavior of these alloys go through two stages: i) from microvoid nucleation and coalescence type ductile fracture to quasi-cleavage type transgranular fracture (ductile-brittle transition) and ii) from transgranular to intergranular fracture. The cause of the transgranular fracture is attributed to the raised ductile-brittle transition temperature resulting from the increased strength and the tendency for deformation twinning which are likely to make the relief of local stress concentration more difficult. The cause of the intergranular fracture is ascribed to the formation of more or less continuous grain boundary precipitate that forms upon further decomposition at lower temperatures (below ∼540‡C). Both types of embrittlement were found to be reversible upon heat treatment at higher temperatures, either within the (α ₁ +α ₂) range or above the miscibility gap.     

Effect of alloying and aging on morphological changes from lamellar to equiaxed microstructure of <Emphasis Type="Italic">α</Emphasis><Subscript>2</Subscript>+<Emphasis Type="Italic">γ</Emphasis> titanium aluminides

The stability of a lamellar structure consisting of α ₂ and γ phases in alloys Ti-48Al, Ti-48Al-2Mo, Ti-48Al-4Nb, and Ti-48Al-1Mo-4Nb has been studied as a function of aging time and temperature. The alloys were solution treated (1400 °C, 30 min, and air-cooled (AC)) and aged at 1000 °C and 1100 °C for 1, 4, and 16 hours, respectively. The results indicate that the kinetics of lamellae to equiaxed transformation depends on alloy chemistry, aging time, and temperature. The Nb decreases and Mo increases the kinetics of transformation. The combined effect of Nb and Mo results in the highest volume fraction of equiaxed microstructure at a given aging time and temperature. The results have been discussed in relation to microstructural features and have been compared with those reported in other α ₂+γ alloys.     

The relationship between toughness and microstructure in Fe-high Mn binary alloys

The influence of Mn content on the ductile-brittle transition in 16 to 36 wt pct Mn steels was investigated and interpreted in light of the evolving microstructure. It was found that when hcp ε martensite is present in the as-quenched condition or forms during deformation, it lowers the toughness. In 25Mn steel, the stress concentrations at e plate intersections result in the formation of planar void sheets along the {111}_γ planes. The deformation-induced α’ martensite in 16 to 20 pct Mn alloys enhances the toughness, but leads to a ductile-to-brittle transition at low temperatures that is due to the intrusion of an intergranular fracture mode. Binary alloys with greater than 31 pct Mn also fracture in an intergranular mode at 77 K although the impact energy remains quite high. Auger spectroscopy of the fracture surfaces shows no evidence of significant impurity segregation, which suggests the importance of slip heterogeneity in controlling intergranular fracture in these alloys.     

Effect of phase morphology on the mechanical behavior of two titanium aluminide composites

Two binary titanium aluminide alloys, Ti-43A1 and Ti-47A1 (atomic percent), were discontinuously reinforced with 6 vol pct titanium diboride, resulting in a two-phase Ti₃Al and TiAl (α₂ and γ, respectively) matrix with a dispersion of TiB₂ particulate. Cast material was successfully ex-truded and subjected to a series of single-step and duplex-step heat treatments. Thermo-mechanical processing was correlated with microstructural changes, and the ambient temperature mechanical properties were measured for the various heat-treated conditions using tensile and hardness testing. Yield stress and plastic elongation to failure and hardness were found to cor-relate well with the fraction of proeutectoid, or primary, TiAl formed during heat treatment within the α/γ phase field. Precipitation of y within proeutectoid α grains during subsequent aging treatments within the α₂ phase field was seen to increase the room-temperature ductility with negligible debits in yield stress. Enhanced ductility and decreases in yield stress and hard-ness are associated with morphologically large regions of the TiAl phase. Incompatibility of slip systems across γ/α₂ and the inherent resistance to slip in hyperstoichiometric Ti₃Al are suggested as possible explanations for the observed phenomena. Formerly with Martin Marietta Laboratories     

High resolution scanning auger microscopic investigation of intergranular fracture in As-quenched Fe-12mn

Previous research in this laboratory led to the conclusion that the low temperature intergranular fracture mode in Fe-Mn alloys is microstructurally determined, and does not require metalloid segregation or other chemical contamination. That conclusion was tested in the present investigation, which used high resolution scanning Auger microscopy to study the intergranular fracture surfaces. The fracture mode at liquid nitrogen temperature was found to be intergranular fracture whenever the alloy was quenched from the austenite field, irrespective of the austenization time or temperature. High resolution chemical analyses of the intergranular fracture surfaces failed to reveal any consistent segregation of P, S, O, or N. The occasional appearance of sulfur or oxygen on the fracture surface was found to be due to a low density precipitation of MnS and MnO₂ along the prior austenite grain boundaries. Excepting these dispersed precipitates, there was no evidence of manganese enrichment of the prior austenite grain boundaries. A slight segregation of carbon was found along the grain boundaries, but does not appear to be implicated in the tendency toward intergranular fracture. The present results hence reinforce the conclusion that the low temperature intergranular fracture of Fe-12Mn is microstructurally determined.     

The effect of oxygen on the structure and mechanical behavior of Aged Ti-8 Wt pct Al

It is shown that for a Ti-8 wt pct Al alloy aged at a temperature high in the two-phase region (695°C) to precipitate the ordered α₂ phase, an increase in oxygen content from 600 ppm to 1200 ppm decreases the fracture strain from 20 to 1 pct elongation at room temperature and slightly increases the yield strength. The fracture mode is changed from dimpled rupture to predominantly cleavage. Further increase in oxygen content to 3000 ppm does not produce significant additional changes in ductility or yield strength. It is demonstrated that oxygen additions alter the position of the α/α + α₂ coherent solvus, resulting in formation of coherent α₂ in specimens containing ⪞ 1000 ppm oxygen aged at 968 K (695°C). For a given aging time the volume fraction of α₂ increases with increasing oxygen up to 1300 wt ppm and then levels off. The changes in mechanical behavior are attributed to the presence of α₂. The experimental evidence suggests that oxygen partitions preferentially into α₂. Formerly a Graduate Student in the Department of Metallurgy and Materials Science, University of Pennsylvania     

Precipitation in lead-calcium alloys containing tin

The effect of Sn addition on the precipitation in Pb-Ca binary alloys has been studied by means of metallographic observations, hardness and resistivity measurements, and transmission electron microscopy (TEM) observations. With increasing Sn content, the cell advance of discontinuous precipitation is retarded and continuous precipitation occurs preferentially in higher Sn content alloys. The amount of Sn necessary for the retardation is higher for higher Ca contents. The retardation effect is attributed to the segregation of Sn at advancing cell boundaries. Precipitates in ternary alloys are L1₂-type ordered (Pb, Sn)₃Ca. Formerly Student, Department of Materials Science and Engineering, Himeji Institute of Technology This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

The influence of austenite stability on the hydrogen embrittlement and stress- corrosion cracking of stainless steel

A study has been made of the HE and SCC of a type 304 and a type 310 austenitic stainless steel, and the results correlated with the presence or absence of α′ martensite, determined by means of a ferrite detector. Hydrogen induced slow crack growth (SCG) was observed at room temperature when type 304 was stressed i) in 1 psig (∼10⁵ N/m²) gaseous hydrogen, ii) after high temperature charging, and iii) while undergoing cathodic charging. The fracture surfaces corresponding to SCG were primarily transgranular and cleavage-like, and were found to be associated with α′. Conditions i) to iii) did not produce SCG in the type 310 steel, in which α′ martensite was not detected, nor did SCG occur when type 304 was stressed in gaseous hydrogen above the M_D temperature (∼110°C). These observations indicated that the formation of the martensitic phase was a prerequisite for SCG under these test conditions. Stressing of type 310 while it was undergoing cathodic charging at room temperature was found to produce shallow, nonpropagating cracks, confirming earlier reports that austenite can be embrittled by hydrogen in the absence of α′. SCC occurred in both alloys in boiling aqueous MgCl₂ (154°C) with no evidence for α′ formation. The results are discussed in terms of the mechanisms of HE and SCC. Formerly Research Associate, Department of Metallurgy and Mining Engineering, University of Illinois. Formerly Corrosion-Control Analyst with the Physical Plant at the University of Illinois.     

Embrittlement of titanium alloys by long time,high temperature exposure

α stabilized titanium alloys are known to exhibit embrittlement after long-time exposures above ∼800°F. The time-temperature dependency of this embrittlement phenomenon in the Ti-6Al-2Sn-4Zr-2Mo and Ti-5Al-6Sn-2Zr-lMo-0.25Si alloys was observed using a substandard fracture mechanics test. Room temperature slow-bend tests of fatigue precracked Charpy specimens were used to monitor toughness degradation after unstressed thermal exposures in the temperature range of 800° to 1100°F for times to 5000 hr. The activation energy for the embrittlement process was found to be ∼25 to 28 kcal per g mole, which approximates that for diffusion of aluminum or tin in α-Ti. The embrittlement is attributed to the Ti₃X (X = Al, Sn) phase with the rate controlling step that of diffusion controlled growth of the Ti₃X phase domains. The embrittlement process is reversible by heat treatment at temperatures above the α + Ti₃X two phase region.     

Fatigue deformation of TiAl base alloys

The fatigue behavior of Ti-36.3 wt pct Al and Ti-36.2 wt pct Al-4.65 wt pct Nb alloys was studied in the temperature range room temperature to 900°C. The microstructures of the alloys tested consisted predominantly of γ phase (TiAl) with a small volume fraction of γ phase (Ti₃Al) distributed in lamellar form. The alloys were tested to failure in alternate tension-compression fatigue at several constant load amplitudes with zero mean stress. Fracture modes and substructural changes resulting from fatigue deformation were studied by scanning electron microscopy and transmission electron miscroscopy respectively. The ratio of fatigue strength (at 10⁶ cycles) to ultimate tensile strength was found to be in the range 0.5 to 0.8 over the range of temperatures tested. The predominant mode of fracture changed from cleavage type at room temperature to intergranular type at temperatures above 600°C. The fatigue microstructure at low temperatures consisted of a high density of a/3 [111] faults and dislocation debris of predominantly a/2 [110] and a/2 [110] Burger's vectors with no preferential alignment of dislocations. At high temperatures, a dislocation braid structure consisting of all 〈110〉 slip vectors was observed. The changes in fracture behavior with temperature correlated well with changes in dislocation substructure developed during fatigue deformation. S. M. L. SASTRY was formerly NRC Research Associate in the Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH     

The effect of ternary additions on the decompositon of metastable beta-phase titanium alloys

The effect of ternary additions of Al, O, Sn, and Zr on the decomposition of metastable Ti-Mo and Ti-V Β-phase alloys has been studied. It is shown that all these additions reduce the volume fraction, upper temperature limit of formation, and time of stability of the Ω phase. These results have been contrasted to earlier work in binary alloys in which misfit was shown to be a primary factor in determining the morphology and range of stability of the Ω phase. In the ternary alloys the particle morphology can still be explained in terms of misfit, but other considerations appear to be predominant in determining the range of stability. Results are also presented on a phase separation reaction which occurs at higher alloy contents and which has an important influence on the morphology of α-phase precipitation. Formerly with North American Rockwell Science Center.     

Site competition between sulfur and carbon at grain boundaries and their effects on the grain boundary cohesion in iron

Grain boundary segregation in iron-sulfur-carbon alloys containing up to 100 wt ppm sulfur and up to 90 wt ppm carbon has been investigated with Auger electron spectroscopy (AES). The results show the site compctition on grain boundaries between the segregation of sulfur and carbon. The segregation energy of sulfur is estimated to be 75 kJ/mol. Impact tests of these alloys were carried out. Iron-sulfur alloys with less than 20 wt ppm carbon fractured by the intergranular mode with high ductile-brittle transition temperatures (DBTT’s). Addition of up to 90 wt ppm carbon to the binary alloys prevented the intergranular fracture caused by the grain boundary segregation of sulfur, and decreased the DBTT. Carbon, when segregated to grain boundaries, drives sulfur away from the boundaries and also increases the grain boundary cohesion. The DBTT values of the iron-sulfur-carbon alloys are analyzed in terms of the degree of grain boundary segregation of sulfur and carbon. It is shown that sulfur decreases the grain boundary cohesion of iron more severely than phosphorus if compared at the same degree of grain boundary segregation. Formerly Graduate Student     

Effects of microstructure on the fracture toughness of Ti3Al-based titanium aluminides

The influence of microstructure on the fracture toughness of Ti-23A1-9Nb-2Mo-1Zr-1.2Si (at. pct) and Ti-23A1-11Nb-0.9Si (at. pct) Ti₃Al-based alloys has been investigated. Basket-weave microstructures comprising different volume fractions of α ₂ and retained β phases were produced by systematic heat treatments. Besides the volume fraction of the retained β phase, the average size of the β laths has also been used to characterize these microstructures. The toughness of both alloys was examined at room temperature, and the brittle transgranular fracture modes were found to be controlled by microstructure. However, the toughness is not determined solely by the volume fraction of the retained β phase, and a linear relationship has been obtained between the fracture toughness and the average size of the retained β laths. It appears therefore that the toughness of Ti₃Al-based alloys at room temperature is controlled primarily by the width of retained β laths rather than by the retained β volume fraction.