The effect of microstructure on the deformation modes and mechanical properties of Ti-6Al-2Nb-1Ta-0.8Mo: Part I. Widmanstätten structures

An alpha + beta Ti-6Al-2Nb-lTa-0.8Mo alloy with an initial Widmanstätten structure was thermally treated to produce a wide range of microstructures. The effects of individual microstructural parameters on deformation behavior and mechanical properties were investigated. The results show that the Widmanstätten colony boundaries are major barriers to slip. However, the slip distance can be decreased to a distance equal to the thickness of acicular alpha by transforming the beta phase in the Widmanstätten structure to martensite by quenching from 950°C. The decrease in slip distance is accompanied by a 25 pct increase in yield strength with no loss in ductility. A large decrease in ductility occurs after excursions above the beta-transus. The development of both equiaxed beta grains during heating in the beta phase field and continuous grain boundary alpha during cooling in the alpha + beta phase field leads to strain localization along prior beta grain boundaries.     

The effect of microstructure on the deformation modes and mechanical properties of Ti-6Al-2Nb-1Ta-0.8Mo: Part II. Equiaxed structures

The Ti-6Al-2Nb-lTa-0.8Mo alloy was processed to develop both near-basal and transverse textures. Samples were annealed at different temperatures to vary the equiaxed alpha grain size and the thick-ness of the grain boundary beta, and subsequently quenched in order to transform the beta phase to either martensite, tempered martensite, or Widmanstätten alpha + beta. The effect of microstructure and texture on tensile properties and on fracture toughness was investigated. In addition, yield locus diagrams were constructed in order to study the texture strengthening effect. The yield strength was found to be strongly dependent on the thickness and Burgers relationship of the transformed beta phase surrounding the alpha grains. A texture hardening effect as large as 60 pct was found for the basal-texture material but only 15 pct for the transverse texture material. These variations are asso-ciated with differences in deformation behavior.     

The mechanism of void formation,void growth,and tensile fracture in an alloy consisting of two ductile phases

An investigation has shown that it is possible to relate void formation, void growth, and tensile ductility to microstructural features in an α-β titanium alloy, Ti-5.25A1-5.5V-0.9Fe-0.5Cu, heat treated to a constant yield strength. Equations relating tensile void growth rates to microstructure for both equiaxed,E, and Widmanstätten plus grain boundaryα, W + ITG. B.,in aged β morphologies have been derived. A mechanism for void formation at α-β interfaces is presented which accounts for the observed fact that voids do not form at Widmanstätten α platelets. Tensile fracture is shown to be intergranular in nature and occurs when a critical crack length-stress relationship is satisfied. The amount of ductility achievable in a specimen depends upon the rate of void growth. If the rate is large, the void reaches a critical size for fracture at a lower applied stress and strain and hence the ductility is less.     

The influence of heat treatment on the structure and properties of a near-α titanium alloy

The microstructure and tensile properties of a near-α titanium alloy, IMI-829 (Ti-6.1 wt pct Al-3.2 wt pct Zr-3.3 wt pct Sn-0.5 wt pct Mo-1 wt pct Nb-0.32 wt pct Si) have been studied after solutionizing (and no subsequent aging) at two different temperatures separately, one above the β transus (1050 °C) and another below the β transus (975 °C) followed by various cooling rates (furnace, air, oil, or water). While 1050 °C treatment resulted in coarse Widmanstätten structures on furnace or air cooling, fine Widmanstätten structure on oil quenching and martensitic structure on water quenching, 975 °C treatment produced duplex microstructures consisting of equiaxed alpha and partially transformed beta phases. Transmission electron microscopy studies revealed the morphology, size, and distribution of the α, β, and martensite phases and also the presence of small ellipsoidal suicide particles and an interface phase with fcc structure at almost all α-β interfaces. The oil quenched structure from 1050 °C has been found to be a mixture of fine Widmanstätten α coexisting with martensite laths and retained beta at the lath boundaries. Silicides with hcp structure of about 0.4 μm size were observed in specimens solution treated at 975 °C. The interface phase is seen in all slowly-cooled specimens. The YS and UTS are superior for 975 °C treatment compared to 1050 °C treatment after water quenching or oil quenching. The tensile ductility values are superior for any cooling rate after 975 °C solution treatment as compared to 1050 °C solution treatment. The specimens failed in tension diagonally by shear after 1050 °C treatment and by cup and cone fracture after 975 °C treatment. In all cases fracture has taken place by microvoid coalescence and in most cases, along the α-β boundaries.     

The structure and properties of thermomechanically treated β-III titanium

Beta-III titanium (Ti-11.5Mo-5.5Zr-4.5Sn) was solutionized above the β-transus, water-quenched and deformed by rolling at room temperature. The deformation accelerated the aging kinetics at all temperatures up to the β transus. The thermomechanically treated (TMT) alloy always had higher strength than the conventionally heat treated (CHT) alloy; the effect being most marked when the aging product was normally α in a β matrix. In addition, the ductility and notched impact resistance of TMT β-III was greater than that of the CHT alloy in the over-aged condition. The TMT did not alter the morphology of the ellipsoidal α phase formed at low aging temperatures and short aging times, respectively. Here the strengthening increase is attributed to strain hardening of the initial β + ω microstructure. The deformation did substantially change the morphology of the Widmanstätten α phase that formed at higher aging temperatures. In particular, the Widmanstätten α plates were much finer and the β grain boundaries were no longer a preferred precipitation site following TMT. At the highest aging temperatures, the TMT material developed a cell structure of about 1 μ in diameter.     

Plastic flow and microstructure evolution during thermomechanical processing of laser-deposited Ti-6Al-4V preforms

Plastic flow behavior and microstructure evolution during hot working and heat treatment of Ti-6Al-4V synthesized via a laser-deposition, Laser Engineered Net Shaping (LENS?), process were established. To this end, isothermal, hot compression tests were conducted on samples in either a deposited + stress relieved condition or a deposited + hot isostatically pressed (hipped) condition. The starting microstructures consisted of columnar grains with fine or coarse Widmanstätten (basketweave) alpha platelets. At subtransus temperatures, the flow curves of both microstructural conditions exhibited a peak stress at low strains followed by extensive flow softening; these curves were almost identical to previous measurements on ingot-metallurgy (IM) Ti-6Al-4V with similar transformed microstructures. In addition, the kinetics of globularization of the alpha phase during subtransus deformation or subsequent static heat treatment were found to be the same as for IM Ti-6Al-4V with comparable alpha-platelet thicknesses. During supertransus heat treatment, moderately fine beta-grain microstructures were developed in samples that had been predeformed below the beta transus. Such a heat treatment for samples previously deformed above the transus gave rise to a nonuniform distribution of coarse beta grains, an effect attributed to critical grain growth.     

Low cycle fatigue of Ti-Mn alloys: Microstructural aspects of fatigue crack growth

Fatigue crack path behavior of a series of Ti-Mn alloys heat treated to produce volume fractions of the alpha phase ranging from 0 to 97.5 pct was investigated. Both Widmanstätten and equiaxed morphologies of the α phase were used in this study. Interior and surface crack paths are discussed in terms of slip behavior and microstructural details.     

Microstructural evolution in laser-deposited multilayer Ti-6Al-4V builds: Part I. Microstructural characterization

The macro and microstructure of laser-deposited Ti-6Al-4V has been investigated to determine the evolution of unique microstructural features in mutilayer builds. The macro and microstructures exhibited in the build include large, columnar prior-beta grains, a gradient in the individual alpha-lath thickness between the deposited layers, and the presence of layer bands within each layer, except for the last three layers deposited. The layer band consists of a colony Widmanstätten alpha morphology, while the nominal microstructure between layer bands exhibits a basketweave morphology. Optical microscopy, hardness, and composition measurements were used to determine that the layer-band and gradient morphologies are resultant from the complex thermal history the build experiences and not a result of segregation or oxidation. The gradient alpha and layer-band morphologies form in layer n after the deposition of layer n+3.     

The influence of microstructure and strain rate on the compressive deformation behavior of Ti-6Al-4V

This article reports on a study of deformation of Ti-6Al-4V in compression. In particular, two different microstructures, the equiaxed microstructure and the Widmanstätten microstructure, were generated from the same parent material and their properties were measured. The results show that at small strains, the mechanical response of samples with these microstructures is similar. The yield strength and the flow stress at a 0.05 true strain have similar values; these increase with increasing strain rate over the range of 0.1 to 1000 s^?1. However, samples with the Widmanstätten microstructure failed at a smaller strain than their counterparts with the equiaxed microstructure, and this difference increased with increasing strain rate. Examination of cross sections of samples deformed to different levels of strain showed that the deformation was inhomogeneous. As the sample barreled, the deformation built up on the surfaces of two cones of material whose apices met in the center of the sample. Cracks formed in the corners of the samples and propagated in toward the center. In samples with the equiaxed microstructure, short cracks and voids formed, but they were usually blunted at the grain boundaries. Long cracks were only observed immediately before failure. In samples with the Widmanstätten microstructure, cracks could grow within the laths more easily, and, as a result, longer cracks formed at lower strains. We propose that this difference leads to the differences in the failure strains for these two microstructures. Finally, examination of data in the literature, along with our own results, indicates that the interstitial content plays an important role in determining the yield stress of the material.     

A study of grain boundary nucleated widmanstätten precipitates in a two-phase stainless steel

The factors which control the nucleation and growth of Widmanstätten precipitates at grain boundaries in a two-phase stainlesss steel, have been studied by transmission electron microscopy. Widmanstätten precipitates usually involves a separate nucleation and growth event at prior austenite films or allotriomorphs, which are the first phase to form at grain boundaries. The Widmanstätten precipitates have a preferred orientation relationship with both the prior grain boundary γ-phase and with the α matrix. The particular γ variant which forms has a near Kurdjumov-Sachs relationship with the α-grain and the invariant line assciated with the variant is nearly normal to the grain boundary plane. The orientation relationship changes during growth, towards an exact K-S relationship, with the growth direction (which is parallel to the invariant line) also rotating to be nearly parallel to the common close-packed direction associated with the transformation.     

Fracture characteristics of Ti-6Al-4V and Ti-5Al-2.5Fe with refined microstructure using hydrogen

The hydrogenation behavior of Ti-6Al-4V, with the starting microstructures of coarse equiaxed α and coarse Widmanstätten α, respectively, was investigated under a hydrogen pressure of 0.1 MPa at temperatures between 843 and 1123 K. The hydrogen content was determined as a function of hydrogenation time, hydrogenation temperature, and hydrogen flow rate. The phases presented in the alloy of after hydrogenation were determined with X-ray and electron diffraction analysis in order to define the effect of Thermochemical Processing (TCP) on the microstructure of the alloy. Mechanical properties and fracture toughness of Ti-6Al-4V and Ti-5Al-2.5Fe subjected to the various TCP were then investigated. Hydrogenation of Ti-6Al-4V with the starting microstructure of coarse equiaxed α at 1023 K, just below hydrogen saturated β (denoted β″ (H)) transus temperature, produces a microstructure of a, orthohombic martensite (denoted α″ (H)) and β (H). Hydrogenation at 1123 K, above β (H) transus, results in a microstructure of α″ (H) and β (H). Microstructure refinement during TCP results mainly from decomposition of α″ (H) and ;β (H) into a fine mixture of α + β during dehydrogenation. An alternative TCP method is below β (H) transus hydrogenation (BTH), consisting of hydrogenation of the alloy below the hydrogenated β (H) transus temperature, air cooling to room temperature, and dehydrogenation at a lower temperature, which is found to improve mechanical properties significantly over a conventional TCP treatment. Compared with the untreated material, the BTH treatment increases the yield strength and increases the ultimate tensile strength significantly without decreasing the tensile elongation in the starting microstructure of coarse equiaxed α or with a little decrease in the tensile elongation in the starting microstructure of coarse Widmanstätten α, although the conventional TCP treatment results in a large decrease in elongation over the unprocessed material in Ti-6Al-4V. In Ti-5Al-2.5 Fe, both conventional TCP and BTH result in a increase in yield strength, ultimate tensile strength, and elongation; however, the BTH gives the best balance between strength and elongation. The TCP-treated Ti-6Al-4V shows smaller fracture toughness compared with the unprocessed material, while TCP-treated Ti-5Al-2.5Fe shows greater fracture toughness compared with the unprocessed material. The BTH treatment results in a improvement in fatigue strength in both Ti-6Al-4V and Ti-5Al-2.5Fe.     

The effect of heat input on the microstructure and properties of nickel aluminum bronze laser clad with a consumable of composition Cu-9.0Al-4.6Ni-3.9Fe-1.2Mn

The effect of heat input in the laser cladding of nickel aluminum bronze was investigated. Nickel aluminum bronze castings were clad with a consumable of the composition Cu-9.0Al-4.6Ni-3.9Fe-1.2Mn and exposed to a variety of heat inputs from 42.5 to 595 J/mm. At the lowest heat input, the deposit microstructure was almost entirely martensitic. Increases in heat input caused the amount of α to increase. Depending upon heat input, the α was present as grain boundary allotriomorphs, secondary Widmanstätten α sideplates, and intragranular Widmanstätten α precipitates. The reheated zones were of lower hardness and, at all heat inputs, consisted of a mixture of grain boundary allotriomorphs and Widmanstätten α and martensite. Laser cladding improved the corrosion- and cavitation-erosion resistance of the surfaces but reduced their ductility. The properties of the clad surfaces depended on heat input.     

Effects of microstructural morphology on quasi-static and dynamic deformation behavior of Ti-6Al-4V alloy

The effects of microstructural morphology on quasi-static and dynamic deformation behavior of a Ti-6Al-4V alloy were investigated in this study. Quasi-static and dynamic torsional tests were conducted using a torsional Kolsky bar for Widmanstätten, equiaxed, and bimodal microstructures, which were processed by different heat treatments, and then, the test data were analyzed in relation to microstructures, tensile properties, and fracture mode. Quasi-static torsional properties showed a tendency similar to tensile properties and ductile fracture occurred in all three microstructures. Under dynamic torsional loading, maximum shear stress of the three microstructures was higher and fracture shear strain was lower than those under quasi-static loading, but the overall tendency was similar. In the Widmanstätten and equiaxed microstructures, adiabatic shear bands were found in the deformed region of the fractured specimens. The possibility of the adiabatic shear band formation under dynamic loading was quantitatively analyzed, depending on how plastic deformation energy was distributed to either void initiation or adiabatic shear banding. It was found to be most likely in the equiaxed microstructure, whereas it was least likely in the bimodal microstructure.     

Morphology of bainite and widmanstätten ferrite

Morphology of bainite and Widmanstätten ferrite in various steels has been investigated by means of microstructural and surface relief observations. It was shown that upper and lower bainite should be classified by ferrite morphology,i.e., lathlike or platelike, and that the morphology of cementite precipitation cannot be the index for the classification. Widmanstätten ferrite formed in the upper C-nose where ferrite grain-boundary allotriomorphs nucleate exhibits quite similar appearance with bainitic ferrite that forms in the lower C-nose of bainitic reaction. The only difference between them exists in the fact that Widmanstätten ferrite laths grow in the temperature range where primary ferrite forms and often terminate at a grain boundary ferrite but that bainitic ferrite has its own C-curve at temperatures belowB _s and nucleates directly at an austenite grain boundary. The mechanisms for their formations are discussed.     

Microstructural change during superplastic deformation of δ-ferrite/austenite duplex stainless steel

Superplasticity of a 25 pct Cr-6.5 pct Ni-3 pct Mo-0.14 pct N δ/γ duplex stainless steel has been studied with particular emphasis on the microstructural change during deformation. Two large superplastic elongations are obtained at temperatures around 1323 K in δ/γ duplex phase region and 1173 K where σ phase particles precipitate dynamically at a strain rate of ~10^?3 s^?1. During deformation in the higher temperature region, fine Widmanstätten γ particles coarsen and coarse γ grains formed during the prior treatments are broken into spherical particles, resulting in a homogeneous dispersion of γ particles within the σ-ferrite matrix. The dynamic recrystallization of soft σ-ferrite matrix occurs locally in the region where the strain reaches some critical value, and the final microstructure consists of equiaxed σ and γ grains. In the case of lower temperature deformation, a eutectoid decomposition of δ-ferrite into γ and σ phases occurs. The relatively soft γ grains which are severely deformed by hard σ particles recrystallize dynamically, and these processes lead to the γ/σ equiaxed duplex structure. The extremely large superplasticity of this alloy can mainly be explained in terms of the above microstructural change during deformation.     

Widmanstätten ferrite plate formation in low-carbon steels

The mechanism by which Widmanstätten ferrite plates nucleate and grow in low-carbon steels has been studied. In-situ laser scanning confocal microscopy (LSCM) observations, optical microscopy, and electron backscattered diffraction (EBSD) techniques have been used to characterize the relationship between grain boundary allotriomorphs and Widmanstätten ferrite plates. The issue of where Widmanstätten ferrite plates nucleate is one of some debate, with theories including morphological instability and sympathetic nucleation. Evidence has been found that supports the theory of a sympathetic nucleation mechanism being responsible for the formation of Widmanstätten ferrite plates. The EBSD measurements have shown that low-angle misorientations of between 5 and 10 deg exist between ferrite allotriomorphs and Widmanstätten ferrite plates.     

Microstructural evolution in laser-deposited multilayer Ti-6Al-4V builds: Part II. Thermal modeling

The thermal history developed in laser metal deposition (LMD) processes has been shown to be quite complex and results in the evolution of an equally complex microstructure. A companion article (Part I. Microstructural Characterization) discussed the LMD of Ti-6Al-4V, where the resultant microstructure consists of a periodic, scale-graded layer of basketweave Widmanstätten alpha and a banding that consists of colony Widmanstätten alpha. In order to understand the microstructural evolution in Ti-6Al-4V, a numerical thermal model based on the implicit finite-difference technique was developed to model LMD processes. The effect of different laser-scan velocities on the characteristics of the thermal history was investigated using an eight-layer single-line build. As the laser-scan speed decreases and the position within a layer increases, the peak temperature increases. The heating rate and the peak thermal gradient within a deposited layer were shown to follow the same trend as the peak temperature after two layers were deposited on top of the substrate. In general, the laser-scan speed or z-position within a layer did not have a significant effect on the cooling rate. The cooling rate in a newly deposited layer decreases as the number of layer additions increases. Given the predicted temperature vs time profile from the thermal model, the evolution of phase transformations occurring in the deposit is mapped as each layer is deposited. As a result of the thermal cycling imposed by the periodic deposition of material, a characteristic layer, consisting of two regions heated above and below the beta transus, forms in layer n due to the deposition of layer n+1.     

Low Cycle Fatigue Behavior of Ti-6AI-2Sn-4Zr-6Mo: Part I. The Role of Microstructure in Low Cycle Crack Nucleation and Early Crack Growth

Widmanstätten + grain boundary (W + GB) α and equiaxed (E) structures of different α particle sizes were produced in smooth bar specimens of a Ti-6Al-2Sn-4Zr-6Mo alloy, heat treated to produce a 0.2 pct yield stress of about 1100 MPa. Specimens were cycled at room temperature under total strain control. For both W + GB and Eα structures at low strains crack initiation occurred at α-β interfaces and in the aged β matrix. Profuse extrusions were found in the W + GBα structure. At higher strains for both structures, crack initiation took place preferentially within slip bands in α. Surface crack nucleation and crack link-up occurred more readily in W + GBα structures, which had a shorter life²⁸ than Eα structures, a phenonmenon thought to be due to the much longer surface lengths of the cracks in W + GBα structures. Larger prior β grain size permitted surface cracks to be developed more readily, but the role of β grain size is an indirect one. At large plastic strains colonies of Wα at large angles to the direction of crack propagation can cause the crack to change directions. An explanation for this behavior is offered.     

Low cycle fatigue behavior of Ti-Mn alloys: Fatigue life

The effect of morphology, particle size, β grain size and volume fraction of β, from 0.025 to 1.0, on the low cycle fatigue life of α-β Ti-Mn alloys, have been studied under total strain control. In general, Widmanstätten plus grain boundary (W+GB) α structures show shorter fatigue lives than equiaxed (E) α structures, and this has been ascribed to the formation of much larger surface cracks and ease of transfer of slip from α to β. For Eα structures, fatigue life increases with decreasing α particle size and when the alloy is single phase β fatigue life increases with decreasing grain size. At high total strains the nearly all α alloy had the longest fatigue life and at lower strains the β alloy, with the higher yield strength, had the longest fatigue life. Fatigue life was correlated with strain hardening. The nearly all α alloy which had the highest strain hardening, over the plastic strains encountered, had the highest fatigue life, while the β alloy, with the lowest strain hardening, had the lowest fatigue life. For a portion of the fatigue life curves, it was found that as the average Baushinger strain (ABS) increased, the Coffin-Manson exponentc decreased. The results are discussed.     

Age-hardening behavior and phase identification in solution-treated AEREX 350 superalloy

This article presents results of an investigation on age-hardening behavior of superalloy AEREX 350. Microhardness testing was employed to evaluate the age-hardening response of the alloy while optical, scanning, and transmission electron microscopy techniques were used to characterize the major phases formed during the aging process. No significant hardening was found in solution-treated samples aged at temperatures up to about 680 °C. Aging at 700 °C up to 950 °C, however, caused a characteristic hardening response. This hardening was concurrent with the formation of γ', an ordered phase with L1₂ structure, as fine precipitate distributed throughout the fcc matrix. In the temperature range of 800 °C to 1055 °C, a new phase called η with D024 structure was formed. Two morphologies of η phase were found: the discrete blocky precipitates mainly at grain boundaries and elongated plates with a Widmanstätten appearance within the grains. The latter morphology was predominant at higher aging temperatures. This was attributed to accelerated diffusion of solute to the incoherent tips of Widmanstätten plates at high temperatures. No evidence of any precipitates was found in the microstructure of samples aged at 1060 °C, implying that this temperature was above the solvus temperatures of all precipitates in the AEREX 350 alloy. Based on the results presented in this investigation, it is suggested that considerable improvement in the properties of the alloy may be achieved through modification of the commercial heat-treatment practice.