Prior-β Grain Research Articles

Microstructure of additive layer manufactured Ti–6Al–4V after exceptional post heat treatments

The microstructure of Ti–6Al–4V components, which were produced by an additive layer manufacturing (ALM) process, typically consists of columnar prior β-grains containing colony and/or basket-weave α+β with martensite. The microstructure is equivalent to microstructures neither of wrought nor of cast parts. However, the post heat treatments applied for the present investigation are derived from heat treatment specifications of those materials. This paper presents the microstructure and hardness of additive manufactured Ti–6Al–4V after heat treatments, which are not standardized or commonly applied, in order to suggest new ways forward. The experiments show that columnar prior β-grains can be transformed to globular prior β-grains without cold or hot working. The hardness and microstructure in a prior β-grain is a function of the cooling rate through β transus temperature rather than of the duration at above β transus. A heating cycle was determined which leads to a microstructure that is homogeneous in each prior β-grain and does not significantly differ from grain to grain.

Microstructural studies and crystallographic orientation of different zones and δ-hydrides in resistance welded Zircaloy-4 sheets

The cold worked stress relieved (CWSR) Zircaloy-4 sheet used as endplate in nuclear fuel bundle is resistance welded with an endcap in argon environment. Later the welded sample is hydrided in a gaseous atmosphere at 400 °C. Optical microscopy (OM), electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) were used to examine the morphology and crystal orientation of the hydrides. The microstructural changes in different areas of the weld zone, heat affected zone (HAZ) and the as-received zone were analyzed using EBSD technique. Optical examination showed complete random morphological orientation of hydrides and predominantly basket-weave structure in the weld zone, with very few colonies of parallel plate structures. Variant selection for α-phase formation inside prior β-grains was identified at the weld centre. As we move from the weld centre to the as-received zone, the variant selection is found to be less probable. The δ-hydride platelets at the weld zone were always found to be growing perpendicular to the α-colonies having angular difference of 60–63° and follow (0 0 0 1) α-Zr//{1 1 1}δ-ZrH 1.5 orientation relationship with the zirconium matrix. Proposed description of complex distribution of hydrides and alloy microstructure at the weld and heat affected zone will contribute to a better understanding of mechanisms of failure of fuel cladding in various types of nuclear reactors.

The Effect of Yttrium on Ti-5111 Gas Tungsten Arc Welds

Much interest has developed in the near-α titanium alloy Ti-5Al-1Sn-1V-1Zr-0.8Mo (Ti-5111) for naval applications. When gas tungsten arc welded with filler metal that has the same chemical composition as the base metal, however, the weld FZ tends to be harder and less ductile than the base metal, which may make the weld susceptible to failure. This behavior may be attributed to the presence of oxygen impurities and the large prior-β grain size in the weld. In this investigation, the addition of a small amount of yttrium to the weld filler metal can decrease hardening and increase the ductility of Ti-5111 welds, which is beneficial for weld performance. Microstructural and chemical analyses of unmodified and yttrium-modified Ti-5111 welds are presented along with results from mechanical testing of the welds.

Effect of cooling rate on transformation texture and variant selection during β → α transformation in Ti–5Ta–1.8Nb alloy

The evolution of texture in a cold rolled Ti–5Ta–1.8Nb alloy sheet, during the α → β → α transformation has been studied using EBSD and XRD techniques, for different cooling rates. The sheet exhibited a basal plane type texture upon cold rolling, and a sharp ‘{1 1 −2 0}||rolling plane’ transformation texture was inherited after the heat treatment. The microtexture analysis of the lamellar α/β structure, suggested that this transformation texture arises from the {1 1 1}〈1 1 0〉 type of high temperature β texture, obeying the Burgers orientation relationship. The strength of the transformation texture was found to sharply increase with decrease in cooling rates, denoting variant selection. Two types of parent β orientations with [1 1 0] parallel to either rolling directions are possible, and among their α product variants, 3 of them are common, and can be preferentially formed at the prior-β grain boundaries. The role of grain boundary-α in influencing variant selection and the transformation texture for different cooling rates is described in this paper.

Segregation and grain refinement in cast titanium alloys

The growth restriction factor is a parameter derived from binary phase diagrams and is a useful predictor for the grain refining response when a solute is added to a base alloy. This work investigates the relevance of growth restriction theory to titanium alloys where solidification rates are an order of magnitude faster than previous studies in aluminum- and magnesium-based systems. In particular, the segregation of Fe and Cr in titanium is investigated and the effects on grain size studied. It was found that the Scheil equation reasonably modeled solidification of titanium where cooling rates approach 120 °C/s, and the growth restriction factors for Fe and Cr were useful in predicting prior-β grain refinement. However, it was found that caution must be used when calculating growth restriction factors from binary phase diagrams.

Microstructure and Mechanical Properties of Laser Solid Formed Ti-6Al-4V from Blended Elemental Powders

Abstract The microstructure and mechanical properties of laser solid formed Ti-6Al-4V from the blend of Ti, Al, and V powders were investigated. It is found that the macrostructure changes from equiaxed to columnar grains with the increase of the laser power, which is very different from that of the samples using pre-alloyed powders as cladding materials. The disturbance effect of the mixing enthalpy during laser deposition is considered responsible for the formation of equiaxed grains at low laser powers. The microstructure within prior-β grains mainly consists of Widmanstatten α laths separated by β-Ti, which is basically identical to those from pre-alloyed powders. Oxygen content of as-deposited samples was measured and the tensile testing was carried out. The results show that the oxygen content of as-deposited Ti-6Al-4V from blended elemental powders is only about 0.1wt.% and the mechanical properties are comparable to or even better than that of conventionally wrought material.

Effects of boron on microstructure in cast titanium alloys

Trace additions of boron to commercial purity titanium result in significant microstructural changes. Changes include the promotion of dendritic growth and refinement in both the prior-β and α-grain size. Boron partitions strongly on solidification and TiB intermetallics are observed throughout the microstructure, confined to interdendritic regions. This strong partitioning of solute causes prior-β grain refinement, and the presence of TiB intermetallics and/or the increased β-grain boundary surface area associated with the change in β-grain morphology assists nucleation of the α-phase.

Investigations of the Microstructure and Mechanical Properties of Prior-β Structure as a Function of the Oxygen Content in Two Zirconium Alloys

Abstract It is now well acknowledged that, after a prototypical loss of coolant accident (LOCA) transient, the resultant mechanical properties of fuel cladding tubes depend strongly on the oxygen content of the residual prior-β layer, as this phase is the only metallic part of the high-temperature oxidized cladding that may show some residual ductility. The aim of this study is to obtain relevant information on the evolution of the mechanical properties, on the one hand, of the prior-β structure as a function of the oxygen content, assuming that there is a critical oxygen content that leads to a ductile-to-brittle failure mode transition at low testing temperatures (20–135°C); and on the other hand, of the α(O) structure as a function of the oxygen content. Sheets of Zircaloy-4, 1 to 3 mm thick, and M5®M5® is a registered trademark of AREVA-NP. advanced alloys from AREVA NP have been studied. To obtain different oxygen contents, they were oxidized at high temperature and then annealed under vacuum in order to reduce the oxide layer. Systematic post-treatment measurements of the oxygen concentration and of its homogeneity within the sheet thickness were performed. The different prior-β and α(O) structures thus obtained have homogeneous oxygen content between ∼0.14 wt. % and 0.9 wt. % and ∼2 wt. % and 7 wt. %, respectively. Such oxygen concentration ranges cover the solubility values that are expected in the β phase and in the α(O) phase at high temperatures typical of LOCA transients. Detailed microstructure investigations were subsequently performed on the prior-β structures since it is considered to be the most important layer when regarding the post-quench mechanical behavior of the material. Continuous cooling temperature (CCT) phase diagrams as a function of the oxygen content were established to correctly interpret the results. Electron backscattered diffraction (EBSD) analysis has then allowed the crystallographic orientations and the morphology of prior-β phase sub-grains to be determined. For each considered prior-β grain, it was possible to interpret the data by taking into account the “Bürgers” crystallographic relationship between the parent β phase and the resultant α phase. Complementary electron probe microanalysis (EPMA) was also used. These last experiments have shown a spatial fluctuation of the oxygen content within the microstructure that depends both on the nominal oxygen content and on the cooling rate. Nanohardness measurements were also performed and correlated with this oxygen spatial partition. These measurements proved to be useful for the understanding of the tensile macroscopic mechanical behavior. Finally, on the one hand, tensile tests were performed on prior-β phase at testing temperatures ranging from −100°C up to 260°C. The ductile-to-brittle temperature transition and the mechanical constitutive laws as a function of the oxygen content were then described. These tests show the existence of a ductile-to-brittle failure mode transition at 20°C for a critical oxygen concentration of ∼0.5 wt. %. A detailed fractographic analysis was performed to assess the failure mechanism. On the other hand, four-point bending tests were conducted on α(O) phase at 25°C and 135°C in order to obtain behavior laws. Preliminary finite element calculations were performed to simulate ring compression tests carried out on multi-layered high-temperature oxidized cladding tubes.

The mechanism of grain refinement of titanium by silicon

A recently developed model of grain refinement was used to clarify the role of silicon as a solute element in the titanium system. A refinement in the as-cast prior-β grain size is observed when silicon is added, and this is attributed to silicon’s high growth-restricting effect. However, it was also determined that the identification and presence of potent nuclei for β-Ti is required to accurately predict the degree of refinement with a given silicon addition.

Microstructure, Tensile, and Creep Behavior of Boron-Modified Ti-15Al-33Nb (at.%)

The effect of boron on the microstructure, tensile, and creep behavior of a Ti-15Al-33Nb (at. pct) alloy was investigated. In addition to the normal constituent phases present in the unmodified alloy, the boron-modified alloys contained borides enriched in titanium and niobium. These borides made up to 9 pct of the volume and were present in the form of needles/laths. Boron additions of 5 at. pct resulted in significant strengthening and stiffening and reduced elongation-to-failure. Smaller boron additions of 0.5 at. pct did not as significantly impact the RT tensile properties, but reduced the 650 °C yield strength by 45 pct. Constant load, tensile-creep experiments were performed in the stress range of 150 to 400 MPa and the temperature range of 650 °C to 710 °C, in both air and vacuum environments. The addition of 5 at. pct boron significantly improved the creep resistance, whereas the addition of 0.5 at. pct boron degraded the creep resistance. In-situ tensile-creep experiments indicated that localized grain boundary cracking was prevalent at the prior-β grain boundaries.

Grain-refinement mechanisms in titanium alloys

Despite the importance of the prior-β grain structure in determining the properties of titanium-based alloys, there are few published studies on methods of controlling the size of these grains in commercial alloys. The existing research raises questions about the relative importance of solute elements in grain-refining mechanisms, particularly the common alloying elements of aluminum and vanadium. The effect of these elements was investigated by producing a series of castings in a nonconsumable arc-melting furnace, and the results were interpreted with the aid of available phase-diagram information and solute-based models of grain refinement. A small reduction in grain size was obtained with increasing solute additions; however, this was not expected from the theoretical analysis. Possible reasons for this discrepancy are discussed.

Microstructure and corrosion characteristics of Zr–1.5Nb–0.4Sn–0.2Fe–0.1Cr alloy with a β-annealing

The corrosion characteristics of Zr-based alloys are very sensitive to their microstructures which are determined by the β-annealing conditions. Specimens of Zr–1.5Nb–0.4Sn–0.2Fe–0.1Cr alloy were investigated in order to obtain their optimized β-annealing conditions such as the β-annealing temperature and the cooling rate from the β-region. From the microstructural analysis, the prior β-grain size of this alloy was increased by increasing the β-annealing temperature. The martensitic structure was formed by a fast cooling rate of a water quenching from a range of 960–1200 °C and the Widmanstätten structure was formed by a slow cooling rate of an air cooling from 1050 °C. The corrosion behavior of the Zr–1.5Nb–0.4Sn–0.2Fe–0.1Cr alloy was not affected by the β-annealing temperature but it was changed by the cooling rate. The increased corrosion resistance was caused by a decreased the supersaturated Nb content in the matrix by a formation of the β-Zr phase. However, the formation of the β-Zr phase is not recommended in the β-solution annealing procedure of Nb-containing Zr-based alloy, because the aim of the β-solution annealing is to obtain a good homogeneity.

Modeling the tensile properties in β-processed α/β Ti alloys

The development of a set of computational tools that permit microstructurally based predictions for the tensile properties of commercially important titanium alloys, such as Ti-6Al-4V, is a valuable step toward the accelerated maturation of materials. This paper will discuss the development of neural network models based on a Bayesian framework to predict the yield and ultimate tensile strengths of Ti-6Al-4V at room temperature. The development of such rules-based model requires the population of extensive databases, which in the present case are microstructurally based. The steps involved in database development include producing controlled variations of the microstructure using novel approaches to heat treatments, the use of standardized stereology protocols to characterize and quantify microstructural features rapidly, and mechanical testing of the heat-treated specimens. These databases have been used to train and test neural network models for prediction of tensile properties. In addition, these models have been used to identify the influence of individual microstructural features on the tensile properties, consequently guiding the efforts toward development of more robust mechanistically based models. Based on the neural network model, it is possible to investigate the influence of individual microstructural features on the tensile properties, and in certain cases these dependencies can point toward unrecognized phenomena. For example, the apparently unexpected trend of increase in tensile strength with increasing prior β-grain size has led to the determination of the pronounced role of the basketweave microstructure in strengthening these alloys, especially in case of larger prior β grains.

Anisotropic plasticity and cavity growth during upset forging of Ti–6Al–4V

Orientation imaging microscopy was used to determine the effect of local crystallographic texture on the nucleation and growth of cavities in Ti–6Al–4V with a colony-α microstructure during upset forging to a 35% height reduction at 815 °C and a strain rate of 0.1 s −1. In contrast to uniaxial deformation, the stress–strain history in the bulged equatorial region was complex. Cavities developed preferentially along prior-β grain boundaries nominally perpendicular to principal stress directions, particularly where there were 90° colony misorientations, even in regions where global secondary-tension stresses were small or even negative. High cavity nucleation and growth rates were strongly correlated with the 90°-misoriented colonies, regardless of the orientations of the adjacent colonies. A rationale was developed to account for the particular sensitivity of 90°-misorientations based on colony orientation, slip systems, Taylor and Schmid factors, strain ratio and deformation history. Colonies with orientations in which prism slip was highly favored were stable and exhibited strong plastic anisotropy. When a neighboring colony had a 90°-misorientation, highly non-uniform strains developed in the boundary region, resulting in large triaxial stress concentrations that facilitated cavity nucleation and plastic strain conditions that favored cavity growth.

Weldability study of Ti-Ta-Nb alloy and pure titanium

Weldability of Ti–4.8%Ta–1.8%Nb alloy, for use in electrolytic dissolver of nuclear fuel reprocessing plant, was evaluated in autogenous bead-on-plate gas tungsten arc welds made at two different heat inputs. Pure titanium was also welded for comparison. A secondary shielding setup was employed for protection of the weldment from gaseous contamination during and after welding. In the weld and heat-affected zone (HAZ) of Ti-Ta-Nb weldments, plate-like martensite, characteristic of cooling rates during welding, was observed. Low heat input and slower welding speed resulted in greater proportion of the desirable fine equiaxed prior-β grains. Lower heat input also reduced HAZ width.

An experimental and theoretical investigation of the effect of local colony orientations and misorientation on cavitation during hot working of Ti-6Al-4V

Orientation in aging microscopy was used to determine the effect of local crystallographic texture on the size of cavities formed during hot tension testing at 815 °C and a strain rate of 0.1 s−1 in Ti-6Al-4V with a colony-α microstructure. Cavities nucleated preferentially in the α-β interface along prior-β grain boundaries that were perpendicular to the tension axis, adjacent to colonies with a (hard) c-axis colony orientation parallel to the tension axis. Cavity growth was most rapid at locations where 20 to 40 pct of the area surrounding the cavity also had colonies with soft orientations (with the c-axis inclined to the tension axis). The constraint of the hard orientations and the strongly incompatible anisotropic deformation by prism and basal slip in the softer orientations appeared to facilitate cavity nucleation and growth in these local regions. To interpret these observations, a simple model was developed to quantify the effect of the misorientation between neighboring colonies on the partitioning of strain between them and the development of a local stress triaxiality. Estimates of the local strains and stress states were then incorporated into a plasticity-controlled cavity-growth model to estimate the cavity-growth rate, and thus cavity sizes. Predicted cavity sizes following initiation were very sensitive to the local strain and the hydrostatic stress through its effect on the cavity-growth parameter. The model was successful in differentiating growth rates according to local values of the Taylor factor.

Study of texture in a TiTaNb alloy using electron back scattered diffraction technique

The development of texture during the β → α transformation of a deformed and β-annealed alloy of Ti–5Ta–1.8Nb has been studied using electron back scattered diffraction (EBSD) technique. The microstructures that develop during the above transformation have been characterized and correlated to their mechanism of formation. The X-ray diffraction studies of the wire-drawn alloy suggested a development of characteristic (1 0.0) type of deformation texture. Diffusion controlled slow transformation of the alloy from above the β-transus resulted in a completely different type of texture. The microstructure of this transformation product consisted of colonies of alternate lamella of coarse-α and fine-β together with α along the prior-β grain boundary. EBSD results show that the crystallographic orientation of α lamellae within a colony remains same and changes across the colonies. The observation of a high full width at half maximum (FWHM) due to scatter of the α poles suggests a deviation from an exact Burgers relation. Comparison of experimentally measured misorientation angle between different α-colonies with calculated values based on Burgers relation enabled the identification of orientation of the parent-β grain. Thus a method to characterise the diffusion controlled β → α + β transformation in the Ti–5Ta–1.8Nb alloy has been developed combining microscopy with EBSD studies.

Measurements and Monte Carlo simulation of grain growth in the heat-affected zone of Ti–6Al–4V welds

Grain size and topological class distributions in the heat-affected zone (HAZ) of gas tungsten arc welded Ti–6Al–4V alloy were measured for various heat inputs in the range of 0.55–4.33 MJ m −1. The evolution of grain structure and topological class distributions were also calculated using a three-dimensional Monte Carlo model utilizing thermal cycles computed from a well tested numerical heat transfer and fluid flow model. Both the experimental data and the calculated results showed that the average prior-β grain size near the fusion plane was about four to twelve times larger than the average grain size in the base plate, depending on the heat input. At locations equidistant from the fusion plane, the grains were larger in the mid-section vertical symmetry plane as compared to those at the top surface due to local variations of the thermal cycles. The normalized grain size distributions were unaffected by the local differences in the thermal cycles. It is demonstrated that the presence of a spatial gradient of temperature in the HAZ significantly impeded grain growth due to thermal pinning effect. Furthermore, the steep temperature gradients near the fusion plane did not introduce any significant texture in the grains. Both the experimental data and the calculated results indicated that the grains in the HAZ of the Ti–6Al–4V alloy were significantly smaller than the grains in the commercially pure titanium for identical welding conditions.

The effect of rapid heat treatment on the high-temperature tensile behavior of superplastic Ti-6Al-4V

The effect of rapid heat treatment on the microstructure and elevated-temperature tensile behavior of a Ti-6Al-4V sheet has been studied. Nonequiaxed secondary α phase, introduced by rapid heat treatment, increased the flow stress and changed the stress-strain behavior. A finer scale of the microstructures produced by rapid heat treatment, however, resulted in a stress-strain behavior that differed from that reported in the microstructures obtained from a slower, furnace heat treatment. These fine structures exhibited higher values of m, greater elongation, and lower flow stresses. The stress-strain behavior was found to be dependent on the prior-β grain size; this effect was not observed when the material was furnace heat treated in the β-phase field. The stress-strain curve gradually changed from exhibiting continuous strain hardening to exhibiting a peak stress followed by strain softening, i.e., the superplastic behavior of the as received material gradually diminished as the prior-heat-treatment temperature increased.

Effect of boron microalloying on microstructure, tensile properties and creep behavior of Ti–22Al–20Nb–2W alloy

The effect of 0.2 at.% boron addition on the microstructure, tensile properties, and creep behavior of Ti–22Al–20Nb–2W were investigated. A TiB phase was observed in the B-doped alloy, which shows that the solubility of boron in this alloy is very low. The addition of 0.2 at.% boron was very effective in reducing the prior β-grain size of the Ti–22Al–20Nb–2W alloy, which is thought to be due to a strong boron segregation at the grain boundaries. The decrease in grain size due to the addition of boron is effective in improving the alloy's room temperature ductility. The addition of a small amount of boron was beneficial in lowering the steady-state creep rate at 650°C/310 MPa, but did not improve the creep behavior at 750°C/310 MPa.