Αp Grains Research Articles

Microstructure‐Dependent Local Fatigue Cracking Resistance of Bimodal Ti–6Al–4V Alloys

The fatigue crack growth behavior of the bimodal Ti–6Al–4V alloys with two different volume fractions of the primary α phase (αp) of 76 and 36% is investigated by the in situ testing technique. The experimental results show that the crack growth rate of the αp = 36% Ti–6Al–4V alloy is lower than that of the αp = 76% one. The local fatigue crack growth rate is evidently decreased by the various boundaries including αp grain boundaries, boundaries between the αp phase and basketweave microstructure, and α/β lamellar interfaces. A criterion associated with the boundary characteristics is obtained to evaluate the grain boundary resistance to the fatigue crack growth in the engineering alloys.

Evolution of equiaxed alpha phase during heat treatment in a near alpha titanium alloy

The evolution of equiaxed primary α phase (αp) during post-deformation heat treatment of near-α titanium alloy Ti-5.6Al-3.8Sn-3.2Zr-1.0Ta-0.5Mo-0.4Nb-0.35Si was studied. This alloy was isothermally compressed at 1019 °C and subsequently heat treatment at 1024 °C for times ranging from 0.5 to 24 h. The recrystallization behavior and boundary splitting within equiaxed αp were analyzed by crystallographic orientation and microstructure observations. The results showed that the formation of internal (sub)boundaries within the αp was by the strong recovery. In most cases, it is difficult to obtain a refinement of big equiaxed αp by boundary splitting and αp grains were still contiguous with “peanut” shape at relatively short heat treatment time (≤24 h, 1024 °C). The groove in the “peanut” structure was indicative of boundary of α/α. Only in a few cases, when an isolated β grain can form within the interior αp along the (sub)boundary, refinement of αp grain may be achieved due to the obvious reduction of boundary splitting distance. These observations were rationalized on the basis of the classical Mullins grooving analysis.

Effect of heat treatment on the crystallographic orientation evolution in a near-α titanium alloy Ti60

The crystallographic orientation of equiaxed primary α (αp) phase is one of the main factors that determine the final texture/microtexture and mechanical properties for near-α titanium alloys. By annealing a forged Ti60 alloy pancake at 1030 and 1040 °C in the α+β two-phase field, two homogeneous bimodal microstructures were obtained, the intensity of secondary α in the {0001} pole figure increasing with the annealing temperature. In this study, the influence of annealing temperature on the crystallographic orientation evolution in the pancake was studied using an electron backscattered diffraction analysis method. In particular, a misorientation angle θ associated with (βΔJβBOR) that could quantitatively evaluate the deviation of the orientation relationship (OR) between αp and the surrounding β grains from Burgers OR was used to understand the role of the αp phase, which is considered as an anisotropic precipitate, in the texture evolution during annealing in the two-phase field. It was found that the αp grains with small θ are more readily preserved during the annealing, while their pinning effect is relatively small, resulting in faster growth of the neighboring β grains. A possible effect of heterogeneous local orientation distribution on the microstructure during the annealing process is also discussed.

Effect of heat treatment on the microstructure and tensile properties of deformed α/β Ti–47Zr–5Al–3V alloy

Microstructure and tensile properties of a new α/β Ti–47Zr–5Al–3V (wt%) alloy are investigated in the present study. Both α/β and β thermal annealing at 650 °C–800 °C for 1 h and subsequent aging at 450 °C for 2 h are introduced to investigate the effect of thermal annealing and aging on the microstructure and tensile properties of the Ti–47Zr–5Al–3V alloy after cold rolling. After thermal annealing, a fully equiaxed microstructure, a bi-modal microstructure and a fully lamellar microstructure are obtained with increasing the annealing temperature (650–800 °C), respectively. The 450 °C/2 h aging leads to further precipitation of extremely fine α platelets from the annealed samples. With increasing the annealing temperature, the tensile strength (σb) of annealed samples increases gradually from ∼1350 to 1420 MPa, whereas the elongation to failure (εf) first increases and then decreases and the highest εf ∼10.6% is attained at 750 °C. For all annealed samples, the 450 °C/2 h aging leads to increased strength and decreased ductility and the 750 °C/1 h annealing plus 450 °C/2 h aging results in the best combination of ultrahigh strength (σb ∼1510 MPa) and good ductility (εf ∼9.3%). The ultrahigh strength results from the strengthening of fine secondary α platelets, while the good ductility is attributed to the formation of large primary αp grains and the fine β grain size.

Simultaneously Enhancing the Ductility and Strength in a Hierarchical and Multiphase Nanolaminated TiZrAlV

High strength and ductility are the prerequisite for structural materials for wide applications. Here, a simultaneous enhancement of both the ductility and strength is reported in a hierarchical and multiphase nanolaminated (HMN) TiZrAlV prepared via thermomechanical processing treatments. An excellent combination of high ultimate tensile strength of σb ~ 1550 MPa and good elongation to failure of ef ~ 8.0% is obtained in an appropriate HMN structure that consists of nanoscale α″ martensites, submicroscale α plates, and large microscale primary αp grains, much better than that (σb ~ 1440 MPa, ef ~ 3.6%) of its coarse-laminated counterpart without primary αp grains. The present study is significant for the enhancement of strength and ductility of engineering materials via the design of hierarchical-laminated structure.

Thermal stability of hierarchical and multiphase nanolaminated TiZrAlV

Abstract Microstructures, thermal stabilities and microstructures and properties evolutions of a new β TiZrAlV alloy have been investigated in this paper. Various hierarchical and multiphase nanolaminated (HMN) structures have been successfully produced in the alloy via appropriate thermomechanical processing treatments. The higher thermal stability (Tp ~ 275 °C), i.e., onset temperature of phase transformation, is achieved in a specific HMN structure consisting of nanoscale acicular isothermal α″ martensites, submicroscale α plates and large microscale primary αp grains, compared with that (Tp ~ 105 °C) of its coarse-laminated counterpart without primary αp grains. For this specific HMN structure, thermal exposures at 300–400 °C lead to further precipitation of isothermal α″ martensites, which enhances evidently the strength and reduces the ductility, while the reverse transformation of α″/α to β and the coarsening of previously formed α plates concur during thermal exposures at 500–600 °C, which leads to dramatically decreased strength and increased ductility.

Enhancing Mechanical Properties of TiZrAlV by Engineering a Multi‐Modal‐Laminated Structure

In the present study, microstructure and mechanical properties of a new β‐metastable TiZrAlV prepared via thermomechanical processing (TMP) treatments have been studied. An excellent combination of ultimate tensile strength (σb ≈ 1 630 MPa) and ductility (ϵf ≈ 6.6%) has been achieved in the alloy after cold rolling, thermal annealing (665 °C/1 h), and two‐step aging (650 °C/0.5 h + 450 °C/2 h). This is attributed to the formation of a refined β microstructure with grain size of ≈10 µm and a multi‐modal‐laminated structure that consists of large primary ap grains (2 µm, 9.5 vol%) and coarse α precipitates (300 nm, 21 vol%) and fine α platelets (50 nm, 65 vol%). The fine α platelets contribute to high strength while the large primary αp grains and the refined β grain size provide ductility.

A hierarchical and multiphase nanolaminated alloy with an excellent combination of tensile properties

Ti alloys that possess a fine fully lamellar structure generally exhibit high strength but limited ductility. In this study, a hierarchical and multiphase nanolaminated structure that consists of large microscale primary αp grains (~1.5μm), sub-microscale α plates (~200nm) and nanoscale acicular isothermal (orthorhombic) α″ martensites (~15nm) is produced in a TiZrAlV alloy with a fine (~12μm) β grain size through the use of severe plastic deformation (SPD) combined with subsequent recrystallisation annealing and aging treatments. This specific structure results in an excellent combination of tensile properties, e.g., an ultimate tensile strength of σb~1545MPa and an elongation to failure of εf~7.9%. The present study demonstrates an alternative route for enhancing the mechanical properties of titanium alloys with laminated structures.

Effect of microstructure on the fatigue properties of Ti–6Al–4V titanium alloys

Through an analysis on microstructure and high cycle fatigue (HCF) properties of Ti–6Al–4V alloys which were selected from literature, the effects of microstructure types and microstructure parameters on HCF properties were investigated systematically. The results show that the HCF properties are strongly determined by microstructure types for Ti–6Al–4V. Generally the HCF strengths of different microstructures decrease in the order of bimodal, lamellar and equiaxed microstructure. Additionally, microstructure parameters such as the primary α (αp) content and the αp grain size in bimodal microstructures, the α lamellar width in lamellar microstructure and the α grain size in equiaxed microstructures, can influence the HCF properties.

Dendrite core grain refining and interdendritic coarsening behaviour in W-containing γ-TiAl based alloys

Two W-containing γ-TiAl based alloys were studied in as-cast ingot condition. Pronounced partitioning of heavy-metals–light-metals were found to occur, causing segregation of W and Nb into dendrite core and Al in interdendritic region. The average Al content in α2+γ lamellae was close to 46% in interdendritic region, and below 44% in dendrite cores, regardless of the nominal Al content of the alloy. About 15% coarse colonies formed in alloy 44Al and 60% in alloy 46Al. Effective grain refinement can be achieved in the dendrite core where both the Al depletion and Nb, W and Ti enrichment ensured a L→β solidification route. In contrast, the solidification in interdendritic regions involves a peritectic reaction L+β→αp. As a result, unusually large αp grains (α2+γ lamellar colonies) formed in the interdendritic regions.

Scandium (Sc) Behavior in High Temperature Ti Alloys

The microstructural evolution and elevated temperature tensile properties of Ti-6.6Al-5.2Sn-1.8Zr-(0~3.8)Sc (wt%) alloys have been investigated. The Sc-added alloys showed improved yield strength at 650°C and 750°C and with the elongation above 10%. Minor addition of Sc was found to significantly reduce the as-cast grain size. Higher amount of Sc additions resulted in the formation of high density of Sc-oxide, which causes the high strength at elevated temperatures and the reduction of ductility. High density of α2-Ti3Al fine precipitates with an average size of about 20 nm were observed inside equiaxed primary α (αp) grains in the Sc-free or minor Sc added alloys. However, precipitation free zone (PFZ) also formed in those alloys, where grain boundaries are free from any precipitates. Higher Sc addition was found to hinder the formation of PFZ and α2–precipitates.

β→α<sub>s</sub> Variant Selection in Sharp hcp Textured Regions of a Bimodal IMI834 Billet

Regions with sharp local textures, called macrozones, have been characterised in a bimodal IMI834 billet, containing 30% of primary αp grains surrounded by secondary αs colonies. It is shown that the αs colonies have been inherited according to a strong variant selection during the β→αs phase transformation. In each observed macrozone, the favoured variants have in average their c-axes in the same macroscopic direction as the αp grains. A detailed analysis of neighbouring αp grains and αs variants clearly shows that the variants favoured at β/αp boundaries are those able to share their c-axes with a neighbouring αp grain. The sharpness of such a variant selection mechanism is strongly related to the local orientation distribution of neighbouring αp/β grains at high temperature. This explains the differences in variant selection sharpness observed from one macrozone to the other.

Local texture and fatigue crack initiation in a Ti‐6Al‐4V titanium alloy

ABSTRACT Fatigue crack initiation was studied in a bimodal TA6V titanium alloy. A ghost structure inherited from the forging process, the scale of which is roughly 100 times the apparent grain size, was found to govern the initiation process. In these macrograins, that we have labelled macrozones, most of the primary alpha grains (αp) are found to display the same crystallographic orientation. Fatigue cracks are initiated on the basal plane or, if basal slip is difficult, on the prismatic plane. Thus in macrozones, where basal or prismatic slip is easy, numerous neighbouring tiny cracks appear over the whole macrozone, which have the size of the primary αp grains. In these macrozones the contribution of crack coalescence to crack growth is consequently very significant. On the contrary, if basal and prismatic slips are both difficult in the macrozone, no crack can be found in the corresponding macrozone. The crack initiation process is thus highly heterogeneous at the scale of the macrozone. Furthermore, this microstructure is found to induce a large scatter in the fatigue life of notched samples.