Alpha Grains Research Articles

Development of Low Anisotropic Commercially Pure Titanium Foils by Asymmetric Rolling and Annealing

This work studies the evolution of microstructure, texture, and anisotropy of pure titanium foils by symmetric rolling (SR) and asymmetric rolling (ASR) and subsequent annealing. The as‐received material shows equiaxed alpha grains and strong transverse texture (S‐T texture), which leads to severe anisotropy. The index of plane anisotropy (IPA) of yield stress (YS) and ultimate tensile stress (UTS) of the as‐received material is 15.2% and 7.5%, respectively. With the increase of deformation, the S‐T texture is mainly transformed into a mixed texture of transverse texture (T texture) and rolling direction texture (R texture), which balances the start‐up of the slip system when loading in the rolling direction (RD) and transverse direction (TD), so that the mechanical stress along RD and TD are close to each other and the anisotropy is improved. Especially for the samples prepared by ASR and annealing, the appearance of more R texture or basal texture (B texture) at the same deformation makes them show lower anisotropy. Therefore, after the same annealing treatment, the IPA of titanium foil with a thickness of 0.2 mm prepared by ASR is the smallest, which is 6.6% and 0.03%, respectively, while the IPA prepared by SR is 10.5% and 2.5%.

Evading the strength-ductility trade-off dilemma in AA2024 alloy by short-term natural re-aging after T351 temper

In this research, the influence of short-term natural re-aging after T351 temper on the microstructural evolution and mechanical behavior of AA2024 aluminum alloy was investigated. Grain growth occurred in the microstructures of the natural re-aged sample and a large number of Al7Cu2Fe particles were located inside the alpha grains. At the re-aging time of 1440 min, the peaks of XRD were shifted strongly to the right due to the formation of θ′′, S″, θ′, and S′. The results revealed that the precipitation rate was high in the AA2024 alloy during natural aging. With increasing the re-aging time, texture parameters remained almost unchanged. The hardness increased slowly within the first 60 min, then enhanced rapidly between 60 and 2880 min, and finally became stable at around 139 HV between 2880 and 11520 min. When the natural re-aging time increased from 240 to 2880 min, the strengthening trended speed up, viz, the yield strength increased from 226.6 to 357.3 MPa, and the ultimate tensile strength enhanced from 452.2 to 535.5 MPa. Compared to the as-received sample (T351 temper), the ultimate tensile strength of the re-aged sheet improved from 455.5 to 535.5 MPa, the ductility remained unchanged, and the hardness increased from 128.8 to 138.2 HV, which was owing to the acceleration of the precipitation caused by the presence of high-content Al7Cu2Fe particles in the interior of the alpha-aluminum grains in the natural re-aged sample. It was found that the Portevin-Le Chatelier instability of AA2024 alloy was effectively postponed after natural re-aging. With increasing the natural re-aging time, the strain hardening rate of the AA2024 sheet increased. The strengthening of the natural re-aged sample for 1440 and 2880 min was a result of a synergistic effect of precipitation hardening due to the formation of θ′′, S″, θ′, and S′ phases, elimination of Portevin Le-Chatelier instability, and highly efficient load transfer from alpha-aluminum to Al7Cu2Fe. Finally, to use the A2024 alloy produced by natural re-aging for 1440 or 2880 min, two methods were proposed.

Impact of process parameters on transitions in the microstructural characteristics and mechanical attributes of Ti–6Al–4V alloy joints during FSW

Abstract Transformations in microstructural characteristics and mechanical attributes of friction stir welded 3 mm thick Ti–6Al–4V alloy plates was investigated by employing distinctive tool rotational and traverse speeds. Impact of these parameters on microstructural transitions, generation of flaws, hardness, and tensile properties of the joints were analyzed. Increase in rotational speed from 1200 rpm to 1600 rpm have contributed for escalation in temperature, even above the β transus temperature. Large sized lamellar alpha grains was found to be transformed into finely refined lamellar alpha + altered beta grains in uppermost portion of nugget zone of joints fabricated at 1600 rpm and 50 mm/min combinations. This transformation have occurred due to the impact of the thermal cycles and stirring mechanism. These joints were found to be free from flaws including volumetric related defects, kissing bond, tunnel flaws. Majority of the fabricated joints possessed lowest value of mechanical properties in their heat affected zone and exhibited fracture in this zone. Properties of Ti–6Al–4V alloy joints were evaluated with respect to pseudo index of heat and it was observed that rotational speed of the tool is a dominant parameter in impacting both the mechanical attributes and microstructural transformations of the joints.

Characterization of the Microstructure of A Breakout Steel Sample

The microstructure of a continuously cast break out of low carbon steel has been studied using optical microscopy; SEM/EDS and EBSD analysis techniques. SEM and EBSD studies were carried out on two different locations on one sample piece and the resulting room temperature grains were found to be mostly BCC α- ferrite phase. Their orientations (using Euler angles) were determined. No meaningful amount of FCC phase was indexed after two patterns indexed on the two locations of the sample piece. The FCC comprised only 0.02% to 0.08% of the total patterns indexed. EBSD maps for the two locations, along with pole figures and grain size charts were deduced for the BCC phase. Volume fraction of the phase was estimated. Reconstruction of the Prior Austenite Grain Boundary was made from the alpha grains data using AztecWave software (AZtecCrystal). Only a small amount of the prior austenite grain boundaries was determined suggesting that the size of the parent grains were significantly larger than the map area in each case.

Ti-Cu積層造形合金における熱処理による析出形態の制御と機構解明

Intermetallic compounds (IMCs), known for their hardness, play an important role in the precipitation strengthening of alloys. However, precipitation in excessive amounts or as large particles can result in embrittlement of the material. Thus, controlling the distribution of IMCs among the matrix is crucial for the design of strong and ductile alloys. In this research, a supersaturated Ti-Cu alloy formed by Laser Powder Bed Fusion (L-PBF) was heat-treated at different phase regions to produce lamellar, network, and dispersed distributions of Ti2Cu IMC precipitates. To comprehensively understand the formation mechanisms of each IMC distribution structure, in-situ microstructure observation was performed during the heat-treatments. The lamellar IMC structure obtained from heat treatment above the beta-transus temperature, was found to derive from the rejection of Cu atoms towards the boundaries of lamellar alpha grains formed during cooling. The network IMC structure obtained from heat-treatment in the α+β phase region involved the formation of a Cu-rich β-Ti layer between α-Ti grains during heating, and subsequent precipitation of Ti2Cu into an α-encapsulating network upon cooling. Finally, the dispersed structure of IMCs was observed to result from regular nucleation and growth of Ti2Cu during heat treatment in the α+IMC region.

Joint addition of isomorphous and eutectoid beta stabilisers for developing ternary Ti alloys

This study considered the simultaneous addition of a eutectoid (i.e. iron) and an isomorphous (i.e. niobium) beta stabiliser to create ternary alpha + beta titanium alloys to fill the gap in literature, which is focused on achieving metastable or stable beta Ti–Fe–Nb titanium alloys. The incremental addition of iron and niobium increases the density, reduces the compressibility, enhances the densification of titanium thanks to the high diffusivity of iron, and permits to develop powder metallurgy ternary Ti–Fe–Nb alloys with a lamellar microstructure whose features, including alpha grains size and interlamellar spacing, is progressively refined. Consequently, the load bearing capability of the sintered Ti–Fe–Nb titanium alloys increases monotonically with the addition of iron and niobium obtaining mechanical properties comparable to those of more heavily alloyed cast Ti–Fe–Nb alloys. The sintered Ti–Fe–Nb titanium alloys share the same work hardening mechanism as a consequence of their common lamellar structure, but their specific work hardening rate depends on the actual chemical composition of the alloy.

Unexpected dynamic transformation from α phase to β phase in zirconium alloy revealed by in-situ neutron diffraction during high temperature deformation

Dynamic transformation from alpha (HCP) to beta (BCC) phase in a zirconium alloy was revealed by the use of in-situ neutron diffraction during hot compression. The dynamic transformation was unexpectedly detected during isothermal compression at temperatures of 900°C and 950°C (alpha + beta two-phase region) and strain rates of 0.01 s−1 and 0.001 s−1, even though equilibrium two-phase states were achieved prior to the hot compression. Dynamic transformation was accompanied by diffusion of Sn from beta to alpha phase, which resulted in changes of lattice parameters and a characteristic microstructure of alpha grains. The lattice constant of alpha phase measured by the in-situ neutron diffraction increased during the hot compression, while the lattice constant of beta phase exhibited an initial increase and subsequent decrease during the hot compression. As a result, the magnitude of lattice (elastic) strain as well as stress (elastic stress, or phase stress) in alpha phase was found to become much greater than those in beta phase. According to an atomistic simulation, the Gibbs free energy of alpha phase increased with hydrostatic compressive pressure more evidently than that of beta phase. It could be concluded from such results that the occurrence of the dynamic transformation from alpha to beta is attributed to an increase in the Gibbs free energy of alpha phase relative to beta phase owing to the difference in the phase stress; i.e., the larger lattice distortion made alpha phase thermodynamically more unstable than beta phase. The present result suggests that deformation of two-phase materials can dynamically make Gibbs free energy of plastically harder phase higher than that of the softer phase through increasing elastic energy in the harder phase, which might lead to dynamic transformation from harder phase to softer phase. [Display omitted]

Microstructure effects on the machinability behaviour of Ti6Al4V produced by Selective Laser Melting and Electron Beam Melting process

Additive Manufacturing (AM) is gaining widespread attention over the last decade in the areas of product development and prototyping, across aerospace and medical domains. Nevertheless, the built parts are near-net shaped that usually require a machining finish to improve the surface quality and operational life.The current study aims to document the machinability behaviour of the Ti6Al4V parts fabricated by the AM process, which is based on powder bed fusion, namely, Selective Laser Melting (SLM) and Electron Beam Melting (EBM). Further, machinability parameters are correlated with the microstructure and mechanical properties of the material. Face milling operation is performed under finishing conditions to study the response on the cutting forces. The effect of Hot Isostatic Pressing (HIP) and the influence of the build direction on microstructure and machinability have been investigated for the two AM processes. The effect of crystallographic textures and microstructure morphology of α and reconstructed prior β phases on the mechanical properties was studied using shear tests. It was observed that the SLM process primarily produced homogenous and fine α lath structures with less grain boundary α (αGB) grains, leading to low shear resistance and hence generated low cutting forces. On the other hand, the EBM process produced microstructures with variable morphologies, including more grain boundary alpha (αGB) grains and widmenstatten structures which resulted in a higher shear resistance and forces. The results show a striking influence of the microstructure induced by the type of the AM process, build orientation, and the subsequent heat treatment on the shear resistance and their ability to be machined. Further, the crystallographic textures were not strong for both processes and seem not to impact the machining process. The results highlight a strong “process – property” link which demands optimization of the cutting process for the type of the AM process and the resulting microstructure.

In-situ observation of temperature effect on the tensile deformation behavior in laser melting deposited TA15 alloy

In this study, laser melting deposited TA15 alloy was examined by in-situ SEM tensile deformation at a range of temperatures; RT, 300℃ and 500℃. The as-fabricated microstructure consists of prior β grains boundaries and fine basketweave microstructure. Due to LMD fabricated alloys’ anisotropic behavior, the tested samples were collected from the same region and direction. The XRD and SEM results showed that the temperature up to 500℃ has no impact on the phase constituent and structure morphology, revealing a stable microstructure. The tensile deformation at room temperature (RT) revealed that slip lines were the primary deformation mechanism generated in the parallel and larger alpha grain. An increase in temperature triggered grain boundary sliding during deformation, which enhanced the elongation in the material at the cost of tensile strength reduction. The grain boundary resistance to dislocation slip was affected by the sliding phenomenon due to increased temperature. Overall, the material showed stable tensile characteristics related linearly to the temperature variation.

Measuring residual stress in Ti-6Al-4V with HR-EBSD, using reference patterns from annealed material

Elastic strain and residual stress values in an engineering material were determined by high-resolution EBSD (HR-EBSD) using reference patterns from annealed, recrystallized material. Specifically, rolled Ti-6Al-4V sheet was analyzed using HR EBSD in both an as-annealed and a strained state, the latter established via a static two-point bend fixture. The commercial software CrossCourt4 (CC4) was used to evaluate the relative intragranular strain tensor components of the alpha grains mapped by HR-EBSD. The intragranular reference patterns identified in each grain by the software were then replaced with similarly-oriented patterns from scans of the as-annealed, recrystallized material. The CC4 analysis was then repeated to provide calibrated strain and residual stress values. Elastic strains on the order of 10−3 were detected, with corresponding residual stress values on the order of 1 GPa. Subsequent analyses using high energy synchrotron x-ray diffraction showed good correlation with the elastic strains measured by HR-EBSD using the annealed reference patterns. Additionally, the maximum macroscopic stress analytically predicted for two-point bending was in agreement with the residual stress observed. Good repeatability of the methodology and insight into the state of individual grains were also observed. The findings provide encouraging evidence that accurate intragranular and long-range residual stress measurements of engineering significance can be made with HR-EBSD using a physical reference pattern library.

Microstructures and Mechanical Properties of Hybrid, Additively Manufactured Ti6Al4V after Thermomechanical Processing.

In the present study, we propose a hybrid manufacturing route to produce high-quality Ti6Al4V parts, combining additive powder laser directed energy deposition (L-DED) for manufacturing of preforms, with subsequent hot forging as a thermomechanical processing (TMP) step. After L-DED, the material was hot formed at two different temperatures (930 °C and 1070 °C) and subsequently heat-treated for stress relief annealing. Tensile tests were performed on small sub-samples, taking into account different sample orientations with respect to the L-DED build direction and resulting in very good tensile strengths and ductility properties, similar or superior to the forged material. The resulting microstructure consists of very fine grained, partially globularized alpha grains, with a mean diameter ~0.8–2.3 µm, within a beta phase matrix, constituting between 2 and 9% of the sample. After forging in the sub-beta transus temperature range, the typical L-DED microstructure was no longer discernible and the anisotropy in tensile properties, common in additive manufacturing (AM), was significantly reduced. However, forging in the super-beta transus temperature range resulted in remaining anisotropies in the mechanical properties as well as an inferior tensile strength and ductility of the material. It was shown, that by combining L-DED with thermomechanical processing in the sub-beta transus temperature range of Ti6Al4V, a suitable microstructure and desirable mechanical properties for many applications can be obtained, with the advantage of reducing the material waste.

Influence of Rapid Solidification and Optimized Process Parameters on the Microstructural Evolution of Additive Manufactured Titanium Alloy Grade 5 Composite

The study experimentally investigates the effects that Ytterbium Laser System process parameters, such as laser power, powder feed rate and traverse speed, has on the resultant microstructure of Ti- 6Al-4V grade 5 alloy. The deposition process was conducted employing a 3kW (CW) Ytterbium Laser System (YLS-2000-TR) machine, coaxial to the reinforcement powder. The laser scanning speed and power were varied between the intervals of 1-1.2 m/min and 900-1000 W. All other parameters kept constant where the rate of gas flow, the spot diameter, and the rate of powder flow. The microstructure was characterized by grain size and morphology by using Optical Microscopy (OM) and Scanning Electron Microscopy (SEM). The microstructural and mechanical properties were ascertained and the relationships with the process parameters were achieved. As a result of rapid cooling, the morphological features of α and α’ are distinctive and appear acicular. The structures appear coarsened. The metallurgy of the samples identifies with a morphology of multi-scale; with the coarsened alpha structures being reduced, plate-like, discrete and finer. The alpha grains closer to the fusion zone grew epitaxially, and the ones above these are acicular and lamellar. The results also indicated that slow traverse speeds increase the scale of columnar grains, while other process parameters were kept constant. Columnar microstructures became prevalent due to the dynamic temperature gradients/spikes, and sustainable cooling rates, pertaining to fabricating direct laser deposited Ti-6Al-4V grade 5 alloy. It was ascertained that by increasing the traverse speeds, the cooling rates increased, which resulted in a decrease in the width of the columnar grains.

PRELIMINARY INVESTIGATION OF BILLET PROCESSING ROUTE ON MICROSTRUCTURE, TEXTURE AND FATIGUE RESPONSE OF Ti834 FORGED DISC PRODUCTS

Non-standard processing routes for the manufacture of industrial scale Ti834 billet have been undertaken to investigate their effect on macrozones in final forged product. Microstructure, texture and dwell fatigue fracture surfaces were characterised from forged disc samples fabricated from these new billets. All processing routes showed a bimodal microstructure consisting of 25pct of primary alpha grains in a transformed beta matrix. Texture analysis has revealed variations in the presence and size of macrozones with relatively weak textures. Quasi-cleavage facets were present in all dwell fatigue samples although the fatigue life was doubled for the sample whose thermomechanical processing has the highest imposed strain.

Microstructure and mechanical property of aluminium based die-cast alloy (ALDC6) for spider arm component

In this paper aims on the development of aluminum alloy (ALDC6) using high pressure die casting (HPDC) for spider arm component in washer. Presently, ALDC3 alloy is commonly used for the fabrication of spider arm but it has poor corrosion resistance. ALDC6 alloy which has good corrosion resistance can be used as an alternative material for spider arm. However due to high Si (Silicon) content of 10wt% in ALDC3 alloy it has good castability compared to ALDC6 alloy with 1wt% of Si. Hence there is a need to modify the ALDC6 alloys by increasing the percentage of silicon and magnesium elements so as to improve the castability, strength and corrosion resistance. The optical microstructure of the developed ALDC6 alloy was investigated and its phases were detected by XRD analysis. It was found out that presence of Mg2Si in ALDC6 alloys is higher than that of the ALDC3 alloy. The microstructure and mechanical properties of ALDC6 alloys were evaluated and compared with existed ALDC3 alloys. The tensile strength and yield strength of the developed ALDC6 alloys has improved but elongation decreased. This implies that the spheroidal primary alpha grains and decrease in grain size due to the presence of Si and Mg within the alloy enhance the strength and yield stress.

Early slip activity and fatigue crack initiation of a near alpha titanium alloy

For titanium alloys, crack initiation as a result of plastic strain accumulation has been shown to govern fatigue life under the high cycle fatigue regime. In this study, the early plastic slip activity and fatigue crack initiation was studied using a cyclic four point bending test at 10 Hz with a load ratio of 0.1, up to 90% of the proof stress. The plastic slip in the high stress area was monitored by interrupting the test and performing optical microscopy. Following fatigue crack initiation, scanning electron microscopy (SEM) combined with electron backscatter diffraction (EBSD) was used to identify the slip and crack initiation mode in a 600 x 600 μm2 area. Using slip trace analysis, it was shown that primary alpha grains offered dominant plastic deformation with basal slip activation. Cracking along basal planes was determined to be the dominant damage mode.

The Influence of Heat Treatment on Tribological Properties of Cu-Al-Fe-Ni Shape Memory Alloy

The effect of heat treatment on tribological properties of Cu-Al-Fe-Ni alloy under dry conditions was investigated. Scanning electron microscopy (SEM) associated with energy-dispersive X-ray spectroscopy (EDX) was used to evaluate the microstructure. Dry sliding wear tests were conducted on the as-cast and heat treated samples using pin on disc machine at different applied loads (10,15,20,25 and 30 N) and sliding speed ( 1.0 , 2.0,3.0 ,4.0 and 5.0 m/s). The results showed that aging the alloy at 350 °C resulted in formation of fine alpha grains and fine κ-phase compared with the cast condition. These fine phases could improve the wear resistance of the investigated alloy. However, the hardness and the wear resistance decreased when the aged temperature increased. Furthermore, the results indicated that the average contact surface temperature in the wear test increased with increment of aging temperature. On the other hand, the worn surfaces revealed that the predominant wear mechanism was abrasive wear.

Dislocation interactions in near-alpha Titanium alloy Ti6242Si under LCF

Dislocation interactions were investigated in near-alpha titanium alloy Ti6242Si after low cycle fatigue. Samples from the gauge section and the crack initiation site on the fracture surface were studied. Grain pairs with different crystallographic orientations were analysed from the gauge section to understand the dislocation interactions near the boundary. Deformation was primarily caused by planar slip, localized into slip bands in primary alpha (αp) grains. Direct slip transfer was observed within grains having similar orientations. In contrast, slip transfer resulted in a different kind of dislocation nucleation in the neighboring hard grain where there was misorientation between the grains, with the observation of cross-slip. Strain transfer was observed between highly misoriented grain pairs. Crack nucleation occurred on an αp grain by basal plane splitting, due to the large tensile stress developed by a double ended pile-up. This kind of pile-up is suggested to result from the incomplete reversibility of dislocation motion during load reversals. The observation of superjogs on the basal dislocations in the crack nucleated αp grain provides a rationale for why cracks nucleate near, rather than on, the basal plane.

Studying the effect of surface conditioning on the corrosion performance of titanium dental implants

The biocompatibility of dental implants is dependent on good interaction between implant-human body tissue and a good osseointegration. The surface roughness plays an important role enhancing bone healing and promoting biomechanical properties at the bone/implant interface. The increase in surface roughness can simultaneously improve cell migration and attachment of an implant, and enhance the osseointegration process. However surface roughness has shown to be detrimental to corrosion, raising the need to achieve a balance between good osseointegration and corrosion performance. This study characterised the corrosion performance of commercially pure titanium grade 4 with different surface conditionings. Implants in the form of 15 mm discs and implant screws were tested in basic Ringers lactate solution of pH 5.5 at 37°C and in modified Ringers of pH 2. Corrosion tests included open circuit potential (OCP), potentiodynamic polarization test (PDP) and mass loss corrosion tests. Metallurgical characterisation showed that titanium Grade 4 consisted of equiaxed alpha (α-Ti) grains. The OCP test showed that the machined implants were nobler than the roughened implants. The corrosion rates obtained were very low, below 0.13 mm/y which is an acceptable corrosion rate for biomaterial design and application. Mass-loss tests showed that the alloys passivated regardless of the surface conditioning. The machined surface had a corrosion resistance higher than that of the roughened surface finish in all conditions.

Effects of Rolling Reduction on Cold Forming Properties of Commercially Pure Titanium Sheet

Different cold rolling reductions were adopted for commercially pure titanium sheet. Cold forming properties were investigated by a microstructural analysis, Vickers microhardness and erichsen value measurements. The results have shown that Cold rolling resulted in refined alpha grains. Alpha grain size was refined further by greater cold reduction. Alpha grain sizes of the specimens of processing 1, 2 and 3 reached 30.90 μm, 26.48 μm and 20.58 μm, respectively. Cold forming properties were affected by different alpha grain sizes. The hardness and erichsen value reached the lowest and the highest values for the specimens in processing 1. The hardness increased and erichsen value decreased due to the finer alpha grain size for the specimen which was cold-rolled at a reduction of 50% in processing 2. Erichsen test results of the specimens of processing 3 had the lowest values due to the deformation of a reduction of 70%. Cold forming properties of the specimens of processing 3 were deteriorated, this is because deformation leads to the high dislocation density and the stored energy increases with accumulated strain after deformation.

Cavitation erosion resistance of a non-standard cast cobalt alloy: Influence of solubilizing and cold working treatments

The wear behavior of a non-standard cast cobalt chromium alloy (Co30Cr19Fe), during vibratory cavitation test, was investigated. Low stacking fault energies (SFE) cobalt alloys deform by complex plastic deformation mechanisms, which enhance the cavitation erosion (CE) resistance, since the onset of localized stress, leading to fatigue failure and material removal, is delayed. The purpose of this work is to characterize the main operating deformation mechanisms during cavitation erosion testing of the non-standard Co30Cr19Fe alloy. As cast, solution treated, and 15% and 30% cold rolled specimens were tested. The as cast microstructure consisted of ~2 mm fcc alpha grains and hcp epsilon-martensite. The 1473 K solubilization treatment led to primary recrystallization and formation of 250 µm new grains. The 15% cold worked microstructure consisted of heavily deformed fcc phase containing deformation twins and epsilon-martensite. Ultrasonic cavitation testing was carried out, during 40 h, according to ASTM G32-09. The solubilized specimens presented the worst behavior, whereas, the 30% cold worked specimens were the most CE resistant. The CE results are discussed based on the microstructural parameters: amount of alpha fcc and epsilon-hcp phases, grain sizes, relative amounts of twinning, slip lines and strain induced martensite formation.