Peak-aged Specimens Research Articles

Strengthening phase and mechanical property analysis of artificially aged Al7075 – Ni coated Al2024 composites

This technical paper demonstrates the possibilities of nickel (Ni) coated Al2024 powder reinforcement in an Al7075 matrix using the liquid stir casting technique. Additionally, the paper focuses on achieving stable properties by implementing artificial aging heat treatment. To apply the Ni coating, the electroless nickel plating technique was utilized, and a minimum coating thickness of 8 μm was determined to effectively prevent the dissolution of Al2024 powder reinforcements within the Al7075 matrix. Stir casting facilitated the uniform dispersion of the coated Al2024 powder up to a weight percentage of 7%. Subsequently, the Al7075 alloy and composites underwent artificial aging through solution heat treatment (SHT) at 450 °C for 4 h, followed by water quenching and aging at temperatures of 120, 150, and 180 °C. Aging at 120 °C was found to yield superior results compared to aging at 150 and 180 °C, thus identified as the optimum aging temperature. When the Ni coating thickness was increased beyond the optimal 8 μm, the resulting enhancements in hardness for both as-cast and peak-aged specimens, as well as the tensile strength, were not significant. The improvements observed were only marginal, ranging between 2 to 3%. Fracture surface analysis revealed that the predominant fracture mode in the Al7075 alloy was ductile, characterized by dimple rupture. In the as-cast Al7075-(7%, 8 μm) Al2024 composite, a mixed fracture mode comprising both brittle and ductile characteristics was observed. In the peak-aged (120 °C) Al7075-(7%, 8 μm)Al2024 composite, the overall fracture mode exhibited a dominant brittle nature. Analytical techniques including XRD, TEM, and EDS confirmed the presence of Mg2Si, MgZn2, CuAl2, and CuAl2Mg phases in the peak-aged (120 °C) Al7075-(7%, 8 μm) Al2024 composite. These phases contributed to the enhancement of the properties of both the Al7075 alloy and its composites. The developed composites can be used in automobile parts and aerospace applications.

Assessment of the post-dynamic recrystallization effects on the overall dynamic recrystallization kinetics in a Ni-base superalloy

Understanding the role of microconstituents, viz., precipitates, and carbides, on dynamic recrystallization (DRX) and on post-dynamic recrystallization (PDRX) is essential to obtain a uniform microstructure with desirable grain size during hot deformation. Thus, the present study was taken up to address the role of carbides on DRX as well as on the PDRXcharacteristics of a Ni-base superalloy XH55 (Ni-17Cr-12Fe-9Mo-2 Nb-1.8Al-0.04 C). Uniaxial hot compression tests were conducted in the temperature range of 950–1100 °C and at a constant strain rate of 10−1 s−1 on the peak-aged specimens. The evolution of DRX without and with post-deformation hold for 15 s was correlated with the flow softening mechanisms across the tested temperature range. Besides, the PDRX fraction and its evolution with temperature was estimated from the electron backscattered diffraction data while using the denoising filters and a grain level misorientation parameter. Further, intermittent post-deformation microstructural characterization was carried out on the specimens deformed at 1050 °C to understand the role of carbides in the initial stages of DRX and PDRX. Overall, the present study illustrates that carbides have inhibited the DRX kinetics in the initial stage, i.e., at low strains (ε<0.40). Whereas, at higher strains (ε>0.40), the role of carbides is limited in inhibiting the growth of the recrystallized (DRX and PDRX) grains present in their vicinity.

Multiscale precipitate evolution and its implications on the tensile deformation behavior in thermomechanically processed and peak-aged lean Mg-Al-Ca-Mn alloy

A cast and solution-annealed Mg-1Al-0.28Ca-0.13Mn-0.05Si (wt. %) alloy is thermomechanically processed using hard plate hot forging (HPHF) at 723 K and subsequently aged at 473 K. Significant grain refinement (∼6 µm) and strong basal fiber texture (∼35 MRD) have been attained through HPHF, which remain unchanged (∼6 µm and ∼32 MRD, respectively) after the peak aging treatment as the grain boundaries remain pinned by nano-sized particles. Some of the coarse (Mg,Al)2Ca particles from the solution-annealed condition are fragmented during HPHF leading to much finer (∼9 µm) particles, while some evolve into eutectic-like clusters of (Mg+Al2Ca). During peak-aging, the (Mg+Al2Ca) clusters transform into fine Al2Ca precipitates, while a few of the fine (Mg,Al)2Ca particles still remains unaltered. Presence of nano-sized Al8Mn5 precipitates (∼29 nm) is also detected in the HPHF specimen, whereas new evolution of finer Al8Mn5 (∼10 nm) particles is observed in the peak-aged specimen. During the tensile tests, the HPHF specimen exhibits relatively better ductility (∼5%) and work hardening than the peak-aged specimen (∼3.7%) due to higher slip and twinning activity. In contrast, finer size and larger fraction of the Al8Mn5 precipitates results in considerably greater resistance to twinning as well as basal-slip in the peak-aged specimen, thereby reducing its ductility. On the other hand, the same precipitates are effective in pinning the dislocations and thus, primarily accountable for the considerably greater strength in the peak-aged specimen.

Hydrogen trapping and hydrogen embrittlement in 15-5PH stainless steel

The hydrogen trapping capability and the effect on hydrogen embrittlement (HE) of Cu precipitations of the aged martensitic stainless steel were studied. The results shows that the hydrogen diffusion coefficient of specimens containing Cu precipitations reduced by an order of magnitude and the hydrogen content greatly increased. The peak-aged specimens (1 h) having the highest HE susceptibility was due to abundant hydrogen trapped by Cu precipitations coherent with matrix and escaped from the traps of Cu precipitations during tensile process. While Cu precipitations semi-coherent with matrix might have higher hydrogen trapping capability and reduce the HE sensitivity. Data AvailabilityThe raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Enhanced electrical, mechanical and tribological properties of Cu-Cr-Zr alloys by continuous extrusion forming and subsequent aging treatment

• A high performance Cu-1Cr-0.1Zr alloy was obtained by continuous extrusion forming and subsequent aging treatment. • The strengthening contributions of the grain refinement, dislocation and precipitation were quantitatively calculated. • The wear behavior was studied by examining the evolution of wear surface morphology and subsurface microstructure. • The microhardness (H), H/E r ratio and H 3 /E r 2 ratio were considered to gage the wear resistance of Cu-Cr-Zr alloy. As a promising material for the new generation of high-speed railway contact wires, the comprehensive optimization of the electrical conductivity, strength, hardness and wear resistance of the Cu-Cr-Zr alloy has received extensive attention. In this paper, a high-performance Cu-1Cr-0.1Zr alloy with an ultimate tensile strength of 599.1 MPa, a uniform elongation of 8.6%, a microhardness of 195.7 HV 0.2 and an electrical conductivity of 80.07%IACS was achieved by the continuous extrusion forming (CEF) and subsequent peak-aging treatment. The grain refinement strengthening, dislocation strengthening and precipitation strengthening are identified to be responsible for the excellent electrical and mechanical properties of Cu-Cr-Zr alloy. The wear behavior of Cu-Cr-Zr alloy was investigated by examining the evolution of worn surface morphology and subsurface microstructure. The microhardness ( H ) and reduced elastic modulus ( E r ) of the subsurface below the worn surface measured by nanoindentation were calculated to gage the tribological performance of Cu-Cr-Zr alloy. Results show that the continuously extruded and subsequently peak-aged specimen has the best wear resistance, which indicates that the tribological properties of Cu-Cr-Zr alloy strongly depend on its strength and hardness. It can be concluded that the CEF and subsequent aging treatment process provides a new and high-efficiency procedure for the continuous preparation of Cu-Cr-Zr alloys.

Effect of Organizational Evolution on the Stress Corrosion Cracking of the Cr-Co-Ni-Mo Series of Ultra-High Strength Stainless Steel.

Different microstructures were obtained under various thermal conditions by adjusting the heat treatment parameters of the Cr-Co-Ni-Mo series of ultra-high strength stainless steel. The effect of organizational evolution on the stress corrosion cracking (SCC) of the Cr-Co-Ni-Mo series of ultra-high strength stainless steel was investigated using potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and other test methods in combination with slow strain rate tensile tests (SSRTs). The results show that the Mo- and Cr-rich clusters and precipitation of the Laves phase reduce the corrosion resistance, while increasing the austenite content can improve the corrosion resistance. The Cr-Co-Ni-Mo series of ultra-high strength stainless steel has a high SCC resistance after quenching at 1080 °C and undergoing deep cooling (DC) treatment at −73 °C. With increasing holding time, the strength of the underaged and peak-aged specimens increases, but the passivation and SCC resistance decreases. At the overaged temperature, the specimen has good SCC resistance after a short holding time, which is attributed to its higher austenite content and lower dislocation density. As a stable hydrogen trap in steel, austenite effectively improves the SCC resistance of steel. However, under the coupled action of hydrogen and stress, martensitic transformation occurs due to the decrease in the lamination energy of austenite, and the weak martensitic interface becomes the preferred location for crack initiation and propagation.

Outstanding enhancement in critical resolved shear stress of basal slip with precipitation in Mg-10Dy-0.7La alloy: A micro-pillar compression study

The strengthening effect of precipitates on the operation of basal < a > slip in Mg-10Dy-0.7La has been studied. Micropillar compression was performed on grains of solution-treated and peak-aged specimens selected to favour basal < a > slip. The CRSS of basal < a > slip was calculated from the stress–strain curves. The peak-aged alloy showed a remarkable increase in the CRSS for basal < a > slip as compared to that of solution-treated specimen: the highest reported increase for any Mg-RE alloy. Increased CRSS can be rationalised by considering network of precipitates.

Microstructural development and aging behavior of Al–Cr–Zr heat-resistant alloy fabricated using laser powder bed fusion

Using laser powder bed fusion (L-PBF), we designed and fabricated specimens of a heat-resistant Al–4mass%Cr–1.5mass%Zr alloy (Al–4Cr–1.5Zr). The Al–4Cr–1.5Zr L-PBF specimen achieved high relative density greater than 99.9% by optimizing the laser scan conditions. The hardness and tensile and 0.2% proof stresses of the Al–4Cr–1.5Zr L-PBF specimens significantly increased after aging heat treatments, and the peak-aged specimens exhibited excellent high-temperature strength. The L-PBF specimens consisted of coarse columnar and fine equiaxed crystal grains owing to heterogeneous nucleation by the primarily crystallized Al3Zr phase. In the peak-aged L-PBF specimen, a finely dispersed Al3Zr granular phase on the order of single nanometers was precipitated in the α-Al matrix by aging. Meanwhile, the chromium was supersaturated in an α-Al solid solution even after peak aging, and partially precipitated into coarse granular Al–Cr-based compounds. These multi-scale microstructural observation results revealed that the dominant strengthening factors in the Al–4Cr–1.5Zr L-PBF specimens were precipitation hardening by the fine L12-Al3Zr phase having a strong crystallographic consistency with the α-Al matrix, solid-solution hardening with chromium, dispersion hardening due to the Al–Cr-based compounds, and hardening by the crystal grain refinement mainly with the fine equiaxed grains.

The role of Gd on the microstructural evolution and mechanical properties of Mg-3Nd-0.2Zn-0.5Zr alloy

This work was undertaken to investigate the specific role of element Gd on the microstructural evolution and mechanical properties of Mg-3Nd-0.2Zn-0.5Zr alloy. It is noticeable that, as the Gd content increases, the average grain size of as-cast alloys is continuously reduced, accompanied by the main secondary phases changing from Mg12Nd to Mg3Gd. Tensile tests reveal that addition of Gd (1.5–4.5 wt%) could lead to substantial enhancements of alloy strength (25–70 MPa) and an excellent combination of strength and ductility is obtained in the alloy with 4.5 wt% Gd addition (YS = 200 MPa, UTS = 343 MPa, EL = 5.4%). Microstructure characterization indicates that the improved solid-solution strengthening effect originated from increasing Gd additions plays a key role in the significant strength improvements obtained in as-quenched alloys. HAADF-STEM observations suggest that Gd is highly enriched in the dominantly disc-like prismatic β″ phases, leading to the strongly enhanced precipitation kinetics and greatly augmented volume fraction of β″ phase. A quantitative microstructural comparison of peak-aged specimens indicates that the significant strength enhancements should be primarily derived from the denser dispersion of β″ phases with higher aspect radio arising from Gd addition.

Dynamic ESPI Evaluation of Deformation and Fracture Mechanism of 7075 Aluminum Alloy.

The deformation and fracture mechanism in 7075 aluminum alloy is discussed based on a field theoretical approach. A pair of peak-aged and overaged plate specimens are prepared under the respective precipitation conditions, and their plastic deformation behaviors are visualized with two-dimensional electronic speckle pattern interferometry (ESPI). The in-plane velocity field caused by monotonic tensile loading is monitored continuously via the contour analysis method of ESPI. In the plastic regime, the peak-aged specimen exhibits a macroscopically uniform deformation behavior, while the annealed specimen exhibits non-uniform deformation characterized by a localized shear band. The occurrence of the shear band is explained by the transition of the material’s elastic resistive mechanism from the longitudinal force dominant to shear force dominant mode. The shear force is interpreted as the frictional force that drives mobile dislocations along the shear band. The dynamic behavior of the shear band is explained as representing the motion of a solitary wave. The observed decrease in the solitary wave’s velocity is accounted for by the change in the acoustic impedance with the advancement of plastic deformation.

Microstructural evolution and its implications on the mechanical properties during age hardening in Mg-1Al-0.28Ca-0.13Mn-0.05Si alloy

A cast Mg-1Al-0.28Ca-0.13Mn-0.5Si (wt%) alloy has been subjected to solution-annealing at 723 K followed by aging treatment at 473 K. The specimens aged for 15 min, 60 min, and 900 min were considered as the under-aged, peak-aged, and over-aged specimens based on the microhardness variations. In comparison to the solution-annealed condition, the peak-aged specimen exhibits excellent strengthening i.e. ~87% increase in the yield strength and ~37% rise in the ultimate tensile strength, at the expense of ~41% decrease in the ductility. The evolution of nano-sized Al-Mn particles and coarse Ca-rich phases takes place with the aging of the alloy specimens. The Al-Mn phase has been identified as the Al8Mn5 phase with the aid of X-ray diffraction, scanning transmission electron microscopy, high-resolution transmission electron microscopy, and 3D atom probe tomography. The changes in the size, fraction, and nature of precipitate-dislocation interactions depending on the coherency of the nano-sized Al8Mn5 precipitates, are accountable for strengthening in the different aging stages of the developed alloy. Precipitate-dislocation interactions involving precipitate-shearing mechanism leads to the excellent strengthening in the peak-aged specimen. Contrarily, the evolution of the crack-initiating coarse (Mg,Al)2Ca phase leads to the loss of ductility in the peak-aged and over-aged specimens.

Environmentally assisted cracking behavior of U-bend specimens of Mg–RE alloys in chloride containing basic solution

Abstract Environmentally assisted cracking (EAC) behavior of two Mg-rare earth (RE) alloys such as Mg–Zn–Gd–Nd–Zr (EV31A) and Mg–Y–Nd (WE43C) alloys was investigated by using U-bend specimens. Open circuit potentials (OCP) of the U-bend specimens were monitored during the EAC tests in 0.1 M NaOH solution with different chloride concentrations at room temperature. EV31A (as-received, and peak aged) and WE43C (peak aged) specimens failed by SCC in 80 ppm chloride containing 0.1 M NaOH solution at OCP. When the EAC initiation occurred, the OCP decreased continuously. Irregular fluctuations of the OCP were observed in the absence of EAC. The OCP versus time profile could be used for monitoring EAC failure of the Mg–RE alloy components in real life service. Applied potentials did not cause cracking of the EV31A alloy in 80 ppm Cl− containing 0.1 M NaOH. Accelerated cracking was observed on the WE43C alloy in peak-aged condition under the applied potentials in the transpassive region when compared to that of OCP condition. Overaging decreased the susceptibility to cracking.

Stress response and microstructural evolution of nickel-based superalloys during low cycle fatigue: Physics-based modelling of cyclic hardening and softening

Low cycle fatigue is one of the main life limiting factors in gas turbine discs. The plastic deformation behaviour that leads to crack initiation is not fully understood, and phenomenological descriptions fail to explain the stress response typical of nickel-based superalloys, which consists of cyclic hardening followed by cyclic softening. In this study, samples of nickel-based superalloy 718Plus with different ageing heat treatments are fatigued for 500 cycles at room temperature, their microstructures characterised and their slip localisation behaviour quantified via electron channelling contrast imaging (ECCI). A physics-based mesoscopic model is developed to investigate the effects of ageing and loading conditions on cyclic deformation behaviour. The formation of slip bands and evolution of the local dislocation density are used to describe cyclic hardening, while continued precipitate shearing from the accumulation of slip irreversibilities is modelled as the source of cyclic softening. Both mechanisms are then coupled via a parameter for the volume fraction of slip bands. The model successfully reproduces the trends observed for the different conditions, with overaged samples eventually surpassing the cyclic stress of the peak-aged specimens due to a slower softening rate. Curves from the literature for superalloy Nimonic PE16 are also reproduced for different ageing conditions and strain amplitudes. Further electron microscopy near surface cracks reveals the presence of precipitate-free deformation bands only in the underaged condition, which is explained in terms of a saturation point for the shearing process.

The combined effects of hydrogen and aging condition on the deformation and fracture behavior of a precipitation-hardened nickel-base superalloy

The effect of hydrogen (H) on the deformation behavior of Monel K-500 in various isothermal heat treatment conditions (non-aged, under-aged, peak-aged, and over-aged) was assessed via uniaxial mechanical testing. H-charged and non-charged specimens were strained to failure to facilitate a comparison of ductility, fracture surface morphology, strength, and work hardening behavior. For all examined heat treatment conditions, H charging leads to a significant reduction in ductility, which is accompanied by a consistent change in fracture surface morphology from ductile microvoid coalescence to brittle intergranular fracture. While H charging led to a systematic enhancement in the yield strength of all heat treatments, the three age-hardened conditions exhibited a more than 2-fold increase relative to the non-aged heat treatment. This suggests that H modifies the dislocation–precipitate interactions, which also manifest themselves through changes in work hardening metrics related to the dislocation storage and recovery rates. In particular, the H-charged peak-aged specimen exhibited a significant increase in initial hardening (dislocation storage) rate relative to the H-charged under-aged specimen. Transmission electron microscopy of these samples revealed the onset of widespread dislocation looping in the H-charged peak-aged sample, in addition to the planar slip bands characteristic of the non-charged condition. This result suggests that hydrogen induces the particle shearing-to-looping transition at smaller particle sizes. Possible mechanistic explanations for this observed behavior are presented.

The effect of isothermal heat treatment on hydrogen environment-assisted cracking susceptibility in Monel K-500

The effect of isothermal heat treatment on hydrogen environment-assisted cracking (HEAC) susceptibility in a single heat of Monel K-500 was evaluated via slow-rising stress intensity (K) testing while immersed in 0.6 M NaCl solution and exposed to applied potentials ranging from −1000 to −1200 mVSCE (vs. saturated calomel electrode). Four heat treatments, corresponding to the non-aged, under-aged, peak-aged, and over-aged conditions, conducted at a constant aging temperature of 923 K, were examined in this study. Crack growth kinetics and fractography demonstrate that alloys heat-treated to the under-aged and peak-aged conditions exhibit increased susceptibility to HEAC relative to alloys treated to the non-aged and over-aged conditions, respectively. Microstructural variables that are expected to vary under this isothermal heat treatment protocol are grain size, grain boundary character, grain boundary impurity concentration, yield strength, strain hardening, and precipitate morphology. A detailed assessment of grain size, grain boundary character, and grain boundary impurity concentration evolution with aging time suggests these variables do not explain the observed variation in HEAC susceptibility of Monel K-500. Differences in the crack tip hydrostatic stress field induced by differences in yield strength and strain hardening were directionally consistent with the observed HEAC susceptibility, but do not capture the enhanced susceptibility of the under-aged heat treatment relative to the peak-aged condition. However, transmission electron microscopy of each heat treatment revealed the presence of planar slip in the under-aged and peak-aged specimens, as compared to wavy slip in the non-aged and over-aged specimens, suggesting that the propensity for strain localization correlates well with HEAC susceptibility.

Mechanical characterization of heat treated Al2219 hybrid composites

Aluminium alloy matrix composites with Al2O3 reinforcements exhibit superior mechanical properties and utilize in several demanding fields’ viz., automobile, aerospace, defense, sports equipment, electronics and bio-medical. The present work emphasizes on improvement of microstructure and mechanical properties of age hardened graphite and alumina reinforced Al alloy matrix hybrid composites. Different composites with a constant carbon content of 1 weight % and 0, 2, 4 and 6 weight % Al2O3 as reinforcements are fabricated by using stir casting technic and tested for hardness, tensile and impact strength. Scanning electron microscopy (SEM) is performed to analyse the failure mode under tensile load. All the composites are subjected to age hardening treatment with solutionising temperature of 530oC and aging temperatures of 100 and 200oC. The peak hardness of the composites at two aging temperatures are noted. Tensile and impact tests are conducted for the peak aged specimens. Results show substantial increase in the hardness of the age hardened specimens in the range of 34-44% in comparison with the as cast specimens. Result analysis shows increase in tensile strength (upto 40%) and decrease in impact resistance (upto 33%) with the increase in weight % of reinforcements. As the aging temperature increases a reduction in tensile strength and impact resistance is observed in each composites.

Age-Induced Precipitation and Hardening Behavior of Ni3Al Intermetallic Alloys Containing Vanadium

L12-type Ni3Al alloys containing vanadium are potential candidates for solid-solution and age-hardenable alloy systems, according to the Ni3Al-Ni3V pseudo-binary phase diagram. Therefore, herein, variations in the microstructure and mechanical properties of Ni-13 at.% Al-12 at.% V-50 ppm B alloy during isothermal aging were investigated. Alloy specimens were solution-treated at 1323 K for 48 h, quenched in water, and aged at 1073 K to 1173 K. The quenched specimens exhibited a single phase of Ni3Al (L12 structure derived from Al (fcc) structure), while in the aged specimens, numerous fine disk-shaped precipitates identified as Ni3V (D022 structure from orthorhombic structure) were formed on {001} planes of the Ni3Al matrix. The size of the disk-shaped Ni3V precipitates increased gradually with increasing aging period. The hardness and strength of the specimens increased initially during aging at 1073 K to 1173 K, reached a maximum, followed by a subsequent decrease. The age-hardening behavior observed for the specimens can be explained in terms of precipitation of the fine disk-shaped Ni3V precipitates in the Ni3Al matrix. Furthermore, the peak-aged specimens exhibited an increase in yield strength with increasing testing temperature, similar to other L12-type intermetallic alloys.

Effect of Light Illumination on the Corrosion Behavior of Mg-RE Alloy EV31A in Chloride-Alkaline Solutions

Mg-RE alloy EV31A is a Mg-Zn-Gd-Nd-Zr alloy used in aerospace and high performance automobile applications where creep and corrosion resistance are required. These alloys can also be used in Mg-air batteries as possible anode materials in alkaline electrolytes in order to minimize the negative difference effect. EV31A is an age hardenable alloy. The precipitation reactions occurring during the aging process follow the sequence of a-Mg →β'' → β' → β (Mg3RE) → Mg41RE5. The alloy showed excellent passivity in the 0.1 M NaOH solution. More than 80 ppm of chloride was required to induce passivity breakdown in any heat treatment condition. Peak aging at 200 °C for 16 h imparted better resistance for localized corrosion than the solution treated or over aged conditions[1]. Light illumination has been reported to increase the resistance to passivity breakdown of various metals and alloys such as iron[2], copper[3], aluminum[4], and stainless steel. During illumination, the band bending of the passive layer is modified by the electron-hole pair generation and the electric field that drives the transport of point defects across the passive layer is considerably decreased. This quenched electric field is considered to improve the localized corrosion resistance of the material. Light illumination also modifies the electronic properties of the passive film. The effect of white light illumination on the corrosion behavior of peak aged EV31A specimens was investigated in 0.1 M NaOH solution containing 0, 80, 100 and 200 ppm chloride ions at room temperature. The passivation current density decreased upon illumination in the 0.1 NaOH solution without chloride addition. On the other hand, increased passive current densities were observed in the chloride containing solutions upon illumination. The white light illumination did not affect the open circuit potential of the EV31A. This result indicated that alloying additions did not significantly alter the band gap of the passive layer or induce band gap states in the passive layer in order to harvest the longer wave lengths of the white light. The incubation time for passivity perturbation decreased with illumination. The charge carrier density increased significantly upon illumination. The exchange current density for hydrogen evolution increased under the illuminated conditions. The Mott-Schottky results pointed out that light illumination induced surface defects of n-type character, possibly oxygen vacancies. These defects could have been potential sites for chloride adsorption that decreased the corrosion resistance. The white light illumination did not result in induced photo potential due to wide band gap of the passive layer. Since the band bending was not altered, the electric field remained the same in both illuminated and non-illuminated condition. Overall, illumination of white light was not beneficial to the localized corrosion resistance of the Mg-RE alloy in alkaline-chloride solutions. [1] J. Ninlachart, K.S. Raja, Acta Metallurgica Sinica (English Letters), 2017, 30(4), 352–366 [2] P. Schmuki, H. Bohni, Electrochimica Acta, Vol. 40, No. 6, pp. 775-783, 1995 [3] C. B. Breslin, D. D. Macdonald, Electrochimica Acta 44 (1998) 643±651 [4] M. A. Amin, Electrochimica Acta 56 (2011) 2518–2531

Controlling the high temperature mechanical behavior of Al alloys by precipitation and severe straining

The aim of this work was to investigate the influence of the precipitate distribution on the microstructure and on the room and high temperature mechanical properties of an age-hardenable aluminium AA6082 alloy following severe straining by high-pressure torsion (HPT). With this goal, specimens in the as-cast and T6 peak-aged conditions were processed by HPT using 0.5, 1 and 5 turns at room temperature. At high strain levels (γ>100), similar saturation grain sizes (~250nm), high-angle boundary fractions (~80%) and hardness values (Hv~160) were obtained for both initial conditions. Grain refinement led to significant strengthening and to good ductility values at room temperature. Analysis by TEM and EDS elemental mapping revealed that HPT processing of the as-cast condition led to fracture of the stable β-phase into many small precipitates located preferentially along grain boundaries and triple junctions. By contrast, HPT processing of the T6 peak-aged specimens revealed a partial dissolution of the needle-shaped nanoprecipitates. The different evolutions of the precipitate distributions following straining in the as-cast and peak-aged conditions gave rise to dramatic differences in the mechanical properties and the operative deformation mechanisms at warm temperatures. These results provide evidence that the high temperature mechanical behavior of age-hardenable Al alloys may be conveniently controlled by altering the precipitate distribution followed by severe straining.

Effect of aging treatment on mechanical properties and fracture behavior of friction stir processed Mg–Y–Nd alloy

In this study, precipitation behavior of Mg–Y–Nd cast alloy during friction stir processing (FSP), and the effect of subsequent artificial aging on mechanical properties and fracture behavior of the FSP alloy were investigated. It is found that the coarse α-Mg grains and large second phases are greatly refined after FSP. Moreover, due to the heat input during processing and the natural cooling, β′ and β1 precipitates are also observed in the FSP alloy. The FSP specimens were subjected to subsequent artificial aging treatment, and the peak hardness is obtained at 150 °C for 54 h and 180 °C for 30 h. Strengths of the peak–aged specimens are further increased, which is attributed to the large quantity of β″ and β1 precipitates, respectively. Meanwhile, elongations of the peak-aged specimens are both decreased. Due to the comprehensive effects of banded structures and fine grains, failure mechanisms of FSP and peak-aged specimens are all mixed ductile–brittle fracture mode. However, compared to the FSP specimens, different fracture paths are exhibited in peak–aged specimens.