Gradient Microstructure Research Articles

Improvement of Surface Properties of 30CrNi2MoVA Steel with Ultrasonic Composite Strengthening Modification

The surface roughness, surface hardness, tensile properties and friction-wear properties were characterized, in comparison with those of the traditional turned cutting, electropulsing treatment (EP) and ultrasonic surface-rolling process (USRP) sample. The surface microstructure was obviously refined after USRP and EP-USRP, with a fine-grain depth of 60 μm and 100 μm, respectively. The surface roughness significantly decreases at first, and then gradually increases after surface-strengthening modification. The lowest roughness of 0.035 μm and 0.040 μm is obtained for the USRP and EP-USRP samples, respectively, which is about 12 times less than that of the turning surface roughness of 0.421 μm. The surface hardness increases from 280 HV to 360 HV after strengthening modification. The super tensile property of 30CrNi2Mo steel is obtained for the USRP, for which the yield strength, tensile strength, elongation and yield-to-strength ratio are 743 MPa, 961 MPa, 11% and 0.773, respectively. The friction coefficients in the turning state, USRP and EP-USRP are 0.37, 0.35 and 0.4, respectively. Ultrasonic composite-strengthening modification can increases the surface hardness, and obtains gradient microstructure on the material surface, which endows the material with better surface properties.

Enhancement of tribological properties by fabricating gradient structure and MoS2 coating on the surface of 06Cr19Ni9 stainless steel

Abstract Stainless steel is extensively used in civil engineering and mechanical industries due to its superior corrosion resistance and mechanical performance. Nevertheless, its surface is prone to wear, which can compromise component functionality and introduce safety concerns. The present study developed a composite structure consisting of a gradient microstructure layer combined with a pure MoS2 coating on the surface of 06Cr19Ni9 stainless steel. The fabrication process involved ultrasonic shot peening using α-Al2O3 particles, followed by thermal spraying. Fretting friction and wear using the ball-on-disc method demonstrated a notable improvement in tribological performance. The composite structure achieved a remarkably low friction coefficient of 0.16 under a normal load of 15 N representing a 74.19% reduction compared to a mechanically polished sample. However, when subjected to a higher normal load of 50 N, the friction coefficient increased significantly. Despite a relatively higher wear rate, the composite structure effectively safeguarded the metal matrix from substantial wear during testing. The enhanced tribological properties were primarily attributed to the synergistic interaction between the gradient microstructure layer and the MoS2 coating. These findings underscore the potential of this composite structure to significantly improve the wear resistance and extend the operational life of 06Cr19Ni9 stainless steel.

Microstructural characteristics and mechanical properties of WAAM components

Abstract Wire arc additive manufacturing (WAAM) is a process enabling the deposition of components with complex geometries while using high deposition rates. The WAAM process is associated with specific thermal process conditions resulting in inhomogeneities in the microstructure and thus, anisotropic properties within a component. Using a WAAM component made of high-alloy steel 1.4370 as an example, correlations between the microstructural gradients and the mechanical properties will be studied based on hardness measurements. To this end, the phases and characteristic features present in the microstructure will be linked to hardness profiles measured in different regions of the WAAM component.

Enhanced strength-ductility synergy of Mg-Gd-Y-Zn-Zr alloy with gradient microstructure and abnormal texture via rotary backward extrusion

Enhanced strength-ductility synergy of Mg-Gd-Y-Zn-Zr alloy with gradient microstructure and abnormal texture via rotary backward extrusion

Hard and Wear-resistant microstructural gradient CrMoNbV MPEA coatings

Hard and Wear-resistant microstructural gradient CrMoNbV MPEA coatings

Microstructure homogenization and strength-ductility synergy improvement of the hybrid additive manufactured dual-phase titanium alloy by sub-critical annealing

ABSTRACT This study employed the effect of sub-critical annealing heat treatment on the microstructure and anisotropy of the hybrid additive manufactured TC11 alloy. The results demonstrate that after sub-critical annealing, a gradient microstructure with the gradual changes in the morphology and content of primary α phases from the laser deposition zone to the substrate zone was formed. Moreover, the tensile anisotropy of the hybrid manufactured specimens was substantially reduced. In particular, when loading along to the deposition direction, the ultimate tensile strength of the heat-treated hybrid manufactured specimen was 1062 MPa, increased by 8.5%. And when loading perpendicular to the deposition direction, the elongation can reach 13.0%, increased by 86% compare to the as-deposited sample. These findings would also shed new insights for the modification of graded structural parts prepared by the hybrid manufacturing technique, laser additive repair and other methods.

Improved properties of wire arc directed energy deposited thin-walled Al-6Mg-0.3Sc component via laser shock peening

ABSTRACT Heat treatment free aluminum alloys have been developed for wire arc directed energy deposition (DED) large thin-walled components, but their further performance enhancement methods remain unexplored. In this study, laser shock peening (LSP) treatment was performed on wire arc DED Al-6Mg-0.3Sc components. The results show that LSP creates a gradient microstructure with submicro-grains near the surface. The effects of LSP on defects were quantified by quasi-in-situ CT, demonstrating a reduction in porosity by over 68% together with decreased size. Simultaneously, tensile strength increases by 39.5% to 295.8 ± 6.4 MPa with only a marginal loss of elongation, as compared to the as-deposited (AD) state. This was due to the accumulation of dislocations and continuous dynamic recrystallization lead to grain refinement, as well as the appearance of a triple heterogeneous microstructure. Therefore, this study offers a novel solution for manufacturing of high-performance heat treatment free large thin-walled components.

Изменение микроструктуры углеродистых прутков после радиально-сдвиговой протяжки и волочения

One of the methods of obtaining gradient structure in long bars is helical rolling and as one of its variants radial-shift rolling. The possibility of combining the processes of helical rolling and drawing has been considered for quite a long time. Since the use of traditional drawing as a final process after various processes is a good option, because it eliminates all the disadvantages of pre-deformation. Therefore, this paper investigates the change in microstructure of carbon steel bars after radial shear broaching and drawing. A gradient microstructure was obtained as a result of the laboratory experiment. Ultra-fine grains of 0.5 μm size were obtained on the surface of the bar, and in the center of the microstructure with an average grain size of 7 μm.

An innovative process for producing high-performance stainless steel/carbon steel composite plate by the controlled rolling

The traditional composite plate rolling process is long with high energy consumption and large microstructure gradient, which restricts the large-scale production seriously. Therefore, this paper proposes an innovative short rolling process, advancing the starting point with the cast billet to produce the high-performance 1Cr13/Q235 composite plates by the controlled rolling. The influence of the reduction ratio on the microstructure and properties of the composite plate was systematically studied. The findings indicate that the tensile strength of the composite plate produced by the innovative process can be up to 897 MPa, while the shear strength can be up to 446 MPa. The maximum diffusion distance of the Cr element can be up to 18.3 µm with the reduction ratio of 45% to 75%. More and more Cr element diffuses from the 1Cr13 side to the Q235 side as reduction ratio increases from 45% to 55%. Meanwhile, the microstructure and its gradient of the composite plates are obviously refined with increasing reduction ratio. Many carbon atoms migrate from the Q235 side to the 1Cr13 side across the bonding interface. Subsequently, a zone of decarburization appeared in the Q235 side. The feasibility of the innovative process provides technical support for the production of stainless steel composite plates characterized by high efficiency, low energy consumption, and environmental protection.

Heterogeneity and Penumbra of White Matter Hyperintensities in Small Vessel Diseases Determined by Quantitative MRI.

White matter hyperintensities (WMHs) are established structural imaging markers of cerebral small vessel disease. The pathophysiologic condition of brain tissue varies over the core, the vicinity, and the subtypes of WMH and cannot be interpreted from conventional magnetic resonance imaging. We aim to improve our pathophysiologic understanding of WMHs and the adjacently injured normal-appearing white matter in terms of microstructural and microvascular alterations using quantitative magnetic resonance imaging in patients with sporadic and genetic cerebral small vessel disease. Structural T2-weighted imaging, multishell diffusion imaging, and dynamic contrast-enhanced magnetic resonance imaging were performed at 3T in 44 participants with sporadic cerebral small vessel disease and 32 participants with monogenic cerebral small vessel disease (cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy; 59±12 years, 41 males) between June 2017 and May 2020 as part of the prospective, multicenter (Edinburgh, the United Kingdom; Maastricht, the Netherlands; and Munich, Germany), observational INVESTIGATE-SVDs study (Imaging Neurovascular, Endothelial and Structural Integrity in Preparation to Treat Small Vessel Diseases). The mean diffusivity, free water content, and perfusion (all derived from multishell diffusion imaging), as well as the blood-brain barrier leakage and plasma volume fraction (derived from dynamic contrast-enhanced magnetic resonance imaging), were compared between deep and periventricular WMH types using paired t tests. Additional spatial analyses were performed inside and outside the WMH types to determine the internal heterogeneity and the extent of the penumbras, that is, adjacent white matter at risk for conversion to WMH. Periventricular WMH had higher mean diffusivity, higher free water content, and more plasma volume compared with deep WMH (P<0.001, P=0.01, and P<0.001, respectively). No differences were observed in perfusion (P=0.94) and blood-brain barrier leakage (P=0.65) between periventricular and deep WMHs. The spatial analyses inside WMH and the adjacent white matter revealed a gradual gradient in white matter microstructure, free water content, perfusion, and plasma volume but not in blood-brain barrier leakage. We showed different pathophysiological heterogeneity of the 2 WMH types. Periventricular WMHs display more severe damage and fluid accumulation compared with deep WMH, whereas deep WMHs reflect stronger hypoperfusion in the lesion's core. URL: https://www.isrctn.com; Unique identifier: ISRCTN10514229.

Study on Microstructure and Corrosion Fatigue Resistance of 14Cr12Ni3Mo2VN Materials Based on the Composite Technology of High-Frequency Induction Quenching and Laser Shock Peening

This study investigated the microstructure, microhardness, and residual compressive stress of 14Cr12Ni3Mo2VN martensitic stainless steel treated with high-frequency induction quenching (HFIQ) and laser shock peening (LSP). Using rotating bending corrosion fatigue testing, the corrosion fatigue performance was analyzed. Results show that a microstructural gradient formed after HFIQ and LSP: the surface layer consisted of nanocrystals, the subsurface layer of short lath martensite, and the core of thick lath martensite. A hardness gradient was introduced, with surface hardness reaching 524 Hv0.1, 163 Hv0.1 higher than the core hardness. A residual compressive stress field was introduced near the surface, with a maximum residual compressive stress of approximately −575 MPa at a depth of 0.1 mm. Corrosion fatigue results indicate that cycle loading times of samples treated with HFIQ and LSP were 2.88, 2.04, and 1.45 times higher than untreated, HFIQ-only, and LSP-only samples, respectively. Transmission electron microscopy (TEM) characterization showed that HFIQ reduced the lath martensite size, while the ultra-high strain rate induced by LSP likely caused dynamic recrystallization, forming numerous sub-boundaries and refining grains, which increased surface hardness. The plastic strain induced by LSP introduced residual compressive stress, counteracting tensile stress and hindering the initiation and propagation of corrosion fatigue cracks.

The immiscible W/Cu composites with heterostructures and excellent bond strength prepared by additively manufactured transient liquid-phase direct bonding: Experiments and simulations

Immiscible metallic composites are a class of promising materials with unique physical properties and excellent comprehensive mechanical properties. However, obtaining high-performance immiscible composites through solid-phase direct/indirect bonding processes has been a great challenge due to the difficulty of bonding and high melting-point difference in immiscible metal/alloy systems, especially immiscible W–Cu systems with the highest positive formation heat and high melting point variations. This work confirms that high-strength bonding between immiscible and highly melting-point-differentiated metals can be achieved under highly non-equilibrium transient liquid-phase direct bonding. Specifically, not only the construction of transient liquid-phase interfaces without interlayers between W and CuCrZr was realized under highly non-equilibrium conditions of melting/solidification and extremely high cooling rates brought about by the laser powder bed fusion additive manufacturing, but also the surface nanocrystallization was simultaneously obtained at the interface, forming the W/CuCrZr composite material with heterogeneous grain size structure. The W/CuCrZr composite materials possessed a gradient microstructure of micron-sized equiaxed tungsten grains, nano tungsten grains and columnar Cu grains in sequence as well as having excellent interfacial bonding strength and a high effective bonding area ratio. By combining experiments and molecular dynamics simulations, it was clarified that the introduction of crystal defects and the formation of tungsten nanograins promote the diffusion of immiscible W and Cu and achieve strong bonding. Our work provides a new strategy and in-depth view into the fabrication of excellent-performance composite materials between immiscible metals/alloys.

Effects of ultrasonic shot peening process on the microstructure and mechanical properties of nickel-based superalloys formed by selective laser melting

Selective laser melting (SLM) is an attractive additive manufacturing technique for fabricating high-performance superalloys engineering components. Unfortunately, these as-built parts generally exhibit unsatisfied mechanical properties because of their natural thermal residual stress and non-equilibrium microstructures. Ultrasonic shot peening (USP) can effectively inhibit defect expansion and even close pores, thereby improving the mechanical properties of alloys. In this study, the internal defects, residual stress and microhardness redistribution, tensile properties, as well as microstructural response of the GH3230 alloy treated by different ultrasonic peening time are systematically studied. USP treatment induces a significant carbides fragmentation and synthesizes surface gradient heterogeneous structure, which displays a microstructure gradient in grain size, carbides size and dislocation density from surface to core. Long-time USP treatment induces work softening of GH3230 alloys. The work softening mechanism of the GH3230 alloys after long-time USP treatment is ascribed to the increase of surface temperature and high stored strain energy. Both the microhardness and residual compressive stress in the softened region of GH3230 alloys are reduced. Work softening effect increases the surface plasticity of the GH3230 sample, which induces deeper gradient structure. The various strengthening mechanisms of gradient heterogeneous structures, as well as the multiple effects of hetero deformation induced (HDI) hardening, and densification strengthening are responsible for achieving strength-ductility synergy. The research findings provide new insights based on USP for improving the mechanical properties of heterogeneous superalloys engineering components.

Elevating alloy performance: the role of bioactive glass in Mg-9Al-Zn matrix composites through friction stir back extrusion

ABSTRACT This study investigates the impact of friction stir back extrusion (FSBE) parameters on the microstructure and mechanical properties of magnesium matrix composites reinforced with bioactive glass particles. Higher rotational speeds (1600 rpm) generate more heat and stirring, promoting finer grains and better particle dispersion, while lower speeds lead to larger grains. Conversely, slower extrusion speeds allow more time for heat dissipation and better mixing, contributing to smaller grains and a more uniform particle distribution. Key findings show that increasing the bioactive glass content enhances both hardness and ultimate tensile strength (UTS), with optimal mechanical properties achieved at 1600 rpm rotational speed, 48.97 mm/min extrusion speed, and 5 vol.% bioactive glass. The introduction of bioactive glass particles restricts grain growth and improves load transfer, thereby reinforcing the composite. Additionally, the study identifies a gradient microstructure in the composite wire, with smaller grains near the surface due to higher plastic strain and temperature, in contrast to the core, which exhibits larger grains.

Fabricating gradient microstructure and fretting wear properties of an as-cast Ti-45Al alloy via warm shot peening and annealing treatment

A gradient microstructure was fabricated in the surface layer of an as-cast Ti-45Al alloy through warm shot peening (WSP) followed by annealing treatment, and its fretting wear properties were investigated. The results showed that the surface integrity of the alloy during WSP was more sensitive to temperature. The gradient microstructure encompassed not only the gradient distribution of grain morphology but also that of grain orientation. The intensity of the texture components on the surface was reduced, which improved the isotropy of the fretting wear properties. The gradient microstructure improved resistance to fretting damage by weakening delamination and abrasive wear. Moreover, a glazed-oxide layer formed exclusively in the gradient microstructure, and its formation mechanism was elucidated.

A novel strategy for preparing gradient grained Mg alloy by normal extrusion process

A novel strategy for fabricating gradient-grained Mg alloys, consisting of equiaxed ultrafine grains (UFG) and bimodal grains, has been developed through normal extrusion processing of the Mg-Mn binary alloy. The gradient-grained structure was created by a gradient strain field from a stepped structure in extrusion die. The gradient strain leads to the foundation for subsequent gradient grain nucleation and also prompts the occurrence of dynamic precipitates at the low angle grain boundaries, which is also distributed in a gradient manner and plays a vital role in impeding grain boundary migration. By combining gradient nucleation and pinning effect, a gradient microstructure is successfully achieved in the Mg-2.0Mn binary alloy using the normal extrusion process. The Mg-2.0Mn alloy with a gradient-grain structure, exhibits exceptional mechanical strength and ductility. The presence of the gradient structure effectively enhances the work-hardening rate, attributed to the synergistic effect of the ultrafine and bimodal grains.

Simultaneously achieving strength-ductility in additive-manufactured Ti6Al4V alloy via ultrasonic surface rolling process

Additively manufactured alloys have a great potential in engineering field, but there still have many issues to be addressed, i.e., the reliability, strength-ductility trade-off of manufactured parts. In this study, a treatment called ultrasonic surface rolling process (USRP) was utilized to achieve superior strength and ductility in the Ti6Al4V alloy prepared via electron beam melting (EBM). The treated additive-manufactured alloy obtained various gradient microstructures and excellent surface quality, as well as its mechanical properties were significantly improved. Especially, the USRP-3 specimen exhibited a high elongation of 17.1 ± 0.9 % and a good ultimate tensile strength of 1043 ± 5.0 MPa, which were both higher than those of untreated specimen; the fine laminated structure with a preferred orientation of (101‾0) direction and gradient structure promoted the hardening capacity and provided the rich dislocations sources, taking a better strength-ductility combination. In addition, the surface microhardness of the multi-pass processed specimen was markedly enhanced. However, excessive USRP treatment would induce micro-cracks in the nano-composite layer, resulting in a significant reduction in ductility. Therefore, appropriate USRP treatment is expected to expand the application range of additive-manufactured metallic materials.

Effect of microstructure and texture gradient on the backscattered ultrasound amplitude of Ti-6Al-4V bars for aeroengine blade

The microstructure and texture gradient from the edge to the center of the Ti-6Al-4V bars, which is related to the deformation modes and processes, affects the backscattered ultrasound amplitude significantly. In this paper, radial forging and rolling were used to process Ti-6Al-4V bars for aeroengine blade. The microstructure and texture of the edge and center areas, as well as the backscattered ultrasound amplitude, were systematically studied. The edge area of the radial forged bar with strong {0002}<10–10> texture corresponds to an abnormal increase in backscattered ultrasound amplitude, while the highly <10–10>//AD textured center area corresponds to an extremely low backscattered ultrasound amplitude. The abnormal increase in backscattered ultrasound amplitude was also observed in the edge of rolled bar, which has a texture similar to the center of radial forged bar. In industrial production, this abnormal increase of backscattered ultrasound amplitude often leads to exceeding the standards of titanium bars for blade. The relationship of process-texture-backscattered ultrasound amplitude was analyzed, and the mechanism of the abnormal increase of backscattered ultrasound amplitude was discussed. The results indicated that the backscattered ultrasound amplitude is related to the shape of the grains, the intensity and type of texture, and the formation of macrozones. Based on these researches, several methods were used to adjust the backscattered ultrasound amplitude of Ti-6Al-4V bars for aeroengine blade, which were effective and met the requirements of standards.

Obtaining Symmetrical Gradient Structure in Copper Wire by Combined Processing

Traditionally, structural wire is characterized by a homogeneous microstructure, where the average grain size in different parts of the wire is uniform. According to the classical Hall–Petch relationship, a homogeneous polycrystalline metal can be strengthened by decreasing the average grain size since an increase in the volume fraction of grain boundaries will further impede the motion of dislocations. However, a decrease in the grain size inevitably leads to a decrease in the ductility and deformability of the material due to limited dislocation mobility. Putting a gradient microstructure into the wire has promising potential for overcoming the compromise between strength and ductility. This is proposed a new combined technology in this paper in order to obtain a gradient microstructure. This technology consists of deforming the wire in a rotating equal-channel step die and subsequent traditional drawing. Deformation of copper wire with a diameter of 6.5 mm to a diameter of 5.0 mm was carried out in three passes at room temperature. As a result of such processing, a gradient microstructure with a surface nanostructured layer (grain size ~400 nm) with a gradual increase in grain size towards the center of the wire was obtained. As a result, the microhardness in the surface zone was 1150 MPa, 770 Mpa in the neutral zone, and 685 MPa in the central zone of the wire. Such a symmetrical spread of microhardness, observed over the entire cross-section of the rod, is a direct confirmation of the presence of a gradient microstructure in deformed materials. The strength characteristics of the wire were doubled: the tensile strength increased from 335 MPa to 675 MPa, and the yield strength from 230 MPa to 445 MPa. At the same time, the relative elongation decreased from 20% to 16%, and the relative contraction from 28% to 23%. Despite the fact that the ductility of copper is decreased after cyclic deformation, its values remain at a fairly high level. The validity of all results is confirmed by numerous experiments using a complex of traditional and modern research methods, which include optical, scanning, and transmission microscopy; determination of mechanical properties under tension; and measurement of hardness and electrical resistance. These methods allow reliable interpretation of the fine microstructure of the wire and provide information on its strength, plastic, and electrical properties.

Influence of the longitudinal welded joint microstructure of X52 and X70 large diameter pipes on resistance to hydrogen embrittlement

The article covers influence of a set of gradient microstructures of large diameter pipes (LDP) on resistance to hydrogen embrittlement. The object of the study were specimens of longitudinal welded joints of X52 and X70 LDP before and after slow strain rate tensile tests (SSRT) in gaseous nitrogen and hydrogen. It was found that fracture of X52 specimens during SSRT occurred with multiple crack initiation centres at the interfaces of the pearlite–ferrite structural constituents, while fracture of X70 specimens resulted in a single crack propagation. Steels Х52 and Х70 fractured not in the heat-affected zone (HAZ) recrystallisation area containing islands of martensite-austenite phase, but in the high-temperature tempering zone, which is characterised by anisotropic ferrite-perlite (Х52) and ferrite-bainite (Х70) microstructure. Finely dispersed uniform structure of bainite ferrite despite the content of martensite-austenite component is not a source of hydrogen crack initiation. Although a welded joint of X70 steel is characterised by higher strength, it is less susceptible to the embrittlement effects of gaseous hydrogen than X52, which is probably due to the finer and more uniform structure of X70 steel. Presence of ferrite-pearlite banding in the structure of X52 steel has a more negative effect on resistance to hydrogen embrittlement than increased hardness of the HAZ high-temperature tempering area of X70 steel.