Rotationally Accelerated Shot Peening Research Articles

Improvement of plasma carbonitriding modified layer on TA15 surface by RASP-assisted DGPSA treatment

In this paper, a dual process combining rotationally accelerated shot peening (RASP) and double glow plasma surface alloying (DGPSA) was designed to prepare Ti(C,N) modified-layer on titanium alloys. The improvement effect of RASP pretreatment on DGPSA technology was analyzed by comparing the surface phase composition, wear resistance and fractography of plasma-carbonitriding (PCN) layers with and without RASP pretreatment. It was found that RASP pretreatment promoted the diffusion and deposition of carbon and nitrogen atoms on the surface of titanium alloy, forming a thick and dense modified layer with a gradient variation in hardness. There is a large residual compressive stress on the surface of the modified layer after RASP-assisted DGPSA treatment. The near-surface fracture morphology indicates that RASP pretreatment can alleviate the adverse effects of surface brittleness and severe grain coarsening caused by heat treatment and improve the surface mechanical properties. The gradient reinforced layer formed by the dual treatment on the subsurface effectively supports the dense compound layer on the surface, resulting in excellent wear resistance.

Formation of nano-grains dominated by twin-twin intersection for a RASP-processed 316L stainless steel

In this work, the formation mechanism of surface nano-grains was explored for a 316L stainless steel processed by a rotationally accelerated shot peening (RASP). The microstructural evolution below the processed surface was observed to reveal the refining behavior during the continuous impact deformation. The results showed that deformation ultrafine-twins were easily generated by one high-speed impact with the strain rate of ∼102–103s−1. As it was impacted by the second ball, ultrafine-twin-twin intersections were appeared in the coupled domain, which was attributed to the accumulated strain induced by twice impacts, activating the bidirectional twinning. Twin–twin intersection can effectively divide the coarse-grains into smaller structures. When the impact number further increased, the average thickness of deformation twins decreased, and resulted in segmentation of original structures. Eventually, nano-grains were formed by nano-twin-twin intersections in the topmost surface. The average grain size of nano-grains is ∼30 nm. The current work clearly exhibited the surface microstructural evolution and revealed that twin-twin intersection was the dominated nanocrystallization mechanism for RASP-processed 316L stainless steel.

Achieving high hetero-deformation induced (HDI) strengthening and hardening in brass by dual heterostructures

• A dual heterostructured brass with superior combination of strength and ductility has been fabricated. • High hetero-deformation induced (HDI) strengthening and hardening is achieved by the design of the microstructure. • Extra interfaces generated by the dual heterostructure become the reason for the high HDI strengthening and hardening. Heterostructured materials have a superior combination of strength and ductility, due to their ability to produce hetero-deform induced (HDI) strengthening and hardening. Therefore, achieving high HDI strengthening and hardening is the primary goal for designing heterostructures. Here we report a dual heterostructure in brass that consists of the heterogeneous lamella and gradient structure, fabricated by rolling, partial annealing, and rotationally accelerated shot peening (RASP). The dual heterostructures are able to generate extra interfaces of heterogeneity compared to the single heterostructures, which could lead to higher HDI strengthening and hardening, and a more superior combination of strength and ductility. This finding presents a new pathway to designing heterostructures in metallic materials.

Effect of shot peening pretreatment on the high-temperature tribological behaviours of a TaN coating prepared via double-cathode glow plasma alloying

Gradient coatings with excellent physical and mechanical properties show great potential for use in engineering applications. In this study, a TaN gradient-modified layer was prepared on TA15 titanium alloy by double-cathode glow plasma alloying (DGPA) with a gradient structure pretreated by rotationally accelerated shot peening (RASP). The effects of the RASP pretreatment at different speeds on the mechanical and tribological properties and microstructure of the TaN coatings were investigated. The results indicated that the crystal size was refined to the nanometre scale (41.8–71.4 nm) by RASP pretreatment, and a gradient structure layer (30–55 μm) formed, which facilitated the adsorption and inward diffusion of Ta and N atoms. The pretreated TaN-modified layer contained an interdiffusion layer with a thin deposition layer consisting of Ta 2 N and TaN. Furthermore, the pretreated TaN-modified layer maintained good wear resistance in ball-on-disc testing at 700 K, and the wear rate was only 36.9% of that without pretreatment, which can be attributed to the high hardness, elastic modulus , toughness and thermal stability of the modified layer. This work provides insights relevant to the development of a mechanical-diffusion composite process to produce titanium alloy coatings with enhanced mechanical properties and wear resistance at elevated temperature. • TaN modified layer with a gradient change of composition was prepared on titanium alloy using RASP and DGPA. • The refined surface promotes the adsorption and diffusion of Ta and N atoms to obtain a thicker gradient-modified layer. • RASP pretreatment can improve the wear resistance of TaN modified layers at high temperatures.

Effects of surface nanocrystallization on the oxide film formed on 316LN stainless steel in a high‐temperature aqueous environment

AbstractA newly developed technology, rotationally accelerated shot peening (RASP), was used to produce the nanocrystals on the surface of 316LN stainless steel. The effect of surface nanocrystallization on the oxide film properties of the steel in a high temperature and high‐pressure aqueous environment was studied. It was found that the outer layer of the oxide film consisted of NiFe2O4 with a loose spinel structure. The inner layer distributed tiny Cr2O3 particles compactly. The oxide films that grew on the RASP‐treated 316LN showed thicker and denser with higher corrosion resistance than on the as‐received steel. The nanostructures produced by RASP enhanced the formation of the superior oxide film. However, the deformation defects caused by RASP were harmful to the corrosion resistance. The results in this paper provide an easy way to enhance the corrosion resistance of the steel served in a high‐temperature aqueous environment.

Significance of surface layer integrity for sustaining the ductility of gradient-structured nickel

Gradient-structured nickel (Ni) was processed by rotationally accelerated shot peening (RASP). The microstructures and mechanical properties of the RASP-processed samples were studied. Nanostructured layers with a thickness of ~50 μm were formed on the sample surfaces. Beneath the surface layer there is a transition region in the depth range of ~50–150 μm. The transition region contains high densities of dense dislocation walls (DDWs), microbands (MBs), sub-grain boundaries and dislocations, and is susceptible to plastic instability. High-speed RASP processing created some small cracks which deteriorate the integrity of the nanostructured surface layer. During tensile deformation, local stress concentrations are created beneath the surface cracks to trigger plastic instability in the nearby transition region. As a result, interior cracks nucleated and grew in the transition region, and eventually agglomerated with surface cracks to cause premature failure of the bulk material.

Hierarchical grain size and nanotwin gradient microstructure for improved mechanical properties of a non-equiatomic CoCrFeMnNi high-entropy alloy

This study explored a multi-mechanism approach to improving the mechanical properties of a CoCrFeMnNi high-entropy alloy through non-equiatomic alloy design and processing. The alloy design ensures a single-phase face-centered cubic structure while lowering the stacking fault energy to encourage the formation of deformation twins and stacking faults by altering the equiatomic composition of the alloy. The processing strategy applied helped create a hierarchical grain size gradient microstructure with a high nanotwins population. This was achieved by means of rotationally accelerated shot peening (RASP). The non-equiatomic CoCrFeMnNi high-entropy alloy achieved a yield strength of 750 MPa, a tensile strength of 1050 MPa, and tensile uniform elongation of 27.5%. The toughness of the alloy was 2.53 × 1010 kJ/m3, which is about 2 times that of the same alloy without the RASP treatment. The strength increase is attributed to the effects of grain boundary strengthening, dislocation strengthening, twin strengthening, and hetero-deformation strengthening associated with the heterogeneous microstructure of the alloy. The concurrent occurrence of the multiple deformation mechanisms, i.e., dislocation deformation, twining deformation and microband deformation, contributes to achieving a suitable strain hardening of the alloy that helps to prevent early necking and to assure steady plastic deformation for high toughness.

Effect of rotationally accelerated shot peening on the microstructure and mechanical behavior of a metastable β titanium alloy

Microstructural evolution and deformation mechanism of a metastable β alloy (Ti-10V-2Fe-3Al) processed by rotationally accelerated shot peening (RASP) were systematically investigated with optical microscopy, X-ray diffraction, electron backscatter diffraction and transmission electron microscopy. Different gradient hierarchical microstructures (gradients in α″ martensite and β phase, and hierarchical twins range from the nanoscale to microscale) can be fabricated by RASP via changing the shot peening time. The hardening behavior and tensile mechanical properties of gradient hierarchical microstructure were systematically explored. Novel deformation twinning systems of {112}α″ and {130}<310>α″ in the kinked α″ martensite were revealed during the tensile deformation. It was found that stress-induced martensitic transformation, twinned α″ martensite and the related dynamic grain refinement contribute to hardness and work hardening ability. Simultaneous improvement of strength and ductility of the metastable β titanium alloy can be achieved by introducing a gradient hierarchical microstructure.

Continuous multi-cycle nanoindentation behavior of a gradient nanostructured metastable β titanium alloy fabricated by rotationally accelerated shot peening

A novel gradient nanostructured structure fabricated by rotationally accelerated shot peening (RASP) has been proven useful in the enhancement of strength and ductility. In this study, a series of nanoindentation tests using continuous multi-cycle (CMC) mode are first used to characterize the nanoindentation behavior of the RASP-processed metastable β titanium alloy with a gradient nanostructured structure. The experimental results indicate that indentation depth and grain size paly a dominant role in the nanoindentation behavior. Inverse indentation size effect (ISE) on the hardness occurs at shallow indentation depth (<50 nm), which is due to the hardening behavior caused by the effect of grain boundaries. While ISE occurs at the indentation depth ranging from 50 nm to 225 nm, then ISE disappears in deep indentation depth (>225 nm), which is dominated by the known strain gradient mechanism. The hardness, reduced module and back stress decrease from the treated surface with a nanostructured grain size of 20 nm to the center zone with a coarse grain size of 216 μm. For the RASP-processed sample, the fine grain zone exhibits a more pronounced inverse and normal ISEs than these of the coarse grain zone. In addition, the Bauschinger effect and back stress strengthening mechanism are also firstly used to explain the variation of gradient micro-mechanical properties.

Effective Surface Nano-Crystallization of Ni2FeCoMo0.5V0.2 Medium Entropy Alloy by Rotationally Accelerated Shot Peening (RASP).

The surface nano-crystallization of Ni2FeCoMo0.5V0.2 medium-entropy alloy was realized by rotationally accelerated shot peening (RASP). The average grain size at the surface layer is ~37 nm, and the nano-grained layer is as thin as ~20 μm. Transmission electron microscopy analysis revealed that deformation twinning and dislocation activities are responsible for the effective grain refinement of the high-entropy alloy. In order to reveal the effectiveness of surface nano-crystallization on the Ni2FeCoMo0.5V0.2 medium-entropy alloy, a common model material, Ni, is used as a reference. Under the same shot peening condition, the surface layer of Ni could only be refined to an average grain size of ~234 nm. An ultrafine grained surface layer is less effective in absorbing strain energy than a nano-grain layer. Thus, grain refinement could be realized at a depth up to 70 μm in the Ni sample.

Achieving High Hetero-Deformation Induced (HDI) Strengthening and Hardening in Brass by Dual Heterostructures

Heterostructured materials have a superior combination of strength and ductility, due to their ability to produce hetero-deform induced (HDI) strengthening and hardening. Therefore, achieving high HDI strengthening and hardening is a goal for designing heterostructures. Here we report a dual heterostructure in brass that includes heterogeneous lamella and gradient structures, fabricated by rolling, partial annealing and rotationally accelerated shot peening (RASP). Extraordinary HDI strengthening and hardening were achieved by the dual heterostructures, which led to a more superior combination of strength and ductility, than the single heterostructures. This finding presents a new pathway to designing heterostructures in metallic materials.

Layer-by-layer corrosion behavior of 316LN stainless steel with a gradient-nanostructured surface

A 316LN stainless steel with a gradient-nanostructured surface layer was fabricated by rotationally accelerated shot peening (RASP). The thickness of this gradient layer was about 210 μm. Layer-by-layer electrochemical experiments confirmed that the layer of the gradient steel with the best corrosion resistance is found at a distance of 110 μm from the surface. The nanostructures and pits generated by RASP produced two opposing effects on its corrosion resistance: the nanocrystals and nano-twins facilitate the formation of a thick, compact passive film which resists corrosion, whereas the pits tend to destroy it. The passive films formed on the pits and on the smoothly nanostructured areas were 5.5 nm and 13.9 nm thick, respectively.

Enhanced Corrosion Resistance of SA106B Low-Carbon Steel Fabricated by Rotationally Accelerated Shot Peening

The corrosion resistance of a SA106B carbon steel with a gradient nanostructure fabricated by rotationally accelerated shot peening (RASP) for 5, 10, 15 and 20 min was investigated. Electrochemical tests were carried out in the 0.05 M H2SO4 + 0.05 M Na2SO4 and 0.2 M NaCl + 0.05 M Na2SO4 solutions. The experimental results showed that the sample RASP-processed for 5 min exhibited the best corrosion resistance among them. TEM analysis confirmed that the cementite dissolution and formation of nanograins, which improved the corrosion resistance of the steel. Prominent micro-cracks and holes were produced in the samples when the RASP was processed for more than 5 min, resulting in the decrease of corrosion resistance.

Improved corrosion resistance of 316LN stainless steel performed by rotationally accelerated shot peening

Rotationally accelerated shot peening (RASP) technology was used to produce nanocrystallines and twins on the surface of 316LN stainless steel. Electrochemical corrosion experiments were carried out on the RASP-processed 316LN SS specimens in the 3.5 wt% NaCl solution. Improved passivity and better electronic properties of passive films were observed on the RASP-processed specimens. The experimental results showed that the structure with nanocrystallines and a high density of twins produced by the RASP process can provide more nucleation sites, adhesion work and homogeneous capillary force to improve the uniformity and compactness of the passive film, resulting in a better corrosion resistance.

Slip, twinning and twin-twin interaction in a gradient structured titanium

Quantitative analysis on the deformation mechanisms of a rotationally accelerated shot peening (RASP) processed gradient structured Ti was studied by using electron back-scattering diffraction technique. A 2000 μm thick gradient structure was introduced from the strain-free center to the treated surface. The low angle grain boundary fraction evolution showed that, with decreasing distance from the surface, dislocation generated continuously in the core region, then became saturated in the twin transition region, and finally decreased in the nano-structured region. Three twinning systems were identified: 112¯1, 101¯2 extension twin (ET) and 112¯2 compression twin (CT). The volume fraction of 101¯2 ET increased firstly and then decreased when approached to the surface, while 112¯2 CT showed an opposite tendency. The volume fraction of 112¯1 ET decreased gradually form the center to the surface. Both ET ⟷ ET and CT ⟷ CT twin-twin interactions were found to accommodate the strain and result in grain refinement. Glissile or sessile dislocations could be generated as a result of twin-twin interactions. With twin-twin interactions proceeding continuously, twin lamellae became narrower and even de-twinning was triggered. Due to different grain refinement levels, a hardness gradient was also found in RASP-processed Ti.

Microstructures and Mechanical Properties of a Gradient Nanostructured 316L Stainless Steel Processed by Rotationally Accelerated Shot Peening

Rotationally accelerated shot peening (RASP) is an innovative surface nanocrystallization technique, which can effectively produce gradient nanostructured layers on bulk metallic materials. In current work, the microstructural evolution and mechanical properties of the RASP‐processed 316L stainless steels are systematically explored. The results show that deformation twinning plays a significant role in fabricating nano‐grains. At the depth of ≈200 µm, unidirectional parallel mechanical twins are observed as the dominated deformation activity in each grain. With the decrease of depth (to ≈100 µm), twin‐twin intersections occur under a higher strain rate, which divide coarse grains into finer blocks. Eventually, nano‐grains form via grain rotation and grain boundary sliding at the topmost surface. Deformation twinning is still acted as an operative grain refinement mechanism in the nano‐grains. Importantly, the strength of the treated 316L stainless steel is improved while the ductility is decent. The improved strength is obtained from grain refinement, a high density of dislocations, deformation twins, and strain‐induced martensitic phase. For the good ductility, it is ascribed to the existence of slightly deformed coarse grains, ultrafine/nano‐twins and strain gradient.

Microstructures and Mechanical Properties of Commercially Pure Ti Processed by Rotationally Accelerated Shot Peening.

Gradient structured materials possess good combinations of strength and ductility, rendering the materials attractive in industrial applications. In this research, a surface nanocrystallization (SNC) technique, rotationally accelerated shot peening (RASP), was employed to produce a gradient nanostructured pure Ti with a deformation layer that had a thickness of 2000 μm, which is thicker than those processed by conventional SNC techniques. It is possible to fabricate a gradient structured Ti workpiece without delamination. Moreover, based on the microstructural features, the microstructure of the processed sample can be classified into three regions, from the center to the surface of the RASP-processed sample: (1) a twinning-dominated core region; (2) a “twin intersection”-dominated twin transition region; and (3) the nanostructured region, featuring nanograins. A microhardness gradient was detected from the RASP-processed Ti. The surface hardness was more than twice that of the annealed Ti sample. The RASP-processed Ti sample exhibited a good combination of yield strength and uniform elongation, which may be attributed to the high density of deformation twins and a strong back stress effect.

Microstructural evolution and mechanical properties of a 5052 Al alloy with gradient structures

Abstract

Gradient Structured Copper by Rotationally Accelerated Shot Peening

A new technology—rotationally accelerated shot peening (RASP), was developed to prepare gradient structured materials. By using centrifugal acceleration principle and large steel balls, the RASP technology can produce much higher impact energy compared to conventional shot peening. As a proof-of-concept demonstration, the RASP was utilized to refine the surface layer in pure copper (Cu) with an average grain size of ~85 nm. The grain size increases largely from surface downwards the bulk, forming an 800 μm thick gradient-structured surface layer and consequently a micro-hardness gradient. The difference between the RASP technology and other established techniques in preparing gradient structured materials is discussed. The RASP technology exhibits a promoting future for large-scale manufacturing of gradient materials.