Mechanical properties of 7475 aluminum alloy sheets with fine subgrain structure by warm rolling

The effect of transition elements on grain refinement of 7475 aluminum alloy sheets produced by warm rolling was investigated. The alloy which contains zirconium instead of chromium showed ultra fine structures with stable subgrains after warm rolling at 350 ℃, followed by solution heat treatment at 480 ℃. The average subgrain diameter was approximately 3 pan. It became clear that zirconium in solution has the effect of stabilizing subgrains due to precipitation of fine Al3Zr compounds during warm rolling. On the other hand, chromium-bearing compounds precipitate before warm rolling and they grow up to relatively large size during warm rolling. The warm rolled sheets with fine subgrains have unique properties compared with conventional 7475 aluminum alloy sheets produced by cold rolling. The warm roiled sheets solution heat treated had subgrain structures through the thickness with a high proportion of low-angle boundary less than 15°. The strength of the warm rolled sheets in T6 condition was about 10% higher than that of conventional 7475 aluminum alloy sheets. As the most remarkable point in the warm rolled sheets, the high Lankford （r） value of 3.5 was measured in the orientation of 45° to rolling direction, with the average r-value of 2.2. The high r-value would be derived from well developed r-fiber textures, especially with the strong {011 }（211） brass component. The warm rolled sheets also had high resistance to SCC. From Kikuchi lines analysis and TEM images, it was found that PFZs were hardly formed along the low- angle boundaries of the warm rolled sheets in T6 condition. This would be a factor to lead to the improvement of resistance to SCC because of reducing the difference in electrochemical property between the grain boundary area and the grain interior.

Variation in Deformation Behaviors Along the Transverse Direction During the Warm Rolling of a 1480-mm-Wide AZ31B Plate

A decrease in the weight of aerospace vehicles,large ships,weapons,and high-speed trains will increase the demand for wide-width magnesium alloy plates and their composite parts to replace steel and plastic.An investigation was conducted to study the variation in deformation behaviors along the transverse direction during the warm rolling of a 1480-mm-wide AZ31B plate.A uniaxial thermal compression test with a 59%reduction was performed at different positions on a 13.7-mm-thick rolled plate along the width direction at a temperature of 220℃ and 270℃ and strain rate of 15 s^(−1).At the same time,the 13.7-mm-thick plate was rolled in a single pass to 5.6 mm on a mill with a 1725-mm-wide roll to confirm the thermal deformation behavior and the dynamic recrystallization(DRX).The results show that the main texture type does not change and the grain size does not have a clear deflection when the magnesium alloy plate reaches a certain value under rolling accumulative reduction.The grain size of a 13.7-mm-thick plate increases with a decrease in the distance to the center layer in the thickness direction.In the width direction,the edge(R6)first decreases and then increases toward the symmetric plane(R1).The critical stress required for dynamic recrystallization in the transition zone R3 of the rolled plate width is minimum,and the average grain size is minimum owing to the relatively complete recrystallization.

Effect of Delay Time on Microstructural Evolution during Warm Rolling of Ti-Nb-IF Steel

The effect of delay time with constant first finishing pass temperature (800℃) has been investigated by means of multi-pass torsion tests on Ti-Nb-IF steel. All the tests have been carried out at a strain rate of 2 s-1 with 11 passes and 0.3 strain each pass. During the final pass, dynamic recrystallization occurs to a degree that depends on the delay time. In short interpass time (1 s) and at these temperatures (T≤800℃) there is not enough time to start static recrystallization, therefore, accumulation of strain occurs and after some passes, strain reaches a critical strain for starting dynamic recrystallization. In this study, the changes of mean flow stress during each pass and also the microstructural observation confirms that dynamic recrystallization occurs after some passes in ferrite phase of this steel. The stress-strain curves with constant temperature obtained by using a kinetic model and compensation of the increasing mean flow stress with decreasing temperature. Thus, this result also confirms that dynamic recrystallization occurs in warm rolling of this IF steel.

Effect of warm rolling process on microstructures and tensile properties of 10 Mn steel

The influence of warm rolling processes on the microstructures and tensile properties of 100 Mn steel was studied.Strength appeared to increase with the rolling temperature but strengthening mechanisms varied.The increase of warm rolling temperature from 250℃ to 600℃ leads to enhanced recrystallization in martensite during the intercritical annealing(IA) at 620℃ for 5 h.As a result,both ultimate tensile strength(UTS) and total elongation(TE) increase.However,the size of relatively coarse recrystallized austenite grains and the resultant yield strength(YS) remain almost constant in this temperature range.The further increase of rolling temperature to 700-800℃ causes a considerable amount of pearlite to be formed during the IA,and then martensite is formed after the IA,resulting in dramatical increases in both YS and UTS but at the great loss of ductility.The warm rolling at 600℃ with 63% thickness reduction can produce the steel with the best mechanical combination of 1.2 GPa UTS and 35% TE,due to the formation of many ultrafine austenite grains and strain-induced cementite precipitates.This demonstrates that the mechanical combination of non-V-alloyed medium Mn steel can be improved to an equivalent level of 0.7% V alloyed 10 Mn steel just via the economic strain-induced cementite precipitation.

Microstructure evolution and mechanical properties of 316L austenitic stainless steel with aluminum addition by warm rolling

Microstructure evolution and mechanical properties of 316L austenitic stainless steel with aluminum addition by warm rolling at 550 ℃ were investigated. It is found that sample is composed of an ashen austenite matrix, a gray black ferrite phase and a small number of NiCx. The average grain sizes are 21.62, 19.66 and 19.49 μm for samples with the rolling deformation of 30%, 50% and 70%, respectively. The yield strength and tensile strength of samples with solid solution time of 30 min and deformation of 70% are higher. The fracture modes are similar and belong to toughness fracture. The fracture surfaces of the samples are composed of relatively large equal-axis ductile dimples （5-15 μm） and fine scattered ones around the dimples （〈 5 μm）. As the rolling deformation increases, the quantity of subgrain boundary increases and the 〈 101 〉 orientation is more prominent. {001 } 〈 110 〉 rotation-cube textures are present in ferrite phase of samples and weak Goss texture is formed in austenite pole images.

Effect of Warm Rolling on Micro-deformation Behavior and Mechanical Properties of Columnar-grained Fe-6.5 mass%Si Alloy

Micro-deformation behavior and mechanical properties of columnar-grained Fe-6.5 mass%Si alloy before and after warm rolling were investigated by means of micro-indentation and three-point bending tests.The results show that the columnar-grained Fe-6.5mass%Si alloy before warm rolling presents sink-in mode of micro-indentation,while pile-up mode with a number of arc-shaped deformation bands exists in the warm-rolled alloy.Compared with that of the alloy before warm rolling,the maximum bending fracture stress and maximum bending fracture deflection of the warm-rolled alloy are increased by 96% and 50%,respectively.The different micro-deformation behavior and mechanical properties of the columnar-grained Fe-6.5mass%Si alloy are ascribed to the changes of dislocation density,dislocation configuration and long-range order degree,which significantly improve the room temperature plasticity of the alloy after warm rolling.

Effect of rolling temperature on microstructure and texture of twin roll cast ZK60 magnesium alloy

Twin roll cast ZK60 alloy strip/sheet with final thickness of 0.5 mm was prepared, and effect of rolling temperature on microstructure and texture development was investigated using OM and XRD technique, microstructure and texture were measured on specimens subjected to rolling experiment at different rolling temperature, and macrotexture was also evaluated by X-ray diffraction method. In addition, the （1010）and （0002） pole figures were measured, and the tensile test was performed to reveal the influence of rolling temperature on mechanical properties. The results show that the microstrucmre of ZK60 alloy sheet consisted of fibrous structure with elongated grains, and shear bands along the rolling direction after warm rolling. Dynamic recrystallization could be found during the warm rolling process at rolling temperature 350℃ and above. And many fine recrystallized grain could be observed in the shear bands area. It is a little difficult to see the recrystallized grain in the sheet warm rolled at 300℃ because of higher density of shear bands. The warm rolled ZK60 alloy sheet exhibited strong （0002） pole texture, the intensity of （0002） pole figure decreases with the increasing of rolling temperature and the basal pole tilted slightly to the transverse direction after warm rolling.

Effect of whisker alignment on microstructure,mechanical and thermal properties of Mg-SiC_(w)/Cu composite fabricated by a combination of casting and severe plastic deformation(SPD)

In this research,microstructure evaluation,mechanical properties and thermal conductivity of the Mg-SiC_(w)/Cu composite with laminar structure were investigated.For this purpose,SiC whiskers were added to magnesium alloy by using stir-casting,then the Mg-SiC_(w)composite was bonded to copper layers by warm accumulative roll bonding(ARB).Based on the results of optical microscopy(OM)and scanning electron microscopy(SEM),SiC whiskers were well distributed in the magnesium matrix and they were aligned parallelly when the composites were plastically deformed at higher rolling passes.Furthermore,all layers remained continuous with localized necking sites.Also,no intermetallic compounds and phases were detected by XRD and EDS analyzes.Apart from the significant effect of severe plastic deformation on mechanical properties,the findings of mechanical tests point to the usefulness of reinforcements in improving up to 60%microhardness,Young’s modulus,yield,and up to 41%tensile strengths.Further,thermal conductivities of composites increased by adding reinforcement and above all by increasing the number of rolling passes.This growth is attributed to the higher thermal diffusivity of copper and whiskers as well as the increased number of conductive layers within composite.

High performance nano-structured stainless steel sheet

High-strength steels have been attracting more and more attention of people,Unfortunately.deterioration of ductility limited their applications.To solve this problem,a nano-structured stainless steel sheet is developed to combine high strength and high ductility.Processing of the surface mechanical attrition treatment(SMAT) was introduced to obtain a nano-grain layer on the double surface of the stainless steel sheet.The microstructure of the nanostructured steel sheet is characterized by an alternate distribution of coarse grained layer and nanocrystalline layer.Then the dual surface nano-crystallized stainless steel sheets were co-warm rolled at 500℃.The experimental results reveal that the mechanical properties of the nanostructured steel exhibit high yield strength in the range of 700 -950 MPa and tensi le strength higher than 930 MPa.Moreover,elongation to fracture reaches to 15%-48%, together with a uniform elongation stabilized to 13%-45%.

Achieving higher strength in Cu–Ag–Zr alloy by warm/hot rolling

High-strength Cu-3Ag-0.5Zr alloy plates were produced by multi-pass rolling in the temperature range of 500-800 ℃. An increase in strength was observed by rolling in the aforementioned range without significant loss in ductility. All the rolled samples show higher strength than solution-treated and aged samples. The maximum strength was observed for plates rolled at 500 ℃ with a yield strength and ultimate tensile strength of 311 and 385 MPa, respectively, and retaining a ductility of 23 %. Transmission electron microscopy （TEM） studies showed uniform distribution of fine silver precipitates and high dislocation density in the rolled samples. Nevertheless, the size of precipitates and dislocation density varied with the rolling temperature. The superior strength achieved in the rolled samples is attributed to grain refinement, dislocation strengthening, and precipitation hardening. This method can be employed to produce high-strength plates of pre- cipitation hardenable copper alloys.

Analyse of Warm Surface Rolling on the Grooved Steel Axle

Warm surface rolling is a working process between room temperature and re-crystallization temperature.With warm surface rolling of steel grade 45 grooved axle,its fatigue lifespan was measured by the endurance bending test.The influence of surface rolling reduction on the axle fatigue life period was experimentally studied at different surface rolling temperatures.The experimental results show that the fatigue life of the steel axles can be significantly improved by the warm surface rolling process.The optimum rolling reductions for the maximum fatigue life at different warm surface rolling temperatures were explored.The microstructures of the steel axles were analyzed.The surface strength improved by refining grains after the warm surface rolling was calculated by the Hall-Petch model.These research achievements could be also valuable to the relevant works.

Effect of Post Cryorolling Treatments on Microstructural and Mechanical Behaviour of Ultrafine Grained Al-Mg-Si Alloy

To investigate the effect of post cryorolling treatments on simultaneous enhancement in strength and ductility of ultrafine grained material （UFG）, AI 6061 alloy was subjected to cryorolling followed by warm rolling （CR ＋ WR） and compared with cryorolling followed by short annealing （CR ＋ SA） at the same temperature. Transmission electron microscopy （TEM） was used to characterize the microstructural features of the processed material. The mechanical properties were investigated through Vickers hardness testing and tensile testing at room temperature. TEM, X-ray diffraction （XRD） and differential scanning calorimetry （DSC） were used to investigate the precipitation evolution in UFG material. Results indicated that the alloy subjected to CR ＋ WR has shown improved mechanical properties （114 HV, ultimate tensile strength （UTS）： 350 MPa） as compared to that in the case of CR ＋ SA （105 HV, UTS： 285 MPa）. The size of the precipitates observed in CR ＋ WR sample after peak ageing treatment is finer than that of peak aged CR ＋ SA sample. The UTS of peak aged CR ＋ WR sample （UTS： 390 MPa） was found to be higher than that of peak aged CR ＋ SA sample （UTS： 355 MPa）, without decrease in ductility.

Effect of Warm-Rolled Pearlite Microstructural Features on Austenitic Transformation

The austenite transformation characteristics for various warm-rolled pearlite during rapid heating were investigated. The results indicate that the start temperature （Ts） is sensitive to the microstructural feature of pearlite, whereas the dislocation plays an important role in the transformation rate; at the same time, the uniformity of austenite grains is more or less affected by the amount of spheroidized pearlite. A critical effect on the state of austenite grain is created through the influence of initial microstructures on the start temperature of transformation.

形貌优化提高富TRIP效应中锰钢的抗氢脆性能

相变诱发塑性(TRIP)效应通常可显著提高中锰钢的加工硬化能力,从而获得出色的强度和塑性组合.然而,变形过程中生成的新鲜马氏体极易发生氢脆,是新型高强中锰钢开发的瓶颈问题.我们在此提出了一种长带状形貌来减轻新鲜马氏体引起的氢脆.温轧制备的长带状中锰钢相比传统临界退火制备的等轴晶形貌中锰钢,不仅具有同等的高加工硬化率,而且抗氢脆性能更好.等轴晶形貌中锰钢由于缺乏有效的阻碍机制,氢裂纹一旦出现便能快速扩展.相反,对于长带状形貌中锰钢,氢裂纹会被晶界和相界所阻碍或偏转.本形貌设计可以提高其他富TRIP效应的钢和合金的抗氢脆性能.

Strengthening and ductilization of laminate dual-phase steels with high martensite content

The steels with excellent strength and ductility are expected to be achieved by tailoring the microstructural features.In this work,laminate dual-phase(DP)steels with high martensite content(laminate HMDP steels)were produced by a combination of warm rolling and intercritical annealing.Influence of rolling strain and annealing temperature on the microstructural evolution and mechanical properties of laminate HMDP steels were systematically studied.The strength of HMDP steels was significantly improved to~1.6 GPa associated with a high uniform elongation of 7%,as long as the laminate structure is maintained.The strengthening and ductilizing mechanisms of laminate HMDP steels are discussed based on the influence of laminate structure and the high martensite content,which promote the development of internal stresses and can be correlated to the Bauschinger effect as measured by the cyclic loadingunloading-reloading experiments.Detailed transmission electron microscopy(TEM)observation was applied to characterize the dislocation structure in the deformed ferrite.

Dependence of tensile properties on microstructural features of bimodal-sized ferrite/cementite steels

A medium-carbon steel was processed through different warm rolling techniques,and the microstructural features with bimodal grain size distribution were found to be different.The combination of strength and ductility was ameliorated in the steel processed through warm rolling characterized by biaxial reduction.The enhanced strength is attributed to the densely distributed fine intragranular cementite particles and the small grain size in the coarse grain regions.The enhanced uniform elongation is due to the improved work hardening behavior at the large-strain stage.This work hardening behavior is predominantly ascribed to the finely dispersed intragranular particles.The relatively small grain size with nearly equiaxed shape in the coarse grain regions helps stabilize the uniform deformation to a large strain.

Alloy design by dislocation engineering

Ultra-high strength alloys with good ductility are ideal materials for lightweight structural application in various industries. However, improving the strength of alloys frequently results in a reduction in ductility, which is known as the strength-ductility trade-off in metallic materials. Current alloy design strategies for improving the ductility of ultra-high strength alloys mainly focus on the selection of alloy composition （atomic length scale） or manipulating ultra-fine and nano-grained microstructure （grain length scale）. The intermediate length scale between atomic and grain scales is the dislocation length scale. A new alloy design concept based on such dislocation length scale, namely dislocation engineering, is illustrated in the present work. This dislocation engineering concept has been successfully substantiated by the design and fabrication of a deformed and partitioned （D＆P） steel with a yield strength of 2,2 GPa and an uniform elongation of 16%. In this D＆P steel, high dislocation density can not only increase strength but also improve ductility. High dislocation density is mainly responsible for the improved yield strength through dislocation forest hardening, whilst the improved ductility is achieved by the glide of intensive mobile dislocations and well-controlled transformation-induced plasticity （TRIP） effect, both of which are governed by the high dislocation density resulting from warm rolling and martensitic transformation during cold rolling. In addition, the present work proposes for the first time to apply such dislocation engineering concept to the quenching and partitioning （Q＆P） steel by incorporating a warm rolling process prior to the quenching step, with an aim to improve simultaneously the strength and ductility of the Q＆P steel. It is believed that dislocation engineering provides a new promising alloy design strategy for producing novel strong and ductile alloys.

Tuning Cr-rich nanoprecipitation and heterogeneous structure in equiatomic CrFeNi medium-entropy stainless alloys

High-/medium-entropy stainless alloys(HESAs/MESAs)are a new kind of alloys with great potential to combine excellent properties from high-/medium-entropy alloys(HEAs/MEAs)and stainless steels.A CrFeNi MESA was chosen to investigate its microstructures and mechanical behaviors.After homogenization,the strength and ductility of CrFeNi MESAs with single-phase face-centered-cubic(fcc)structure were higher compared with those of Fe_(100−x-y)Cr_(x)Ni_(y)austenitic stainless steels.Cr-rich body-centered-cubic(bcc)precipitates and heterogeneous structure were introduced by cold rolling and annealing at 800℃.Rolling at 700℃ results in higher dislocation density and the occurrence of lamellar Cr-rich bcc precipitates.High-density dislocations and fcc grains with heterogeneous structure,together with Cr-rich bcc precipitates,contribute to a yield strength improvement of about 50 MPa,and appreciable tensile yield strength of~540 MPa and fracture strain of~20%are obtained.It reveals that not only compositional variations but also grain size and phase structure tuning can be utilized for achieving desired mechanical properties.