Influence of fine-grained structure and superplastic deformation on the strength of aluminium alloys

Samples of 1560 (Al-Mg-Mn) and 1960 (Al-Zn-Mg-Cu) alloys have been used to investigate the nature of the effect of grain size and superplastic treatment on the strength of aluminium alloys. The observed increase in the work needed for crack formation with the transition from coarse-grained (CG) to fine-grained (FG) structure is connected to a greater homogeneity of the plastic deformation in the material volume. This leads to a reduction in local stress concentrations at the sites of preferential crack initiation. The easier crack growth in FG alloys is mainly caused by a reduction in the energy for plastic deformation at the head of a long crack and also for the formation of free fracture surfaces.Deceased.     

Annealing Effect on Wear Resistance of Nanostructured 316L Stainless Steel Subjected to Dynamic Plastic Deformation

Bulk nanostructured 316L austenitic stainless steel(SS) samples with nano-scale twin bundles embedded in nano-sized grains were synthesized by using dynamic plastic deformation(DPD).Subsequent thermal annealing of the as-DPD sample leads to a single austenitic structure with static recrystallized(SRX) grains in nanostructured matrix.Oil-lubricated sliding tests of ball-on-disc type were carried out for the as-DPD and the as-annealed DPD steel samples in comparison with coarse grained(CG) steel samples.Experimental results show that the as-DPD 316L steel exhibits a little enhanced wear resistance under a load of 10 N,and nearly identical wear resistance under a load of 30 N relative to that of the CG sample.After annealing,the wear resistance roughly follows the Archard equation under a load of 10 N.However,the wear resistance increases with increasing hardness,and decreases with a further increase in hardness under a load of 30 N.The highest wear resistance can be found in the DPD sample annealed at 750 C for about 20 min,which is more than 46% higher than that of the CG steel sample.This phenomenon is originated from the microstructure with an optimized combination of strength and ductility as a result of moderate plastic deformation in SRX grained regions.     

High strain rate superplasticity in a nano-structured Al–Mg/SiCP composite severely deformed by equal channel angular extrusion

High strain rate superplastic deformation potential of an Al–4.5%Mg matrix composite reinforced with 10% SiC particles of 3 μm nominal size was investigated. The material was manufactured using powder metallurgical route and mechanical alloying which was then processed by equal channel angular extrusion (ECAE). The composite showed a high resistance to static recrystallization. The manufacturing operations atomized SiC particles to nanoscale particles and the severe plastic deformation process resulted in a dynamically recrystallized microstructure with oxide dispersoids distributed homogeneously throughout the matrix. These particles stabilized the ultra-fine grained microstructure during superplastic (SP) deformation. Testing under optimum conditions at constant strain rates led to tensile elongations >360%, but it could be further increased by control of the strain rate path. Transmission electron microscope (TEM) studies showed that the low angle boundary sub-grain structure obtained on heating to the SP deformation temperature developed on straining into a microstructure containing high angle boundaries capable of sustaining grain boundary sliding.     

Superplasticity and superplastic forming processes

Abstract

Superplasticity, first observed some seventy years ago, remained a scientific curiosity until about twenty years ago. It is now recognized as a property which can be utilized in forming processes. There are two types of superplastic behaviour, known as fine–grained (or fine–structure) and internal–stress superplasticity. Fine–grained superplastic materials have a strain–rate sensitivity exponent of 0·5, and deform principally by a grain–boundary sliding mechanism. In this paper the microstructural features important in the development of fine structure super plasticity are discussed, and phenomenological equations for describing superplastic flow are presented. The superplastic properties of fine–grained materials can be optimized by promoting grain–boundary sliding and inhibiting slip. A number of fine–grained superplastic materials have been developed for commercial use, and their number is increasing. Internal–stress superplastic materials can have a strain–rate sensitivity exponent as high as unity, i.e. they can exhibit Newtonian viscous behaviour. Internal stresses can be generated by thermal cycling in materials that consist of two phases, or are anisotropic in their thermal–expansion coefficients, or are polymorphic. No commercial applications have yet been found for the superplastic forming of materials by generating internal stress.

MST/169     

S–N plots and related phenomena of ultrafine grained copper with different stages of microstructural evolution

To elucidate the effect of the number of passes in equal channel angular pressing on the fatigue strength of ultrafine grained copper, fatigue tests of cylindrical specimens were conducted and the formation behavior of surface damage during cyclic stressing was studied. With the exception of extremely high- and low-stress amplitudes, the fatigue life depended on the number of processing passes and decreased according to the following sequence of pass-numbers: 8-4-12. The difference in fatigue life resulted from the crack initiation life. The physical background of different crack initiation lives among samples is discussed from the viewpoint of slip band formation and the growth of dynamically recrystallized grains.     

Computational Investigation of Effects of Grain Size on Ballistic Performance of Copper

Numerical simulations were conducted to compare ballistic performance and penetration mechanism of copper (Cu) with four representative grain sizes. Ballistic limit velocities for coarse-grained (CG) copper (grain size ≈ 90 µm), regular copper (grain size ≈ 30 µm), fine-grained (FG) copper (grain size ≈ 890 nm), and ultrafine-grained (UG) copper (grain size ≈ 200 nm) were determined for the first time through the simulations. It was found that the copper with reduced grain size would offer higher strength and better ductility, and therefore renders improved ballistic performance than the CG and regular copper. High speed impact and penetration behavior of the FG and UG copper was also compared with the CG coppers strengthened by nanotwinned (NT) regions. The comparison results showed the impact and penetration resistance of UG copper is comparable to the CG copper strengthened by NT regions with the minimum twin spacing. Therefore, besides the NT-strengthened copper, the single phase copper with nanoscale grain size could also be a strong candidate material for better ballistic protection. A computational modeling and simulation framework was proposed for this study, in which Johnson–Cook (JC) constitutive model is used to predict the plastic deformation of Cu; the JC damage model is to capture the penetration and fragmentation behavior of Cu; Bao–Wierzbicki (B-W) failure criterion defines the material's failure mechanisms; and temperature increase during this adiabatic penetration process is given by the Taylor–Quinney method.     

Effect of microstructure of simulated heat‐affected zone on low‐ to high‐cycle fatigue properties of low‐carbon steels

To clarify the effect of microstructural changes on the fatigue property of the weld heat‐affected zone (HAZ), low‐ to high‐cycle fatigue tests were conducted on 16 types of simulated HAZ specimens that had been prepared using thermal processes. The results showed the fatigue S‐N curves of the HAZ to be widely scattered as a function of strength level. These fatigue data were divided into two groups: coarse grain (CG) and fine grain (FG) HAZ, when strain amplitude was used to represent S‐N curves. The fatigue data for the CGHAZ group showed a relatively short fatigue life. Based on surface observations, the initiated fatigue crack size of CGHAZ was larger than that of FGHAZ as a function of microstructural unit size. Hence, fatigue crack growth life, which is almost the same as total fatigue life of CGHAZ, decreased.     

Effect of Concurrent Pulse in Holding Duration on Superplasticity of 2091Al-Li Alloy

The effects of concurrent pulses in holding duration on superplasticity of 2091 Al-Li alloy have beeninvestigated in this paper.The results of superplastic deformation showed that concurrent pulses inholding duration decreased the optimum holding time for superplastic deformation(ε=3.33×10~(-2) s~,T=500℃)from 15 to 5 min and increased elongation(δ)from 530 to 550%.Themetallographic observations showed that the specimens appliedly by concurrent pulse were com-pletely recrystallized after holding 5 min at 500℃,and grains refined to about 2μm,and those spec-imens unapplied current pulses were partly recrystallized even after holding 15 min at 500℃.theelectron probe analyses indicated that concurrent pulses promoted atomic diffusion.It is pointed outthat concurrent pulses accelerate atom diffusion and dislocation motion.increase nucleation rate ofrecrystallization and decrease the optimum holding time for superplastic deformation and increasesuperplastic properties.     

Fatigue behaviour and crack growth rate of cryorolled Al 7075 alloy

The effects of cryorolling (CR) on high cycle fatigue (HCF) and fatigue crack growth rate behaviour of Al 7075 alloy have been investigated in the present work. The Al 7075 alloy was rolled for different thickness reductions (40% and 70%) at cryogenic (liquid nitrogen) temperature and its tensile strength, fatigue life, and fatigue crack growth mechanism were studied by using tensile testing, constant amplitude stress controlled fatigue testing, and fatigue crack growth rate testing using load shedding (decreasing ΔK) technique. The microstructural characterization of the alloy was carried out by using Field emission scanning electron microscopy (FESEM). The cryorolled Al alloy after 70% thickness reduction exhibits ultrafine grain (ufg) structure as observed from its FESEM micrographs. The cryorolled Al 7075 alloys showed improved mechanical properties (Y.S, U.T.S, Impact energy and Fracture toughness are 430 Mpa, 530 Mpa, 21 J, 24 Mpa m^1/2 for 40CR alloy) as compared to the bulk 7075 Al alloy. It is due to suppression of dynamic recovery and accumulation of higher dislocations density in the cryorolled Al alloys. The cryorolled Al alloy investigated under HCF regime of intermediate to low plastic strain amplitudes has shown the significant enhancement in fatigue strength as compared to the coarse grained (CG) bulk alloy due to effective grain refinement. Fatigue crack growth (FCGR) resistance of the ufg Al alloy has been found be higher, especially at higher values of applied stress intensity factor ΔK The reasons behind such crack growth retardation is due to diffused crack branching mechanism, interaction between a propagating crack and the increased amount of grain boundaries (GB), and steps developed on the crack plane during crack-precipitate interaction at the GB due to ultrafine grain formation.     

Effects of weld strength undermatch on fracture toughness of HAZ notched weldments in a HSLA steel

In the present work the effects of weld strength undermatch on fracture toughness of heat affected zone (HAZ) have been studied. In the investigation a high strength low alloyed steel (HSLA) with 800 MPa strength class was used, and the undermatched welded joints were made with two weld strength mismatch levels. Three-point bending test specimens with crack depth to specimen width ratio a/W ranging from 0.05 to 0.5 were extracted from the welded joints. The test results show that strength mismatching gives an obvious influence on the fracture toughness of coarse grained HAZ for the undermatched joints. The lower the weld strength mismatching, the higher the fracture toughness of the HAZ. In addition the tendency of fracture toughness change with crack depths is much the same as in previous studies on base metals or weld metals, that is, fracture toughness of the HAZ is increased with reduction of crack depths. From the measured results it shows that the macroscopically mechanical heterogeneity of the welds may have more important influence on the fracture toughness of the HAZ than the meso-heterogeneity in the reheated coarse grained HAZ. Furthermore, numerical verification indicates that the stress triaxiality at crack tip may be the essential reason for the change of fracture toughness of HAZ. It is also shown that the yield strength of HAZ determined by the limit load in the three-point bend test represents the combinative effects of HAZ and its surrounding materials. This revised version was published online in August 2006 with corrections to the Cover Date.     

超细晶超高碳钢的研究现状及展望

超细晶超高碳钢是国外近年来发展起来的一种新型的、并具有重要发展前景的高性能钢铁材料。在系统总结超细晶超高碳钢的化学成分设计、制备工艺、室温组织性能及超塑性等各方面研究现状的基础上，对超细晶超高碳钢的发展提出展望。     

THE EFFECT OF GRAIN SIZE ON FATIGUE CRACK GROWTH IN AN ALUMINIUM MAGNESIUM ALLOY

Abstract Fully reversed uniaxial fatigue tests were performed on aluminium magnesium alloy Al 5754 with four different grain sizes in order that the effect of grain size on fatigue crack growth could be examined. Surface cracks were monitored by a plastic replication technique. Fatigue strength was shown to improve with a decrease in grain size. The endurance stress is a function of the inverse square root of the grain size and is described empiricdty by a Hall-Petch type relation. The effect of grain size on fatigue crack growth is most significant when the crack length is of the order of the microstructure. Fluctuations in the growth rate of microstructurally short cracks are most marked in a fine grained microstructure and may be related to the need to transfer slip to adjacent grains. Crack path deviation is greatest in the coarsest grained microstructure and SEM fractography reveals a more pronounced crack surface roughness in the coarser grained alloy than in the finer grained alloy.     

Cellular Mechanisms of Enhanced Osteoblasts Functions via Phase‐Reversion Induced Nano/Submicron‐Grained Structure in a Low‐Ni Austenitic Stainless Steel

We elucidate here the fundamental principles underlying the modulation of osteoblasts functions in stainless steel biomedical devices achieved by nanoscale/submicron grain structure obtained through the novel concept of phase reversion in a low Ni bearing 15Cr–9Mn–1.7Cu steel. Interestingly, a comparative investigation of nano/submicron (N‐SM) and coarse‐grained (CG) structure under identical conditions indicated that cell attachment, proliferation, and viability are favorably enhanced in N‐SM grained structure and significantly different from the CG structure. These observations were further confirmed by expression levels of vinculin and associated actin cytoskeleton. Computational analysis of immunofluorescence micrographs suggested increased vinculin concentration associated with actin stress fibers in the outer regions of the cells and cellular extensions, implying enhanced cell–substrate interactions on the N‐SM grained substrate. The favorable enhancement of osteoblasts functions and cellular attachment on N‐SM grained surface is attributed to ultrafine grain size, i.e., the availability of greater open lattice in the position of high angle grain boundaries, and high hydrophilicity. The integration of cellular and molecular biology with material science and engineering as described here provides a route to modulate cellular and molecular reactions in promoting osteoinductive signaling of surface adherent cells. The end outcome of the study is that stainless steels with low Ni contents in comparison to the conventionally used bioimplant with 10–13 wt%Ni, as specially processed, exhibit desired, enhanced cell functions, and bulk properties.     

超塑处理对喷射成形GCr15钢超塑性的影响

研究了超塑处理对喷射成形GCr15钢超塑性的影响，超塑性拉实验结果表明，未经超塑处理的铸态试样，延伸率为119％，经过二次油淬和二次油淬＋高温回火超塑处理的两种试样，延伸率分别为328％和671％，变形温度和应变速率对GCr15钢的超塑性有一定的影响，但材料的微观组织对其超塑性具有决定性作用，超塑处理改变了材料试样的微观组织，导致其超塑性发生变化，经过二次油淬＋高温回火超塑处理后试样具有球化组织，其超塑性最好，未经超塑处理的铸态试样具有珠光体组织，超塑性最差，经过二次油淬超塑处理后试样的组织是马氏体和少量碳体物的混合，其超塑性介于上述两种试样之间，喷射成形工艺使GCr15钢获得均匀细化的稳定组织，这对于细晶超塑性是必要的，超塑处理材料的超塑性得到更大的提高。     

Investigation of Subboundaries Evolution in Superplastically Deformed NiAl by Positron Lifetime

In order to study the subboundaries evolution in superplastically deformed NiAl,the positron lifetime change during superplastic deformation process was measured.It is shown that the superplastic deformation of NiAl has not influence on its τ2,the newly recrystallized grain boundaries formed during entire superplastic deformation process belong to the calegory of subboundaries and have not contribution to the superplastic strain.     

Rheological Model of Flow of a Material with a Variable Structure under Superplastic Deformation

Versions of rheological models describing the phenomenological behavior of materials in the superplasticity state with allowance for the elastic component and without allowance for it have been presented. Consideration has been given to the basic indications of manifestation of superplastic properties from the viewpoint of the influence of the dimensions of the structural components on them. Analytical dependences of the governing relations of the mechanics of a deformable rigid body, which allow for the influence of the applied stresses, the structure, and the temperature on the rate of superplastic deformation (SPD) and enable one to calculate the shear viscosity and the rheological dependences of SPD for deformable aluminum alloys AMg4 and AMg6 with a prepared ultrafinegrained structure, have been given.     

Fatigue properties of ultrafine grained low carbon steel produced by equal channel angular pressing

Ultrafine grained low carbon (0.15 wt.% C) steel produced by equal channel angular pressing (ECAP) was tested for investigating fatigue properties, including cyclic softening and crack growth rate. Emphasis was placed on investigating the effect of load ratio on the fatigue crack growth rates of ultrafine grained microstructure. The ECAPed steel exhibited cyclic softening. After the first cycle, the tension and compression peak stresses decreased gradually with the number of cycles. Fatigue crack growth resistance and the threshold of ECAPed ultrafine grained steel were lower than that of an as-received coarse grained steel. This was attributed to a less tortuous crack path. The ECAPed steel exhibited slightly higher crack growth rates and a lower ΔK_th with an increase in R ratio. The R ratio effect on growth rates and ΔK_th was basically indistinguishable at a lower load ratio (R>0.3) compared with other alloys, indicating that the contribution of the crack closure vanished. This was explained by the fact that finer grained materials produce a lower opening load P_op due to a relatively less serrated crack path. Consequently, K_min can reach K_op readily with a smaller increment of load ratio. The crack growth rate curve for the ECAPed ultrafine grained steel exhibited a linear extension to the lower growth rate regime than that for the coarse grained as-received steel. This behavior can be explained by a reverse crack tip plastic zone size (r_p) that is always larger than the grain size.     

Fatigue behavior of CoCrFeMnNi high-entropy alloy under fully reversed cyclic deformation

Bulk ultrafine-grained (UFG) CoCrFeMnNi high-entropy alloy (HEA) with fully recrystallized microstructure was processed by cold rolling and annealing treatment. The high-cycle fatigue behaviors of the UFG HEA and a coarse-grained (CG) counterpart were investigated under fully reversed cyclic deformation. The fatigue strength of the UFG HEA can be significantly enhanced by refining the grain size. However, no grain coarsening was observed in the UFG HEA during fatigue tests. Mechanisms for the superior mechanical properties of the UFG HEA were explored.     

Impulsive responses of functionally graded material bars due to collision

We investigated the ability of functionally graded materials (FGMs) to absorb impact energy by mathematically analyzing the impulsive responses of functionally graded (FG) bars colliding with a homogeneous bar on the basis of Laplace transformation and calculated by using numerical transformation and its inversion. Young’s modulus in the FG bar was assumed to be proportional to the square of its density, which was similar to foam materials. Results showed that maximum impact loads were not strongly dependent on the distribution of material property in the FG bar. In the FG bar with increasing modulus from the impact end to the fixed end, much larger compressive stress and even large tensile stress occurred near the fixed end compared with the ones in the homogeneous bar. In the FG bar with decreasing modulus from the impact end, the compressive stress was approximately the same as the one in the homogeneous bar, and the history of the stress varied regularly.