Grain refinement in as cast Al–Mg–Mn alloy during high temperature equal channel angular pressing

Abstract

An as cast Al–Mg–Mn alloy with coarse equiaxed grain structure was processed by equal channel angular pressing (ECAP) at 350°C up to eight passes. Systematic studies were made on the microstructural evolution during ECAP by optical microscopy, electron backscattered diffraction and TEM. Equal channel angular pressing led to a considerable grain refinement, resulting in an average cell size of about 1 μm and a fraction of high angle boundaries of 75% after eight pressing passes. Deformation bands were not developed during the ECAP process, and a reasonably equiaxed substructure was obtained even after one pass. The main mechanism of grain refinement was attributed to the continuous dynamic recrystallisation based on the motion of deformation induced dislocations. Discontinuous recrystallisation at grain boundaries and triple junctions also contributed to the refinement, which played an important role especially at high strain of eight passes.     

Significantly enhanced tensile strength and ductility in nickel aluminium bronze by equal channel angular pressing and subsequent heat treatment

Nickel aluminium bronze (NAB) was subjected to equal channel angular pressing (ECAP) at 400 °C for up to 4 passes in routes B_A and C, respectively, followed by isothermal heat treatment with a view to improving the κ phase structures and tensile properties. The lamellar κ_III structure was completely broken after 4 passes in route B_A although route C was less efficient. Spheroidisation and coarsening of the highly deformed κ_III continued during heat treatment especially at ≥600 °C. At 800 °C, both the lamellar structure and the fine κ_IV particles transformed completely into a coarse globular morphology with no distinction between the primary and eutectoid α. Significant increases in strength were achieved by ECAP, reaching a maximum yield strength of 960 MPa with a good ductility of ~14 %. Heat treatment after ECAP was shown to considerably improve tensile ductility to >30 % while keeping the strength high at ~700 MPa, a significant enhancement compared to the as-received NAB.     

Grain refinement and mechanical behavior of a magnesium alloy processed by ECAP

Equal-channel angular pressing (ECAP) is an effective tool for refining the grain structure of magnesium alloys and improving the ductility at moderate temperatures. However, grain refinement in these alloys differs from other metals because new grains are formed along the boundaries of the initial structure and these newly formed grains slowly spread to consume the interiors of the larger grains in subsequent passes. A model is presented for grain refinement in magnesium alloys processed by ECAP based on the principles of dynamic recrystallization where new fine grains are formed along the initial boundaries and along twin boundaries. This model provides an explanation for a wide range of experimental data and introduces the concept of grain size engineering for achieving selected material properties in magnesium alloys.     

The effects of alloying and processing on the fracture characteristics of aluminum sheet

The effects of composition and processing on microstructure and fracture behavior of Al-Zn-Mg-Cu (7075 type) alloys were investigated. The microstructural variations involved changes in second phase particle size and distribution and in grain structure. Second phase particles in three size ranges were investigated: (1) coarse insoluble particles involving Fe at various levels, (2) finer intermediate particles involving Cr or Zr varied in size by choice of homogenizing practice, and (3) fine aging precipitates varied in size by choice of aging practice. The use of high temperature, long-time homogenizing practices to produce coarse intermediate particles also resulted in recrystallization and grain growth, particularly in the Zr-bearing Al-Zn-Mg-Cu alloys. Grain size and shape were found to strongly affect toughness; plane stress fracture toughness [of 12-in. (0.30 m) wide center notched panels] at a given yield strength level differed by a factor of three between a fully recrystallized, coarse equiaxed grain structure to an unrecrystallized lamellar structure. Smaller effects of second phase particles distribution were found in the Al-Zn-Mg-Cu alloys. The trends observed in a fatigue crack growth rate study of the same materials will be discussed.     

Relationship of particle stimulated nucleation,recrystallization and mechanical properties responding to Fe and Si contents in hot-extruded 7055 aluminum alloys

The variations of coarse intermetallic particles in hot-extruded 7055 aluminum alloys with 0.041 wt%Fe and 0.024 wt% Si increasing to 0.272 wt% Fe and 0.134 wt% Si were investigated.The particle stimulated nucleation(PSN) behaviors for different kind of coarse particles were detailly analyzed by EBSD.Moreover,the effect of PSN responding to Fe and Si contents on recrystallization and tensile properties of 7055 alloys was evaluated.With increasing Fe and Si contents,the size and number density of coarseη/S particles are reduced,while the number densities of coarse Al_7Cu_2 Fe and Mg_2Si particles are both increased and the coarse Al_7Cu_2 Fe particles transform from rod-like to irregular.More PSN recrystallized grains with predominant orientations deviated from the extruded fiber textures are stimulated by the irregular Al_7Cu_2 Fe and Mg_2Si particles,because a higher degree of local non-uniform deformation is produced.The rod-like Al_7Cu_2 Fe particles cause the greatest degree of local non-uniform deformation owing to the largest aspect ratio,but the shape also restricts the area of particle deformation zone(PDZ) resulting in fewer PSN recrystallized grains.The irregular η/S particles give rise to the lowest degree of local non-uniform deformation and fewest PSN recrystallized grains with the major orientations close to the extruded fiber textures.Consequently,despite the number and size of coarse η/S particles are reduced,the proportion of high angle grain boundaries(HAGBs) is increased and the extruded fiber textures are weakened with Fe and Si contents increasing,because of the increased Al_7Cu_2 Fe and Mg_2Si particles.The strength is slightly declined by the weakened 111//ED(extrusion direction) fiber texture,while the elongation is reduced for a larger number of coarse particles and more HAGBs with higher Fe and Si contents.     

Influence of particles on recrystallisation textures and microstructures of aluminium alloy 3103

Abstract

The evolution of recrystallisation textures in the commercial aluminium alloy 3103 has been investigated by means of a three-dimensional orientation distribution function (ODF) analysis. The global texture measurements were supplemented by local texture measurements by means of the electron backscattering pattern (EBSP) technique in SEM and optical microscopy, inparticular grain size measurements. The evolution of recrystallisation textures was determined by the competition between particle stimulated nucleation (PSN) and nucleation from cube bands. Precipitated particles were found to have a higher retarding effect on the nucleation from deformation zones around particles than on the nucleation from cube bands. The result of this is a strong cube texture and a large grain size in cases of a strong precipitation reaction. This phenomenon has also been discussed theoretically in terms of a semiquantitative model.     

Effects and mechanisms of grain refinement in aluminium alloys

Grain refinement plays a crucial role in improving characteristics and properties of cast and wrought aluminium alloys. Generally Al-Ti and Al-Ti-B master alloys are added to the aluminium alloys to grain refine the solidified product. The mechanism of grain refinement is of considerable controversy in the scientific literature. The nucleant effects i.e. which particle and its characteristics nucleate α-Al, has been the subject of intensive research. Lately the solute effect i.e. the effect of dissolved titanium on grain refinement, has come into forefront of grain refinement research. The present paper attempts to review the literature on the nucleant effects and solute effects on grain refinement and addresses the importance of dissolved titanium in promoting nucleation of α-Al on nucleant particles.     

The effect of particle content and matrix grain size on the recrystallisation of two-phase aluminium-iron alloys

The recrystallisation of aluminium alloys in the composition range 0.002 to 1.08 wt% iron has been followed metallographically and by hardness measurements. The recrystallisation of solid-solution alloys was retarded as the iron content increased, whereas that of the two-phase alloys was accelerated. This latter effect was associated with a marked increase in nucleation rate, with no significant change of growth rate, as the Al₃Fe particle spacing decreased from 15 to 4m. Decreasing the grain size of the two-phase alloys, at constant composition, also caused an acceleration of recrystallisation, the magnitude of which decreased as the Al₃Fe content increased. These effects have been shown to be independent of the size of the Al₃Fe particles, within the range studied (diameters 0.6 to 2.2m). The results are discussed in terms of a model (which is based on metallographic observations) of nucleation at matrix grain boundaries and at particle/ matrix interfaces. It is suggested that the retardation of nucleation, observed by other workers, in more closely spaced dispersions, occurs because nucleation at particle/ matrix interfaces becomes difficult when the inter-particle spacing becomes of the same order as the diameter of the sub-structure of the deformed matrix.     

Interactions between precipitation and recrystallisation in an Al–Mg–Si alloy

Abstract

Work has been carried out to study a broad range of possible behaviours in a simple model, high-purity, ternary Al–Mg–Si alloy. The starting conditions investigated, before deformation, included a supersaturated solid solution, a very coarse β (Mg₂Si)particle distribution, and a range of preaged conditions with a combination of β′ rods and β platelets, obtained by isothermal treatments at 300°C. On annealing at the same temperature of 300°C, after cold rolling to a 70% reduction, the recrystallisation times were found to vary from 10 min to 1600 h, depending on the initial material condition. The fastest recrystallisation time was observed with the coarse β particles, where nucleation of recrystallisation took place rapidly by particle stimulated nucleation. In the solution treated sample, recrystallisation took the intermediate time of 10 h and the recrystallisation rate was found to be dominated by the transformation of the metastable β′ phase, to the equilibrium β phase at the migrating grain boundaries. By far the longest recrystallisation time of 1600 h occurred for a preaging time of 1 h. This remarkable stability was attributed to the recrystallisation kinetics being controlled by discontinuous coarsening of a very fine distribution of stable β precipitates at the recrystallisation front. These precipitates formed early in the annealing treatment, by the transformation of the metastable β′ rods, broken up, and made partially incoherent by the deformation process.     

Influence of second phases on fatigue crack growth behavior of nickel aluminum bronze

The influence of phases with different morphology and mechanical properties on fatigue crack growth behavior in nickel aluminum bronze (NAB) has been investigated. Annealing at 675 °C and normalizing at 920 °C heat treatments were used to produce different morphologies and fractions of second phases. This analysis shows that the coarse dendritic κ_II particles and κ_III lamellae as hard brittle phases in as-cast and annealed NAB have an accelerative effect on the fatigue crack propagation where by cracks propagate through α and κ_II/κ_III interface. Fatigue cracks in normalized NAB prefer to propagate through the ductile α grains, form fatigue striations and have the lowest crack growth rate. The uniformly distributed, fine κ_IV precipitates in the α grains improves fatigue crack growth resistance. This work identifies the role of NAB second phases on propagation of fatigue cracks, and provides suitable heat treatment for improving fatigue crack resistance in terms of controlling second phase type, distribution and percentage.     

The tensile behaviour of gamma titanium aluminide intermetallic

The titanium aluminide alloys based on gamma have, in recent years, engendered considerable scientific and technological interest because they offer several useful properties that are either comparable to or better than those of the conventional alloys of titanium and nickle-base superalloy families. In this paper, the tensile behaviour of a gamma-TiAl intermetallic is reported. The intermetallic has a near lamellar microstructure comprising regions of single-phase grains, coarse twin-related lamellae and fine lamellae. The tensile properties of the intermetallic containing chromium and niobium are inferior to those of the binary counterpart. Tensile fracture was primarily by intergranular rupture with microcracking along the grain boundaries.     

孪晶片层结构在室温轧制过程中的微观结构演变

研究了一种具有纳米孪晶片层结构的电解沉积铜的微观结构特征及其在室温轧制形变后的微观结构演变.结果表明,电解沉积制备的纯铜样品由柱状晶组成,柱状品内含有平行于样品沉积表面的纳米量级厚度的高密度孪晶片层结构,在孪晶界上缺陷很少,为共格孪晶界.形变后,孪晶片层的微观结构特征与片层厚度密切相关.粗大的孪品片层的形变行为以全位错运动为主,而细小的孪晶片层的形变行为以肖克莱(Shockley)位错在孪晶界上的滑移为主,从而导致几个纳米厚的超细孪晶片层消失.     

The influence of sulphur content on the static recrystallisation of cold worked low carbon aluminium-killed strip steels

The static recrystallisation (SRX) after cold work was studied in four low carbon Al-killed steels with varying sulphur content ranging from 2 to 140 ppm. The sulphur had an indirect but significant effect on the SRX behaviour by the presence of coarse MnS particles on which the AlN precipitated heterogeneously during coiling after hot rolling. The mean size of the AlN/MnS compound particles was generally too coarse to be effective for Zener-pinning of moving boundaries during recrystallisation thereby rendering the AlN ineffective in influencing the SRX process. In the low sulphur content steels, however, with virtually no MnS present, very fine AlN nucleated homogeneously in the matrix or heterogeneously on grain boundaries or dislocations during coiling, with size often less than 30 nm in diameter. These AlN particles were more effective in Zener-pinning of dislocations and moving recrystallisation fronts, thereby retarding the start of SRX by up to 100 times at low coiling temperatures of about 600 °C.     

Influence of ultrasonic melt treatment on the formation of primary intermetallics and related grain refinement in aluminum alloys

Ultrasonic melt treatment (UST) is known to induce grain refining in aluminum alloys. Previous studies have clearly shown that in Al–Zr–Ti alloys, the primary Al₃Zr intermetallics were dramatically refined by cavitation-assisted fragmentation, and a good refinement effect was achieved. In this article, Al–Ti, Al–Ti–Zr alloys, and some commercial aluminum alloys are used to analyze the effect of UST on primary intermetallics and grain refinement. The addition of a small amount of Al–3Ti–B master alloy is also studied in order to compare with the addition of Ti and Zr in commercial aluminum alloys. Experimental results show that the ultrasonic grain refining effect is not only related to the size of particles which are refined and/or dispersed by UST, but also related to an undercooling available for activation of these particles in the solidification process. Athermal heterogeneous nucleation theory is considered to explain the effect of size and distribution of substrate particles on the grain structure with different undercoolings. The distribution of primary particle sizes results in the distribution of required undercoolings. Grain refining occurs when the undercooling is large enough to activate the refined primary intermetallics or dispersed inoculants.     

Effects of Heat Treatment on Microstructure and Mechanical Properties of an ECAPed Al–Zn–Mg–Cu Alloy

Equal‐channel angular pressing (ECAP) has a considerable advantage in the preparation of bulk fine‐grained alloys. To investigate the effect of solid solution treatment (SST) on the microstructure and mechanic properties of an Al–Zn–Mg–Cu alloy after ECAP, a comparative study is conducted using experimental techniques. It is shown that ECAP processing introduces a strong grain refinement, while the SST induces precipitation of skeleton‐like second phases distributed discontinuously at the grain boundary and needle‐like second phases in the grain. In addition, SST can also improve significantly the fractions of both high angle grain boundaries and recrystallization. The {110}<001> texture is introduced and the polar density is reduced during SST. Microstructural evolution involves three typical characteristics, namely, shear bands, substructure, and precipitates. The corresponding mechanism of microstructure evolution is proposed, considering the effect of dislocations, precipitates, and grain boundaries. After SST, the improvement of strength and hardness is not obvious, but significant in plasticity by 33.3%. Different strengthening mechanisms are also examined during ECAP and subsequent SST.

     

Evolution of secondary phase particles during deformation of Al-5Ti-1B master alloy and their effect on α-Al grain refinement

Addition of Al-5Ti-1B alloy to molten aluminum alloys can refine α-Al grains effectively and thereby improve their strength and toughness. TiAl₃ and TiB₂ in Al-5Ti-1B alloy are the main secondary-phase particles for refinement, while the understanding on the effect of their sizes on α-Al grain refinement continues to be fragmented. Therefore, Al-5Ti-1B alloys with various sizes and morphologies of the secondary-phase particles were prepared by equal channel angular pressing (ECAP). Evolution of the secondary-phase particles during ECAP process and their impact on α-Al grain refinement were studied by X-ray diffraction and scanning electron microscope (SEM). Results show that during the ECAP process, micro-cracks firstly appeared inside TiAl₃ particles and then gradually expanded, which resulted in continuous refinement of TiAl₃ particles. In addition, micro-distribution uniformity of TiB₂ particles was improved due to the impingement of TiAl₃ particles to TiB₂ clusters during deformation. Excessively large sizes of TiAl₃ particles would reduce the number of effective heterogeneous nucleus and thus resulted in poor grain refinement effectiveness. Moreover, excessively small TiAl₃ particles would reduce inhibitory factors for grain growth Q and weaken grain refinement effectiveness. Therefore, an optimal size range of 18–22 μm for TiAl₃ particles was suggested.     

Microstructure and strengthening mechanism of bimodal size particle reinforced magnesium matrix composite

One kind of (submicron + micron) bimodal size SiCp/AZ91 composite was fabricated by the stir casting technology. After hot deformation process, the influence of bimodal size particles on microstructures and mechanical properties of AZ91 matrix was investigated by comparing with monolithic A91 alloy, submicron SiCp/AZ91 and micron SiCp/AZ91 composites. The results show that micron particles can stimulate dynamic recrystallized nucleation, while submicron particles may pin grain boundaries during the hot deformation process, which results in a significant grain refinement of AZ91 matrix. Compared to submicron particles, micron particles are more conducive to grain refinement through stimulating the dynamic recrystallized nucleation. Besides, the yield strength of bimodal size SiCp/AZ91 composite is higher than that of single-size particle reinforced composites. Among the strengthening mechanisms of bimodal size particle reinforced composite, it is found that grain refinement and dislocation strengthening mechanism play a larger role on improving the yield strength.     

Effects of homogenisation treatment on microstructure and grain refinement of Al–1.2Mn alloy

The effects of a homogenisation treatment on precipitation, recrystallisation, grain refinement and texture in an Al–1.2Mn alloy were investigated. Based on hardness and electrical conductivity measurements, a precipitation–recrystallisation diagram was generated, which described the sequence of recrystallisation and precipitation. The results showed that recrystallisation was significantly retarded during the annealing treatment for the cold-rolled alloys with a supersaturated solid solution or with dense pre-existing dispersoids. If precipitation occurred prior to recrystallisation, the precipitates caused a strong resistance to recrystallisation, resulting in a coarse and elongated grain structure with a dominant normal direction-rotated cube texture. In contrast, when recrystallisation was completed before precipitation, recrystallisation was little affected by the precipitates, leading to a fine-grained structure with a weak cube texture.     

Superplasticity of Al–Mg–Li alloy prepared by thermomechanical processing

Abstract

Grain refinement of Al–Mg–Li alloys for superplasticity prepared by thermomechanical processing has been a difficult task due to the cracking of these alloys when rolled at low temperatures. Raising the rolling temperature resulted in enhanced rollability of these sheets with no cracks but very coarse grains after recrystallisation. To solve this problem, a cross rolling schedule was developed to hinder fracture and simultaneously provide enough stored energy for following recrystallisaiton coupled by lowering the reheating temperature. Fine, equiaxed grains of ～7 μm was achieved by this new approach and maximum total elongation of about 915% was obtained when deformed at a temperature of 525°C and an initial strain rate of 1×10^?3 s^?1.     

Processing by equal-channel angular pressing: Applications to grain boundary engineering

Equal-channel angular pressing (ECAP) is a processing technique in which a sample is pressed through a die constrained within a channel so that an intense strain is imposed without incurring any change in the cross-sectional dimensions of the sample. This procedure may be used to achieve considerable grain refinement in pure metals and metallic alloys with as-pressed grain sizes lying typically within the submicrometer range. Careful experiments reveal only a minor change in the grain size with increasing numbers of passes through an ECAP die but there is a significant change in the distribution of grain boundary misorientations as a function of the total imposed strain. In practice, the microstructure evolves with increasing strain from an array of grains where the boundaries are predominantly in low-angle misorientations to an array of grains where a high fraction (typically ≥60%) is in high-angle misorientations. This evolution has a significant effect on the characteristics of the as-pressed materials including the high temperature mechanical properties and the measured rates of diffusion. In addition, the evolution provides an opportunity to use Grain Boundary Engineering in order to optimize the behavior of the material.