Abnormal Growth of Goss Grains in Grain-oriented Electrical Steels

With the help of electron back scattering diffraction techniques and field emission microscope, the misorienta- tion and the precipitation environment of Goss grains in conventional grain-oriented steel were observed and investigated at the initial stage of secondary recrystallization. It reveals that the abnormal Goss grains have a high fraction of high angle boundaries ranging from 25 to 40 deg. The most important observation is that some of {110}〈001〉 grains in matrix indicated higher particle density than their neighbor grains during final annealing at 875℃ before secondary recrystallization, which could create a favorable environment for their abnormal grain growth. Based on misorientation and precipitation results, the abnormal growth mechanism of Goss grains was sketched.

Enhancement of ductility and improvement of abnormal Goss grain growth of magnetostrictive Fe–Ga rolled alloys

The influences of initial microstructures on the mechanical properties and the recrystallization texture of magnetostrictive 0.1 at% Nb C-doped Fe83 Ga17 alloys were investigated. The directionally solidified columnar-grained structure substantially enhanced the tensile elongation at intermediate temperatures by suppressing fracture along the transverse boundaries. Compared with tensile elongations of 1.0% at 300℃ and 12.0% at 500℃ of the hot-forged equiaxed-grained alloys, the columnar-grained alloys exhibited substantially increased tensile elongations of 21.6% at 300℃ and 46.6% at 500℃. In the slabs for rolling, the introduction of 〈001〉-oriented columnar grains also promotes the secondary recrystallization of Goss grains in the finally annealed sheets, resulting in an improvement of the saturation magnetostriction. For the columnar-grained specimens, the inhomogeneous microstructure and disadvantage in number and size of Goss grains are improved in the primarily annealed sheets, which is beneficial to the abnormal growth of Goss grains during the final annealing process.

Strategy for suppressing abnormal grain growth of ZK60 Mg alloy during solution by pre-compression:A quasi-in-situ study

Abnormal grain growth(AGG)easily takes place in Mg alloys during high-temperature solutions,result-ing in deterioration of mechanical properties.Hence,the compression prior to solution(pre-compression)was conducted to suppress AGG,and the microstructure evolution as well as suppressing mechanisms was investigated based on quasi-in-situ analysis.After compression along the transverse direction,<11-20>//ED grains preferentially nucleated and rapidly grew up,and the initial<10-10>//ED texture was weakened.Two grain growth modes of heat-induced and strain-induced grain boundary migrations were found.The former was attributed to the high interfacial energy of grain boundaries with large curvature.The latter consumed the adjacent grains with high storage energy,forming abnormal grains with irregular shapes.The compression with a reduction>6%could obviously suppress AGG.The suppressing effects were mainly attributed to weakening the size advantage of<11-20>//ED grains,increasing nucleation,reducing grain boundary character distribution,and redistributing storage energy distribution.After 12%compression along the transverse direction,30°misorientation of<11-20>//ED grains and high energy grain boundaries were reduced.The{10-12}tensile twins and{10-15}high index twins induced by com-pression increased the nucleation of static recrystallization.Beside,compression introduces high-density dislocations,which also contributed to suppressing AGG behavior during solution.

Understanding Growth Hormone Secretion and Short Stature

Short stature is a clinical challenge in the daily practice of pediatric endocrinology, regarding the several technical, cultural and economic factors associated with its approach. This article intends to review the physiology of growth hormone secretion, the endocrine regulation of human growth and the clinical aspects of the diagnosis and treatment of short stature. It specifically analyses the treatment of short stature with growth hormone, along with its side effects, cost/benefit analysis and possible risks. A clinical case from a medical school is also described, intending a better understanding of this frequent ambulatory situation in endocrinology and pediatrics.

Suppressing abnormal grain growth and switching precipitation behaviors in ZK60 Mg profile by inducing pre-tension

The pre-tension was induced to suppress the abnormal grain growth(AGG)of ZK60 Mg profile during solution treatment.The effects of pre-tension on the microstructure evolution and mechanical properties were studied,and the suppressing mechanism was discussed.If the pre-tension strain was larger than 10%,AGG during solution could be effectively inhibited,resulting in a sharp decrease in the grain size.The suppression effects were realized by restricting the orientation dependent of AGG and promoting static recrystallization.The pre-tension reset the distributions of stored energy and sizes of the grains with〈11–20〉and〈10–10〉orientations,and thus retarded the orientation dependent of AGG.Moreover,the pre-tension introduced a mass of dislocations,twins,and stacking faults,all of which promoted the occurrence of static recrystallization,and the grain structure was further refined.The pre-tension accelerated the precipitation kinetics during aging,resulting in fine and dense precipitates.With the increase of pre-tension strain,the strength of ZK60 Mg profile monotonically increased.

Inhibition of Abnormal Grain Growth in Stir Zone via In‑Situ Intermetallic Particle Formation During Friction Stir Welding of AA6061

Abnormal grain growth(AGG)has been widely observed in many friction stir welded(FSWed)joints during post-weld heat treatment(PWHT).The coarse grain structure not only reduces the strength of the joint but also limits its usage in superplastic forming.Several methods have been reported in previous studies to inhibit AGG,but all of them can only mitigate AGG.Complete inhibition of AGG was not achieved.In the current research,AGG was widely observed during the PWHT of friction stir welded AA6061.Multi-pass FSW enhanced the thermal stability of the as-welded grain structure but did not eliminate the occurrence of AGG.A new welding method was developed with the ball-milled Al–Ti powder mixture introduced into the stir zone and proved effective in inhibiting AGG in FSWed AA6061 during PWHT.The adoption of 5-pass FSW with an alternate rotation mode succeeded in producing an AGG-free sample.Microscopic characterizations conducted in the stir zone showed an evolution of Al–Ti powder mixture into different particle formations and Al3Ti new phase.Quantitative analysis of the second phase particles(SPPs)in the stir zone confirmed the increases in both particle number and average size.The quantitative results fit well with Humphreys’grain growth model,which theoretically explains the mechanism for AGG inhibition,i.e.,the significantly enhanced particle pinning effect.

Abnormal grain growth behavior and mechanism of 6005A aluminum alloy extrusion profile

Peripheral coarse grain(PCG)structure is a common microstructural defect appearing in the aluminum alloy extrusion process,which seriously affects the mechanical properties of the profiles.In this work,a series of extrusion experiments and numerical simulations were conducted to investigate the influence of billet temperature and ram speed on the microstructure,mechanical properties and thickness of PCG layers of 6005A aluminum alloy profiles.The mechanism of abnormal grain growth(AGG)occurring on the surface and in the core of profiles was revealed.The result showed that lower ram speed could sup-press the formation of coarse grains.The AGG on the surface of the profiles was activated by the shear deformation and lattice distortion derived from the friction on the interface between the profile and die.When the billet was heated to a relatively high temperature,dynamic recrystallization(DRX)was dominant,and the Cube{100}<100>and R-Cube{100}<110>grains underwent abnormal growth to form surface coarse grains.When the billet was heated to a relatively low temperature,the degree of static recrystallization(SRX)became stronger,and the Goss{110}<100>and R-Cube{100}<110>grains under-went abnormal growth to form surface coarse grains.The AGG in the core of profiles was activated by the large grain boundary misorientation and a strain gradient formed because the Cube{100}<100>re-crystallized grains were surrounded by the Copper{112}<111>and Brass{110}<112>deformed grains.The second phases in the 6005A aluminum alloy extrusion profiles were mainlyβ(Mg_(2) Si)and AlFeMnCrSi.As the billet temperature increased,moreβphases dissolved into the aluminum matrix,thus enhancing the strength and hardness of the profiles.As the ram speed decreased,the thickness of PCG layers reduced,thus resulting in higher strength and hardness of the profiles.Due to the integrated effect of solution strengthening and grain refinement strengthening mechanisms,the combination of extrusion parameters for the profile to obtain the best mechanical properties was determined as 540℃×0.5 mm/s.

Effect of cyclic heat treatment on abnormal grain growth in Fe-Mn-Al-based shape memory alloys with different Ni contents

Cyclic heat treatment that can continuously promote abnormal grain growth is widely used for the prepa-ration of single-crystal Fe-Mn-Al-based shape memory alloys.However,it takes a long time to prepare large-size Fe-Mn-Al-based alloy single crystals via the reported cyclic heat treatments.Meanwhile,the long-time cyclic heat treatment at high temperatures leads to the development of defects including oxidation and a decrease in Mn,which would deteriorate superelasticity in the Fe-Mn-Al-based shape memory alloys.To shorten the fabrication time of single crystals,the effect of the cyclic heat treatment process on the abnormal grain growth in the Fe-Mn-Al-based alloys with different Ni contents was systematically investigated.It is found that the abnormal grain growth of Fe-Mn-Al-based alloys was not significantly affected by the Ni contents(within 2.1 at.%-6.2 at.%).In addition,the abnormal grain growth could be promoted by 1-2℃ min^(-1) cooling rate,high solution temperature,and multiple cycles,while it was insensitive to other processes including heating rate,dual-phase time as well as long-time solution treat-ment.These findings can guide optimizing the fabrication process of single crystals by cyclic heat treat-ment.Finally,the Fe_(41.9)Mn_(37.8)Al_(14.1) Ni_(6.2) single crystal prepared by the optimized cyclic heat treatment showed a recoverable strain of about 4%.

Statistical model of magnetization reversal in Nd-Fe-B sintered magnets

Statistical model of magnetization reversal was used to simulate the magnetization reversal behavior in the sintered Nd-Fe-B magnets with double grain-size distributions due to the abnormal grain growth (AGG). The magnetic properties and mechanical properties due to the formation of AGG grains in Nd-Fe-B sintered magnets were tested. The results show that the magnetic properties, especially the rectangularity were severely deteriorated after the formation of the AGG grains and a step was shown on the demagnetization curve, and the occurrence of AGG may account for the poor rectangularity and existence of the step on demagnetization curve according to the statistical model of magnetization reversal. The fracture toughness and bending strength are lowered because of the stress concentration in the AGG grains. The SEM images show that the formation of AGG grains is caused by the solid sintering due to the absence of RE-rich phase. Statistical model of magnetization reversal can qualitative by explain the dependence of the magnetization reversal behavior on the grain size in the Nd-Fe-B sintered magnets.

Factors Affecting the Squareness of Hysteresis Loops of Sintered NdFeB Magnets

Investigation into the magnets with different squareness of hysteresis loop(SHL) reveals that the microstructure of sintered NdFeB magnets has great effects on the SHL of the magnets. The abnormal grain growth deteriorates the SHL seriously. The shape of the grain and the grain boundary affect the intensity of demagnetization field, and consequently on the SHL. The added elements have effects on the phase structures and distributions in the magnets, which influences the uniform of demagnetization field.

Development of microalloyed steels for high temperature carburizing

The potential for use of microalloy additions to suppress abnormal austenite grain growth and produce steels with enhanced bending fatigue resistance after high temperature vacuum carburizing was investigated in a series of Ti-modified SAE 8620 steels with w(niobium) additions up to 0.1%.Results are considered from a series of papers at the Advanced Steel Processing and Products Research Center on the effects of Nb content,heating rate, rolling history,and processing temperature on the evolution of austenite grain structures in carburizing steels. Emphasis is placed on understanding the effects of alloying and processing on each stage in the annealing process including the as received laboratory rolled conditions,during the onset of carburizing after annealing at different heating rates,and after annealing for various times at carburizing temperatures up to 1 100℃.Heating rate to the carburizing temperature was shown to be an influential variable and suppression of abnormal grain growth was dependent on the development of a critical distribution of fine NbC precipitates,stable at the austenitizing temperature.The importance to industrial carburizing practice of heating rate effects on precipitates and austenite grain size evolution are discussed and correlated to selected data on fatigue performance.

A general mechanism of grain growth-Ⅱ: Experimental

The formed microstructure inside polycrystalline materials strongly influences their practical performances,which process is mostly dominated by grain growth behaviors.However,the general evolution of grain growth behaviors,especial for the occurrence of abnormal grain growth and stagnant growth,remains ambiguous despite decades of efforts.Here,we investigate systematically the general evolution of grain growth behaviors by combining a new grain growth theory with grain growth experiments in SrTiO_(3) polycrystalline materials.The results demonstrate that the observed evolution of grain growth behaviors is in accord with the theoretical predictions,which reveals that the abnormal and stagnant behaviors of grain growth may intrinsically occur in polycrystalline systems due to the existence of nonzero step free energy for grain growth.Furthermore,the general growth theory reveals that normal grain growth results from the roughening transition of grain boundaries which corresponding to step free energy equal to zero.Besides the lower GB energy as commonly believed,the narrower grain size distribution is revealed to play an important role on the thermal stability of grains,which may lead to the counter-intuitive phenomenon of smaller nano-sized grains with higher thermal stability as recently reported in the literature.The general,quantitative growth theory may offer an accurate guidance for the microstructural design with optimal physical properties in polycrystalline materials.

A general mechanism of grain growth-Ⅰ.Theory

The behaviors of grain growth dominate the formation of the microstructure inside polycrystalline materials and thus strongly influence their practical performances.However,grain growth behaviors still remain ambiguous and thus lack a mathematical formula to describe the general evolution despite decades of efforts.Here,we propose a new migration model of grain boundary(GB)and further derive a mathematical expression to depict the general evolution of grain growth in the cellular structures.The expression incorporates the variables influencing growth rate(e.g.GB features,grain size and local grain size distribution)and thus reveals how the normal,abnormal and stagnant behaviors of grain growth occur in polycrystalline systems.In addition,our model correlates quantitatively GB roughening transition with grain growth behavior.The general growth theory may provide new insights into the GB thermodynamics and kinetics during the cellular structure evolution.

Microalloyed Steels for Heat Treating Applications at Higher Process Temperatures

The potential is considered for use of microalloyed bar steels,in conjunction with thermomechanical processing,to enhance the properties of steels heat treated at higher process temperatures than have been used historically.Two examples are highlighted:microalloyed spring steels with enhanced resistance to tempering and Nb-modified gear steels for high temperature vacuum carburizing,e.g.on the order of 1050℃ versus 930℃ for a typical gas carburizing operation.In the spring steel example,the Nb+V steel results in significantly finer prior austenite grain sizes than the other steels considered,enhanced fatigue performance,and improved toughness.In the Nb-modified carburizing steel,Nb additions up to 0.1 wt pct to a Ti-modified 8620 steel,in conjunction with thermomechanical processing to control initial precipitate distributions prior to carburizing,are shown to lead to materials with improved resistance to abnormal austenitic grain growth at the higher process temperatures.Alloy content and heating rate to the carburizing temperature were shown to be important variables and suppression of abnormal grain growth was correlated with the development of a critical distribution of fine NbC precipitates,stable at the austenitizing temperature leading to improved fatigue performance in steels with fine and uniform grain structures.Opportunities for extending the results of this study to alloy design and controlled rolling in bar mills are assessed.