The microstructures of pearlitic steel wire rods and steel wires are commonly characterized by secondary electron imaging (SEI)technique using scanning electron microscopy(SEM).In this work,a back-scattered electron imaging(BSEI)method is proposed to determine the microstructures of undeformed and deformed pearlitic steels with nanometer scale pearlite lamellae.The results indicate that BSEI technique can characterize the pearlite lamellas veritably and is effective in quantitative measurement of the mean size of pearlite interlamellar spacing.To some extent,BSEI method is more suitable than SEI technique for studying undeformed and not severely deformed pearlitic steels.
The relationship between microstructure and tensile behaviors of fatigued AZ31 magnesium alloy was investigated. Axial fatigue tests were performed on PLG-100 fatigue machine at stresses of 50 and 90 MPa. Tensile samples were cut from the fatigued samples, named as L-sample and H-sample respectively, and the O-sample was cut from original rolled AZ31 alloy. The EBSD and TEM were used to characterize the microstructure. It is found that the twinning-detwinning was the main deformation mechanism in high stress fatigue test, while dislocation slipping was dominant in low stress fatigue test. After fatigue tests, the average grain size of the L-sample and H-sample decreased to 4.71 and 5.33 μm, and the tensile and yield strength of the L-sample and H-sample increased slightly. By analyzing SEM images, the ultimate fracture region of the L-sample consisted of dimples, while there were many microvoids in the ultimate fracture region of the H-sample. Consequently, the tensile behaviors of fatigued magnesium have a close relationship with microstructure.
Yang SHUXi-yan ZHANGJiang-ping YULi TANRui-sen YINQing LIU
The influence of torsion deformation on textures of cold drawing pearlific steel wires was investigated by twisting the wires to different number of revolutions. Macro-texture （over the entire wire cross section） associated with torsion deformation was investigated by X-ray diffraction, while micro-texture （near the wire surface） was characterized by EBSD. The results show that the （110） macro-texture increases at the beginning of torsion and then decreases with increasing of torsion strain, while the （110） micro-texture decreases linearly with increasing of torsion strain. The relationships between the （110） fiber texture and the microhardness of the wires are also discussed.
The fatigue fracture was characterized and the fracture behavior was analyzed,using scanning electron microscope(SEM)and electron back-scattered diffraction(EBSD),the fatigue tests of two strain amplitude at room temperature were 0.5%and 1.0%respectively,and the results showed that the fatigue deformation of different strain amplitude produced two typical fatigue fracture morphology,and when the strain amplitude was 1.0%,fatigue fracture mechanism of AZ31 magnesium alloy induced by{1012}twins,when the strain amplitude is 0.5%,it was induced by{1012}-{1012}double twins.In the present study,the average thickness of primary twin is^20μm at amplitude of 0.5%and^80μm at amplitude of 1.%.The thickness of{1012}primary twins was large enough to activate{1012}-{1012}secondary twins at a high strain amplitude,while the thickness of{1012}primary twins was too narrow to activate{1012}-{1012}secondary twins at a low strain amplitude.
The evolution of morphology of pearlite and crystallographic texture of ferrite matrix in fully pearlitic steels during wire drawing were quantitatively investigated. The study revealed that a fiber structure of the pearlite morphology and a 〈110〉 fi- ber texture of the ferrite matrix begin to take shape and develop gradually with increasing strain. The growth rates of the fiber structure and the 〈110〉 texture are different in different regions within the wires with increasing drawing strain. There is a close relationship between the pearlite morphology and the crystalline texture during wire drawing. The pearlite interlamellar spacing （ILS） and thickness of cementite lamellae （To） decrease gradually both in longitudinal and transverse sections. The definition of pearlite colony should be reconsidered for describing microstructure of the wire drawing deformed pearlitic steels.
In order to investigate the microstructure and properties evolution of the AZ31magnesium alloy in different cycles(including104(sample1),106(sample2)and none(sample3,no fatigue tests on the samples))of high cycle deformation,thescanning electron microscopy(SEM)and electron back-scattered diffraction technique(EBSD)were fully used.It is found that many{1012}tension twins(near21%,volume fraction),a few{1011}compression twins and{1011}?{1012}secondary twins aregenerated during tension?compression cyclic deformation.The volume fractions of{1012}twins in samples1,2and3are8%,21%and4%,respectively.By analyzing the fatigue fracture of the samples,larger area of fatigue striations and cleavage fracture insamples are also observed,as well as the shallower dimples.The twin laminae are likely to occur in the crack initiation zone whiledimples are in the final fracture region and the dimple size increases with the increase of the cycle number.The dimple diameters ofsamples1,2and3are14.8,31.2and8.5μm,respectively.It turns out that both the fatigue strength and elongation of the samplesslightly decrease with increasing the cycle number.
The microstructures of the as-rolled magnesium alloy subjected to dynamic plastic deformation along the rolling direction have been investigated. Mostly one { 101-2} twin variant or a twin variant pair is activated in a grain, leading to a parallel {101-2} twin lamellar structure. At the stage of twinning-dominated deformation （ε 〈 -8%）, lamellar thickness decreases significantly with strain, from 5.55 to 2.49 pro. The evolution of lamellar thickness during deformation is directly related to {1012} twin activity. When plastic strain is greater than - 8%, the twin lamellar structure disappears because the volume fraction of twins almost saturates at a value of -90%.
Effect of {10-12} twins on the mechanical properties of magnesium alloy has received considerable research interest. A hot-rolled AZ31 Mg alloy sheet was subjected to dynamic plastic deformation with the aim of introducing {10-12} twin lamellar structure. It has been found that higher strength and better ductility are obtained when tensile loading is perpendicular to the c axis of twin region of the twin lamellar structured sample, indicating that the plasticity improvement caused by twins depends on the special strain path. The fracture morphology of the twin lamellar structured sample shows a dimple fracture mode under tensile loading perpendicular to the c axis, while the cleavage fracture with river pattern has been observed in other fractured samples. Above experimental results indicate that the interaction of dislocations and twin lamellae may play an important role in improving mechanical properties of Mg alloy.
