A NEW METHOD FOR MEASURING LONG-TERMCREEP DEFORMATION IN A SMALL REGION

Creep deformation localization is generally found in structures at high temperamture,typically in weldments. As the heat affected zone (HAZ) in a weldment is very narrow, deformation in HAZ region can hardly be measured by conventional displacement gauge. A new method for measuring long-term local creep deformation was developed by quartz optical fiber and technique of digital image analysis. The creep deformations of base metal, weld metal and HAZ in weldments are thereby determined with crossweld specimens.

TENSILE CREEP DEFORMATION AND DAMAGE BEHAVIOR IN A NICKEL-BASE SUPERALLOY AT 900℃

The tensile creep deformation and damage evolution in a Ni-base superalloy at 900℃/170MPa were investigated. At the first creep stage, abnormal creep occured due to the resolution of fine particles, and the deformation initiated from grain boundary areas. It is evident that nearly all of the dislocations were in γ matrix channels in form of dislocation pairs and the dislocations were impeded at γ/γ' interfaces, thus the dislocation networks developed deformation. At the steady creep stage, impeded dislocations atγ/γ' interfaces climbed over γ' phases by diffusion-dominant mechanism. At the last creep stage, voids were formed around carbides at grain boundary which leaded to accumulated damage and caused creep rate accelerated. With the dislocation networks being broken, the voids connected and grew into micro-cracks gradually. Finally the cracks propagated along grain boundary area and resulted in failure.

ISOCHRONOUS STRESS-STRAIN CURVES OF LOW ALLOY STEEL CROSS-WELD-SPECIMEN AT HIGH TEMPERATURE

In this work, a parametric approach is presented and utilized to determine the creep properties of weldments; then the model of creep strain for cross weld specimen is given. On the basis of the experimental results, attempt has been made to establish equations of the isochronous stress-strain for weld joint that can predict the function of loading and service time in use of the creep data of base metal and weld metal.

Effects of brachial plexus injury anastomosis simulation on biomechanical properties of adult brachial plexus

BACKGROUND： Previous studies of peripheral nerve mechanical properties in animals have utilized one-dimensional drawing methods. OBJECTIVE： To analyze the effects of brachial plexus injury anastomosis simulation on biomechanical properties of adult brachial plexus by observing tensile mechanical properties, stress relaxation, and creep deformation of the brachial plexus in normal human cadavers and brachial plexus from simulated brachial plexus injury anastomosis samples. DESIGN, TIME AND SETTING： The in vitro experiment was performed at the Mechanics Experimental Center, Jilin University, China from April to May 2007. MATERIALS： A total of six adult, male cadavers, who had died from acute trauma, and were aged 20-29 years, were supplied by the Research Room of Anatomy, Medical Department, Jilin University, China. AG-10TA Universal Material Testing Machine （Shimadzu, Japan） was used in this study. METHODS： A total of 36 samples of fresh brachial plexus were collected from the cadavers, comprising 12 C5 nerve roots, 12 C6 nerve roots at the left and right sides of the superior truck, and 12 C7 nerve roots at the middle truck. The C5 and C6 nerve roots were processed into 50 samples and the C7 nerve roots into 24 samples. A total of 36 C5 and C6 nerve root samples were randomly assigned to a non-surgery control group （n = 18） and brachial plexus injury anastomosis simulation group （n = 18）. Brachial plexus injury simulation anastomosis samples underwent an incision in the middle, and then received anastomosis. Samples in both groups underwent a tension test at 5 mm/min on the AG-10TA universal material testing machine. A total of 24 samples from the C6 superior trunk and C7 middle trunk of the brachial plexus were subjected to stress relaxation and creep tests. Test duration was 7 200 seconds. A total of 100 data points were collected and analyzed using a normalization method. MAIN OUTCOME MEASURES： The following parameters were measured： tension maximum displacement, maximum load, maximum stress, maximum strain and stress-strain curve, stress relaxation at 7 200 seconds, creep deformation at 7 200 seconds, stress relaxation, and creep curve in the non-surgery control group and brachial plexus injury simulation anastomosis group. RESULTS： The tension maximum load of brachial plexus was （140.36 ± 30.50） N, maximum stress was （10.67 ± 2.52） MPa, maximum displacement was （7.78 ± 1.48） mm, and maximum strain was （31.64 ± 5.32）% in the non-surgery control group. The tension maximum load of brachial plexus was （93.23 ± 20.65） N, maximum stress was （7.09 ± 1.57） MPa, maximum displacement was （6.13 ± 0.86） mm,and maximum strain was （24.55 ± 3.45）% in the brachial plexus injury simulation anastomosis group. The above-mentioned indices were greater in the non-surgery control group than in the brachial plexus injury simulation anastomosis group （P 〈 0.01）. Stress relaxation at 7 200 seconds was 2.07 MPa and 2.11 MPa, respectively, in the non-surgery control and brachial plexus injury simulation anastomosis groups. Creep deformation at 7 200 seconds was 4.68% and 3.52%, respectively, in the non-surgery control and brachial plexus injury simulation anastomosis groups. CONCLUSION： Decreased tension maximum load, maximum displacement, maximum stress, maximum strain, and creep deformation at 7 200 seconds affected the biomechanical properties of the brachial plexus following brachial plexus injury.

The slope creep law for a soft rock in an open-pit mine in the Gobi region of Xinjiang, China

The lithology of the strata in the Gobi region of the Xinjiang autonomous region of China is mainly composed of mudstone,silty mudstone,and other soft rocks.Because of the low strength of the rock mass and the serious effects of physical weathering in this area,the slope stability in open-pit mines is poor,and creep deformation and instability can readily occur.Taking the Dananhu No.2 open-pit mine as a typical example,the creep test of a mudstone sample under different stress levels was studied.Then,based on a bottom friction experiment and a FLAC3D numerical simulation,the deformation and failure processes of the slope were analyzed.The stress–displacement curve and the displacement–time curve for the monitoring points were plotted to obtain the relationship between the stress and displacement for the slope of the soft rock.The results showed that the long-term strength of the mudstone was between 8.0 and 8.8 MPa,and that stable creep occurred when the slope was under low stress.The potential failure mode for this type of slope is that the front edge creeps along the weak layer and then a crack is formed at the trailing edge of the slope.When the crack penetrates the weak layer,cutting bedding and bedding sliding occur.The deformation process of the stable creep slope includes an initial deformation stage,an initial creep stage,a constant velocity creep stage and a deceleration creep stage.

LONG-TERM CREEP AND RUPTURE PROPERTIES OF HEATRESISTING STEELS AND ALLOYS

The present status of NRIM Creep Data Sheet Project and the recent activities of long-term creep and rupture studies on heat resisting steels are described. The project has been continued to produce long-term data such as 100 000h-creep rupture strength for 47 kinds of principal heat resisting steels and alloys, including welded joints. The long-term creep deformation behavior and microstructural evolution during creep have been shown to be complicated.

Practice of extending the shell life of Baosteel BOF vessels

With the wide application of the magnesia-graphite refractory having a high-thermal-conductivity in BOF vessels for extending the furnace campaign, furnace shell deformation has become increasingly serious. It shortened the furnace campaign and reduced the production. The paper presents the work on increasing the life-span of the furnace shell at Baosteel, which includes the shell renovation, the study on the creep deformation resisting shell plates and the application of the air cooling method to the BOF shell. The results and limitations of these technologies are discussed.

Dislocation reconfiguration during creep deformation of an Al-Cu-Li alloy via electropulsing

Creep mechanism was well-known to be mainly dominated by the dislocation sliding and climbing during creep deformation. Here we study the creep deformation of an Al-Cu-Li alloy with the assistance of electropulsing and subsequent microstructural observations. We find that creep strain increased drastically under electropulsing and was almost twelve times as much as that of the non-pulsed sample. Microstructural observations confirmed that dislocation reconfiguration happens via electropulsing, namely helical dislocations being opened rapidly. This opened dislocation structure can possess a much higher mobility than the initial helical dislocation, which mostly responsible for the greatly increased creep strain. Our results revealed a new mechanism accountable for the distinctly electroplastic creep deformation.

Creep deformation behavior during densification of ZrB_(2)-SiBCN ceramics with ZrO_(2) additive

ZrB_(2)-SiBCN ceramics with ZrO_(2) additive are hot-pressed under a constant applied pressure.The densification behavior of the composites is studied in a view of creep deformation by means of the Bernard-Granger and Guizard model.With determination of the stress exponent(n)and the apparent activation energy(Q_(d)),the specific deformation mechanisms controlling densification are supposed.Within lower temperature ranges of 1300-1400℃,the operative mechanism is considered to be grain boundary sliding accommodated by atom diffusion of the polymer-derived SiBCN(n=1,Q_(d)=123±5 kJ/mol)and by viscous flow of the amorphous SiBCN(n=2,Qd=249±5 kJ/mol).At higher temperatures,the controlling mechanism transforms to lattice or intra-granular diffusion creep(n=3-5)due to gradual consumption of the amorphous phase.It is suggested that diffusion of oxygen ions inside ZrO_(2) into the amorphous SiBCN decreases the viscosity,modifies the fluidity,and contributes to the grain boundary mobility.

Influence of Temperature on Stacking Fault Energy and Creep Mechanism of a Single Crystal Nickel-based Superalloy

The influence of temperatures on the stacking fault energies and deformation mechanism of a Re- containing single crystal nickel-based superalloy during creep at elevated temperatures was investigated by means of calculating the stacking fault energy of alloy, measuring creep properties and performing contrast analysis of dislocation configuration. The results show that the alloy at 760 ℃ possesses lower stacking fault energy, and the stacking fault of alloy increases with increasing temperature. The defor- mation mechanism of alloy during creep at 760 ℃ is 7＇ phase sheared by 〈110〉 super-dislocations, which may be decomposed to form the configuration of Shockley partials plus super-lattice intrinsic stacking fault, while the deformation mechanism of alloy during creep at 1070 ℃ is the screw or edge super- dislocations shearing into the rafted 7＇ phase. But during creep at 7（50 and 980 ℃, some super- dislocations shearing into 7＇ phase may cross-slip from the {111} to {100} planes to form the K-W locks with non-plane core structure, which may restrain the dislocations slipping to enhance the creep resis- tance of alloy at high temperature. The interaction between the Re and other elements may decrease the diffusion rate of atoms to improve the microstructure stability, which is thought to be the main reason why the K-W locks are to be kept in the Re-containing superalloy during creep at 980 ℃.

Growth Kinetics of Laves Phase and Its Effect on Creep Rupture Behavior in 9Cr Heat Resistant Steel

The effects of Laves phase formation and growth on creep rupture behaviors of P92 steel at 883 K were studied.The microstructural evolution was characterized using scanning electron microscopy and transmission electron microscopy.Kinetic modeling was carried out using the software DICTRA.The results indicated Fe_2（W,Mo）Laves phase has formed during creep with 200 MPa applied stress at 883 Kfor 243 h.The experimental results showed a good agreement with thermodynamic calculations.The plastic deformation of laths is the main reason of creep rupture under the applied stress beyond 160 MPa,whereas,creep voids initiated by coarser Laves phase play an effective role in creep rupture under the applied stress lower than 160 MPa.Laves phase particles with the mean size of 243 nm lead to the change of creep rupture feature.Microstructures at the vicinity of fracture surface,the gage portion and the threaded ends of creep rupture specimens were also observed,indicating that creep tensile stress enhances the coarsening of Laves phase.

What is going on in magnesium alloys？

China has been developed into one of the most active regions in terms of both fundamental and applied research on magnesium （Mg） and its alloys in the world from a solid base laid by its prominent metallurgist and materials scientists over the past decades. Nowadays, a large number of young-generation researchers have been inspired by their predecessors and become the key participants in the fields of Mg alloys, which consequently led to the establishment of China Youth Scholar Society for Magnesium Alloys Research in 2015. Since then, the first two China Youth Scholars Symposiums on Mg Alloys Research had been held at Harbin （2015） and Chongqing （2016） China, respectively. A number of crucial research inter- ests related to fundamental and applied Mg research were discussed at the conferences and summarized in this short perspective, aiming to boost far-reaching initiatives for development of new Mg-based materials to satisfy the requirements for a broad range of industrial employments. Herein, four main aspects are included as follows： i） Plastic deformation mechanism and strengthening strategy, ii） Design and development of new Mg-based materials, iii） Key service properties, and iv） New processing technologies.

Numerical study of creep effect on purging plug performance under cyclic service

The service life of the purging plug is one of the fundamental factors that determine the downtime and usage efficiency of the whole ladle.The creep behaviour of the purging plug was thus investigated to identify the possible failure mechanism.At first,the creep parameters of the Norton–Bailey strain hardening rule were inversely identified via the results of the creep test.Then,the thermal-solid coupling model approach was employed to predict the creep behaviour of the purging plug,in which the Norton–Bailey strain hardening rule was applied.The numerical results show that the temperature of the purging plug presents a cyclic trend after the first service period since the preheating temperature is lower than the temperature of molten steel.Furthermore,the distribution of the creep strain intensity in a layered form could also contribute to a gradual spalling of the purging plug end in service.Besides,the creep strain concentration around the slit can be responsible for the clogging of the purging plug.