A new preparation technology of structural- gradient materials without a brittle transition zone in a Mn-Si-Cr-B bainitic cast steel

In the present work, a unique gradient cooling heat treatment process(GCHT) for a Mn-Si-Cr-B bainitic cast steel was developed, and microstructure and mechanical properties were examined by OM, SEM, EBSD and a uniaxial tensile test. The results showed that the structural-gradient-material(SGM) with a gradient microstructure from granular bainite to martensite was successfully produced, and it exhibited a good ductility(~13.8%) at one end and an excellent ultimate strength(~1,720 MPa) at the other end. In between the bainite and martensite, a transition region with a superior combination of tensile strength and ductility(1,700 MPa and 11.1%) was obtained, which is different from the normal knowledge of a brittle transition region. Moreover, through changing the gradient of cooling rate, the optimized SGM with a new gradient microstructure from pearlite to martensite showed a more stable structural gradient and an improved ductility(22.8%) at one end. The microstructure variation in the sample was mainly related to the carbon diffusion rate during heat treatment, and the diffusion rate could be controlled by regulating the cooling velocity. Therefore, the SGMs with different gradient microstructures could be designed to meet the needs of different properties. As a result, this work provides a new approach for preparation of the gradient structured steel, which has potential for practical application for dual-property automobile parts.

Austenite Grain Refinement by Reverse α′→γ Transformation in Metastable Austenitic Manganese Steel

Microstructure of metastable austenitic manganese steel after reverse transformation treatment was investi gated using optical microscopy, X ray diffraction （XRD）, electrical resistivity and hardness testing. Austenite grain refinement was successfully achieved by a two-step heat treatment. First, martensite was produced by cooling the so- lution-treated samples to --196 ℃. Then, the deep cryogenic treated samples were heated to 850 ℃ upon slow or rapid heating. The mean size of original austenite grain was about 400 fire. But the mean size of equiaxed reversion austenite was refined to 50 μm. Microstructure evolution and electrical resistivity change showed that martensite plates underwent tempering action upon slow heating, and the residual austenite was decomposed, resulting in the formation of pearlite nodules at the austenite grains boundaries. The refinement mechanism upon slow heating is the diffusion-controlled nucleation and growth of austenite. However, the reverse transformation upon rapid heating was predominated by displacive manner. The residual austenite was not decomposed. The plate α-phase was carbon-super- saturated until the starting of reverse transformation. The reverse transformation was accompanied by surface effect, resulting in the formation of plate austenite with high density dislocations. The refinement mechanism upon rapid heating is the recrystallization of displacive reversed austenite.

Microstructure Refinement and Property Improvement of Metastable Austenitic Manganese Steel Induced by Electropulsing

Grain refinement efficiency of electropulsing treatment （EPT） for metastable austenitic manganese steel was investigated. The mean grain size of original austenite is 300 ptm. However, after EPT, the microstructure ex hibits a bimodal grain size distribution, and nearly 70vol. % grains are less than 60 /Lm. The refined austenite results in ultrafine martensitic microstrncture. The tensile strengths of refined austenitic and martensitic microstructures were improved from 495 to 670, and 794 to 900 MPa respectively. The fine grained materials possess better fracture toughness. The work hardening capacity and wear resistance of the refined austenitic microstructure are improved. The reasonable mechanism of grain refinement is the combination of accelerating new phase nucleation and restraining the growth of neonatal austenitic grain during reverse transformation and rapid recrystallization induced by electropulsing.

Correlation of circulating tumor DNA EGFR mutation levels with clinical outcomes in patients with advanced lung adenocarcinoma

Background:Circulating tumor DNA(ctDNA)is a promising biomarker for non-invasive epidermal growth factor receptor mutations(EGFRm)detection in lung cancer patients,but existing methods have limitations in sensitivity and availability.In this study,we used theΔCt value(mutant cycle threshold[Ct]value-internal control Ct value)generated during the polymerase chain reaction(PCR)assay to convert super-amplification-refractory mutation system(superARMS)from a qualitative method to a semi-quantitative method named reformed-superARMS(R-superARMS),and evaluated its performance in detectingEGFRm in plasma ctDNA in patients with advanced lung adenocarcinoma.Methods:A total of 41 pairs of tissues and plasma samples were obtained from lung adenocarcinoma patients who had knownEGFRm in tumor tissue and were previously untreated.EGFRm in ctDNA was identified by using superARMS.Through making use ofΔCt value generated during the detection process of superARMS,we indirectly transform this qualitative detection method into a semi-quantitative PCR detection method,named R-superARMS.Both qualitative and quantitative analyses of the data were performed.Kaplan-Meier analysis was performed to estimate the progression-free survival(PFS)and overall survival(OS).Fisher exact test was used for categorical variables.Results:The concordance rate ofEGFRm in tumor tissues and matched plasma samples was 68.3%(28/41).At baseline,EGFRm-positive patients were divided into two groups according to the cut-offΔCt value ofEGFRm set at 8.11.A significant difference in the median OS(mOS)between the two groups was observed(EGFRmΔCt≤8.11vs.>8.11:not reachedvs.11.0 months;log-rankP=0.024).Patients were divided into mutation clearance(MC)group and mutation incomplete clearance(MIC)group according to whether theΔCt value ofEGFRm test turned negative after 1 month of treatment.We found that there was also a significant difference in mOS(not reachedvs.10.4 months;log-rankP=0.021)between MC group and MIC group.Although there was no significant difference in PFS between the two groups,the two curves were separated and the PFS of MC group tended to be higher than the MIC group(not reachedvs.27.5 months;log-rankP=0.088).Furthermore,EGFRm-positive patients were divided into two groups according to the cut-off of the changes inΔCt value ofEGFRm after 1 month of treatment,which was set at 4.89.A significant difference in the mOS between the two groups was observed(change value ofΔCt>4.89vs.≤4.89:not reachedvs.11.0 months;log-rankP=0.014).Conclusions:DetectingEGFRm in ctDNA using R-superARMS can identify patients who are more likely sensitive to targeted therapy,reflect the molecular load of patients,and predict the therapeutic efficacy and clinical outcomes of patients.