X RAY DIFFRACTION STUDY OF THE THERMALLY INDUCED γ ε MARTENSITIC TRANSFOR MATIONSINFe31 6Mn6 5Si ALLOY

Thethermal induced γ εmartensitic transformation in a Fe 31 6 Mn 6 5 Si ( wt% ) alloy wasstudied by meansof X ray diffraction throughtwothermalcyclesin the83 480 Ktem perature range.Itisshownthatduringcoolingtheγ→εtransformationterminatesnotat TγNbutatalowertemperatureandtheγphaseexhibitsstrongstabilityevencoolingto83 K;like wise,εmartensiteis noticeably stable even heating up to 480 K. Atthe temperature about 275 K, an anomalousdecreaseintheintensityofthe(200)γreflections wasobserved on heat ing, whereasthe(101)εreflectionsshow ageneralvariationinintensity.Itisconsideredthat theabnormalphenomenonis probably related tothe magnetictransition of the γphase fromantiferromagnetism to paramagnetism .

High Strength-Ductility Nb-microalloyed Low Martensitic Carbon Steel:Novel Process and Mechanism

A low-carbon Nb-microalloyed Fe-Mn-Si-based steel was treated by a novel quenching-partitioning-tempering（Q-P-T） process as a modified quenching and partitioning（Q＆P） process. After processed by Q-P-T treatment,this steel exhibits excellent mechanical properties such as high product of strength and elongation. The addition of Nb markedly raises both the yield strength and tensile strength of Q-P-T martensitic steel, especially the yield strength, which can be attributed to the strong grain refinement strengthening and precipitation strengthening of Nb. The Nb addition can also lead to a little increase in ductility. The Nb-microalloyed steel treated by Q-P-T process displays much higher ductility than that treated by traditional quenching and tempering（Q＆T） process. The mechanisms of Q-P-T process on ductility enhancement were fully analyzed and can be attributed to high quenching temperature and considerable amount of retained austenite.

Design and fabrication of a low modulus β-type Ti-Nb-Zr alloy by controlling martensitic transformation

In this paper, high density of dislocations, grain boundaries and nanometer-scale α precipitates were intro- duced to a metastable Ti-36Nb-5Zr alloy （wt%） through a thermo-mechanical approach including severe cold rolling and short-time annealing treatment. The martensitic trans- formation was retarded, and the β phase with low content of β stabilizers was retained at room temperature after the thermo-mechanical treatment. As a result, both low mod- ulus （57 GPa） and high strength （950 MPa） are obtained. The results indicate that it is a feasible strategy to control martensitic transformation start temperature through microstructure optimization instead of composition design, with the aim of fabricating low modulus β-type Ti alloy.

Shape memory behavior of Ti–20Zr–10Nb–5Al alloy subjected to annealing treatment

The effects of annealing temperature on microstructures, phase transformation, mechanical properties, and shape memory effect of Ti-20Zr-10Nb-5Al alloy were investigated. X-ray diffraction（XRD） patterns show that the alloy is composed of single hexagonal ɑ＇-martensite phase for both as-rolled sample and sample annealed at773 K for 30 min, while single orthorhombic ɑ＇＇ phase exists in the samples annealed at 873 and 973 K for30 min. The optical observations indicate that the alloy is recrystallized when annealed at 873 K, and the grain size of the sample annealed at 973 K is about five times larger than that annealed at 873 K. Both of the samples annealed at 873 and 973 K show almost the same reverse martensite transformation start temperature of 483 K as demonstrated by thermal dilatation tests. The critical stress values for martensite reorientation（σ_M） are 392 and 438 MPa for the alloys annealed at 873 and 973 K, respectively. The maximum shape memory strain is 2.8 %, which is obtained in the alloy annealed at 873 K due to the lower σ_M. Moreover,the sample annealed at 873 K exhibits larger tensile stress and tensile strain due to the smaller grain size.