Rong Lijian

Development of the technique for fabricating submicron moire gratings on metal materials using focused ion beam milling

A focused gallium ion (Ga+) beam is used to fabricate micro/submicron spacing gratings on the surface of porous NiTi shape memory alloy (SMA). The crossing type of gratings with double-frequency (2500 l/mm and 5000 l/mm) using the focused ion beam (FIB) milling are successfully produced in a combination mode or superposition mode. Based on the double-frequency gratings, high-quality scanning electron microscopy (SEM) Moire patterns are obtained to study the micro-scale deformation of porous NiTi SMA. The grating fabrication technique is discussed in detail. The experimental results verify the feasibility of fabricating high frequency grating on metal surface using FIB milling.

HYDROGEN EMBRITTLEMENT RESISTANCE OF AUSTENITIC ALLOYS AND ALUMINIUM ALLOYS

Hydrogen embrittlement is of the technological importance in which the hydrogen in metallic materials can cause the loss in tensile ductility, cracking or damage and degradation of other mechanical properties. The common hydrogen resistant alloys are austenitic stainless steels, precipitation-strengthened austenitic alloys, low alloy steels and aluminum alloys etc.. As austenitic alloys have high hydrogen resistance properties and their strength can be improved by precipitation strengthening, they are commonly used in hydrogen conditions. IMR had developed a series hydrogen resistant steels, such as HR-1, HR-2, HR-3, J75 and J100 as well as Al-6Mg-0.2Sc-0.15Zr hydrogen resistant aluminum alloy. These alloys posses comprehensive mechanical properties and high hydrogen resistant ability. In addition, alloy smelting, casting, forging, welding and heat treatment should be followed the laws of the preparation. It is important to control the microstructures in order to improve the performance of resistant hydrogen.

Internal friction peak associated with dislocations in Mg-0.6%Zr alloy

The internal friction (IF) of Mg-0.6%Zr alloy is measured in the present study and two IF peaks of (P(1) (97 degrees C, 1 Hz) and P(2) (230 degrees C, 1Hz)) are found, respectively. It is shown that the novel P(1) peak is frequency independent anal the peak temperature increased with the increase of strain amplitude or heating rate. P(1) peak could be repressed by the reheating-up measurements and represented by heat treatment. The mechanism of P(1) peak is clarified by optical microscope (OM) and transmission electron microscope (TEM) observation, and considered to be caused by the creation and expansion of dislocations during plastic deformation. The P(2) peak is a thermally activated relaxation IF peak, and activation energy is 1.46 eV, which is caused by the relaxation of the grain boundaries in Mg-0.6%Zr alloy.

SERRATED FLOW IN A FeNi–BASED AUSTENITIC ALLOY

Tensile tests on a FeNi-base austenitic alloy,with different amount of twin boundaries, were conducted at different temperatures and three strain rates,respectively.The results show that serrated flow occurs at temperatures from 300 to 700℃.This serrated flow exhibits bulge-like serrations at temperatures from 300 to 600℃and stress-loss serrations at 700℃,which manifests the nature of thermal activation,i.e.higher temperatures boost serrations and higher strain rates depress them.Investigations on samples deformed at room temperature(no serrated flow) and 400℃(prominent serrated flow) indicate that twin boundaries are strong enough to block slip deformation at 400℃.As a result,stress accumulates on twin boundaries and bulge-like serrations appear on the tensile curves.Effect of twin boundary amount on the morphologies of serrations testified this mechanism.