The behaviour of α+γ region treated microstructure of Ni9 steel during-196℃ cryogenic impact and tensile deformation has been studied by using both a quasi-dynamic method in which the low temperature(-196℃) impact cracks stopped in the specimen and the tensile deformation halted,and a dynamic method to observe the surface structure under deformation to fracture.The results show that if more(about 10 v.-%) thermally stable γ'-phase occurred in specimens,the excellent cryogenic toughness is resulted in γ'to α'transformation and very dense cavities formed in the impact fracture effected zone(IFEZ);in case of littl γ'-phase,high cryogenic toughness is caused by the high density of hindered tiny cracks in IFEZ:while no γ'-phase,the cryogenic toughness may be poor owing to some sparse and longer cracks only.The tensile deformation ahead of the crack tip makes the lathy packet turn toward the direction being beneficial to crack tip passivation and to toughness.It is shown that the structure of materials with high toughness can adjust oneself to resist the impact load.

The effect of hydrogen on the cracking of type 310 stainless steel was studied by TEM in situ observation of hydrogen-charged specimens and comparison with the hydrogen-free specimens.The results show that 1)in precharged specimens,many dislocations can emit from a crack tip followed by forming a dislocation free zone(DFZ) after equilibrium:2)nanocracks nucleate preferable in DFZs;3)microcracks connect with each other,resulting in the brittle cracks,instead of blunting into voids.The effect of hydrogen on the change from ductile to brittle is to prevent nanocracks in DFZs from blunting into voids.

Hydrogenation of a Ti3Al-based intermetallic(Super-α_2) was carried out to systematically investigate the microstructural variations of this compound.The experimental results showed that evident variations have taken place in microstructures of Super-α2 due to the hydrogenations.Acicular or Widmanstatten α2 was found to form in the hydrogenated microstructure under continuously cooling with slower rate,instead of nearly equiaxed α2 under the normal treatment.The precipitations in the hydrogenated microstructures undergoing ageing treatment showed more regular and homogeneous in morphology and distribution. Furthermore,the formation of O-phase in the microstructure was found to be promoted by the hydrogenation.The essential reason for these microstructural variations of Super-α2 caused by hydrogenation can be explained by the fact that hydrogen,as a strongβ-stabilising element,can significantly lower the α2/βtransition temperature of the compound and thus strongly affect,thermodynamically and dynamically,the transformation process of β→α2(or O-phase).

Based on a fractal model for ductile fracture,a theoretical formula of elongation to failure is established,i.e.,whereδstands for elongation,k for a constant,rs for surface energy,B,l and W for thickness,length and width of the specimen,respectively,ρ for area density of inclusions,d for average grain size and DF for the fractal dimension of the fracture surface.Bothδand DF for precharged 310 austenitic stainless steel decreased with the increase of charging time.Experiment showed that lnδ decreased linearly with the decrease of DF,which is consistent with above-mentioned formula.On the other hand, the formula also shows that the smallar the d,ρ and l,and the larger the W and B, the larger theδ,which conforms to the experimental results.

A fractal model of critical crack extension force is proposed to indicate that the fractal dimension of a fractured surface is incompatible with the elastic fracture mechanics.Consequently,only the mechanical parameters characterizing the dissipation in crack propagation process can be employed in fracture research using the fractal.

Dynamic restoration mechanism of Fe_3Al during hot compression at 900-1300℃ and strain rates of 1-10-3s-1 was studied.As the parameter Z larger or lower than the critic value,the dynamic recrystallization or recovery only may take place respectively.And dynamic recrystallization takes place through subgrain nucleation.The true stress increases with the increase of true strain during hot compression.The jerky flow appears in all true stress-true strain curves and more obvious with the decrease of the strain rate.

Bi/MnBi eutectic alloy was unidirectionally solidified with growth rate varing in a wide range to produce in situ composite.It is found that a new kind of leaf like Bi/MnBi eutectic structure appeared in the origination and even in the whole length of the sample.The morphology,crystallographical relationship and the formation mechanism of this unexpected Bi/MnBi eutectic structure are studied in detail.

The solidification behaviour of Al-33% Cu eutectic alloy under the influnence of an electromagnetic fluid flow has been studied.It is found that with the increase of flow velocity,the Al-Al2Cu eutectic morphologies changes from regular laminae to unregular laminae,unregular rods up to regular rods and the laminar spacingsλ have a little increase.The solute redistribution caused by fluid flow is a main factor for the formation of unregular and other eutectic morphologies.