Kink-band formation in directionally solidified Mg/Mg_(2)Yb and Mg/Mg_(2)Ca eutectic alloys with Mg/Laves-phase lamellar microstructure

For the development of high-strength Mg alloys,active use of Laves phases such as C14-type Mg_(2)Yb and Mg_(2)Ca is strongly expected.However,the brittleness of the Laves phases is the biggest obstacle to it.We first found that kink-band formation can be induced in directionally solidified Mg/Mg_(2)Yb and Mg/Mg_(2)Ca eutectic lamellar alloys when a stress is applied parallel to the lamellar interface,leading to a high yield stress accompanied with ductility.That is,microstructural control can induce a new deformation mode that is not activated in the constituent phases,thereby inducing ductility.It was clarified that the geometric relationship between the operative slip plane in the constituent phases and the lamellar interface,and the microstructural features that provide kink-band nucleation sites are important factors for controlling kink-band formation.The obtained results show a possibility to open the new door for the development of novel high-strength structural material using the kink bands.

Effect of carbon addition on creep behavior of cast TiAl alloy with hard-oriented directional lamellar microstructure

Two TiAl alloys,Ti-47.5Al-3.7(Cr,V,Zr)and Ti-47.5Al-3.7(Cr,V,Zr)-0.1C(at.%),were prepared by cold crucible levitation melting to couple the hard-oriented directional lamellar microstructure with carbon microalloying strengthening.The creep behavior and mechanism for the improvement in creep properties by carbon addition were investigated by mechanical tests and electron microscopy characterizations.The results show that obvious improvements on the creep properties at 760°C and 276 MPa are achieved by 0.1 at.%C addition into TiAl alloy with directional lamellar microstructure,which promotes the creep strain and minimum creep rate decreasing with a large content.The minimum creep rate is reduced from 4.37×10^(-8) to 3.97×10^(-9) s^(-1),and the duration entering into creep acceleration is prolonged for more than 10 times.The mechanism for creep property improvement by 0.1%C addition is attributed to two aspects.The first one is that Ti_(2) AlC is found to be strong obstacles of 1/2[110]dislocations when moving across the lamellar interface in the carbon containing alloy.The other one is that the in terfacial dislocatio ns are effectively impeded and the release process is hindered by dynamic precipitation of Ti_(3) AlC,which is proposed to be the special mechanism for creep resistance improvement of this hard-oriented directional lamellar microstructure.

Microstructure and Mechanical Properties of Two-phaseTiAl Alloys in Lamellar Form

In two-phase TiAl alloys, the lamellar structures are of special interest and importance since they are so common and persistent. not only under as-cast conditions but also after thermal treatment. However. the lamellar structures are still poor in ductility,although they are beneficial for toughness and high temperature strength. This article will review the recent progress made in understanding the basic mechanical properties of the γ and α2 phases which comprise the two-phase alloys in Iamellar form, and discuss how an improved balance of strength and ductillty in the lamellar form may be achieved

Lamellar Disintegration Mechanism of TC11 Titanium Alloy during Hot Compression

TC11 titanium alloy samples with lamellar microstructrue were compressed on a Gleeble 1500D Simulator.Compression tests were carried out at 950 ℃ and a strain rate of 0.1 s-1 with height reduction of 20%,40% and 60%,respectively.Microstruture of the compressed TC11 alloy was obeserved and analyzed by optical microscopy(OM),transmission electron microscope(TEM),electron back-scattered diffraction(EBSD).The lamellar disintegration mechanism of the TC11 titanium alloy was deduced.The results indicated that the compressive deformation promoted the phase transformation in bi-phase area.βphase layers were formed along the gliding planes inα phase,and α slivers were disintegrated into many small flakes through theα/βinterface slipping.

Superior strength-plasticity synergy in a heterogeneous lamellar Ti_(2)AlC/TiAl composite with unique interfacial microstructure

Improving the plasticity of TiAl alloys at room temperature has been a longstanding challenge for the de-velopment of next-generation aerospace engines.By adopting the nacre-like architecture design strategy,we have obtained a novel heterogeneous lamellar Ti_(2)AlC/TiAl composite with superior strength-plasticity synergy,i.e.,compressive strength of∼2065 MPa and fracture strain of∼27%.A combination of micropil-lar compression and large-scale atomistic simulation has revealed that the superior strength-plasticity synergy is attributed to the collaboration of Ti_(2)AlC reinforcement,lamellar architecture and heteroge-neous interface.More specifically,multiple deformation modes in Ti_(2)AlC,i.e.,basal-plane dislocations,atomic-scale ripples and kink bands,could be activated during the compression,thus promoting the plas-tic deformation capability of composite.Meanwhile,the lamellar architecture could not only induce sig-nificant stress redistribution and crack deflection between Ti_(2)AlC and TiAl,but also generate high-density SFs and DTs interactions in TiAl,leading to an improved strength and strain hardening ability.In addi-tion,profuse unique Ti_(2)AlC(1¯10¯3)/TiAl(111)interfaces in the composite could dramatically contribute to the strength and plasticity due to the interface-mediated dislocation nucleation and obstruction mecha-nisms.These findings offer a promising paradigm for tailoring microstructure of TiAl matrix composites with extraordinary strength and plasticity at ambient temperature.

Effect of electropulsing on anisotropy behaviour of cold-rolled commercially pure titanium sheet

The specimens cut from the cold-rolled pure titanium sheet at 0°,45°and 90°to the rolling direction were treated by high density electropulsing(maximum current density J=(7.22-7.96)×10^(3)A/mm^(2),pulse period t_(p)=110μs).The mechanical properties and microstructures of the cold-rolled,electropulsed and conventional annealed commercially pure titanium sheet were examined by using uniaxial tension test machine and optical microscope(OM),respectively.The results show that the deformation behavior of the electropulsed pure titanium sheet is significantly different from that of conventional annealed pure titanium sheet.The difference of the mechanical properties between the 0°,45°and 90°direction specimens is almost diminished.It is mainly due to the increase in dislocation mobility and formation of lamellar microstructure after the electropulsing.

CREEP BEHAVIOR OF γ-TiAl POLYSYNTHETICALLY TWINNED CRYSTALS

Creep behavior of γ-TiAl polysynthetically twinned (PST) crystals has been investigated at temperature between 700 and 800℃. The results show that the creep behavior of the crystals is strongly dependent on the angle between the lamellar boundaries and loading axis. The samples with the angle φ=45 deg. have the lowest creep resistance, while the samples with φ=0 deg., rather than φ=90 deg.,show the strongest creep resistance. The stress exponent and creep activation energy for the power law creep vary with the orientations of samples, which indicates that the creep mechanisms of the samples with different angle φ are quite different.The deformation substructure has been examined by transmission electron microscopy (TEM),which shows that both gliding, perhaps also climbing, of dislocations and twinning contribute to the creep deformation with some particular observation in the samples with φ=90 deg. in which rotation of the γ plates across a true twin boundary was observed, which indicates the deformation mechanism of the samples is different from the samples in other orientations.

A COMPONENT-SPECIFIC ALLOY DESIGN OF TiAl INTERMETALLICS

The metallographic observation and analyses of TiAl alloy cast ingots revealed that the preferably arranged γ/α_2 lamellar microstructure can be obtained in columnar dendritic cast ingot through controlling the Ti/Al atomic ratio. The experiments conf irmed that the preferably arranged γ/α_2 lamellar microstructure has excellent tensile strength and fracture toughness and tolerant tensile plasticity when the stress is applied parallel to the γ/α_2 interface.Based on these results and the working condition of the turbine blades,a component-specific alloy design has been suggested.