Kinetics and mechanism of titanium hydride powder and aluminum melt reaction

Based on the measurement of the released hydrogen gas pressure （PH2）, the reaction kinetics between TiH2 powder and pure aluminum melt was studied at various temperatures. After cooling the samples, the interface of TiH2 powder and aluminum melt was studied. The results show that the-time curves have three regions; in the first and second regions, the rate of reaction conforms zero and one order, respectively; in the third region, the hydrogen gas pressure remains constant and the rate of reaction reaches zero. The main factors that control the rate of reaction in the first and second regions are the penetration of hydrogen atoms in the titanium lattice and the chemical reaction between molten aluminum and titanium, respectively. According to the main factors that control the rate of reaction, three temperature ranges are considered for the reaction mechanism： （a） 700-750°C, （b） 750-800°C, and （c） 800-1000°C. In the first temperature range, the reaction is mostly under the control of chemical reaction; at the temperature range of 750 to 800°C, the reaction is controlled by the diffusion and chemical reaction; at the third temperature range （800-1000°C）, the dominant controlling mechanism is diffusion.

Thermodynamic Calculation on the Formation of Titanium Hydride

A modified Miedema model, using interrelationship among the basic properties of elements Ti and H, is employed to calculate the standard enthalpy of formation of titanium hydride TiHx （1≤x≤2）. Based on Debye theories of solid thermal capacity, the vibrational entropy, as well as electronic entropy, is acquired by quantum mechanics and statistic thermodynamics methods, and a new approach is presented to calculate the standard entropy of formation of TiH2. The values of standard enthalpy of formation of TiHx decrease linearly with increase of x. The calculated results of standard enthalpy, entropy, and free energy of formation of TiH2 at 298.16 K are -142.39 kJ/mol, -143.0 J/（mol·K） and -99.75 kJ/mol, respectively, which is consistent with the previously-reported data obtained by either experimental or theoretical calculation methods. The results show that the thermodynamic model for titanium hydride is reasonable.

Ballistic performance of titanium-based layered composites made using blended elemental powder metallurgy and hot isostatic pressing

Metal matrix composites tiles based on Ti-6Al-4V(Ti64)alloy,reinforced with 10,20,and 40(vol%)of either TiC or TiB particles were made using press-and-sinter blended elemental powder metallurgy(BEPM)and then bonded together into 3-layer laminated plates using hot isostatic pressing(HIP).The laminates were ballistically tested and demonstrated superior performance.The microstructure and properties of the laminates were analyzed to determine the effect of the BEPM and HIP processing on the ballistic properties of the layered plates.The effect of porosity in sintered composites on further diffusion bonding of the plates during HIP is analyzed to understand the bonding features at the interfaces between different adjacent layers in the laminate.Exceptional ballistic performance of fabricated structures was explained by a significant reduction in the residual porosity of the BEPM products by their additional processing using HIP,which provides an unprecedented increase in the hardness of the layered composites.It is argued that the combination of the used two technologies,BEPM and HIP is principally complimentary for the materials in question with the abilities to solve the essential problems of each used individually.

Thermal decomposition kinetics of titanium hydride and Al alloy melt foaming process

A temperature programmed decomposition (TPD) apparatus with metal tube structure, in which Ar is used as the carrier gas, is established and the TPD spectrum of titanium hydride is acquired. Using consulting table method (CTM), spectrum superposition method (SSM) and differential spectrum technique, TPD spectrum of titanium hydride is separated and a set of thermal decomposition kinetics equations are acquired. According to these equations, the relationship between decomposition quantity and time for titanium hydride at the temperature of 940 K is obtained and the result well coincides with the AI alloy melt foaming process.

Gelcasting of titanium hydride to fabricate low-cost titanium

In this work, low-cost titanium was fabricated by gelcasting of titanium hydride powder. The effects of morphology and grain composition of powder raw material and solid loading on the rheological behavior of gelcasting slurry were studied. The degreasing, dehydriding and sintering behaviors of gelcasted green body were investigated by differential thermal analysis （DTA） and dilatometer. The results show that the solid loading of titanium hydride slurry reaches 50 vol%. Combination of dehydriding and sintering in one process accelerates the densification, and the relative sintered density of the final part achieves 96.5 %. In order to test the ability of gelcasting process for fabricating structural materials, a resin handle produced by 3D printing technology was used as a model and a titanium handle was successfully fabricated. Higher solid loading and better sinterability of titanium hydride powder promote manufacture of bulk titanium with high relative density, complex shape and well-defined microstructure.

Effect of Decomposition Kinetics of Titanium Hydride on the Al Alloy Melt Foaming Process

The gas released from the titanium hydride decomposition is one of the key factors to influence the Al alloy melt foaming process.In this study,a set of decomposition kinetic equations of titanium hydride was acquired by separating its temperature programmed decomposition(TPD) spectrum,which was acquired by a special designed TPD apparatus with argon used as carrier gas and thermal conductivity cell as the detector.According to these equations,the decomposition and hydrogen release characteristics of titanium hydride at a fixed/elevated temperature are described quantitatively,which can be applied to forecast the Al alloy melt foaming process and furnish the theoretical basis for fabrication of three-dimensional complex shaped Al alloy foam.

Decomposition behavior of titanium hydride treated by surface oxidation

Titanium hydride attracts more attention as foaming agent in the fabrication of cellular metal materials. In order to meet most aluminum casting alloy＇s melting properties, the heat treatment processes for Till2 particles were investigated in a rotating device. In the present work, the most two important dynamic parameters, the treating temperature and oxidation interval, were taken under consideration. The decomposition behavior of titanium hydride was measured by differential scanning calorimetry （DSC） and the residual hydrogen content, morphologies and phase conversion were also characterized by hydrogen determinator, scanning electron microscopy （SEM）, and X-ray diffractometer （XRD）, respectively. The results show that the effect of temperature on the formation of oxidation film and decomposition behavior of Till2 is more significant than that of oxidation time. The onset temper- ature and peak value of Till2 decomposition shift from left to right through elevating temperature and extending time. Heat treatment process for Till2 at 500 ℃ between 1 and 5 h in air is favorable for preparing aluminum foam.

A Quantitative Study on the Effect of Heat Treatment on the Microstructure and Orientation of Titanium Hydride in Electrochemically Hydrogenated Pure Titanium

The electrochemical hydrogen charging of pure titanium and its alloys has been investigated previously,while how a subsequent annealing treatment aff ects the type of hydride and its orientation relationship with matrix is not clear.In the present study,a quantitative study on the microstructure and orientation of titanium hydrides during electrochemical hydrogen charging and subsequent annealing treatment was carried out using scanning electron microscopy,transmission electron microscopy and electron backscatter diff raction.The results show thatδ-hydride is the main in both the electrochemically hydrogenated sample and the subsequent annealing treated sample.After electrochemical hydrogen charging for 48 h,the surface is mainly composed of denseδ-hydride with a thickness of approximately 42μm,the orientation relationship betweenα-matrix andδ-hydride follows only the orientation relationship of OR2,{0001}_(α)//{111}_(δ),<1210>_(α)//<110>_(δ) and an interface plane{1013}_(a)//{110}_(δ).Besides OR2,a part of hydrides show an orientation relationship of OR1 with the matrix after annealing,{0001}a//{001}_(δ),<1210>_(a)//{110}_(δ) and an interface plane of {1010}_(δ)//{110}_(δ).It is further found that the relative frequency of OR1and OR2 is closely related to annealing duration.Under an argon atmosphere at 450℃,the frequencies of OR1 and OR2 are nearly balance with an annealing time of 12 h,while OR1 becomes to be the predominant one with a relative frequency of 96.5%after annealing for 96 h.The mechanism for the evolution of orientation relationship of hydrides with annealing time was discussed.

STRUCTURE OF HYDRIDES IN HIGHLY HYDROGENATED Ti-6Al-4V ALLOY

The microstrueture and various hydrides precipitated in Ti-6A1-4V alloys containing hydrogen 0.16,0.58,0.87,1.49 wt-%,respectively,have been studied by means of TEM and X-ray diffraction.The Ti_3Al phase may precipitate when H over 0.58 wt-%.In the same time,the morphology of hydrides gradually changed from rugged sheets to narrow laths as H contents increased.The microstructure of highly H-doped alloys is obviously fine.A mas- sive hydride and the hydride with tetragonal lattice were observed in the specimen containing 1.49 wt-%H.The twin hydrides were found in the alloys with different H contents and the electron diffraction patterns of the twin hydrides can be served as a simple criterion for distin- guishing the cubic and tetragonal structures.

Developing nacre-inspired laminate-reticular 2024Al/B_(4)C composites with high damage resistance by adjusting compositional wettability

To address the issue that B_(4)C ceramics are difficult to be wetted by aluminum metals in the composites,TiB_(2)was introduced via an in-situ reaction between TiH_(2)and B_(4)C to regulate their wettability and interfacial bonding.By pressure infiltration of the molten alloy into the freeze-cast porous ceramic skeleton,the 2024Al/B_(4)C-TiB_(2)composites with a laminate-reticular hierarchical structure were produced.Compared with 2024Al/B_(4)C composite,adding initial TiH_(2)improved the flexural strength and valid fracture toughness from(484±27)to(665±30)MPa and(19.3±1.5)to(32.7±1.8)MPa·m^(1/2),respectively.This exceptional damage resistance ability was derived from multiple extrinsic toughening mechanisms including uncracked-ligament bridging,crack branching,crack propagation and crack blunting,and more importantly,the fracture model transition from single to multiple crack propagation.This strategy opens a pathway for improving the wettability and interfacial bonding of Al/B_(4)C composites,and thus produces nacre-inspired materials with optimized damage tolerance.

Formation mechanism of hydride precipitation in commercially pure titanium

Since titanium has high affinity for hydrogen and reacts reversibly with hydrogen,the precipitation of titanium hydrides in titanium and its alloys cannot be ignored.Two most common hydride precipitates in α-Ti matrix areγ-hydride and δ-hydride,however their mechanisms for precipitation are still unclear.In the present study,we find that both γ-hydride and δ-hydride phases with different specific orientations were randomly precipitated in the as-received hot forged commercially pure Ti.In addition,a large amount of the titanium hydrides can be introduced into Ti matrix with selective precipitation by using electrochemical treatment.Cs-corrected scanning transmission electron microscopy is used to study the precipitation mechanisms of the two hydrides.It is revealed that the γ-hydride and δ-hydride precipitations are both formed through slip+shuffle mechanisms involving a unit of two layers of titanium atoms,but the difference is that the γ-hydride is formed by prismatic slip corresponding to hydrogen occupying the octahedral sites of α-Ti,while the δ-hydride is formed by basal slip corresponding to hydrogen occupying the tetrahedral sites ofα-Ti.

Preparation of spherical tungsten and titanium powders by RF induction plasma processing

Spherical tungsten and titanium powders were prepared by radio frequency inductively coupled plasma torch with irregular shaped tungsten and titanium hydride （TiH2） powders, respectively. The effect of the feed rate on spheroidization efficiency was investigated. The phase composition, oxygen content, morphology, and particle size distribution of the powders before and after spheroidization were characterized by X-ray diffraction （XRD）, nitrogen/oxygen analyzer, scanning electron microscopy （SEM）, and laser micron sizer （LMS）. The results show that both kinds of plasma-processed powders have good dispersity, smooth surface, and single phase. A maximum of 100 % of spheroidization efficiency can be achieved at a lower feed rate. The spherical titanium powder obtained by the plasma treatment consists of particles with mean diameter of 33.34 gin, while the mean diameter of ori- ginal Till2 powder is 136.56 μm. The apparent density and flowability of the spherical tungsten powder are 6.3 g.cm-3 and 0.16 s.g-1, respectively. The apparent density and flowability of the spherical titanium powder are 2.8 g.cm-3 and 0.52 s.g-1, respectively. With the increase of the feed rate, the sphemidization efficiency of raw powders drops gradually.

Preparation of TiAl alloy powder by high-energy ball milling and diffusion reaction at low temperature

In this paper, TiAl alloy powders were prepared successfully by high-energy ball milling and diffusion reaction in vacuum at low temperature. The titanium powder, aluminum powder, and titanium hydride powder were used as raw materials. The samples were characterized by scanning electron microscopy（SEM）, X-ray diffraction（XRD）, field-emission scanning electron microscopy（FESEM）, and differential thermal analysis（DTA）. The results show that the alloy powders with the main intermetallic compounds of TiAl are obtained using Ti-Al powders and TiH2-Al powders after heated for 2 h at 500 ℃,3 h at 600 ℃,and 3 h at 750 ℃,respectively.The average grain sizes of alloy powder are about 45 and20 μm with irregular shape, respectively. The prepared TiAl alloy powders are relatively pure, and the average quality content of oxygen in the alloy powders is0.33 wt%. The forming process of alloy powder contains both the diffusion reaction of Ti and Al,which gives priority to the diffusion reaction of aluminum.